Schrage Motor Operation Principle and Characteristics of Schrage Motor

A Schrage motor is essentially a combination of a wound rotor induction motor and a frequency converter. Unlike a standard induction motor, the primary winding of a Schrage motor is on the rotor, and the secondary winding is on the stator. There is also a tertiary winding connected to the commutator. The primary and tertiary windings are housed in the same rotor slots and are mutually coupled. The secondary winding terminals connect to the commutator through three sets of movable brushes (A₁A₂, B₁B₂ and C₁C₂). The brush positions can be adjusted with a wheel at the back of the motor, controlling the injected emf for speed and power factor regulation.

Operation Principle of Schrage Motor

At standstill, three-phase currents in the primary winding produce a rotating field. This rotating field cuts the secondary winding at synchronous speed (n_s).
Therefore according to Lenz’s law the rotor will rotate in a direction so as to oppose the cause i.e. to induce slip frequency emfs into secondary. Therefore the rotor rotates opposite to the direction of rotation of synchronously rotating field. Now air gap field is rotating at slip speed n_s – n_r with respect to secondary. Therefore the emf collected by the stationary brushes is at slip frequency and hence suitable for injection into secondary.

Speed Control of Schrage Motor

Speed control of schrage motor is possible by varying the injected emf into the motor which can be controlled by changing the angular displacement between the two brushes. To understand the speed control of schrage motor let us understand the speed control in WRIMs using injected emf method.

Consider the following rotor circuits (values are only for illustration purpose).

Let initially electrical torque (T_e) = load torque (T_l) = 2Nm
Rotor current I_r = 2A.
Let sE₂ = slip emf induced in the rotor ckt.
And E_j = emf injected in the rotor ckt.

Case 1: When E_j is in phase opposition to sE₂

Now the rotor current becomes I_r = 1A. Therefore T_e < T_l due to which motor decelerates. Therefore ω_r decreases. That implies slip increases. Therefore ω_r decreases till sE₂ becomes 15V and I_r = 2A i.e till T_e = T_l again.

Case 2: When E_j is in phase with sE₂

Now the rotor current becomes 3A. Therefore T_e > T_l due to which motor accelerates. Therefore ω_r increases. That implies slip decreases. Therefore ω_r increases till sE₂ becomes 5V and I_r = 2A i.e till T_e = T_l again.
From the above analysis it can be seen that to increase speed the injected emf should be in phase with slip emf in the rotor. To decrease speed the injected emf should be out of phase with the slip emf in the rotor.
Now based on the above principles we shall take a look at the speed control of schrage motor.

In the above figure
E₂₀ = standstill emf induced in the secondary.
sE₂₀ = induced emf at any slip s.
a, b = brush terminals.
In fig (a) both the brushes are connected to the same commutator segment and hence are short circuited. The injected emf in this case is zero. Therefore rotor rotates with speed close to synchronous speed.
In fig(b) the brushes a and b are separated by an angular displacement θ such that the tertiary winding axis between brushes a and b is coincident with secondary winding axis. Now on tracing the path BAabB we find that injected emf E_j is in phase opposition to E₂₀. Therefore from above discussed principles speed of the motor shall decrease from what it was in case a. Hence the motor operates at sub synchronous speeds i.e. n_r < n_s.
In fig(c), the brush positions are interchanged. Tracing the path BAabB shows that the injected emf is in phase with the standstill emf (E₂₀). Therefore, the motor speed increases from what it was in case (a), operating at a super-synchronous speed (n_r > n_s).
For any brush separation θ the injected emf is given by

From the equation it can be seen that minimum value of injected emf E_j = 0 at θ = 0 (i.e. when the brushes are short circuited). And maximum value of injected emf is E_j = E_jmax at θ = 90 degrees (i.e. when the brushes are one pole pitch apart).

Power Factor Control

To improve the power factor, an angular displacement (ρ) is introduced between the tertiary and secondary winding axes. The flux (φ) cuts the tertiary winding axis after an angular displacement of ρ degrees, causing the emf phasor (–E_j) to lag by an angle ρ.
The phasor diagram for the two cases is as shown below

The phasor diagram has been constructed based on the following equations:

I₂ lags I₂Z₂ by some angle θ. I₂’ is draw opposite to I₂. The resultant of I₂’ and magnetizing current I₀ gives the primary current I₁.
From the phasor diagram it is clear that if the tertiary winding axis and secondary winding axis are displaced by an angle ρ then power factor improves.

Characteristics of Schrage Motor

If we apply KVL to secondary circuit then we get

Under no load conditions I₂ value is very small and hence the can be neglected.
Therefore we have,

Where, s₀ is the no load slip

Where,
E_jmax is the transformer emf induced in the Tertiary winding.
φ_m = max flux linkage
f_s = supply frequency
Z = number of conductors in tertiary
A = number of parallel paths
Also

Where,
E₂₀ = the transformer emf induced in the secondary.
N₂’ = effective number of turns in the secondary
Now on substituting these values in the expression of no load slip we get

This implies values of slip depend completely on machine constants and the brush separation.


This shows that two different speeds are possible at no load depending on the phase of injected emf. The magnitude of these speeds can be controlled by adjusting the brush separation.
Under load conditions


Applications
Used in drives requiring variable speed such as cranes, fan, centrifugal pumps, conveyors etc.