Train Motor Operations Background

highest torque is below the base speed of the motor. Base speed is the speed attained by the motor when operating at rated armature motoring voltage and full-rated field flux. The typical ratio of top to base speed for urban service is approximately 3 to 1.

Two types of dc motors currently in use are the series-wound motor and the separately excited motor. These motor types differ in the way which the motor armature and motor field are electrically connected. As the name implies, the series-wound motor has the motor armature and field connected in series; hence, the same current passes through both components. The separately excited motor has the motor armature and motor field connected in parallel; therefore, the field current can be modulated independently of the armature current.

Base Speed 3 x Base Speed

0 0 0.2 0.4 0.6 0.8 1.0

Per Unit Torque

0 1.0 2.0 3.0

Per Unit Horsepower

Resistance Motoring

Horsepower

Braking Moto

ring

Braking Torque

Figure 8.1 Typical Torque, Power and Speed Curves

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When starting a dc motor, the full line voltage cannot be initially imposed across the motor without overloading it. It is necessary to control the voltage across the motor until it comes up to the base speed. In conventional trains with dc series motors, the motor voltage is controlled by inserting external

resistances in the circuit. The resistances are then notched out of the circuit by a device such as a cam controller as the motor-generated back voltage (or back EMF) increases with speed. More recently, the advent of solid-state thyristors with large power capabilities is providing a practical alternative to the cam controller equipment which had been almost universally used in rapid-transit cars. The thyristor chopper is a semi-conductor device which can be described as a switch which opens and closes very rapidly. By varying the switch “on” to “off” time ratio, the average motor voltage can be regulated. The speed with which the thyristor chopper can regulate and control the line voltage with all its fluctuations also allows the use of separately-excited traction motors.

During braking, the motor is reconnected to act as a generator whose armature is driven by the train. The generated power is dissipated at a controlled rate producing a uniform rate of deceleration, a process known as dynamic braking. Two methods for dissipating this current are in use: rheostatic braking and regenerative braking.

Rheostatic braking is the more established method and consists of dissipating the generated electricity in the form of heat by passing it through a bank of resistors (known as “resistor grids”).

Regenerative braking, unlike rheostatic braking, tries to recover as much of the generated energy as possible and put it to useful work. Part of the generated current is used to power on-board auxiliary equipment and the remainder is available to the current distribution (or third rail) system for use by other trains operating in the vicinity of the regenerating train. The ability of the direct current distribution system to accept power from a regenerating train, known as the “receptivity,” is a function of the distribution network circuitry and the positioning of current-drawing trains relative to the regenerating train. In the event that the line is not receptive, the excess energy is dissipated by on-board resistor grids. Other schemes have been contemplated which utilize wayside resistor banks to expel the heat outside the subway system as well as methods of energy storage such as a flywheel.

In general, a conventional train utilizing a cam controller will be equipped for rheostatic braking.

Though not impossible, there are serious technical difficulties in trying to regenerate with dc motors having resistance control. The difficulties arise because the mechanical contactors performing the switching between the resistance and regeneration circuits cannot respond quickly enough to variations in line voltage (which is a measure of the receptivity of the line), leading to unstable operation.

The SES program has the capability of simulating both methods of speed control (i.e., cam control and chopper control) as well as rheostatic braking and regenerative braking. The program is structured to simulate either a train with cam controller and rheostatic braking or a train with chopper control and either regenerative or rheostatic braking. Also, since the SES program does not consider the details of the traction motor but uses the overall motor characteristics (i.e., tractive effort versus speed, armature current versus speed, etc.), both series-wound and separately-excited motors can be simulated.

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Train Speed Control

Cam Controller - the electrical switching which takes place as a rapid transit vehicle accelerates from a standstill is complex and may vary with the particular control system used. The following sequence is for a cam controller. In a typical 600 volt traction system, where each car employs four 300-volt series-wound dc motors, all four traction motors are initially connected in series for acceleration in order to limit current and to reduce the size and weight of resistances (refer to Figure 8.2). As the train accelerates, the current limit control sequentially removes (or notches) all the resistors out of the circuit until each motor is running at 150 terminal volts. The function of the current limit control is to permit notching advance of the control system to the next increment of voltage across the motors. This occurs only after the current has decreased to a predetermined value. In this way an approximately constant average amperage is maintained through the traction motors resulting in a constant average tractive effort. This results in a constant rate of acceleration. At this point, the voltage to each motor must be increased if the train is to continue acceleration. The control system then reconnects the motors from four in series to two parallel groups of two motors in series — a switching sequence known as “transition” (refer to Figure 8.3).

Resistance is again introduced into the system, this time with an imposed voltage of 600 volts per parallel circuit. The resistance is gradually notched out under current limit control until each motor is running at 300 terminal volts.

Transition ordinarily occurs at a train speed in the neighborhood of one-half of base speed. The series-parallel connection is used throughout the remainder of the cycle. The motors which have operated at full field strength throughout the series connection, continue to do so until all external resistance has been notched out. By definition, this would occur at base speed. At this point, the control system reduces the field strength of the motors as dictated by the current limit control. The base speed varies among motor designs, but a representative value would be 25 mph. When the field strength has been reduced to the minimum value, the train continues to accelerate according to the available tractive effort as indicated on the motor characteristic curves.

MOTOR 2

MOTOR 3

MOTOR 4

GROUND MOTOR 1 RESISTOR GRID +600 V

Figure 8.2 Traction Motors in Series

MOTOR 4 MOTOR 3

MOTOR 2 MOTOR 1 +600 V

RESISTOR GRID

GROUND

Figure 8.3 Traction Motors in Series/Parallel Connection

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Chopper Controller - A typical chopper control arrangement containing the major components is shown in Figure 8.4. The system shown has four series-wound motors per car operating from a 600-volt line. Unlike the cam controller which reconnects the motors, the chopper controller is assumed to retain the motors in series-parallel. The additional components required by the chopper controller are the main thyristor and logic circuit, the line filter (capacitor and inductor), the main motor reactor (inductance), and the free-wheeling diode.

Figure 8.4 Typical Chopper Control Motor Armature

Motor Field

Motor Reactor

Chopper

Capacitor FWD Line Reactor Third Rail

1 2

3

Ground

1 2

3 4

4

The heart of the chopper is a form of thyristor known as a silicon-controlled rectifier (SCR). The SCR is a uni-directional semi-conductor which may turn “on” by the application of a low-power signal to its “gate.” Once “on,” the SCR remains in a conducting mode. To turn the SCR “off” it is necessary to interrupt the power flow for a specified amount of time, which is accomplished by reversing the voltage across the SCR. This process of turning the SCR off is called force commutation, and it must be used when the power source is dc

It is sufficient to treat the chopper inner working as a “black box” operation. Hence, reference to

motors to ground. When the chopper is turned “off,” the energy stored in the motor reactor as well as in the collapsing motor field continues to drive current through the motor circuits by way of the loop formed by the free wheeling diode (FWD).

The average voltage applied to the motors is controlled by adjusting the ratio of the chopper “on”

to “off” time. This adjustment is made by the chopper control logic to maintain the desired average motor current and hence, motor torque. The input to the control logic is from the train operator or from automatic control equipment.

A train accelerating from a standstill uses chopper control of the armature voltage (and thus torque) up to base speed. For the cam controller, this operation was accomplished by progressively notching the external resistance out of the motor circuit. By eliminating the need for external resistance, the chopper demonstrates a savings in heat input to the tunnel air. The high-power portion of the accelerating cycle is extended beyond base speed by motor field weakening, as in the case of the cam-controlled train.

Braking Methods

Rheostatic Braking - Rheostatic braking refers to the method of slowing down a train by dissipating its kinetic energy in a controlled manner. The kinetic energy is converted to electrical energy which is dissipated in the form of heat by resistor losses. This is accomplished by reconnecting the traction motors as generators in series with resistor grids. The generators are driven by the motion of the train. Some of the kinetic energy is directly converted to heat due to irreversible losses from air friction and turbulence, mechanical friction of bearings and gears, electrical windage and chopper losses, etc.

The remainder is released by the deceleration resistor grids.

Regenerative Braking - A suitably equipped chopper-controlled train has the potential for feeding energy back into the “line” during braking. This regenerated energy could be used to power on-board equipment as well as satisfy the power needs of a train operating in the vicinity of the regenerating train. Regeneration into the third rail is sometimes not possible because of a third rail gap or the absence of a load on the third rail. In that event, the chopper control logic provides an almost instantaneous shift to rheostatic braking (use of on-board resistances in conjunction with regeneration is termed “natural”

regenerative braking). The control logic continually tests the receptivity of the line and if at a later time it determines the line to be receptive, regenerative braking will be resumed.

Other methods have been proposed for handling the regenerated energy. One feasible design consists of providing “wayside resistors” at electrical substations to dissipate the excess electrical energy which could not be absorbed by other trains. The heat generated is then released outside the subway system. Another scheme proposes recycling the regenerated energy via dc-to-ac inverters to be fed-back to the utility feeders, and yet another proposes storing the energy in a flywheel device making the energy available for future use.

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The amount of energy regenerated varies from one subway system to another. Among the variables affecting regeneration are: track profile, train headway, the type of traction motor used, electrical distribution network circuitry, and the type of regeneration scheme. Because of the complex dependence on various system parameters, the results obtained for a particular system cannot be universally applied. In addition, the sparse measured data available is not always presented in a usable way. For example, measurements were taken on eight Class C7 cars on the Stockholm Subway System during off peak periods. The cars were equipped with chopper-controlled, separately excited dc traction motors and natural regenerative braking. Regeneration was found to be around 25 percent, the lowest value being 20 percent and the highest more than 30 percent. The percentage of regeneration was defined as:

R W

W

r Total

= × 100 percent

where Wr was the energy regenerated to the line during braking, and WTotal is the total energy supplied from the line during the whole run without regeneration. Unfortunately, these results cannot be directly applied to the SES, and sufficient information is not available for transforming this data to a usable form.

The following convention will be established for determining the percentage of regeneration during braking:

The Regenerative Effectiveness Total Energy Regenerated Total Energy Available for Regeneration

=

where the total energy available for regeneration equals the total mechanical energy stored in the train (kinetic plus potential energy relative to the stopping point) minus all the losses. Note that this

regenerative effectiveness pertains to a particular train during a braking cycle and does not reflect the total system efficiency. An average value for all the stations in the portion of the system being simulated is more representative of the overall benefit of regeneration. Therefore, the average regeneration effectiveness is the input required by the SES.

When running an SES simulation for an existing subway system with regenerative capabilities, the value of the regenerative effectiveness can be determined through a field testing program. On the other hand, when evaluating a system which is still in the preliminary design stage, the only resource available is to gain access to a computer program which simulates the current flow from a chopper-

consisting of three subway stations and their connecting tunnels. Each of the stations was identical and the distances between stations were approximately 3800 ft and 6300 ft. The maximum train speed attained was 50 mph and the maximum rates of acceleration and deceleration were 3.0 mph/sec. The train comprised eight cars with each car weighing 50 tons. The results are given in Table 8.2.

Table 8.2 Regeneration Effectiveness

Headway (sec) Natural Regeneration Assured Regeneration

90 46% 73%

150 47% 75%

The values given in Table 8.2 reflect the average of six values (two per station - one value for each direction). The use of an average effectiveness in SES applications is justified by an examination of the overall thermal behavior of the train/tunnel system. In real systems with natural regenerative braking, the energy regenerated by successive trains following the same route will vary because of unavoidable changes in the relative positioning of decelerating and accelerating trains. In terms of vehicle heat release, this variation in regenerated energy is reflected first in an altered electrical input to the on-board rheostatic brakes and subsequently as heat release to the subway air. The thermal time lag of both the rheostatic braking system and the subway air temperature acts to smooth the effects of train-to-train variations in regeneration effectiveness. Hence, an average value for effectiveness will reflect the actual benefit of regeneration with reasonable accuracy.

In general, the regeneration effectiveness for a real system should increase with shorter headways since the probability of having a favorable situation for regeneration (finding an accelerating train in the vicinity of a regenerating train) is increased. The effectiveness data in Table 8.2 violates this generalization, showing a slight increase in effectiveness with increased headway. This discrepancy is explained by the limited number of relative train situations examined by the study and serves to emphasize the statistical nature of average regeneration effectiveness.

Motor Operating Characteristics

The rate of acceleration depends on the net tractive effort (i.e., the propulsive force applied at the rim of the wheel). The net tractive effort is defined as the tractive effort available from the train's traction motors minus the total drag on the vehicle. The total drag on the vehicle is a function of track grade and curvature, train speed, air velocity relative to the train, and blockage ratio. The tractive effort which is available from the motors is a function of the performance characteristics of the motors. The performance

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characteristics of the motor system can be obtained from the manufacturer in a graphic form as shown in Figure 8.5.

Series-Wound DC Motor - The relationship among the train speed, the tractive effort, and the current drawn may be taken from the motor characteristic curves similar to those shown in Figure 8.5.

This set of curves, which is typical of series-wound motors, shows two basic relationships: train speed versus motor current at a given motor terminal voltage and tractive effort versus motor current. The curves marked “Train Speed” show the maximum current drawn by the motor as a function of the train speed. Six mph curves are shown, labeled FS1 through FS6, and these correspond to six field strength settings for this motor. FS1 shows the relationship at the maximum field strength which is used at low speeds and FS6 shows the relationship at the minimum field strength which is used at higher speeds.

The curves marked “Tractive Effort” show the tractive effort produced by the motor as a function of motor current. The six curves, marked FS1 to FS6, correspond to the six train speed curves, respectively.

The curves are interpreted by first choosing a train speed, entering the graph at that speed, and continuing horizontally to the “Train Speed versus Motor Current” curve for the proper field strength. From this point, a corresponding motor current can be read from the scale at the bottom of the graph. This is the maximum current that would pass through the motor at the chosen train speed and field strength. This motor current is what would be observed if the motor was connected directly to a power source of the rated voltage. However, this current could be reduced if an external resistance is placed in series with the motor.

For a given motor current, the graph is entered at the bottom moving vertically until the tractive effort curve with the proper field strength is intersected. From this point, the corresponding tractive effort can be read from the scale on the right. This is the tractive effort that would be produced at the given motor current. If a smaller motor current were used, a smaller tractive effort would be produced according to the relationship shown on the graph. In summary, if no external current limiting resistances are used, the graph is entered at the left for a given train speed. The intersection with the mph curve of the proper field strength allows the motor current to be read from the scale at the bottom of the graph. If the mph line does not intersect a field strength curve, then it should terminate at the maximum amperage allowed as shown in Figure 8.6. This current is then used with the tractive effort curve corresponding to the proper field strength to determine the tractive effort from the scale on the right side of the graph. In a similar manner, the graph may be entered with a motor current or a tractive effort, and the corresponding values of the other parameters can be evaluated.