What is a Repulsion Motor : Construction and Its Working

A motor is an electrical device that converts electrical input into mechanical output, where electrical input can be in current or voltage form and the mechanical output can be in torque or force form. Motor consist of two main parts namely stator and rotor, where the stator is a stationary part of the motor and the rotor is a rotatory part of the motor. A motor that works on the principle of repulsion is known as a repulsion motor, where the repulsion takes place between two magnetic fields of either stator or a rotor. Repulsion motor is a single-phase motor.

What is Repulsion Motor?

Definition: A repulsion motor is a single-phase electric motor that operates by providing input AC (alternating current). The main application of repulsion motor is electric trains. It starts as a repulsion motor and runs as an induction motor, where the starting torque should be high for repulsion motor and very good running characteristics for induction motor.

Construction of Repulsion Motor

It is a single-phase AC motor, which consists of a pole core that is the north pole and south pole of a magnet. The construction of this motor is similar to the split-phase induction motor and DC series motor. The rotor and stator are the two main components of the motors which are inductively coupled. The field winding (or a distributed type winding or the stator) is similar to the main winding of the split-phase induction motor. Hence the flux is evenly distributed and the gap between stator and rotor is decreased and the reluctance is also decreased, which in turn improves the power factor.

The rotor or armature is similar to the DC series motor which is provided with a drum-type winding connected to the commutator, where the commutator is in-turn connected to carbon brushes which are short-circuited. A brush holder mechanism provides variable crankshaft to change the direction or alignment of brushes along the axis. Hence the torque produced during this process helps to control the speed. The energy in the repulsion motor is transferred through the transformer action or by the induction action (where the emf is transferred between stator to the rotor).

Working Principle

Repulsion motor works on the principle of repulsion where two poles of a magnet repel. The working principle of repulsion motor can be explained from 3 cases of α, depending on the position of the magnet as follows.

Case(i) : When α = 90⁰

Assume brushes ‘C and D’ are aligned vertically at 90 degrees and rotor aligned horizontally along the d-axis (field axis) which is the direction of current flow. From the principle of Lenz’s law, we know that the emf induced mainly depends on the stator flux and the current direction (which is based on the alignment of brushes). Therefore, the net emf of brush from ‘C to D’ is ‘0’ as shown in the diagram, which is represented as ‘x’ and ‘.’ there is no current flow in the rotor, so Ir = 0. When no current passes in the rotor,  then it acts as an open-circuited transformer. Therefore, stator current Is = less. The direction of the magnetic field is in along brush axis direction, where the stator and rotor field axis are 180 degrees phase-shifted, the torque generated is ‘0’ and the mutual induction induced in the motor is ‘0’.

Case (ii) : When α = 0⁰

Now the brushes ‘C and D’ are oriented along the d-axis and are short-circuited. Therefore the net emf induced in the motor is very high, which generates the flux between windings. The net emf can be represented as ‘x’ and ‘.’ as shown in the figure. It is similar to a short-circuited transformer. Where the stator current and the mutual induction are maxima which means Ir = Is = maximum. From the figure, we can observe that the stator and rotor fields are  180 degrees opposite in phase, which means the torque generated will oppose each other, so the rotor cannot rotate.

Case (iii) : When α = 450

When the brushes ‘C and D’ are inclined at some angle (45 degrees) and the brushes are shorted. Let us assume the rotor (brush axis) is fixed & the stator is rotated. The stator winding is represented as ‘Ns’ number of effective turns and the current passing is ‘Is’, the field produced by stator is in the direction ‘Is Ns’ which is the stator MMF as shown in the figure. The MMF (magnetomotive force) is resolved into two components(MMF1 and MMF2), where MMF1 is along with the brush direction (Is Nf ) and MMF2 is perpendicular to the brush direction (Is Nt) which is the transformer direction, and ‘α’ is the angle between ‘Is Nt’ and ‘Is Nf’. Hence the flux produced by this field into two components is ‘Is Nf’ and ‘Is Nt’. The emf induced in the rotor produces flux along the q-axis.

The field produced by the rotor along the brush axis is mathematically represented as follows

Is Nt = Is Ns cos α  ……….. 1

Nt = Ns Cos α  …………2

Nf = Ns Sin α …………3

Since the magnetic axis ‘T’ and brush axis coincides with the rotor MMF which is along the brush axis is equal to the flux generated by the stator.

The equation of torque is given as

Ґ α (stator d-axis MMF) * ( rotor q-axis MMF)  ……….4

Ґ α ( Is Ns Sin α) ( Is Ns cos α )  ………..5

Ґ α I 2s N 2s Sin α cos α [we know that Sin2 α = 2 Sin α cos α]  ……….6

Ґ α ½( I 2s N 2s Sin2 α) …….7

Ґ α K I 2s N 2s Sin2 α [ When α = 0 Torque = 0 ………..8

K = constant value α = π/4 Torque = maximum

Graphical Representation

Practically this is a problem this can be shown in a graphical format, where the x-axis is represented as ‘α’ and the y-axis is represented as  ‘current’.

• From the graph, we can observe that the current is directly proportional to α
• The current value is 0 when α =90⁰ which is similar to open circuit transformer
• The current is maximum when α =0⁰ which is similar to the short circuit transformer as shown in the graph.
• Where Is is the stator current.
• The torque equation can be given as Ґ α K I 2s N 2s Sin2 α.
• Practically it is observed that torque is maximum if α is ranging between 150 – 300.

Classification of Repulsion Motor

There are three types of repulsion motor they are,

Compensated Type

It consists of an additional winding namely compensating winding and an additional pair of brushes are placed between the (short-circuited) brushes. Both compensating winding and a pair of brushes are connected in series for improving the power and speed factors. A compensated type motor is used where there is required for high power at the same speed.

Repulsion Start Induction Type

It starts with the repulsion of coils and runs with the induction principle, where speed is maintained constant. It has a single stator and rotor similar to DC armature and a commutator where a centrifuge mechanism short-circuits the commutator bars and has higher torque (6 times) than the current in the load. The operation of repulsion can be understood from the graph that is, when the frequency of synchronous speed increases, the percentage of full torque load starts decreasing, where at a point the magnet poles experience a repulsive force and switches into induction mode. Here we can observe the load that is inversely proportional to speed.

Repulsion Type

It works on the principle of repulsion and induction, which consists of a stator winding, 2 rotors winding (where one is squirrel cage and other DC winding). These windings are shorted to commutator and two brushes. It operates in a condition where the load can be adjustable and whose starting torque is 2.5-3.

Advantages

The advantages are

• The high value of starting torque
• The speed is not limited
• By adjusting the value of ‘α’ we can adjust the torque, where we can increase the speed based on adjustment of torque.
• By adjusting the position brushes, we can control the torque and speed easily.

Disadvantages

The disadvantages are

• Speed varies with variation in the load
• The power factor is less except for high speeds
• The cost is high
• High maintenance.

Applications

The applications are

• They are used where there is a need for starting torque with high-speed equipment’s
• Coil Winders: Where we can adjust speed flexibly and easily and direction can also be changed by reversing the brush axis direction.
• Toys
• Lifts etc.

FAQs

1). What is the angle the repulsion motor experience repulsion?

At 45 degrees angle, it experiences repulsion.

2). Repulsion motor is based on which principle?

It is based on the repulsion principle

3). What are the two main components of Repulsion motor?

The stator and rotor are the two main components of the motor.

4). How can the torque be controlled in repulsion motor?

The torque can be controlled by adjusting the primary brushes of the motor

5). Classification of repulsion motor

They are classified into 3 types

• Repulsion type
• Repulsion start induction run motor
• Compensated type

Thus, this is an overview of repulsion motor which works on the principle of repulsion. It has two important components namely stator and rotor. The working principle of the motor can be understood in three cases of angles (0, 90,45 degrees) that are based on brushes position and the fields generated. The motor experience a repulsive effect only at 45 degrees. These motors are used where starting torque is highly required. The main advantage is that torque can be controlled by adjusting the brushes.