List of electric motors used for traction: 1. DC Series Motor 2. DC Shunt Motor 3. AC Series Motor 4. Three Phase Induction Motor 5. Linear Induction Motor.
1. DC Series Motor:
In this article dc series motor will be analysed from the point of view of requirements of traction service.
1. The dc series motor develops high torque at start which is an essential requirement of traction service.
2. The series motor is amenable to simple speed control methods.
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3. In case of dc series motors-
assuming flux proportional to exciting current and applied voltage constant.
But since power output of a motor is proportional to the product of torque and speed
Thus power drawn from supply mains varies as the square root of the load torque in case of dc series motors i.e., the series motors to some extent are self-relieving ones.
4. Series motors are not suitable for regenerative braking as these are not electrically stable unless certain measures are taken.
5. In case of dc series motors commutation is excellent up to twice full load, therefore, replacement of brushes etc. is not required frequently.
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6. In case of dc series motors the flux varies as, the armature current, torque corresponding to a given armature current, therefore, is independent of line voltage and thus unaffected by variations in the supply voltage.
With the sudden increase in supply voltage, the armature current tends to increase, but with the increase in armature current the flux also increases which causes increase in back emf and thus armature current restores to its initial value. Thus series motors are less susceptible to sudden change in supply voltage.
7. In case of dc series motors, up to the point of magnetic saturation, torque developed is proportional to the square of the armature current. Therefore, dc series motor requires comparatively less increased power input with the increase in load torque. Thus the series motors are capable of withstanding excessive loads.
8. Because of low time constant of field in case of series motor, the field flux dies away in very short time with the result that back emf of the motor ceases with temporary interruption of supply. In case of dc series motors, therefore, there is undue rush of initial current on temporary interruption of supply.
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9. The dc series motors, when operated in parallel to drive a vehicle by means of different driving axles, share load almost equally even there is unequal wear of different driving wheels. This is explained as below. Consider two identical motors of series type (say of speed- current characteristic shown in Fig. 12.3) connected to different driving axles of the vehicle.
For a given speed of vehicle, the peripheral speed of all the driving wheels will be same, but should there be a slight difference in the diameters of the driving wheels (which is possible on account of difference in wear) there will be a corresponding difference in the speeds of the driving axles to which they are connected and also, therefore, a difference in speed of the motors. The speed of the motors will be given by the relation N2/N1 = D1D2.
When the motors are working in parallel the current drawn by each motor (determined from speed-current characteristic) due to slight difference in the speed does not differ much. As voltage across each motor is same, therefore motors share the load almost equally.
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10. When the series motors are operating in series (Fig. 12.4), the current drawn by each motor will be same but due to slight difference in speeds, the back emfs will differ slightly. This will result in slight difference in voltage acting across each motor and also, therefore, slight difference in load shared by each motor.
Let V1 and V2 and V be the voltage drops across motors I and II and supply voltage respectively (Fig. 12.4). Since N ∝ (V- IR) where I is the load current and R is the resistance of each motor
Also V1 + V2= V …(12.11)
Solving above Eqs. (12.10) and (12.11) we have
11. The dc series motor is simple and robust in construction.
The dc series motor, owing to its characteristics, is most suitable for all types of traction services but more particularly for suburban and urban services where high rate of acceleration is essential. Motors of 45 kW and 75 kW, mass 5.4 to 7.2 kg per kW are available for trams and trolley buses although larger motors required for railway work are usually of masses of 12 to 18 kg per kW.
In 1,500 V dc system the dc series motors insulated for 1,500 V may be operated either at 1,500 V or at 750 V by connecting two of them permanently in series. In 3,000 V dc system the dc motors are insulated for 3,000 V but operated at 1,500 V by connecting two in series, as shown in Fig. 12.4.
The main drawbacks of dc series motors are owing to commutator such as restricted speed, voltage and current. There is a risk of flash-over and the brush gear requires considerable maintenance. The speed is also limited due to centrifugal forces acting on the rotor winding.
2. DC Shunt Motor:
DC shunt motors are not suitable for traction purpose, the reasons are given below:
1. The dc shunt motor is a constant speed motor and its speed-torque characteristic does not meet with the requirements of traction.
2. In case of dc shunt motors, where the speed is approximately constant and independent of load torque, the power output ∝ TN ∝ T
i.e., the power drawn from supply mains varies directly as the load torque. Hence a given overload will require a greater amount of power when shunt motors are employed than when dc series motors are employed.
3. With the increase in load, the dc shunt motor besides getting overloaded develops commutation troubles also.
4. In case of dc shunt motors the flux is not independent of supply voltage, torque developed corresponding to a given armature current, therefore, not independent of supply voltage. Thus the torque developed is considerably affected by variation in supply voltage.
Due to large inductance of field winding, the rate of change of current in this winding is low. Thus the current increases with the increase in supply voltage. With the buildup of current in the field winding due to increase in supply voltage, the back emf increases and hence the value of increased current starts decreasing. The current inrush in case of shunt motors will, therefore, be of long duration and high initial value in comparison to that in series motors.
5. In case of dc shunt motors, the field flux remains constant and torque developed is directly proportional to the armature current. Hence for a given increase in torque developed a greater amount of current will be drawn from supply mains when dc shunt motor is used than when dc series motor is used.
6. Because of the large time constant of field winding of dc shunt motor and because it is connected across the armature, the back emf during interruption of supply is maintained even though it is decreasing. Now when the supply voltage is restored, the inrush current in the shunt motor will be determined by the resultant voltage in the armature circuit and impedance of armature circuit. Thus, inrush current in the shunt motor due to interruption of supply will be less in comparison to dc series motor.
7. In case of two identical shunt motors connected in parallel, the difference in currents drawn from supply mains due to small difference in speeds will be considerable, as obvious from Fig. 12.5, and therefore, motors will be loaded very unequally.
In case of two identical shunt motors connected in series (Fig. 12.6) there will be a slight difference in the voltage acting across each motor due to slight difference in the speeds and thus the motors will share the load almost equally.
From the above discussions it is obvious that shunt motor is not suitable for traction purposes.
3. AC Series Motor:
Many single phase ac motors have been developed for traction purposes but only compensated series type commutator motor is found to be best suited for traction. Single phase induction motors have been abandoned as they are not capable of developing high starting torque.
The construction of an AC series motor is very similar to a dc series motor except that some modifications (such as whole magnetic circuit laminated, series field with as few turns as possible, large number of armature conductors, use of high resistance carbon brushes, numerous poles with lesser flux per pole, very short air gaps etc.) are incorporated so as to obtain better performance. Compensating windings are provided to neutralize armature reaction and commutating or interpoles are provided for better performance in terms of higher efficiency and a greater output from a given size of armature core.
A schematic diagram of a single phase series motor with interpole and compensating windings is illustrated in Fig. 12.9.
The average value of the torque on the motor shaft is given as-
Where, I is the effective value of current, φmax is the peak value of the flux per pole and 0 is the phase angle between phasors φ and I.
The phasor diagram of a single phase ac series motor is given in Fig. 12.10. Flux φ is produced by current I. Flux φ lags behind the current I by an amount too small to be important, so it has been shown in phase with current I. The resistive drops IRse, IRp, IRc and IRa due to resistances of series field, interpole circuit, compensating winding and of armature respectively are in phase with current I. The reactive drops IXse, IXp, IXc and IXa due to reactances of series field, interpole circuit, compensating winding and of armature respectively are in quadrature with current I and leading.
Here it is noteworthy that reactive drop due to series field is much greater than that of either armature or the compensating field even all the measures to reduce series field inductance, are taken. Unlike the armature cross-flux φ’, the flux that causes this reactive drop cannot be neutralized because it is the necessity for the development of torque. The interpole circuit consists of the interpole winding in parallel with the non-inductive shunt, as illustrated in Fig. 12.9. Rp and Xp are the respective equivalent resistance and reactance of the interpole parallel circuit.
The phase angle of E, in the phasor diagram is most easily determined by considering instantaneous values. When the alternating field flux φ is at its peak value, the armature conductors cut the maximum flux, and the rotational or speed emf is, therefore, a maximum. When the field flux is zero, the rotational or speed emf, E is zero.
Since the graphs of flux φ and generated emf E pass through zero at the same instant, and also through maximum at the same instant, the phasor E must be either exactly in phase with the phasor of flux φ or exactly in phase opposition. The first possibility is readily eliminated by noting that in case of a motor the generated voltage is in opposition to the direction of flow of current, and, therefore, E is in phase opposition with current and is called the counter emf.
The terminal voltage V is the phasor sum of the counter- emf reversed and all the resistive and reactive volt drops.
Series Motor Characteristics:
i. Power-Output Characteristic:
Mechanical power developed is given by the product of counter emf E and the armature current I. Though counter emf E decreases slightly with the increase in current, but if it is neglected in comparison with the magnitude of counter emf E, mechanical power developed increases almost in proportion with current I.
Power available at shaft or power output is equal to the mechanical power developed less rotational losses. Power output characteristic is shown in Fig. 12.14.
ii. Power Factor Characteristic:
The cosine of the phase angle φ is the power factor of the motor.
From phasor diagram shown in Fig. 12.10
In order to have high power factor the reactance drops must be low and counter emf high. The reactance drops are lowest and the counter emf is highest at light loads, and therefore, the power factor of the single phase series motor is highest (nearly unity) at light loads and falls with the increase in current. This is the reason that power factor of a series motor is low at overloads and at starting. This is the reverse of the power factor relation that exists in case of the induction motor and transformer.
iii. Speed-Current Characteristic:
Speed of a commutator motor is proportional to counter emf E (Er or Eb) or proportional to supply voltage less voltage drops because
E α φ N and E = V – voltage drops
The current causes resistive drops in series field and armature in case of dc operation but in case of ac operation it also causes large reactance drops in series field, armature, compensating winding and interpole circuit in addition to resistive drops in these windings. Hence with ac operation, counter emf E developed is much less than that with dc operation. It means speed-current characteristic for ac series motors are more drooping as compared to those for dc series motors, as illustrated in Fig. 12.11.
Let the same series commutator motor be operated on dc as well as on ac and with same current I.
The back emf developed in case of dc operation is given-
Neglecting saturation, φmax = √2φ as the peak value of ac flux φmax is set up by the peak value of current √2 while dc flux φ is set up by current I. So from above two equations
if the supply voltage is same in both of the cases and resistive drops IR are neglected in comparison with supply voltage V
where, φ is the phase angle between supply voltage and current.
iv. Torque-Current Characteristic:
From Eq. (12.14) T α φI if phase angle between flux φ and current I is neglected or Tα I2 neglecting saturation. Under saturated condition, flux φ becomes constant and, therefore, torque becomes directly proportional to current I. Torque-current characteristic for an ac series motor is shown in Fig. 12.12.
v. Torque-Speed Characteristic:
The torque-speed characteristic for an ac series motor can be derived from speed- current and torque-current characteristics given in Figs. 12.11 and 12.12 respectively.
For given value of torque T and applied voltage V, the armature current is same but voltage drop in case of ac series motor is much more than that in case of a dc series motor so speed of an ac series motor for a given developed torque is less than that of a dc series motor, as illustrated in Fig. 12.13.
The single phase ac series motor has practically the same operating characteristics as the dc series motor. The torque, or tractive effort, varies nearly as the square of the current and the speed varies inversely as the current. This is illustrated in Fig. 12.14.
However, in case of an ac series motor:
(i) Power factor is very low at starting and on overloads on account of high inductive nature of the series field and armature circuits
(ii) Efficiency is not as good as in a corresponding dc machine due to eddy current losses and effects of power factor and
(iii) Starting torque is low due to poor power factor at starting. For a given kilowatt rating ac series motor is 1.5-2 times in size and weight of the corresponding dc series motor. The construction cost of an ac series motor is much more than that of a dc series motor.
The speed of an ac series motor may be controlled efficiently by taps on a transformer, which is not possible in case of a dc series motor.
The torque-speed characteristic of the single phase series motor is similar to that of the dc series motor i.e., high starting torque and decrease in speed with increase in load making it to have self-relieving property from heavy excessive load, so such a machine is particularly useful for traction services.
These motors are used for main line services. These are not suitable for urban and suburban services because of low starting torque and poor power factor at start.
However, single phase series motors have better performance (improved power factor, higher efficiency, improved commutation, better starting and fewer poles for a given output) on reduced supply frequency (say 16or 25 Hz). The higher frequency results in higher leakage reactance and hence a relatively poor power factor. The weight per kW output is also greater for the higher frequency because of larger dimensions of the motor.
The 50 Hz motors have steeper speed-torque characteristics as they operate at lower flux densities than a 16 2/3 or 25 Hz motors. This results in fewer voltage steps at starting and the division of load between parallel connected motors will be less affected by wear of the driving wheels.
The operating voltage is kept low (300 to 400 V) in order to reduce the inductance. Because high voltage motor with proportionately low current would require a large number of turns to produce the given flux.
4. Three Phase Induction Motor:
The three phase induction motors have the advantages of simple and robust construction, trouble free operation, less maintenance, high voltage operation consequently requiring reduced amount of current and automatic regeneration. But due to their flat speed-torque characteristics, constant speed operation, developing low starting torque, drawing high starting current, complicated speed control system and complicated overhead feeding systems they are not suitable for electric traction work.
It is also impossible to employ 3-phase induction motors for a multiple unit system in which two or more locomotives are used for propelling a heavy train. In traction system employing 3-φ induction motors it becomes necessary to couple all the driving axles through a connecting rod so that there is no possibility of a difference in speeds.
With the development of thyristorised inverter circuits, it has now been possible to invert the supply and obtain a variable frequency supply which could be used for the 3- phase induction motor and a very smooth speed control can be obtained.
Induction motor gives good power factor and good efficiency at the speed near to the synchronous speed of the motor. Now in conventional methods of speed control, change of slip method is normally used. But in this method at low speeds of motor or at high value of slip, rotor losses will be more so good efficiency cannot be obtained.
To get good efficiency and good power factor, synchronous speed of the motor itself can be brought to lower speed near to the desired actual speed of the motor. This can be done by supplying the motor with the help of variable frequency supply.
Starting current of motor also decreases when motor is started at low frequency.
When 3-phase induction motor is used, distribution system consists of two overhead wires and track rail for the third phase to feed power to locomotive. This gives a complicated overhead structure and also a person who comes in contact of third live rail might be in danger. This drawback is removed by employing Kando system.
In this system single phase HV supply is given to locomotive with the help of single overhead wire. In locomotive, this single phase supply is converted into three phase supply with the help of phase converters and fed to the motor. Kando system is in use in Hungary and in some sections of Italian State Railway.
5. Linear Induction Motor:
It is a special type of induction motor which gives linear motion instead of rotational motion as in the case of a conventional induction motor. It operates on the same principle on which a conventional induction motor operates i.e., “whenever there occurs a relative motion between the field and the short-circuited conductors, currents are induced in them which results in electromagnetic forces and under the influence of these forces, according to Lenz’s law, the conductors try to move in such a way as to eliminate the induced currents”.
In case of a conventional induction motor, movement of field is rotary about an axis so the movement of the conductors is also rotary. But is case of a linear induction motor, the movement of the field is rectilinear and so the movement of conductors.
In its simplest form, a linear induction motor consists of field system having a 3-phase distributed winding placed in slots as shown in Fig. 12.15. The field system may be a single primary system or double primary system (Fig. 12.15). The secondary of the linear induction motor is normally a conducting plate made of either copper or aluminium in which interaction currents are induced. Either member can be the stator, the other being the runner in accordance with the particular requirements imposed by the duty for which motor is intended.
In a single primary system a ferromagnetic plate is usually placed on the other side of the conducting plate to provide a path of low reluctance to the main flux. However, the ferromagnetic plate gets attracted towards the primary on energisation of the field and this causes unequal gap length on two sides of the conducting plate. This problem can be overcome by employing double primary system. [Fig. 12.15 (b)].
Which of the two primary and secondary will be shorter in length compared to the other depends upon the use of the motor. When the operating distance is large, the primary is made shorter than the secondary because it is uneconomical to wind a very long 3-phase primary. The short secondary form [Fig. 12.15 (c)] is useful with limited operating distance.
When the three phase primary winding of a linear induction motor is energized from a balanced three phase source, a magnetic field moving in a straight line from one end to the other at a linear synchronous speed v is developed. The linear synchronous speed is given as-
vs = 2τf metres/second
where τ is the pole pitch in metres and/is the supply frequency in Hz. It is to be noted here that the synchronous speed does not depend on the number of poles, but only on the pole pitch and the stator supply frequency.
As the flux moves linearly, it drags rotor plate alongwith it in the same direction. This reduces the relative speed of travel of the flux with respect to rotor plate. If the speed of the rotor plate is equal to that of the magnetic field, latter would be stationary when viewed from rotor plate. This is corresponding to the synchronous speed of induction motor. If the rotor plate is moved faster than this speed, the direction of the force would be reversed and a form of regenerative braking based on the principle of induction generator will come into being.
Slip of a linear induction motor is given as-
Where, v is the actual speed of rotor plate.
Thrust or force or tractive effort is given as-
Where P2 is the actual power supplied to the rotor
Active power flow is similar to that in a conventional rotary induction motor i.e.,
Copper losses in rotor = sP2 …(12.20)
and mechanical power developed, Pmech = (1 – s) P2 …(12.21)
Tractive effort will be a function of slip s i.e., (vs – v). The trust (or tractive effort)-speed characteristics of a linear induction motor, as shown in Fig. 12.16 are similar to the torque speed characteristics of a conventional rotary induction motor.
Tractive effort, F can be controlled by varying both frequency and voltage simultaneously so that induction density remains constant.
There are two peculiar effects, which are encountered in a linear induction motor but not in a conventional rotary induction motor. These effects are transverse edge effect and end effect.
The paths of the induced currents in the secondary are not well defined because the secondary of a linear induction motor is a solid conducting plate. The portion of the current paths parallel to the direction of motion of secondary does not make any contribution towards the production of useful thrust but only contributes towards losses. This effect reduces the effective thrust and increases the losses and is known as the transverse edge effect because the current paths parallel to the direction of motion are more towards the edges of the conducting plate.
In the case of linear induction motors with short primary, the current paths towards the end of field structure on the conducting plate go beyond the field structure and such portions of current paths do not contribute to useful thrust but only towards motor losses. This is called the end effect. The end effect can be effectively reduced by increasing the number of poles on the motor.
The advantages of linear induction motors are:
(i) Low initial cost
(ii) Low maintenance cost because of absence of rotating parts
(iii) Simplicity
(iv) No limitation of tractive effort due to adhesion between the wheel and the rail
(v) no limitation of maximum speed due to centrifugal forces
(vi) No overheating of rotor because the motor moves continuously over cool rotor plate leaving behind heated rotor portion and
(vii) Better power to weight ratio.
The disadvantages of linear induction motors are:
(i) Poor utilisation of motor due to transverse edge effect and end effect
(ii) Larger air gap and non-magnetic reaction rail (rotor plate) need more magnetizing current resulting in poor efficiency and low pf
(iii) Very high capital cost of reaction rail fixed along the centre line of the track
(iv) Complications and high cost involved in providing three phase collector system along the track
(v) Difficulties encountered in maintaining adequate clearances at points and crossings.
Applications:
Linear induction motors can be used in conveyors, travelling cranes, haulers, electromagnetic pumps, high-speed rail traction etc. Such motors may be employed in applications in which the field system moves and the conducting plate remains stationary such as travelling crane motors. These motors can also be used in applications where the field system remains stationary and the conducting plate moves such as in automatic sliding doors in electric trains, metallic belt conveyors etc. It can be used on trolley cars for internal transport in workshop, as booster accelerator for moving heavy trains from rest or up the inclines or on curves or as a propulsion unit in marshalling yards in place of shunting locomotives.
Linear induction motor has superiority over conventional rotary motor for speeds over 200 kmph. Linear induction motor provides excellent source of motive power for magnetically suspended trains where conventional rotary motor fails because the latter depends for torque conversion to linear tractive force upon the adhesive weight on driving wheels. The use of linear induction motor is limited to only a few applications till now owing to design difficulties and economic considerations.