Schrage Motor Operation Principle and Characteristics of Schrage Motor

A Schrage motor combines an inverted wound-rotor induction motor with a commutator frequency converter. The line-fed primary winding is on the rotor and receives three-phase power through slip rings. The induction-motor secondary is on the stator. A regulating, or tertiary, winding shares the rotor slots with the primary and connects to the commutator. Three pairs of stationary brushes connect the stator phases to selected commutator segments (A₁A₂, B₁B₂ and C₁C₂). The brush gear changes the magnitude and phase of the voltage injected into the secondary circuit, which controls speed and can adjust input power factor.

Operating Principle of Schrage Motor

At standstill, three-phase current in the rotor primary produces a rotating magnetic field at synchronous speed n_s relative to the rotor.
Under Lenz’s law, electromagnetic torque drives the rotor opposite to that field’s rotation relative to the rotor. If rotor speed is nr, the field crosses the stationary secondary at the relative speed n_s – n_r. The secondary emf is therefore at slip frequency. The regulating winding and commutator produce a voltage at the same frequency for injection into that secondary circuit.

Speed Control of Schrage Motor

Speed control of a Schrage motor comes from varying the injected slip-frequency emf. Moving each paired set of brushes changes the commutator span, which changes voltage magnitude. Crossing the brush connection reverses its phase relative to the secondary slip emf. The following simplified wound-rotor examples show how series voltage injection shifts the steady operating speed.

The circuit values below are illustrative rather than a general motor rating.

Initially, electromagnetic torque (T_e) equals load torque (T_l) at 2 N·m.
Rotor current I_r is 2 A.
sE₂ is the slip emf induced in the secondary circuit.
E_j is the emf injected in series with that circuit.

Case 1: E_j opposes sE₂ in phase.

In this illustrative circuit, rotor current I_r falls to 1 A. Electromagnetic torque T_e is then below load torque T_l, so the motor decelerates and ω_r falls. As ω_r decreases, slip increases until the larger sE₂ restores I_r to 2 A and T_e again equals T_l.

Case 2: E_j is in phase with sE₂.

The illustrative current rises to 3 A. Electromagnetic torque T_e exceeds load torque T_l, so the motor accelerates and ω_r rises. As ω_r increases, slip decreases until sE₂ falls to the value that restores I_r to 2 A and makes T_e equal T_l.
The example shows the basic direction of speed change: an injected emf that reinforces the slip emf raises the operating speed, while an opposing emf lowers it.
The actual steady speed is the point where motor torque again equals load torque; winding impedance and losses also affect it.

In the diagram:
E₂₀ is the standstill emf induced in the secondary.
sE₂₀ is its value at slip s.
a and b are brush terminals.
In figure (a), both brushes touch the same commutator segment, so their injected emf is zero. The machine then operates like an inverted induction motor at a speed close to synchronous speed.
In figure (b), brushes a and b are separated by θ so the injected emf E_j opposes E₂₀. The motor then settles below the zero-injection speed, at sub-synchronous speed n_r < n_s.
In figure (c), the brush connections are crossed. The injected emf reinforces the secondary emf E₂₀, so the motor can settle above synchronous speed with n_r > n_s.
For the ideal winding represented here, brush separation determines the injected-emf magnitude:

The equation gives E_j = 0 at θ = 0, when the paired brushes share one segment. Its maximum E_j = E_jmax occurs at the separation specified by the winding design, shown here as 90 electrical degrees or one pole pitch.

Power Factor Control

Brush separation mainly controls injected-voltage magnitude and speed. A collective angular shift ρ of the brush sets changes the phase of that voltage relative to the secondary power factor conditions. Equivalently, the selected regulating-winding axis is displaced from the secondary axis. As the rotating flux φ reaches that selected axis later, the injected-emf phasor –E_j is shifted by ρ.
The two phasor cases are shown below.

The diagram uses the following circuit equations:

The voltage drop I₂Z₂ has the impedance angle relative to I₂. The referred current I₂’ is drawn opposite to I₂ in the transformer convention. Combining I₂’ with magnetising current I₀ gives primary current I₁.
A suitable collective brush shift introduces a reactive component of injected voltage that reduces the reactive current drawn from the supply. The shift must be set for the speed, load and machine connection; displacement alone does not guarantee improvement in every operating condition.

Characteristics of Schrage Motor

Applying KVL to the secondary equivalent circuit gives

At no load, I₂ is small, so the secondary impedance-drop term is neglected in this approximation.
The balance then becomes

Here s₀ is no-load slip.

In the winding expression:
E_jmax is the maximum transformer emf induced in the regulating winding.
φ_m is maximum flux linkage.
f_s is supply frequency.
Z is the number of regulating-winding conductors.
A is the number of parallel paths.
The corresponding secondary expression is

Here:
E₂₀ is the standstill emf induced in the secondary.
N₂’ is the effective number of secondary turns.
Substitution gives the approximate no-load-slip expression

Within the assumptions of this model, no-load slip depends on the winding constants, brush separation and fast or slow brush connection.


The two signs represent sub-synchronous and super-synchronous no-load settings. Brush separation changes their magnitude. Under load, speed shifts until electromagnetic torque balances the load torque and losses.
With load current included


Applications
Schrage motors were used where continuous adjustable speed was needed, including cranes, fans, centrifugal pumps and conveyors. Their commutator, slip rings and brush gear require maintenance, so modern electronic variable-frequency drives have replaced them in many installations.