In this article we will discuss about how to start and control the speed of traction motors.
Starting and Speed Control of DC Traction Motors:
Only series and compound dc motors are suitable for traction work. With a dc series motor, the current and torque produced at standstill may be reduced by strengthening the field or lowering the terminal voltage or both. Motors may be placed in series, reducing the terminal voltage of each without loss in external resistance.
External resistance may be placed in series with the motors to limit the starting current to any desired value, and by varying the resistance the current may be kept constant during notching up period as desired, as the back emf is being built up. Since maximum torque while starting demands full field strength, any shunt or reduced field connections are usually thrown out of action in starting.
With the dc compound motor the start may be made with full armature current in the series field coils and maximum current in the shunt field coils. A starting resistance inserted in the armature circuit is reduced in steps until the armature and series field are connected across the line. Further speed increase is affected by reducing the shunt field current in steps to the point where the shunt field winding is disconnected, and the action is then identical with that of a plain series motor.
ADVERTISEMENTS:
Plain Rheostatic Starting (Notching):
In this method the voltage across the motor is gradually increased from zero to full (normal) voltage and the current is kept constant at normal rated value by reducing the external resistance gradually. Let the starting period be T seconds, constant current I amperes and line voltage V volts.
At the instant of switching on the supply, the back emf is zero and it starts building up immediately the motor starts rotating. At any instant the supply voltage is equal to emf developed by motor plus voltage drop in the series field and armature of motor plus voltage drop in starting rheostat.
i.e., V = Eb + IRm + IR …(13.1)
ADVERTISEMENTS:
At the end of starting period, the back emf becomes approximately equal to line voltage and is given as-
Eb = V – IRm …(13.2)
... external resistance is reduced to zero, and therefore, voltage drop in the external resistance is zero. Energy drawn from the supply during starting
= VIT watt-seconds
ADVERTISEMENTS:
Energy lost in external resistance
= Average voltage drop across the external resistance × current × time
if voltage drop in series field and armature of motor is neglected.
Energy utilized in driving the motor
= Energy supplied – energy wasted in external resistance
= VIT – ½ VIT = ½ VIT watt-second
Starting effeciency,
The energy lost and the voltage drop can be illustrated with the help of illustrations shown in Fig. 13.6.
Series-Parallel Starting:
This method is usually employed in electric traction as motors employed for traction work are usually two, four, six or more than this even.
First consider two motors. Let the current be constant and equal to I amperes throughout, the line voltage be V volts and accelerating period be T seconds.
At the starting instant, both the motors are connected in series along with the starting resistance, the voltage across the motors (neglecting voltage drop in armature and series field resistance) is approximately equal to zero and voltage drop across external resistance is maximum and equal to V volts. As the motors speed up, the external resistance is gradually reduced to zero. During this period the voltage acting across each motor gradually increases from 0 to V/2 volts. Let the time required behalf of total accelerating period i.e., T/2 seconds.
Energy utilised by each motor
= Average voltage acting across each motor × current × time.
When the motors are changed from series grouping to parallel grouping external resistance is again inserted in the motors circuit. The voltage across each motor is equal to V/2 volts, the current per motor is I amperes and current drawn from the line is 21 amperes. As the motors speed up, the external resistance is gradually reduced to zero and voltage across each motor increases gradually from V/2 to V volts.
Energy utilised in driving the motors
= Number of motors × average voltage across each motor × current × time
In the case of 4 motors all the four motors are connected in series with starting resistance first. The voltage across each motor is gradually increased from 0 to V/4 volts by reducing the external resistance gradually to zero. The current per motor is equal to I amperes. The current drawn from the line is also equal to I amperes. The time required for increasing the voltage across each motor from 0 to V/4 volts is one-fourth of the total accelerating period i.e., T/4 seconds.
In second step two groups, each consisting of two motors connected in series are put in parallel. The voltage across each motor increases from V/4 to V/2 volts. The current per motor is equal to I amperes and current drawn from the line is 2I amperes. The period is T/4 seconds.
In case of 6 motors all the six motors are connected in series with starting resistance. The voltage across each motor is gradually increased from 0 to V/6 volts by reducing the external resistance gradually to zero. The current drawn per motor is equal to I amperes. The time required for increasing the voltage across each motor from 0 to V/6 volts is one-third of the total accelerating period i.e., T/3 seconds.
In the third step three groups each consisting of two motors connected in series are put in parallel. The voltage across each motor increases from V/3 to V/2 volts. The current per motor is equal to I amperes and current drawn from the line is 3I amperes. The time required will be again T/3 seconds. Energy utilized in driving the motors-
The system of double series-parallel control is used on heavy locomotives requiring a large number of rheostatic steps so as to provide smooth starting. With suburban trains, however, the slight saving in energy of double series-parallel control compared with series-parallel control is entirely offset by the additional cost, weight and maintenance of the double series parallel control equipment.
Advantages of Series-Parallel Starting:
(i) Higher starting efficiency. 66.67% with two motors, 72.72% with four motors and 75% with six motors whereas only 50% efficiency is obtained with plain rheostat starting.
(ii) Different economical speeds. Two with two motors in the ratio of 1 : 2 : three with four motors in the ratio of 1:2:4 and three with six motors in the ratio of 1 : 2 : 3.
(iii) For the same power input the torque of different magnitude is obtained. Torque in parallel combination is 1/4th of that in series combination but speed is four times of that with series combination. For the same motor current, the torque remains the same but speed in parallel combination becomes double the speed of that of series combination.
(iv) Owing to low energy loss in the starting rheostats, they are not of cumbersome size.
The main difficulty associated with series-parallel control is in obtaining a satisfactory method of transition from series to parallel without interrupting the torque or permitting any heavy rush of current. Leaving smaller units it is quite inadmissible to open the circuit to affect the change-over.
Practical Requirements:
Starting resistance is inserted in series with the motors connected in series or series-parallel control at the start and then gradually reduced to zero with the increase in speed of motors. In practice it is not possible to cut off the resistance from the circuit gradually but it is affected with a limited number of steps in the starting rheostats so as to avoid excessive cost and complication of the controller. Hence the motor current during starting cannot be maintained absolutely constant as assumed but it varies between two definite upper and lower limits as the sections of the rheostat are cut out (Fig. 13.7).
The number of steps to be provided proportional increase in the tractive effort taking place when a section of resistance is cut out. Because of heavy loads involved, shock tolerance limit is lower in the cases of locomotive trains than in the case of multiple unit trains. This, therefore, requires more rheostat steps in the case of controllers for locomotive trains than for multiple unit trains.
Fig. 13.7. Representation Current Variation during Starting Period
In order to maintain the average value of starting current constant it becomes necessary to change-over from series grouping to parallel grouping when all the starting rheostat has been cut out and the starting current is at its minimum value or at the lower limit. It is also essential to reinsert the starting resistance in the circuit while changing-over from series grouping to parallel grouping so that the motor current is limited to the prescribed value.
Drum Controller:
A series-parallel controller (with shunt transition) for two series motors is shown in Fig. 13.11.
In addition to its primary function of providing series-parallel operation of dc series traction motors it also controls:
(i) Speed and direction of motion of the vehicle
(ii) Magnitude of retardation during braking period and
(iii) Provides means to remove faulty motor if any fault occurs during the operation
(iv) If possible it must also provide prevent its moving backward when stopped on a steep gradient.
The controller is in the form of a rotating drum having insulated and interconnected segments in the form of strips, which makes contact with the fixed points (known as fingers). The controller has 8 positions, 4 positions for series and four for parallel running. The working positions of the controller (known as notches) are shown by vertical dotted lines.
The segments on the rotating drum are shown by black rectangles. Fixed contacts, known as fingers, are shown by vertical row of large circular dots on the left. Across these fingers, starting resistances and reversing contacts are connected. The reversing arrangement is provided by providing an additional rotating drum known as reversing drum.
Now when the reversing drum is put in forward position and controlling drum is set in position 1, the connections with fixed points are along vertical line 1. In this position segments make contact with fixed points 1 and 2, 8 and 9. In this position current flows from + ve fixed point 1 through the drum segment, fixed point 2, all the starting resistances 2-6, fixed point 7, and reversing drum to armature of motor No. I, series field of motor No. I, fixed points 9 and 8, armature and series field of motor No. II and returns back to the -ve terminal.
In the second position the segments of the controller drum come in contact with fixed points 1 and 3, 8 and 9 and thereby bringing some of the starting resistances out of circuit, therefore, speed of the motors increases. In the subsequent 3rd position additional step of resistance is brought out of the circuit and finally in fourth position starting resistance is reduced to zero and half of the supply voltage acts across each motor.
Further movement of the drum reinsert some of the starting resistance in the circuit and short circuit the motor No. II as the current from +ve fixed point 1 passes to fixed point 3, starting resistance, fixed points 6 and 7, reversing drum, armature and series field of motor No. I, fixed points 9 and 10 and returns to the -ve terminal through lower two segments on the controller drum (on lines 5 to 8).
As the controller drum reaches vertical line No. 5 the motor No. II is again inserted in the circuit and operates in parallel with motor No. I. As the controller drum moves further the starting resistance is reduced and finally in vertical position 8 of controlling drum, both the motors are directly connected across the supply mains and are in full parallel.
For reversing the direction of rotation of motor, it is essential that the direction of current of either of the two (armature or field) is changed. Generally it is achieved by reversing the armature current. In the forward position of reversing drum, the current enters the armatures and series field of motors No. I and II through reversing drum from the left but when the reversing drum is placed in reverse position, the current enters the armatures of motors No. I and II from the right while the direction of field currents remains the same. Thus the directions of rotation of motors are reversed by changing the position of the reversing drum which has two fixed positions.
When the position of controller drum is changed, the arcing will take place. For suppression of the arc magnetic blow-outs are employed. The blow-out coil system consists of flat iron inserted in the movable part of the rotating drum. The reversing drum is mechanically interlocked with the controlling drum so that it may not be operated unless the controlling drum is in the ‘OFF’ position.
2. Contactor Type Controller:
Schematic power diagram of a 1,500 V dc train equipment consisting of 4 dc series motors wired for series-parallel operation during starting, in its simplified form is shown in Fig. 13.12. Motors M1 and M2 are connected in series and have three breaks L1 and L2 and D. Motors M3 and M4 are also connected in series and have three breaks L3, L4 and C. Bridging contactors B1 and B2 have full line voltage of 1,500 V across them when the motors are in parallel and for this reason there are two breaks. Resistance X is inserted in the circuit to protect the system at switch on in case of fault in any motor.
In the starting position line switches L3, L4 and bridging contractor B close and then line switches L1 and L2. Now motors M1, M2, M3 and M4 are in series with full starting resistance in the circuit. Then contactors R operate on the following notches and takes the starting resistances out of circuit in steps, the motors are running in series on the full line voltage.
Then bridging contactors B1 and B2 close and B opens, the motors still operate in series but with no external resistance in the circuit. The contactors R then open, B1 and B2 open, C and D close, placing the motors in parallel with full starting resistance in the circuit. The contactors R then take the starting resistances out of circuit in steps and finally the required arrangement is obtained.
Buck and Boost Method of Speed Control:
The circuit diagram is shown in Fig. 13.15. The armatures of both the traction motors I and II and M-G set are connected in series. The whole series combination is connected across the supply. When the generator terminal voltage is equal to supply voltage in magnitude but of opposite polarity and the main contactor MC is closed, the voltage across the traction motors is zero and thus their speed is also zero.
On reducing the generator voltage, voltage across the traction motors will start increasing and so their speed too. When the generator voltage becomes equal to zero, full line voltage will appear across both the motors i.e., one-half of the supply voltage across each motor. In case polarities of both supply source and generator are same and generator voltage is equal to supply voltage, voltage across each traction motor will be equal to supply voltage. Thus the voltage across traction motors can be adjusted (bucked or boosted up) by adjusting the excitation of the generator.
The above methods of speed control have the following advantages:
(i) Traction motors can be operated on any speed, while in case of resistance control only a few speeds are possible.
(ii) There is no energy loss in the starting resistance of the traction motors. However, there is a loss of energy in the starting resistance of the M-G set.
(iii) In case of temporary interruption in the supply, the kinetic energy of the flywheel can be utilized in generating energy from the M-G set and supplied to the traction motors.
The main drawback of this method of speed control is requirement of M-G set. There is a loss of energy in M-G set.
2. Metadyne Control of Traction Motor:
The metadyne control system is based on constant current system of speed control. In resistance control or series- parallel control a great deal of energy is dissipated in the starting resistance and jerks are experienced when the controller of the starter moves from one position to another position. In metadyne control, current throughout the accelerating period remains constant, therefore, uniform tractive effort is developed and very smooth control, without causing any wastage of energy in the starting resistance, is achieved.
The essential part of the metadyne control is metadyne converter. The metadyne converter is a cross-field machine which behaves like a transformer on direct current. The transformation ratio of a metadyne can be varied continuously. It takes power at constant voltage and variable current and delivers the same at constant current and varying voltage.
The metadyne converter essentially consists, in its simplest form, of a 2 pole dc armature with two pairs of brushes and a four pole field magnet, as shown in Fig. 13.16. One pair of the brushes (say A and C) are connected across constant voltage dc supply while the other pair (B and D) are connected to the load (normally a dc series motor).
For understanding the working of a metadyne converter consider first an ordinary dc machine with two poles and two brushes supplied with a current flowing in the direction shown in Fig. 13.17(a). It will cause armature current distribution, as illustrated in the figure with corresponding cross flux, mainly confined to the poles.
Now consider that metadyne converter (a dc machine with two pairs of brushes and two pairs of poles) is running at constant speed and drawing a current I, from the dc supply main, which flows through the armature conductors via the brushes A and C, as shown in Fig. 13.17 (b). An armature reaction flux φ1 , set up in usual way is provided with a fairly low reluctance path through the yoke by the four poles, as shown in the figure. Due to rotation of armature conductors in this primary flux, and emf E2 = KI1 is set up between the brushes B and D. When these brushes B and D are connected to a load, a current I2 flows through the load. The load current I2 sets up another flux φ2 known as secondary flux, at the right angles to the first, the distribution is shown in Fig. 13.17 (c). This secondary flux φ2 causes an emf, E1 = KI0 between brushes A and C opposing the applied voltage. As the applied voltage is constant, the resistance drop is negligible so the back emf E1 opposing applied voltage and the current I2 producing E1 are also constant.
Since input = E1I1 = KI2I1 = KI2 × E2/K = E2I2 = output, therefore, power required to drive the metadyne is very small being equal to the running losses of the machine.
This simple metadyne converter transforms the constant voltage supply into a constant current variable voltage supply to feed the load. The arrangement, therefore, is quite suitable for starting dc traction motors. With this arrangement the load current I2 and supply voltage V remain constant and as the load increases on account of building of back emf in dc traction motor E2 and I1 increases to meet with the increased load.
The metadyne described above has no winding on the poles and is capable of delivering only a single value of constant current but for supplying dc traction motors, after the motor has gained speed, the load current I2 has to be reduced to the running value. For this purpose the field magnet poles are provided with variator and regulator windings, as shown in Fig. 13.18.
The variator winding sets up a flux in -the same direction as that set up by the load current I2. Total flux φ2 required in this axis being constant in order to produce a back emf E1 equal to the constant supply voltage, therefore if some of this flux is set up by a separate winding, known as variator winding, the load current I2 will decrease and can be of a smaller constant value. Similarly the load current can be increased by causing the current to flow in the variator winding in the opposite direction.
If the output current I2 is, say, reduced in this way, the voltage remaining the constant, the total output will be reduced but input will remain the same and therefore, the set will speed up. In order to keep the speed of the metadyne converter constant, an additional winding known as regulator winding is provided. By adjusting the current in the regulator winding, the input current can be varied and therefore, input power can be adjusted equal to output power, the speed of the converter remaining the same.
The regulator winding is supplied from a small dc shunt generator mounted on the shaft of the metadyne. Any tendency towards a change in speed of the metadyne will cause corresponding change in the emf set up by the shunt machine and as it acts in the opposite direction of the supply voltage, so corresponding change in the regulator winding current will result in.
The variator winding is supplied excitation from an exciter mounted on the same shaft, as shown in Fig. 13.18.
With metadyne converter, regenerative braking can be accomplished very easily by reversing the field of the traction motor. This causes the reversal of direction of induced emf E2 which in turn will change the direction of current I1. Thus current I1 can be supplied back to the supply source. By controlling the magnitude of reversed excitation of traction motors supplied by metadyne, the magnitude of regenerative braking can be regulated.
The metadyne is employed whenever control of dc motors is required. The control provided by the metadyne is smooth and does not require any switching. Thus switchgear and arcing are avoided. In some cases it is cheaper than the Ward Leonard system in initial cost. In traction it provides smooth acceleration without skill on the part of driver and regenerative braking down to very slow speeds. The savings due to these items may easily counterbalance the additional cost of the more complicated equipment required and its additional maintenance cost. It is already being employed in the underground railway.
3. Thyristor Control of Traction Motors:
The modern trend is towards the use of dc motors (both separately excited and dc series motors) equipped with thyristor chopper control. The operating voltages are 600 V or 1,000 V. Brakings employed are mechanical, rheostatic and regenerative. Thyristorised converters provide accurate control and fast response. Main advantages of thyristor control are the absence of bulky on-load tap changer and electromagnetic devices, saving of energy, notchless control, increase in pulling ability of the motive power, and minimum wear and tear because of absence of conventional moving parts in the motor control circuits.
In addition to ordinary phase control methods, cycle selection methods of control of SCR for varying the voltage applied to the traction motors are also employed. In this method the required average voltage is obtained by accepting or rejecting a certain number of complete half cycles. In practice, at the start only one half cycle out of eight is accepted and as the speed builds up, it is gradually raised to 2/8, 3/8 and finally 8/8 for full-power operation. This method is advantageous due to low frequency harmonics, low rate of rise of current, better power factor etc.
In chopper control of traction motors, at start, the ‘on’ period of pulse is kept very short which lengthens during the period of controlled acceleration. Thus the average voltage applied across the traction motors is gradually increased keeping the mean value of the input current close to the desired value.
Figure 13.19 shows a typical thyristorised dc traction system supplying a group of four separately excited motors. The armatures are supplied from half controlled bridge converters. However, it is desirable to feed the field windings through fully controlled bridge converters so as to reduce the ripple in the field current. Low ripple in the field current ensures low iron losses in the machines. However, if regenerative braking is required then the armatures should be supplied from fully controlled bridges.
Freewheeling diodes are connected as illustrated to ensure good waveform of armature current. The armatures are connected in series-parallel arrangement to ensure good starting and running characteristics. It is seen that armatures are supplied by three bridges connected in series.
For starting first only bridge A is triggered and firing angle is advanced as speed builds up. When bridge A is fully conducting (i.e., when a = 0), bridge B is triggered and then bridge C is triggered. During starting field currents are set to maximum to provide high starting torque. The use of three bridges ensures better power factor than would be possible with a single bridge.
Speed Control and Starting of Single Phase Series Motors:
The control of speed and torque in case of single phase ac series motor is carried out by varying the applied voltage. As the operating voltage for a single phase series motor is very low (about 300 volts), thus, transformer forms an essential part of the locomotive equipment and, therefore, variable voltage for both starting and speed control can be obtained by means of tappings on the transformer without requiring any additional equipment for this purpose.
The transformer tappings are usually provided on it side of the transformer in order to avoid insulation difficulties but 11,000 volt locomotives have been built with hv tappings, which enables a lighter construction to be used on account of much lower currents to be dealt with. As no external resistance is required for starting purpose, so there is no loss of power and each control point becomes a running point. The regulation of voltage is affected by the contactor method of tap changing, as shown in Fig. 13.20.
A preventive coil is employed for reducing the circulating current flowing on account of short circuiting of a section of the winding of the transformer between the tappings. Preventive coil (choking coil having high resistance) is inserted between the terminals of two groups consisting of tappings 1, 3, 5 and 2, 4, 6 as shown in the Fig. 13.20. The motor is connected to the centre point of the preventive coil.
Two contactors, connected to the adjacent tappings (1 and 2 or 2 and 3 or 3 and 4 or 4 and 5 or 5 and 6) are closed on each notch of the controller. Each contactor, therefore, carries approximately one half of the total current required. For changing-over voltage from one value to another one contactor is opened and another contactor belonging to the same group is closed.
An important advantage of this method is that each notch provides a running position, so that many running speeds are available. Between 6 and 12 tappings are usually provided, giving a corresponding number of economical speeds.
In case larger currents are to be handled by the contactors (for large capacity traction motors), a larger number of contactors are to be employed simultaneously so that the current is divided between them and each contactor has not to deal more than a pre-specified design value.
Contactor method of tap changing for larger currents (over 1,000 A) is shown in Fig. 13.21. In this method four adjacent contactors are connected to the same notch of the controller. There are three preventive coils, also called ‘bridging coils’ arranged in such a fashion as to facilitate the correct division of current among the contactors.
The ac contactors are quite different from dc contactors. In general, dc contactor is much lighter than the ac contactor and needs less power for its operation.
Other special features of ac contactors are:
(i) Laminated magnetic circuit
(ii) Provision of shading coil in order to avoid chattering of contactors.
Starting and Speed Control of 3-Phase Induction Motors:
The method employed for speed control of 3-phase induction motors are:
(i) Rheostatic control
(ii) Pole changing control
(iii) Cascade control and
(iv) Combination of cascading and pole changing control.
Rheostatic control is the simplest but the least efficient method of speed control of 3-phase induction motors. This method of speed control is employed in light locomotives and motor coaches where a single economical speed is sufficient and energy consumption is of no importance.
Pole-changing control is the simplest of the multi-speed control methods. This method has the advantage of simplicity, good speed regulation for each setting, high operation efficiency, and moderate first cost and maintenance. The choice of the number of poles on a pole-changing winding is in the ratio of 2 : 1, 3 : 2, 4 : 3.
Cascade control is employed for the two-speed goods locomotives (24 kmph and 50 kmph).
In combination of cascading and pole-changing control method, both of the motors employed are wound for the same number of poles and windings so arranged that the number of poles can be changed. With equal number of poles on both the machines one speed is obtained and then more speeds are made available by changing the number of poles. This method has been employed for the four-speed passenger locomotives (37, 51, 74 and 102 kmph) in some types of locomotives.