Schrage Motor

What is Schrage Motor

Schrage motor is Essentially a Frequency Converter and wound rotor Induction motor Combined. An Inverted Polyphase Induction motor can be used to describe the Schrage motor. Schrage motor’s primary winding, in Contrast to an Induction motor, is located on the rotor. Three slip rings are used to provide the primary with a three phase supply. The stator has the secondary winding. A third type of winding, known as Tertiary winding, is in addition to primary and Secondary Windings and is connected to the Commutator.

The primary and Tertiary are Mutually coupled and housed in the same rotor slots. Three sets of movable brushes, A1A2, B1B2, and C1C2, join the commutator to the secondary winding terminals. A wheel located at the motor’s back can be used to adjust the brush position. The Injected emf into the secondary winding, which is necessary for speed and power factor control, is determined by the angle between the brushes.

Operation Principle of Schrage Motor

A rotating field is created at a standstill as a result of three phase currents flowing in the primary winding. With a synchronous speed of ns, this rotating field abrades the secondary.
The rotor will therefore rotate in a way that opposes the cause, which is to cause slip frequency emfs to enter the secondary, in accordance with Lenz’s law. As a result, the rotor rotates in the opposite direction to the synchronously rotating field. The air gap field is currently rotating with respect to the secondary at a slip speed of ns – nr. The emf that the stationary brushes have collected is therefore at slip frequency and suitable for injection into secondary.

Speed Control of Schrage Motor

By adjusting the injected emf into the motor, which is controlled by altering the angular displacement between the two brushes, the speed of a schrage motor can be changed. Let’s first examine the speed control in WRIMs using the injected emf method in order to comprehend the speed control of the Schrage motor.

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.

The injected emf needs to be in phase with the rotor’s slip emf in order to increase speed, as can be seen from the analysis above. The slip emf in the rotor should be out of phase with the injected emf in order to slow down the speed.
We will now examine the speed control of the Schrage motor based on the aforementioned principles.

In the above figure
E₂₀ = standstill emf induced in the secondary.
sE₂₀ = induced emf at any slip s.
a, b = brush terminals.

Both brushes in Fig. (a) are short-circuited because they are both connected to the same commutator segment. In this instance, no injected emf exists. Rotor spins at a speed that is nearly synchronous as a result.
In figure (b), the brushes a and b are separated by an angle θ such that the secondary winding axis and the tertiary winding axis are parallel to each other. The injected emf Ej is now in phase opposition to E20 when we trace the path BAabB. Therefore, according to the principles mentioned above, the motor’s speed should be lower than it was in case a. The motor therefore runs at sub-synchronous speeds.

i.e. n_r < n_s.
In fig(c) the brush positions are interchanged. Now on tracing the path BAabB we find that the injected emf is in phase with the standstill emf E₂₀. Therefore speed of motor should increase from what it was in case a. Hence the motor operates at super synchronous speed i.e. 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

An angular displacement of ρ is added between the secondary and tertiary winding axes in order to improve power factor. Now, flux φ moves an additional angular distance of ρ degrees before cutting the tertiary winding axis. Because of this, the emf phasor-Ej in case a lags the emf phasor-Ej in case b by an angle ρ of.
The two cases’ phasor diagrams are displayed 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 demonstrates that depending on the phase of the injected emf, two different speeds are possible at no load. By changing the brush separation, it is possible to control the magnitude of these speeds.
when there is load

Schrage Motor Application

Utilised in drives that need variable speed, such as those for conveyors, fans, centrifugal pumps, and cranes.