EMERGENCY STOP devices
3 Starting and switching motors
3.9 Electronic soft starters
Soft starters serve for a continuous adjustment of the starting characteristic of three-phase asynchronous motors to the requirements of the load by controlling the voltage across the motor and enable for an optimum integration of the drives in process control by means of various complementary functions.
While when star-delta starters are used, the starting torque and starting current can be fix reduced to around a third, with electronic soft starters the reduction can be set within a wide range. It should be noted that the motor torque of a soft starter falls with the square of the voltage and current reduction. With the same starting current as with a star-delta starter in star connection (= 1/3 IAΔ), with a soft starter the motor torque falls to 1/9 TAΔ in comparison to 1/3 TAΔ for star-delta. See also Section 1.7.1.3.
With the conventional starting procedures such as direct on line starters, starting transformers or star-delta starters, the motor, supply and the entire drive chain is loaded by switching transients. Each switching procedure also means a rapid current change (transient current peaks) and hence generates high torque peaks in the motor. Electronic equipment with power
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semiconductors can prevent these transient effects and reduce the loading of power supply and drive.
The following features and options are characteristic in the use of soft starters:
Extended setting range of the starting characteristic or selection of various starting character-istics for an optimum adjustment to the requirements of the working machine
Infinite variable characteristic of current, voltage and torque. No transient current peaks
Motor connection with only three lines with control in the motor supply lines
Increased rated power of the soft starter (factor 1.73) with control in the windings circuit and motor connection with six lines
By-passing of power semiconductors after motor start to reduce the permanent losses
Limited number of starts per hour depending on starting conditions and thermal specifica-tions of the soft starter
Extended coasting to stop and braking of drives
Crawl speed for positioning
Diagnostic and early warning functions such as overload, underload, locked rotor etc.
Integration in a communication network
Integrated (motor) protection functions
Current harmonics during the starting time by phase control
Drives with soft starters require for maintenance work on the motor a series disconnecting device (for example disconnector switches, circuit breakers with isolating function).
Soft starters are available in a variety of different designs, each with specific technical charac-teristics. For the selection of a device for a specific application the technical literature of the manufacturer and its technical support have to be observed. See also IEC 60947-4-2 [5] and [17]. For specific aspects of high efficiency motors see 1.7.1.2.1.
3.9.1 Voltage ramp versus current limitation
The basic mode of operation of soft starters is to control the voltage across the motor by phase control. Usually the phase control is performed in 3 phases and in both current half-cycles by means of triacs or antiparallel connected thyristors. Economical solutions use controlled semiconductors in only two or even only one (1) phase. The resulting asymmetries create disadvantages with respect to the available torque related to the current consumption and for example an increasing loading of the motor bearings because of torque asymmetries. The 1-phase controller corresponds to the stator-resistance starting circuit (see Section 3.5).
The voltage across the motor can for example be controlled by
a (selectable) voltage ramp or
a fixed (reduced) voltage (quasi current-limiting)
in relation to a feedback variable such as o motor current (current limitation) or
o speed (start following a speed characteristic)
Depending on the method chosen, typical torque and speed characteristics for starting are produced (Fig. 3.9-1). When starting with a voltage ramp and especially when starting with current limitation, large acceleration torques in the range of the breakdown torque are generated towards the end of the starting period.
Direct start Current at direct
start T/Te
Current limit Current at soft start
with voltage ramp Current at soft start
with current limit Voltage ramp
Load
n/ns
Fig. 3.9-1
Current and torque characteristics for starting
In the following a more detailed discussion of the characteristics of various available soft starter functions is presented.
3.9.2 Voltage ramp
The voltage across the motor is linearly increased during a settable time, starting from an adjustable initial value (Fig. 3.9-2). The starting current and the starting torque, and hence the acceleration, adjust themselves in accordance with the voltage ramp and the torque characteris-tic of the load. This method is especially suitable for load-free start-ups and for working ma-chines with increasing torque requirement at increasing speed (drives with larger inertial masses, fans etc.).
Soft start with voltage ramp
For drives with variable loading at the start – for example processing machines that normally start up in a load-free condition, but which can be under load due to a fault – soft starters with two voltage ramps are available (Fig. 3.9-3). The initial voltages and starting times of ramps are separately adjustable and hence can be adapted to both operating states. It is possible to switch between both ramps as required.
Ramp #2
Soft starter with changeable voltage ramp for various loading states at start.
3.9.3 Kickstart
Many drives have a high breakaway torque at rest, because for example bearings surfaces may generate high initial friction. This requires a short period of increased starting voltage at the
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beginning of the start-up. As soon as the drive begins turning, the torque requirement decreases strongly and the start can be continued with a lower voltage. Soft starters with kickstart function offer the required functionality (Fig. 3.9-4).
Percent Voltage 100%
Initial Torque
Kickstart
Start Run
Time (seconds)
Fig. 3.9-4
The kickstart function briefly increases the voltage at the beginning of a start-up to overcome the breakaway torque of the drive.
3.9.4 Current limitation
For starts with current limitation, the starting current is adjustable. The supply load can hence be adjusted to local requirements – for example, of the electricity utility. The motor current is
measured in the soft starter and the motor voltage adjusted in accordance with the set current limitation. The available motor torque falls with the square of the voltage or current. With strong current limitation, a small starting torque is thus available (Fig. 3.9-5 and Fig. 3.9-1).
A simplified version just sets a fixed reduced voltage during starting.
600%
Percent Full Load Current
Start Time (seconds)
Fig. 3.9-5
During starting with current limitation the starting current can be adjusted to the supply conditions
3.9.5 Soft stop
With some drives it is desirable to also control the stoppage of the motor with a soft stop instead of a possible abrupt stop when the motor voltage is disconnected. This may for example be the case with conveyor equipment, in which sudden stopping could result in the transported goods being displaced or falling over. Soft starters with an adjustable voltage ramp upon stopping are one suitable solution. For pump controllers, see Section 3.9.6.
100%
Percent Voltage
Initial Torque
Kickstart
Start Run Soft Stop
Soft Stop
Time (seconds)
Coast-to-rest
Fig. 3.9-6
Softstop function with adjustable coasting time
3.9.6 Soft starters for pump controls
In the case of rapid changes of the speed of liquids – whether at acceleration or braking – hydraulic hammer and cavitation effects can arise in large centrifugal pump systems that subject the systems to heavy mechanical stress and generate corresponding acoustic side effects. The mechanical impacts result from the fact that liquids cannot be compressed and the pressure changes in the pipe systems resulting from changes in the speed of the liquid. In the accelera-tion process the inertial mass of the medium to be accelerated (or decelerated) must be taken into account.
To avoid the problems described above, a slow change in the flow rate of the fluid is required during starting and stopping. During starting, this requires control of the motor voltage so that a small, constant acceleration torque matched to the specific plant requirements is produced that towards the end of starting makes a gentle transition to the operating point. The characteristics of normal soft starters with voltage ramp or with current limitation only partially fulfill these requirements. The special pump control characteristic of Rockwell Automation soft starters offer an appropriate solution (Fig. 3.9-7).
T/Te
Fig. 3.9-7
The pump control function ensures that the liquid is accelerated in a gentle manner
When a centrifugal pump is allowed to naturally come to a halt, the counter-pressure of the pumped medium usually results in abrupt braking and the associated hydraulic surges, which subject the system to heavy mechanical stress. A normal soft stop by linear reduction of the motor voltage would reduce the problem somewhat but not completely remove it. The Rockwell Automation pump control function with soft stop reduces the motor voltage so that the flow-rate of the liquid continuously decreases and hydraulic impacts are avoided.
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Fig. 3.9-8
The Rockwell Automation pump control function for soft starters continuously controls the flow of the medium during start-up and stopping and prevents hydraulic impacts with their adverse consequences.
3.9.7 Motor braking
For applications in which the natural coasting to a halt of the motor takes too long – for example with drives with large inertial masses – the braking function of soft starters can be useful. By appropriate control of thyristors, a braking torque is generated in the motor that results in accelerated braking. The coming to rest of the motor is detected by measurements of the back e.m.f. so that no additional devices such as standstill-sensors, contactors etc. are required. This method is not suitable for EMERGENCY STOP shutdowns.
100%
Motor Speed
Start Run Brake
Time (seconds)
Smart Motor Braking
Coast-to-rest
Automatic Zero Speed Shut-off
Fig. 3.9-9
The shutdown time of a drive can be reduced with the braking function
3.9.8 Positioning speed and controlled braking
Precise positioning is frequently required in engineering applications, for which the motor speed is temporarily reduced. Soft starters provide various options for accurate positioning, in one or two senses of rotation and in combination with controlled braking. See Fig. 3.9-10 to
Fig. 3.9-12.
100%
Motor Speed
Forward 15% - High
20% - High 7% - Low
10% - Low
Time (seconds)
Reverse
Start Run
Fig. 3.9-10
Soft starters with crawl speed in both directions for positioning purposes
100%
Motor
Speed 7 or 15%
Slow
Speed Start Run Stop
Braking Coast-to-rest
Time (seconds)
Fig. 3.9-11
Soft starters with positioning speed in one direction and controlled braking
100%
Motor
Speed 7% or 15%
Slow Speed
Start Run
Time (seconds) Brake Slow Speed
Braking
Slow Speed Braking Coast-to-rest
Fig. 3.9-12
The Accu-Stop™ function enables precise (accurate) positioning in one direction and precise stopping. In the braking phase, the motor is in a first phase broken until the positioning speed is reached. Complete braking happens as soon as a stop command is issued.
3.9.9 Linear acceleration and deceleration by speed feedback
Feedback of the motor speed can be used for linear acceleration and stoppage of a drive. The motor voltage is adjusted – largely independently of the torque requirement of the drive – in accordance with the speed feedback so that starting and braking follow the selected character-istic. The resulting motor current is a function of the required voltage.
100%
Percent Speed
Start Run Stop
Time (seconds)
Fig. 3.9-13
Linear acceleration and deceleration of a drive by speed feedback via tachometer.
3.9.10 Direct start with full voltage
Just as with solid state (semiconductor) contactors, a start with full voltage and hence with full motor torque can be performed with soft starters.
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100%
Percent Voltage
Time (seconds)
Fig. 3.9-14
Direct start with soft starters. The motor voltage is raised in a short period to the supply voltage.