OBJECTIVE
To understand the basic concepts of different types of electrical machines and their performance. To study the different methods of starting D.C motors and induction motors.
To study the conventional and solid-state drives.
DRIVE MOTOR CHARACTERISTICS 9
Mechanical characteristics – Speed-Torque characteristics of various types of load and drive motors – Braking of Electrical motors – DC motors: Shunt, series and compound - single phase and three phase induction motors.
STARTING METHODS 8
Types of D.C Motor starters – Typical control circuits for shunt and series motors – Three phase squirrel cage and slip ring induction motors.
CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES 10
Speed control of DC series and shunt motors – Armature and field control, Ward-Leonard control system - Using controlled rectifiers and DC choppers –applications.
CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVES 10
Speed control of three phase induction motor – Voltage control, voltage / frequency control, slip power recovery scheme – Using inverters and AC voltage regulators – applications.
UNIT-I
INTRODUCTION TO ELECTRICAL DRIVES
Drives are employed for systems that require motion control – e.g. transportation system, fans, Robots, pumps, machine tools, etc. Prime movers are required in drive systems to provide the Sources: di esel engines, petrol engines, hydraulic motors, electric motors etc.
movement or motion and energy that is used to provide the motion
can come from various Drives that use electric motors as the prime movers are known as electrical drives
There are several advantages of electrical drives:
a. Flexible control characteristic – This is particularly true when power electronic Converters are employed where the dynamic and steady state characteristics of the motor can be controlled by controlling the applied voltage or current.
b. Available in wide range of speed, torque and power
c. High efficiency, lower noi se, low maintenance requirements and cleaner operation d. Electric energy is easy to be transported.
A typical conventional electric drive system for variable speed application employing multi-machine system is shown in Figure 1. The systems are obviously bulky, expensive, and inflexible and require regular maintenance. In the past, induction and synchronous machines were used for constant speed applications – this was mainly because of the unavailability of variable frequency supply.
With the advancement of power electronics, microprocessors and digital electronics, typical electric drive systems nowadays are becoming more compact, efficient, cheaper and versatile – this is shown in Figure 2. The voltage and current applied to the motor can be changed at will by employing power electronic converters. AC motor is no longer limited to application where Only AC source is available, however, it can also be used when the power source available is DC or vice versa
An electric drive is multi-disciplinary field. Various research areas can be sub-divided from electric drives as shown in Figure 3.
COM PONENTS OF ELECTRICAL DRIVES
The main components of a modern electrical drive are the motors, power processor, control unit and electrical source. These are briefly discussed below
a) Motors
Motors obtain power from electrical sources. They convert energy from electrical to mechanical - therefore can be regarded as energy converters. In braking mode, the flow of power is reversed. Depending upon the type of power converters used, it is also possible for the power to be fed back to the sources rather than dissipated as heat
There are several types of motors used in electric drives – choice of type used depends on applications, cost, environmental factors and also the type of sources available.. Broadly, they can be classified as either DC or AC motors they can be classified as either DC or AC motors:
DC motors (wound or permanent magnet) AC motors
motors – wound field, permanent magnet Brushless DC motor – require power electronic converters Stepper motors – require power electronic converters
Synchronous reluctance motors or switched reluctance motor – require power electronic converters
b) Power processor or power modulator
Since the electrical sources are normally uncontrollable, it is therefore necessary to be able to control the flow of power to the motor – this i s achieved using power processor or power modulator. With controllable sources, the motor can be reversed, brake or can be operated with variable speed. Conventional methods used, for example, variable impedance or relays, to shape the voltage or current that is supplied to the motor – these methods however are inflexible and inefficient. Modern electric drives normally used power electronic converters to shape the desired voltage or current supplied to the motor.
In other words, the characteristic of the motors can be changed at will. Power electronic converters have several advantages over classical methods of power conversion, such as
1) More efficient – since ideally no losses occur in power electronic converters
2) Flexible – voltage and current can be shaped by simply controlling switching functions of the power converter.
3) Compact – smaller, compact and higher ratings solid–state power electronic devices are continuously being developed – the prices are getting cheaper
Converters are used to convert and possibly regulate (i.e. using closed-loop control) the available sources to sui t the load i.e. motors. These converters are efficient because the switches operate in either cut-off or saturation modes
b)Control Unit
The complexity of the control unit depends on the desired drive performance and the type of motors used. A controller can be as simple as few op-amps and/or a few digital ICs, or it can be as complex as the combinations of several ASICs and digital signal processors (DSPs).
The types of the main controllers can be
• Analog - This is noisy, inflexible. However analog circuit ideally has infinite bandwidth.
• DSP/microprocessor – flexible, lower bandwidth compared to above. DSPs perform faster operation than microprocessors (multiplication in single cycle). With DSP/microprocessor., complex estimations and observers can be easily implemented. d) Source
Electrical sources or power supplies provide the energy to the electrical motors. For high efficiency operation, the power obtained from the electrical sources need to be regulated using power electronic converters Power sources can be of AC or D C in nature and normally are uncontrollable, i.e. their magnitudes or frequencies are fixed or depend on the sources of energy such as solar or wind. AC source can be either three-phase or single-phase; 3-phase sources are normally for high power applications There can be several factors that affect the selection of different configuration of electrical drive system such as
a) Torque and speed profile - determine the ratings of converters and the quadrant of operation required.
b) Capital and running cost – Drive systems will vary in terms of start-up cost and running cost, e.g. maintenance
c) c) Space and weight restrictions d) Environment and location
3. Selecting a Drive
Often drive selection is straight forward, as a motor is already installed and the speed range requirement is not excessive. However, when a drive system is selected from first principles, careful consideration may avoid problems in installation and operation, and may also save significant cost.
3.1 Overall Considerations.
• Check the Current rating of the inverter and the motor. Power rating is only a rough guide
• Check that you have selected the correct operating voltage. 230V three phase input MICROMASTERs will operate with single or three phase inputs; MIDIMASTERs will operate with three phase only. Single phase input units can be more cost effective in some cases, but note that 230V units will be damaged if operated at 400V.
• Check the speed range you require. Operation above normal supply frequency (50 or 60Hz) is usually only possible at reduced power. Operation at low frequency and high torque can cause the motor to overheat due to lack of cooling
•Synchronous motors require de-rating, typically by 2 -3 times. This is because the
power factor, and hence the current, can be very high at low frequency.
• Check overloads performance. The inverter will limit current to 150 or 200 % of full current very quickly - a standard, fixed speed motor will tolerate these overloads.
• Do you need to stop quickly? If so, consider using a braking resistor (braking unit on MIDIMASTERs) to absorb the energy.
• Do you need to operate with cables longer than 50m, or screened or armoured cables longer than 25m? If so, it may be necessary to de-rate, or fit a choke to compensate for the cable capacitance.
3.2 Motor limitations
For more information concerning calculation of Power requirements, Torque, and Moment of Inertia, see later.
The motor speed is determined mainly by the applied frequency. The motor slows down a little as the load increases and the slip increases. If the load is too great the motor will exceed the maximum torque and stall or „pull out . Most motors and inverters will‟ operate at 150% load
Thermal considerations
The losses in the machines contribute to the temperature increase in the machine. The various parts of the machine use different type of insulation materials which have different temperature limits. Particularly important is the insulation used for the windings which give rise to the different classes of machines. Allowable power losses are higher for materials which can withstand higher temperature which translates to higher costs. Three main cause of power losses are:
Conductor losses :
Exist in the windings, cables, brushes, slip rings, commutator, and etc. Core losses:
Mainly due to eddy current and hysteresis losses Friction and windage losses:
Mainly due to ball bearings, brushes, ventilation losses
The constructions of the machines are very complex; normally built from various types of materials (heterogeneous) with complex geometrical shapes. To exactly predict the heat flow and hence the temperature distribution is extremely difficult. Based on the assumptions that the temperature limits of all parts does not exceed the temperature limits under certain operating conditions, the motors can therefore adequately modeled as homogeneous bodies. Obviously, this assumption cannot determine the specific internal thermal conditions for the motors
Let us assume that a homogeneous body shown in Figure 12 represents a motor which has a thermal capacity C. The input power, which is the losses incurred in the motor, is represented by p1 whereas the output power, which is the power released as heat by convection, is represented by p2. The output power due to radiation is assumed negligible because of the low operating temperature and back radiation. Under a steady state condition, the input power equals the output power; this is when the steady state temperature is reached. The equation describing the power balance is given by
The heat dissipated by convection is given by where is the coefficient of heat transfer
If we let equation (12) can be written as
where is the thermal time constant. With and a step change in the power input p1 from 0 to ph at t=0, the solution for is
At steady state, \
During cooling, i.e. when heat is removed at t=0, the temperature of the body decays to the ambient temperature.
If the thermal time constant is large, a temporary overload is therefore possible without exceeding the temperature limits. Three typical modes of operation are:
- Continuous duty
- Short time intermittent duty - Periodic intermittent duty\
Ratings of converters and motors
In order to accelerate to a given reference value, the motor torque has to be larger than the load torque. According to (1), the difference between T1 and Te determines how fast the angular acceleration is. For example, the speed and torque responses for a closed-loop speed control DC drive with two different torque limit setting (10 Nm and 15 Nm) is shown in Figure 7. The higher the torque during the speed transient, the faster is the speed gets to its reference
In most cases, the torque during this transient condition can be up to 3 times the rated torque of the motor and for servo motor, it can be as high as 8 to 10 times the rated value. This momentary high torque is possible due to the large thermal capacity of the motor with suitable insulators used for the winding. The converter, which conducts the motor current, must be able to sustain this condition. However since the thermal capacity of the converter is small, the current cannot be higher than its rated value. Consequently, the current rating of the converter is normally set to equal the maximum allowable motor current and this can be as high as the 3 times the motor rated current. The maximum allowable torque during transient of a drive system is determined by the current rating of the converter used whereas the continuous torque limit depends on the current rating of the motor. The operating area of a 4-quadrant motor drive is shown in Figure 8. The converter is normally protected from the over-current condition by the current limiter mechanism within the converter system, which means that sustained
Overloads on the motor have to be protected by an additional thermal protection mechanism. Above the base speed ωb, the toque is limited by the maximum allowable power, which depends on whether the transient or continuous torque limit is considered. The speed limit basically depends on the mechanical limitation of the
Fig. Limits for torque, speed and power for drive system
Steady-state stability
The motor will operate at the steady-state speed (point where T1 = Te) provided that the speed is of stable equilibrium. The stable equilibrium speed is investigated using steady-state torque- speed characteristics of the load and motor. A disturbance in any part of the drive will result in a speed to depart from the steady state speed. However, if the steady-state speed is of stable equilibrium, the speed will return to the stable equilibrium speed. On the other hand, if the speed i s not of the stable equilibrium, the disturbance will results in the speed to drift away from the equilibrium speed. It can be shown that the condition for stable equilibrium is:
UNIT-3,4
Torque speed characteristics of a shunt motor:
A constant applied voltage V is assumed across the armature. As the armature current Ia, varies the armature drop varies proportionally and one can plot the variation of the induced emf E. The mmf of the field is assumed to be constant. The flux inside the machine however slightly falls due to the effect of saturation and due to armature reaction.
The variation of these parameters is shown in Fig. Knowing the value of E and flux one can determine the value of the speed. Also knowing the armature current and the flux, the value of the torque is found out. This procedure is repeated for different values of the assumed armature currents and the values are plotted as in Fig. (a). From these graphs, a graph indicating speed as a function of torque or the torque-speed characteristics is plotted Fig. (b)(i).
As seen from the figure the fall in the flux due to load increases the speed due to the fact that the induced emf depends on the product of speed and flux. Thus the speed of the machine remains more or less constant with load. With highly saturated machines
the on-load speed may even slightly increase at over load conditions. This effect gets more pronounced if the machine is designed to have its normal field ampere turns much less than the armature ampere turns. This type of external characteristics introduces instability during operation Fig. (b)(ii) and hence must be avoided. This may be simply achieved by
providing a series stability winding which aids the shunt field mmf.
Load characteristics of a series motor
Following the procedure described earlier under shunt motor, the torque speed Characteristics of a series motor can also be determined. The armature current also happens to be the excitation current of the series field and hence the flux variation resembles the magnetization curve of the machine. At large value of the armature currents the useful flux would be less than the no-load magnetization curve for the machine. Similarly for small values of the load currents the torque varies as a square of the armature currents as the flux is proportional to armature current in this region. As the magnetic circuit becomes more and more saturated the torque becomes proportional to Ia as flux variation becomes small.
Fig. (a) shows the variation of E1, flux, torque and speed following the above procedure from which the torque-speed characteristics of the series motor for a given applied voltage V can be plotted as shown in Fig.(b) The initial portion of this torque-speed curve is seen to be a rectangular hyperbola and the final portion is nearly a straight line. The speed under light load conditions is many times more than the rated speed of the
commutator can destroy them giving rise to a catastrophic break down. Hence series motors are not recommended for use where there is a possibility of the load becoming zero. In order to safeguard the motor and personnel, in the modern machines, a 'weak' shunt field is provided
on series motors to ensure a definite, though small, value of flux even when the armature current is nearly zero. This way the no-load speed is limited to a safe maximum speed. It is needless to say, this field should be connected so as to aid the series field.
Load characteristics of a compound motor
Two situations arise in the case of compound motors. The mmf of the shunt field and series field may oppose each other or they may aid each other. The first configuration is called differential compounding and is rarely used. They lead to unstable operation of the machine unless the armature mmf is small and there is no magnetic saturation. This mode may sometimes result due to the motoring operation of a level-compounded generator, say by the failure of the prime mover. Also, differential compounding may result in large negative mmf under overload/starting condition and the machine may start in the reverse direction. In motors intended for constant speed operation the level of compounding is very low as not to cause any problem.
Cumulatively compounded motors are very widely used for industrial drives.
High degree of compounding will make the machine approach a series machine like characteristics but with a safe no-load speed. The major benefit of the compounding is that the field is strengthened on load. Thus the torque per ampere of the armature current is made high. This feature makes a cumulatively compounded machine well suited for
Intermittent peak loads. Due to the large speed variation between light load and peak load conditions, a y wheel can be used with such motors with advantage. Due to the reasons provided under shunt and series motors for the provision of an additional series/shunt winding, it can be seen that all modern machines are compound machines. The difference between them is only in the level of compounding.
Braking the d.c. Motors
When a motor is switched off it `coasts' to rest under the action of frictional forces.
Braking is employed when rapid stopping is required. In many cases mechanical braking is adopted. The electric braking may be done for various reasons such as those mentioned below:
1. To augment the brake power of the mechanical brakes. 2. To save the life of the mechanical brakes.
3. To regenerate the electrical power and improve the energy efficiency. 4. In the case of emergencies to step the machine instantly.
5. To improve the throughput in many production processes by reducing the stopping time.
In many cases electric braking makes more brake power available to the braking process where mechanical brakes are applied. This reduces the wear and tear of the mechanical brakes and reduces the frequency of the replacement of these parts. By recovering the mechanical energy stored in the rotating parts and pumping it into the supply lines the overall energy efficiency is improved. This is called regeneration. Where the safety of the personnel or the equipment is at stake the machine may be required to stop instantly.
Extremely large brake power is needed under those conditions. Electric braking can help in these situations also. In processes where frequent starting and stopping is involved the process time requirement can be reduced if braking time is reduced. The reduction of the
1. Dynamic 2. Regenerative
3. Reverse voltage braking or plugging
These are now explained briefly with reference to shunt, series and compound motors. Dynamic braking
Shunt machine
In dynamic braking the motor is disconnected from the supply and connected to a dynamic braking resistance RDB. In and Fig. 49 this is done by changing the switch from position 1 to 2. The supply to the field should not be removed. Due to the rotation of the armature during motoring mode and due to the inertia, the armature continues to rotate. An emf is induced due to the presence of the field and the rotation. This voltage drives a
current through the braking resistance. The direction of this current is opposite to the one which was owing before change in the connection. Therefore, torque developed also gets reversed. The machine acts like a brake. The torque speed characteristics separate by excited shunt of the machine under dynamic braking mode is as shown in Fig. (b) For a particular value of RDB. The positive torque corresponds to the motoring operation. Fig. shows the dynamic braking of a shunt excited motor and the corresponding torque-speed curve. Here the machine behaves as a self-excited generator. Below a certain speed the self-excitation collapses and the braking action becomes Zero. Process time improves the throughput.
Basically the electric braking involved is fairly simple. The electric motor can be made to work as a generator by suitable terminal conditions and absorb mechanical energy.
This converted mechanical power is dissipated/used on the electrical network suitably.
Figure: Dynamic braking of shunt excited shunt machine
Series machine
In the case of a series machine the excitation current becomes zero as soon as the armature is disconnected from the mains and hence the induced emf also vanishes. In order to achieve dynamic braking the series field must be isolated and connected to a low voltage high current source to provide the field. Rather, the motor is made to work like a separately excited machine. When several machines are available at any spot, as in railway locomotives, dynamic braking is feasible. Series connection of all the series fields with parallel connection of all the armatures connected across a single dynamic braking resistor is used in that case.
Compound generators
In the case of compound machine, the situation is like in a shunt machine. A separately excited shunt field and the armature connected across the braking resistance are used.
A cumulatively connected motor becomes differentially compounded generator and the braking torque generated comes down. It is therefore necessary to reverse the
Regenerative braking
In regenerative braking as the name suggests the energy recovered from the rotating masses is fed back into the d.c. power source. Thus this type of braking improves the energy efficiency of the machine. The armature current can be made to reverse for a constant voltage operation by increase in speed/excitation only. Increase in speed does not result in braking and the increase in excitation is feasible only over a small range, which may be of the order of 10 to 15%. Hence the best method for obtaining the regenerative braking is to operate, the machine on a variable voltage supply. As the voltage is continuously pulled below the value of the induced emf the speed steadily comes down. The field current is held constant by means of separate excitation. The variable d.c. supply voltage can be obtained by Ward-Leonard arrangement, shown schematically in Fig. .
Braking torque can be obtained right up to zero speed. In modern times static Ward-Leonard scheme is used for getting the variable d.c. voltage. This has many advantages over its rotating machine counterpart. Static set is compact, has higher efficiency, and requires lesser space and silent in operation; however it suffers from drawbacks like large ripple at low voltage levels, unidirectional power flow and low over load capacity. Bidirectional power flow capacity is a must if regenerative braking is required. Series motors cannot be regenerative braked as the characteristics do not extend to the second quadrant.
Plugging
The third method for braking is by plugging. Fig. shows the method of connection for the plugging of a shunt motor. Initially the machine is connected to the supply with the switch S in position number 1. If now the switch is moved to position 2, then a reverse voltage is applied across the armature. The induced armature voltage E and supply voltage V aid each other and a large reverse current flow through the armature. This produces a large negative torque or braking torque. Hence plugging is also termed as reverse voltage braking. The machine instantly comes to rest. If the motor is not switched off at this instant the direction of rotation reverses and the motor starts rotating the reverse direction. This type of braking therefore has two modes viz. 1) plug to reverse and 2) plug to stop. If we need the plugging only for bringing the speed to zero, then we have to open the switch S at zero speed. If nothing is done it is plug to reverse mode. Plugging is a convenient mode for quick reversal of direction of rotation in reversible rives. Just as in starting, during
Plugging also it is necessary to limit the current and thus the torque, to reduce the stress on the mechanical system and the commutator. This is done by adding additional resistance in series with the armature during plugging.
Series motors
In the case of series motors plugging cannot be employed as the field current too gets reversed when reverse voltage is applied across the machine. This keeps the direction of the torque produced unchanged. This fact is used with advantage, in operating a d.c. series motor on d.c. or a.c. supply. Series motors thus qualify to be called as `Universal motors'.
Compound motors
Plugging of compound motors proceeds on similar lines as the shunt motors. However some precautions have to be observed due to the presence of series field winding. A cumulatively compounded motor becomes differentially compounded on plugging. The mmf due to the series field can 'over power' the shunt field forcing the flux to low values or even reverse the net field. This decreases the braking torque, and increases the duration of the large braking current. To avoid this it may be advisable to deactivate the series field at the time of braking by short-circuiting the same. In such cases the braking proceeds just as in a shunt motor. If plugging is done to operate the motor in the negative direction of rotation as well, then the series field has to be reversed and connected for getting the proper mmf. Unlike dynamic braking and regenerative braking where the motor is made to work as a generator during braking period, plugging makes the motor work on reverse motoring mode.
Deducing the machine performance. (Single phase Induction motor)
From the equivalent circuit, many aspects of the steady state behavior of the machine can be deduced. We will begin by looking at the speed-torque characteristic of the machine. We will
Consider the approximate equivalent circuit of the machine. We have reasoned earlier that the power consumed by the 'rotor-portion' of the equivalent circuit is the power transferred across the air-gap. Out of that quantity the amount dissipated in R0 r is the rotor copper loss and the quantity consumed by R0r(1 + s)=s is the mechanical power developed. Neglecting mechanical losses, this is the power available at the shaft. The torque available can be obtained by dividing this number by the shaft speed.
The complete torque-speed characteristic of Induction motor
In order to estimate the speed torque characteristic let us suppose that a sinusoidal voltage is impressed on the machine. Recalling that the equivalent circuit is the per-phase representation of the machine, the current drawn by the circuit is given by
Where Vs is the phase voltage phasor and Is is the current phasor. The magnetizing current is neglected. Since this current is owing through , the air-gap power is given by
The mechanical power output was shown to be (1_s) Pg (power dissipated in R0r=s). The torque is obtained by dividing this by the shaft speed .Thus we have,
Where! S is the synchronous speed in radians per second and s is the slip. Further, this is the torque produced per phase. Hence the overall torque is given by
The torque may be plotted as a function of `s' and is called the slip (or torque-speed, since slip indicates speed) characteristic | a very important characteristic of the induction machine. Equation 16 is valid for a two-pole (one pole pair) machine. In general, this expression should be multiplied by p, the number of pole-pairs. A typical torque-speed characteristic is shown in _g. 22. This plot corresponds to a 3 kW, 4 pole,60 Hz machine. The rated operating speed is 1780 rpm.
We must note that the approximate equivalent circuit was used in deriving this relation. Readers with access to MATLAB or suitable equivalents (octave, scilab available free under GNU at the time of this writing) may find out the difference caused by using the `exact' equivalent circuit by using the script found here. A comparison between the two is found in the plot of fig. The plots correspond to a 3 kW, 4 pole, 50 machine, with a rated speed of 1440 rpm. It can be seen that the approximate equivalent circuit is a good approximation in the operating speed range of the machine. Comparing the two figures. We can see that the slope and shape of the characteristics are dependent intimately on the machine parameters.
Further, this curve is obtained by varying slip with the applied voltage being held constant. Coupled with the fact that this is an equivalent circuit valid under steady state, it implies that if this characteristic is to be measured experimentally, we need to look at the torque for a given speed after all transients have died down. One cannot, for example, try
Torque, Nm to obtain this curve by directly starting the motor with full voltage applied to the terminals and measuring the torque and speed dynamically as it runs up to steady speed.
Another point to note is that the equivalent circuit and the values of torque predicted is valid when the applied voltage waveform is sinusoidal. With non-sinusoidal voltage waveforms, the procedure is not as straightforward.
With respect to the direction of rotation of the air-gap flux, the rotor maybe driven to higher speeds by a prime mover or may also be rotated in the reverse direction. The torque-speed relation for the machine under the entire speed range is called the complete speed-torque characteristic. A typical curve is shown in fig for a four-pole machine, the synchronous speed being 1500 rpm. Note that negative speeds correspond to slip values greater than 1, and speeds greater than 1500 rpm correspond to negative slip. The plot also shows the operating modes of the induction machine in various regions. The slip axis is also shown for convenience.
Restricting ourselves to positive values of slip, we see that the curve has a peak point. This is the maximum torque that the machine can produce, and is called as stalling torque. If the load torque is more than this value, the machine stops rotating or stalls. It occurs at a slip ^s, which for the machine of fig is 0.38. At values of slip lower than ^s, the curve falls steeply down to zero at s = 0. The torque at synchronous speed is therefore zero. At values of slip higher than s = ^s, the curve falls slowly to a minimum value at s = 1. The torque at s = 1 (speed = 0) is called the starting torque.
The value of the stalling torque may be obtained by differentiating the expression for torque with respect to zero and setting it to zero to find the value of ^s. Using this method,
Substituting ^s into the expression for torque gives us the value of the stalling torque ^ T
the negative sign being valid for negative slip.
The expression shows that ^ Te is the independent of R0 r, while ^s is directly proportional to R0 r. This fact can be made use of conveniently to alter ^s. If it is possible to change R0 r, then we can get a whole series of torque-speed characteristics, the maximum torque remaining constant all the while. But this is a subject to be discussed later.
We may note that if R is chosen equal to becomes unity, which p means that the maximum torque occurs at starting. Thus changing of R r, wherever possible can serve as a means to control the starting torque.
While considering the negative slip range, (generator mode) we note that the maximum torque is higher than in the positive slip region (motoring mode).
Operating Point
Consider a speed torque characteristic shown in fig. For an induction machine, having the load characteristic also superimposed on it. The load is a constant torque load i.e., the torque required for operation is fixed irrespective of speed. The system consisting of the motor and load will operate at a point where the two characteristics meet. From the above plot, we note that there are two such points. We therefore need to find out which of these is the actual operating point.
To answer this we must note that, in practice, the characteristics are never fixed; they change slightly with time. It would be appropriate to consider a small band around the curve drawn where the actual points of the characteristic will lie. This being the case let us considers that the system is operating at point 1, and the load torque demand increases slightly. This is shown in fig, where the change is exaggerated for clarity. This would shift the point of operation to a point 10 at which the slip would be less and the developed torque higher.
The difference in torque-developed 4Te, being positive will accelerate the machine. Any overshoot in speed as it approaches the point 10 will cause it to further accelerate since the developed torque is increasing. Similar arguments may be used to show that if for some reason the developed torque becomes smaller the speed would drop and the effect is cumulative. Therefore we may conclude that 1 is not a stable operating point.
Let us consider the point 2. If this point shifts to 20, the slip is now higher (speed is lower) and the positive difference in torque will accelerate the machine. This behavior will tend to bring the operating point towards 2 once again. In other words, disturbances at point 2 will not cause a runaway effect. Similar arguments may be given for the case where the load characteristic shifts down. Therefore we conclude that point 2 is a stable operating point.
torque, Nm From the foregoing discussions, we can say that the entire region of the speed-torque characteristic from s = 0 to s = ^s is an unstable region, while the region from s = ^s to s = 0 is a stable region. Therefore the machine will always operate between s = 0 and s = ^s.
Modes of Operation
The reader is referred to fig which shows the complete speed-torque characteristic of the induction machine along with the various regions of operation.
Let us consider a situation where the machine has just been excited with three phase supply and the rotor has not yet started moving. A little reaction on the definition of the slip indicates that we are at the point s = 1. When the rotating magnetic field is set up due to stator currents, it is the induced emf that causes current in the rotor, and the interaction between the two causes torque. It has already been pointed out that it is the presence of the non-zero slip that causes a torque to be developed. Thus the region of the
curve between
Figure : Stability of operating point
s = 0 and s = 1 is the region where the machine produces torque to rotate a passive load and hence is called the motoring region. Note further that the direction of rotation of the rotor is the same as that of the air gap flux.
Suppose when the rotor is rotating, we change the phase sequence of excitation to the machine. This would cause the rotating stator field to reverse its direction | the rotating stator mmf and the rotor are now moving in opposite directions. If we adopt the convention that positive direction is the direction of the air gap flux, the rotor speed would then be a negative quantity. The slip would be a number greater than unity. Further, the rotor as we know should be "dragged along" by the stator field. Since the rotor is rotating in the opposite direction to that of the field, it would now tend to slow down, and reach zero speed.
Therefore this region (s > 1) is called the braking region. (What would happen if the supply is not cut-off when the speed reaches zero?) . There is yet another situation. Consider a situation where the induction machine is operating from mains and is driving an active load (a load capable of producing rotation by itself). A typical example is that of a windmill, where the fan like blades of the windmill are connected to the shaft of the induction machine. Rotation of the blades may be caused by the motoring action of the machine, or by wind blowing. Further suppose that both acting independently cause rotation in the same direction. Now when both grid and windact, a strong wind may cause the rotor to rotate faster than the mmf produced by the stator excitation. A little reaction shows that slip is then negative.
Further, the wind is rotating the rotor to a speed higher than what the electrical supply alone would cause. In order to do this it has to contend with an opposing torque generated by the machine preventing the speed build up. The torque generated is therefore negative. It is this action of the wind against the torque of the machine that enables wind-energy generation. The region of slip s > 1 is the generating mode of operation. Indeed this is at present the most commonly used approach in wind-energy
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generation. It may be noted from the torque expression of equation that torque is negative for negative values of slip.
Braking of d.c shunt motor: basic idea
It is often necessary in many applications to stop a running motor rather quickly. We know that any moving or rotating object acquires kinetic energy. Therefore, how fast we can bring the object to rest will depend essentially upon how quickly we can extract its kinetic energy and make arrangement to dissipate that energy somewhere else. If you stop pedaling your bicycle, it will eventually come to a stop eventually after moving quite some distance. The initial kinetic energy stored, in this case dissipates as heat in the friction of the road. However, to make the stopping faster, brake is applied with the help of rubber brake shoes on the rim of the wheels.
Thus stored K.E now gets two ways of getting dissipated, one at the wheel-brake shoe interface (where most of the energy is dissipated) and the other at the road-tier interface. This is a good method no doubt, but regular maintenance of brake shoes due to wear and tear is necessary.
If a motor is simply disconnected from supply it will eventually come to stop no doubt, but will take longer time particularly for large motors having high rotational inertia. Because here the stored energy has to dissipate mainly through bearing friction and wind friction. The situation can be improved, by forcing the motor to operate as a generator during braking. The idea can be understood remembering that in motor mode electromagnetic torque acts along the direction of rotation while in generator the electromagnetic torque acts in the opposite direction of rotation. Thus by forcing the machine to operate as generator during the braking period, a torque opposite to the direction of rotation will be imposed on the shaft, thereby helping the machine to come to stop quickly. During braking action, the initial K.E stored in the rotor is either dissipated in an external resistance or fed back to the supply or both.
Rheostatic braking
Consider a d.c shunt motor operating from a d.c supply with the switch S connected to position 1 as shown in figure. S is a single pole double throw switch and can be connected either to position 1 or to position 2. One end of an external resistance Rb is connected to position 2 of the switch S as shown.
Let with S in position 1, motor runs at n rpm, drawing an armature current Ia and the back emf is Note the polarity of Eb which, as usual for motor mode in
opposition with the supply voltage. Also note Te and n have same clockwise direction.
Now if S is suddenly thrown to position 2 at t = 0, the armature gets disconnected from the supply and terminated by Rb with field coil remains energized from the supply. Since speed of the rotor can not change instantaneously, the back emf value Eb is still maintained with same polarity prevailing at t = 0-. Thus at t = 0+, armature current will be Ia = Eb/(ra + Rb) and with reversed direction compared to direction prevailing during motor mode at t = 0-.
Obviously for t > 0, the machine is operating as generator dissipating power to Rb and now the electromagnetic torque Te must act in the opposite direction to that of n
since Ia has changed direction but has not As time passes after
switching, n decreases reducing K.E and as a consequence both Eb and Ia decrease. In other words value of braking torque will be highest at t = 0+, and it decreases progressively and becoming zero when the machine finally
come to a stop.
Plugging or dynamic braking
This method of braking can be understood by referring to figures 39.25 and 39.26. Here S is a double pole double throw switch. For usual motoring mode, S is connected to positions 1 and 1'.
Across terminals 2 and 2', a series combination of an external resistance Rb and supply voltage with polarity as indicated is connected. However, during motor mode this part of the circuit remains inactive.
To initiate braking, the switch is thrown to position 2 and 2' at t = 0, thereby disconnecting the armature from the left hand supply. Here at t = 0+, the armature current will be Ia = (Eb + V)/(ra + Rb) as Eb and the right hand supply voltage have additive polarities by virtue of the connection. Here also Ia reverses direction-producing Te in opposite direction to n. Ia decreases as Eb decreases with time as speed decreases. However, Ia cannot become zero at any time due to presence of supply V. So unlike rheostatic braking, substantial magnitude of braking torque prevails. Hence stopping of the motor is expected to be much faster then rheostatic breaking.
the reverse direction operating as a motor. So care should be taken to disconnect the right hand supply, the moment armature speed becomes zero.
Regenerative braking
A machine operating as motor may go into regenerative braking mode if its speed becomes sufficiently high so as to make back emf greater than the supply voltage i.e., Eb > V. Obviously under this condition the direction of Ia will reverse imposing torque which is opposite to the direction of rotation. The situation is explained in figures 39.27 and 39.28. The normal motor operation is shown in figure 39.27 where armature motoring current Ia is drawn from the supply and as usual Eb < V. Since
The question is how speed on its own become large enough to make Eb <
V causing regenerative braking. Such a situation may occur in practice when the
mechanical load itself becomes active. Imagine the d.c motor is coupled to the wheel of locomotive which is moving along a plain track without any gradient as shown in figure. Machine is running as a motor at a speed of n1 rpm. However, when the track has a downward gradient (shown in figure 39.28), component of gravitational force along the track also appears which will try to accelerate the motor and may increase its speed to n2 such that Eb In such a scenario, direction of Ia reverses, feeding power back to supply.
Regenerative braking here will not stop the motor but will help to arrest rise of dangerously high speed.
UNIT-III
STARTING METHODS STARTING OF D.C. MACHINES:
For the machine to start, the torque developed by the motor at zero speed must exceed that demanded by the load. Then TM _ TL will be positive so also is di=dt, and the machine accelerates. The induced emf at starting point is zero as the i = 0 The armature current with rated applied voltage is given by V=Ra where Ra is armature circuit resistance.
Normally the armature resistance of a d.c. machine is such as to cause 1 to 5 percent drop at full load current. Hence the starting current tends to rise to several times the full load current. The same can be told of the torque if full flux is already established. The machine instantly picks up the speed. As the speed increases the induced emf appears across the terminals opposing the applied voltage. The current drawn from the mains thus decreases, so also the torque. This continues till the load torque and the motor torque are equal to each other. Machine tends to run continuously at this speed, as the acceleration is zero at this point of operation. The starting is now discussed with respect to specific machines.
If armature and field of d.c. shunt motor are energized together, large current is drawn at start but the torque builds up gradually as the field flux increases gradually. To improve the torque per ampere of line current drawn it is advisable to energize the field first. The starting current is given by V=Ra and hence to reduce the starting current to a safe value, the voltage V can be reduced or armature circuit resistance Ra can be increased. Variable voltage V can be obtained from a motor generator set. This arrangement is called Leonard arrangement. A schematic diagram of Ward-Leonard arrangement is shown in Fig. By controlling the field of the Ward-Ward-Leonard generator one can get a variable voltage at its terminals, which is used, for starting the motor. The second method of starting with increased armature circuit resistance can be obtained by adding additional resistances in series with the armature, at start. The current and the torque get reduced. The torque speed curve under these conditions is shown in Fig. (a). It can be readily seen
from this graph that the unloaded machine reaches its final speed but a loaded machine may crawl at a speed much below the normal speed. Also, the starting resistance wastes large amount of power. Hence the starting resistance must be reduced to zero at the end of the starting process. This has to be done progressively, making sure that the current does not jump up to large values. Starting of series motor and compound motors are similar to the shunt motor. Better starting torques are obtained for compound motors as the torque per ampere is more. Characteristics for series motors are given in fig.
Grading of starting resistance for a shunt motor
If the starting resistor is reduced in uniform steps then the current peaks reached as we cut down the resistances progressively increase. To ascertain that at no step does
Starting of D.C shunt motor
1. Problems of starting with full voltage
We know armature current in a d.c motor is given by
At the instant of starting, rotor speed n = 0, hence starting armature current is
Since, armature resistance is quite small, starting current may be quite high (many times larger than the rated current). A large machine, characterized by large rotor inertia (J), will pick up speed rather slowly. Thus the level of high starting current may be
maintained for quite some time so as to cause serious damage to the brush/ commutator and to the armature winding. Also the source should be capable of supplying this burst of large current. The other loads already connected to the same source, would experience a dip in the terminal voltage, every time a D.C motor is attempted to start with full voltage.
This dip in supply voltage is caused due to sudden rise in voltage drop in the source's internal resistance. The duration for which this drop in voltage will persist once again
depends on inertia (size) of the motor. Hence, for small D.C motors extra precaution may not be necessary during starting as large starting current will very quickly die down because of fast rise in the back emf. However, for large motor, a starter is to be used during starting.
2. A simple starter
To limit the starting current, a suitable external resistance Rext is connected in series (Figure (a)) with the armature so that At the time of starting, to have sufficient starting torque, field current is maximized by keeping the external field resistance Rf, to zero value. As the motor picks up speed, the value of Rext is gradually decreased to zero so that during running no external resistance remains in the armature circuit. But each time one has to restart the motor, the external armature resistance must be set to maximum value by moving the jockey manually.
Imagine, the motor to be running with Rext = 0 (Figure (b)).
Now if the supply goes off (due to some problem in the supply side or due to load shedding), motor will come to a stop. All on a sudden, let us imagine, supply is restored. This is then nothing but full voltage starting. In other words, one should be constantly alert to set the resistance to maximum value whenever the motor comes to a stop. This is one major limitation of a simple rheostatic starter.
3. 3-point starter
A “3-point starter” is extensively used to start a D.C shunt motor. It not only overcomes the difficulty of a plain resistance starter, but also provides additional
3-point starter connected to a shunt motor is shown in figure . Although, the circuit looks a bit clumsy at a first glance, the basic working principle is same as that of plain resistance starter The starter is shown enclosed within the dotted rectangular box having
three terminals marked as A, L and F for external connections. Terminal A is connected to one armature terminal Al of the motor. Terminal F is connected to one field terminal F1 of the motor and terminal L is connected to one supply terminal as shown. F2 terminal of field coil is connected to A2 through an external variable field resistance and the common point connected to supply (-ve). The external armatures resistances consist of several resistances connected in series and are shown in the form of an arc. The junctions of the resistances are brought out as terminals (called studs) and marked as 1,2,.. .12. Just beneath the resistances, a continuous copper strip also in the form of an arc is present.
There is a handle which can be moved in the clockwise direction against the spring tension. The spring tension keeps the handle in the OFF position when no one attempts to move it. Now let us trace the circuit from terminal L (supply + ve). The wire from L passes through a small electro magnet called OLRC, (the function of which we shall discuss a little later) and enters through the handle shown by dashed lines. Near the end of the handle two copper strips are firmly connected with the wire. The furthest strip is shown circular shaped and the other strip is shown to be rectangular. When the handle is moved to the right, the circular strip of the handle will make contacts with resistance terminals 1, 2 etc. progressively. On the other hand, the rectangular strip will make contact with the continuous arc copper strip. The other end of this strip is brought as terminal F after going through an electromagnet coil (called NVRC). Terminal F is finally connected to motor field terminal Fl.
4. Working principle
Let us explain the operation of the starter. Initially the handle is in the OFF position. Neither armature nor the field of the motor gets supply. Now the handle is moved to stud number 1. In this position armature and all the resistances in series gets connected to the supply. Field coil gets full supply as the rectangular strip makes contact with arc copper strip. As the machine picks up speed handle is moved further r to stud number 2. In this position the external resistance in the armature circuit is less as the first resistance is left out. Field however, continues to get full voltage by virtue of the continuous arc strip. Continuing in this way, all resistances will be left out when stud number 12 (ON) is reached. In this position, the electromagnet (NVRC) will attract the soft iron piece attached to the handle. Even if the operator removes his hand from the handle, it will still remain in the ON position as spring restoring force will be balanced by the force of attraction between NVRC and the soft iron piece of the handle. The no volt
release coil (NVRC) carries same current as that of the field coil. In case supply voltage
goes off, field coil current will decrease to zero. Hence NVRC will be deenergised and will not be able to exert any force on the soft iron piece of the handle. Restoring force of the spring will bring the handle back in the OFF position.
The starter also provides over load protection for the motor. The other electromagnet, OLRC overload release coil along with a soft iron piece kept under it, is used to achieve this. The current flowing through OLRC is the line current IL drawn by the motor. As the motor is loaded, Ia hence IL increases. Therefore, IL is a measure of loading of the motor. Suppose we want that the motor should not be over loaded beyond rated current. Now gap between the electromagnet and the soft iron piece is so adjusted that for the iron piece will not be pulled up. However, if rated I I force of attraction will be sufficient to pull up iron piece. This upward movement of the
iron piece of OLRC is utilized to de-energize NVRC. To the iron a copper strip (Ä shaped in figure) is attached. During over loading condition, this copper strip will also move up and put a short circuit between two terminals B and C. Carefully note that B and C are nothing but the two ends of the NVRC. In other words, when over load occurs a short circuit path is created across the NVRC. Hence NVRC will not carry any current now and gets deenergised. The moment it gets deenergised, spring action will bring the handle in the OFF position thereby disconnecting the motor from the supply.
Three-point starter has one disadvantage. If we want to run the machine at higher speed (above rated speed) by field weakening (i.e., by reducing field current), the strength of NVRC magnet may become so weak that it will fail to hold the handle in the ON position and the spring action will bring it back in the OFF position. Thus we find that a false disconnection of the motor takes place even when there is neither over load nor any
sudden disruption of supply.
DIFFERENT TYPES OF STARTERS FOR 3 PHASE INDUCTION
MOTOR (IM)
•Need of using starters for Induction motor
• Two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage Induction motor
•Starter using additional resistance in rotor circuit, for Wound rotor (Slip-ring) Induction motor
Introduction
In the previous, i.e. fourth, lesson of this module, the expression of gross torque developed, as a function of slip (speed), in IM has been derived first. The sketches of the different torque-slip (speed) characteristics, with the variations in input (stator) voltage and rotor resistance, are presented, along with the explanation of their features. Lastly, the expression of maximum torque developed and also the slip, where it occurs, have been derived. In this lesson, starting with the need for using starters in IM to reduce the starting current, first two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage IM and then, the starter using additional resistance in rotor circuit, for Wound rotor (Slip-ring) IM, are presented along with the starting current drawn from the input (supply) voltage, and also the starting torque developed using the above starters.
Keywords:
Direct-on-Line (DOL) starter, Star-delta starter, auto-transformer starter, rotor resistance starter, starting current, starting torque, starters for squirrel cage and wound rotor induction motor, need for starters.
Direct-on-Line (DOL) Starters
Induction motors can be started Direct-on-Line (DOL), which means that the rated voltage is supplied to the stator, with the rotor terminals short-circuited in a wound rotor (slip-ring) motor. For the cage rotor, the rotor bars are short circuited via two end rings. Neglecting stator impedance, the starting current in the stator windings
The input voltage per phase to the stator is equal to the induced emf per phase in the stator winding, as the stator impedance is neglected (also shown in the last lesson (#32)). In the formula for starting current, no load current is neglected. It may be noted that the starting current is quite high, about 4-6 times the current at full load, may be higher, depending on the rating of IM, as compared to no load current. The starting
torque is which shows that, as the starting current increases, the starting torque also increases. This results in higher accelerating torque (minus the load torque and the torque component of the losses), with the motor reaching rated or near rated speed quickly.
The main problem in starting induction motors having large or medium size lies mainly in the requirement of high starting current, when started direct-on-line (DOL). Assume that the distribution line is starting from a substation (Fig.), where the supply voltage is constant. The line feeds a no. of consumers, of which one consumer has an induction motor with a DOL starter, drawing a high current from the line, which is higher than the current for which this line is designed. This will cause a drop (dip) in the voltage, all along the line, both for the consumers between the substation and this consumer, and those, who are in the line after this consumer. This drop in the voltage is more than the drop permitted, i.e. higher than the limit as per ISS, because the current drawn is more than the current for which the line is designed. Only for the current lower the current for which the line is designed, the drop in voltage is lower the limit. So, the supply authorities set a limit on the rating or size of IM, which can be started DOL. Any motor exceeding the specified rating, is not permitted to be started DOL, for which a starter is to be used to reduce the current drawn at starting.
Starters for Cage IM
The starting current in IM is proportional to the input voltage per phase to
the motor (stator), i.e. , where, as the voltage drop in the
stator impedance is small compared to the input voltage, or if the stator impedance is neglected. This has been shown earlier. So, in a (squirrel) cage induction motor, the starter is used only to decrease the input voltage to the motor so as to decrease the starting current. As described later, this also results in decrease of starting torque.
This type is used for the induction motor, the stator winding of which is nominally delta-connected (Fig. 33.2a). If the above winding is reconnected as star (Fig. 33.2b), the voltage per phase supplied to each winding is reduced by )1/√3(.577). This is a simple starter, which can be easily reconfigured as shown in Fig. 33.2c. As the voltage per phase in delta connection is Vs, the phase current in each stator winding is
, where is the impedance of the motor per phase at standstill or start (stator impedance and rotor impedance referred to the stator, at standstill). The line current or
the input current to the motor is which is the current, if the
motor is started direct-on-line (DOL). Now, if the stator winding is connected as star, the phase or line current drawn from supply at start (standstill) is which is of the starting current, if DOL starter is used. The voltage per phase in each stator winding is now (. 3 / s V ). So, the starting current using star-delta starter is reduced by 33.3%. As for starting torque, being proportional to the square of the current in each of the stator windings in two different connections as shown earlier, is also reduced by ( 2 ) 3 / 1 ( 3 / 1 = ), as the ratio of the two currents is ( 3 / 1 ), same as that (ratio) of the voltages applied to each winding as shown earlier. So, the starting torque is reduced by 33.3%, which is a disadvantage of the use of this starter. The load torque and the loss torque, must be lower than the starting torque, if the motor is to be started using this starter. The advantage is that, no extra component, except that shown in Fig. 33.2c, need be used, thus making it simple. As shown later, this is an
auto-transformer starter with the voltage ratio as 57.7%. Alternatively, the starting current in the second case with the stator winding reconnected as star, can be found by using
star-delta conversion as given in lesson #18, with the impedance per phase after converting to delta, found as ( s Z · 3 ), and the starting current now being reduced to (1/3 ) of the starting current obtained using DOL starter, with the stator winding connected in delta.
Auto-transformer Starter
An auto-transformer, whose output is fed to the stator and input is from the supply (Fig. 33.3), is used to start the induction motor. The input voltage of IM is , which is the output voltage of the auto-transformer, the input voltage being Vs. The output
voltage/input voltage ratio is x , the value of which lies between 0.0 and 1.0
Let be the starting current, when the motor is started using DOL starter, i.e applying rated input voltage. The input current of IM, which is the output current of auto-transformer, when this starter is used with input voltage as . The input current of auto-transformer, which is the starting current drawn from the supply, is, obtained by equating input and output volt-amperes, neglecting losses and assuming nearly same power factor on both sides. As discussed earlier, the starting torque, being proportional to the square of the input current to IM in two cases, with and without auto-transformer (i.e. direct), is also reduced by , as the ratio of the two currents is same as that (ratio) of the voltages applied to the motor as shown earlier. So, the starting torque is reduced by the same ratio as that of the starting current.
If the ratio is , both starting current and torque are %) 80 ( 8 . 0 = x %) 64 ( 64 . 0 ) 8 . 0 ( 2 2 = = x times the values of starting current and torque with DOL starting, which is nearly 2 times the values obtained using star-delta starter. So, the disadvantage is that starting current is increased, with the result that lower rated motor can now be started, as the current drawn from the supply is to be kept within limits, while the advantage is that the starting torque is now doubled, such that the motor can start against higher load torque. The star-delta
starter can be considered equivalent to an autotransformer starter with the ratio, %) 7 . 57 ( 577 . 0 = x . If %) 70 ( 7 . 0 = x , both starting current and
torque are times the values of starting current and torque with DOL starting, which is nearly 1.5 times the values obtained using star delta starter.
Rotor Resistance Starters for Slip-ring (wound rotor) IM
In a slip-ring (wound rotor) induction motor, resistance can be inserted in the rotor circuit via slip rings (Fig. 33.4), so as to increase the starting torque. The starting current in the rotor winding is
where = Additional resistance per phase in the rotor circuit.
The input (stator) current is proportional to the rotor current as shown earlier. The starting current (input) reduces, as resistance is inserted in the rotor circuit. But the
starting torque increases, as the total resistance in
the rotor circuit is increased. Though the starting current decreases, the total resistance increases, thus resulting in increase of starting torque as shown in Fig. 32.2b, and also obtained by using the expression given earlier, for increasing values of the resistance in the rotor circuit. If the additional resistance is used only for starting, being rated for intermittent duty, the resistance is to be decreased in steps, as the motor speed increases. Finally, the external resistance is to be completely cut out, i.e. to be made equal to zero (0.0), thus leaving the slip-rings short-circuited. Here, also the additional cost of the external resistance with intermittent rating is to be incurred, which results in decrease of
starting current, along with increase of starting torque, both being advantageous. Also it may be noted that the cost of a slip-ring induction is higher than that of IM with cage rotor, having same power rating. So, in both cases, additional cost is to be incurred to obtain the above advantages. This is only used in case higher starting torque is needed to
start IM with high load torque. It may be observed from Fig. 32.2b that the starting torque increases till it reaches maximum value, the external resistance in the rotor circuit is increased, the range of total resistance being The range of external resistance is between zero (0.0) and 2 r x -). The starting torque is equal to the maximum value, i.e. if the external resistance inserted is equal to if the external resistance in the rotor circuit is increased further,
the starting torque decreases. This is, because the
This is, because the starting current decreases at a faster rate, even if the total resistance in the rotor circuit is increased.
In this lesson - the fifth one of this module, the direct-on-line (DOL) starter used for IM, along with the need for other types of starters, has been described first. Then, two types of starters - star-delta and autotransformer, for cage type IM, are presented. Lastly, the rotor resistance starter for slip-ring (wound rotor) IM is briefly described. In the next (sixth and last) lesson of this module, the various types of single-phase induction motors, along with the starting methods, will be presented.
STARTING METHODS FOR SINGLE-PHASE INDUCTION MOTOR
Instructional Objectives
• Why there is no starting torque in a single-phase induction motor with one (main) winding in the stator?
• Various starting methods used in the single-phase induction motors, with the introduction of additional features, like the addition of another winding in the stator, and/or capacitor in series with it.
Introduction
In the previous, i.e. fifth, lesson of this module, the direct-on-line (DOL) starter used in three-phase IM, along with the need for starters, has been described first. Two types of starters - star-delta, for motors with nominally delta-connected stator winding, and autotransformer, used for cage rotor IM, are then presented, where both decrease in starting current and torque occur. Lastly, the rotor resistance starter for slip-ring (wound rotor) IM has been discussed, where starting current decreases along with increase in starting torque. In all such