Different Types Of Traction Motors

A traction motor is basically an electric motor that generates the power to rotate the wheels of the train. The rotating force produced by the traction motor is transmitted to the wheels of the vehicle through the pinion and gear wheel. The various types of traction motors used are,

  • DC Series Motor,
  • AC Series Motor,
  • Three Phase Induction Motor,
  • Linear Induction Motor.

Different Types Of Traction Motors

DC Series Motor :

The aspects pertaining to the suitability of a dc series motor for traction duty are given below,
  • It generates high torque at low speeds and low torque at high speeds which is required to accelerate the vehicle.
  • Speed-torque and speed-current characteristics of a dc series motor are steep. So, the difference in speed of motors due to different wear of driving wheels is less.
  • As field flux is directly proportional to armature current, torque for a given current will not be affected by voltage fluctuations.
  • Series speed torque curve gives stable operation because as the speed increases, torque decreases.
  • If armature current increases (due to heavy load torque), then speed decreases. Therefore, the emf induced decreases and spark-less commutation can be obtained.
  • Since torque is proportional to the square of armature current, less current is needed to increase the torque. Therefore, the series motor can withstand heavy load torque.
  • For a given increase in torque, the horsepower (power = torque x speed, as torque increases, speed decreases) remains almost constant. This indicates the self-relieving property of the series motor.
  • Series motor is amenable to various speed control methods.
  • Without special arrangements, a series motor cannot be used for regenerative braking.
  • Since the series field time constant is low, the back emf becomes zero in case of power failure. So, the initial rush of current on temporary interruption of supply is more in the series motor.

Applications of Series Motor :

Due to low weight and high starting torque, series motors can develop high starting torque. Therefore, they are suitable for urban and suburban services where a high rate of acceleration is required which can be met by a series motor. In a 1500V dc system, the dc series motor may be operated either at 1500V or 750V by connecting them in series permanently.

DC Shunt Motor :

Shunt motors are not suitable for traction purposes due to the following reasons.
  • A dc shunt motor is a constant speed motor. But for traction purposes, the speed should fall with an increase in load. Thus, the dc shunt motor does not meet this requirement.
  • In the case of a shunt motor, speed is constant and independent of load torque. As power output is directly proportional to torque, thus for a given overload, a shunt motor requires a greater amount of power as compared to a series motor.
  • With the increase in load, the shunt motor besides getting overloaded and develops commutation problems.
  • In a shunt motor, field flux depends upon supply voltage. Thus the torque developed is considerably affected by supply voltage variations.
  • Speed-torque and speed-current characteristics of shunt motors are flat and if they are run in parallel the difference in currents drawn from supply mains due to small difference will be considerable.
  • In the case of dc shunt motors, the field flux remains constant thus torque developed is directly proportional to the armature current. Hence for a given increase in torque developed current drawn from the supply by dc shunt motor is more as compared to dc series motor.

Single Phase AC Series Motor :

If an ordinary dc series motor is fed from an ac supply, it would operate as ac series motor but not very satisfactorily owing to the following reasons,
  • Since the field and armature currents both reverse every half cycle, the torque would be exerted at a double frequency in one direction.
  • The alternating flux set up by the field winding due to alternating current causes excessive eddy current losses in fixed core and yokes, thereby increasing the motor temperature and decreasing the operating efficiency.
  • The inductance of field and armature winding decreases the power factor and causes some abnormal voltage drops which in turn affects the performance of the motor.
  • There will be heavy sparking at the brushes undergoing commutation.
Hence, some modifications are necessary for a dc series motor for the satisfactory operation with ac supply.
  • In order to reduce the reactance of the series field, ac series motors are built with as few turns as possible.
  • Reduction in the number of turns on the field winding results in the reduction of flux per pole leading to an increase in the speed for a given current and therefore there would be a reduction in the load torque available for a given current. Hence to develop the required load torque, the number of armature conductor have to be increased proportionately.
  • The increase in armature conductors would increase inductive reactance of the armature which can be neutralized by providing the compensating winding (compensating winding neutralizes completely the armature MMF).
  • The air gap is made very small because of a very weak field which is necessary to obtain a high power factor.
  • The yoke and field of the motor are laminated in order to reduce eddy current losses.
  • To reduce sparking, brush width is decreased.
  • Series inductive reactance is directly proportional to the frequency, so ac series motor characteristics are better at low frequencies.

Applications of Single Phase AC Series Motor :

The operating characteristics of an ac series motor are similar to a series motor. In which speed is inversely proportional to the armature current and the torque produced will be equal to the square of the armature current.

In a traction system, an ac series motor of several hundred KW is usually employed. Due to poor power factor at starting, ac series motor has low starting torque compared to dc motor. So, they are not suitable for urban and suburban services where high starting torque is needed. So they can be employed for mainline traction service.

Three-Phase Induction Motor :

Earlier three-phase induction motor was considered a constant speed machine. But with the advent of power electronics, many characteristics of a three-phase induction motor can be modified to suit the requirements of traction. The factors governing the suitability of a three-phase induction motor for traction applications are given below.

  • It has many advantages as simple, robust construction, trouble-free operation, less maintenance, and high voltage operation requiring reduced current and easy braking.
  • With the development of power electronic inverter circuits, the variable output frequency can be obtained. This can be used to control the speed of a three-phase induction motor.
  • With variable-frequency input to the induction motor, good efficiency and power factor can also be obtained by lowering the synchronous speed of the motor.
  • Starting current of the motor can be decreased by starting the motor at a low frequency using semiconductor converters.

Applications of Three-Phase Induction Motor :

The traction systems employing three-phase induction motors consist of two overhead conductors for two phases and a track rail for the third phase. This makes the overhead structure complicated and may also lead to electric shock if any person gets in contact with the third rail.

This can be overcome by using the Kando system. In this system, a single-phase high voltage supply is given by a single overhead conductor. The locomotive has phase converters that will convert the single-phase to three-phase at the desired frequency and feed it to the three-phase induction motor.

Linear Motor :

The principle of linear induction motor is the same as that of rotary induction motor. In the case of a rotary induction motor, the movement of the field is rotary about an axis, so the movement of the conductor is also rotary but in a linear induction motor the movement of the field is rectilinear, and therefore the movement of conductors is also rectilinear.

Constructionally, the linear motor can be derived from the conventional induction motor by cutting it on the plane through its axis and opening it out at right angles to that plane. But the main drawback with this construction was the magnetic pull between rotor and stator. If the rotor is considered as made of a cup of conducting material having a fixed magnetic core and if this motor is cut and opened out, linear induction motor construction can be obtained.

The most preferred arrangement of the linear motor is the one in which the rotor is fixed along the centerline of the track and is embraced by two stator windings. The currents in the two windings flow in such a way that poles of opposite polarity are produced on their face to face with the result that the current loading is doubled and stator losses are reduced to one-fourth. The stator is attached to the vehicle so as to have a small air gap as possible.

Tractive effort in a linear induction motor can be controlled either by varying voltage and keeping the frequency constant or by varying both frequency and voltage simultaneously. The disadvantages of the former method are poor utilization, low efficiency, and poor power factor resulting increase in the size of supply installation and heavier handling for some effective power.

Applications of Linear Motor :

  • On trolley cars for internal transport in workshops.
  • As booster accelerator for moving heaving trains from rest on up the inclines or on curves.
  • As propulsion unit in marshaling yards.
  • In magnetically suspended trains.

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