What is Schrage Motor : Circuit Diagram, Advantages & Its Applications

In 1911, Mr. H. K. Schrage designed the Schrage motor. This motor is one type of an induction motor, where the maintenance of this motor is less, cheap, and rugged. This is a 3-phase commutator, brush shifting, rotor fed and shunt type motor. This motor has three types of windings, out of three windings, two are placed in the rotor and the remaining one is placed in the stator. The primary winding, secondary winding, and the regulating winding are the three types of windings present in this motor. These induction motors are used for high, medium and low powers of the commutators. The supply voltage of this Schrage motor doesn’t exceed 600V. In this article, a brief explanation of this motor is discussed.

What is Schrage Motor?

Definition: The Schrage motor is one type of induction motor, which has three types of windings they are primary, secondary and tertiary windings. This motor is a combination of frequency convertor and the wound rotor induction. The motor primary winding is placed on the rotor with the help of three slip rings & the phase supply is given to primary winding. The secondary winding is placed on the stator & it is required for PF control (Power Factor) and speed, and the third winding that is tertiary which is connected to the commutator.

Schrage Motor Circuit Diagram

The equivalent circuit diagram of the variable speed commutator type 3-phase induction motor (Schrage motor) is shown in the below figure.

Where

‘r_1′ is the resistance of stator per phase

‘X_1′ is the stator leakage reactance per phase

‘X_0′ and R_{0 ­}are the core loss components per phase

‘V_1′ is the supply voltage,

‘E_1′ is the EMF per phase

‘I’₀ is the no-load current per phase

‘I’_w is the ‘I’_{0 ­}working component

‘I’_m is the ‘I’₀ is magnetizing component per phase.

The approximate equivalent circuit diagram of the Schrage induction motor or three-phase induction motor is shown in the below figure.

In the above figure, ‘I’₂ is the reflected rotor current in the stator and this current flows through all the components r₁, r₂^’, X₁^’, and X₂^’. The r₂^’(1-S)/S is an electrical equivalent of mechanical load. At the no-load condition of three-phase induction motor, N=N_s, when ‘Ns’ is equal to zero and slip (S) is also equal to zero.

Now put S=0 in ‘r’₂, then ‘r’₂ becomes infinity. If ‘r’₂is treated as infinity in no-load condition, then no current flows through the electrical equivalent of mechanical load. At this time, the secondary winding is open-circuited. When N=0, S=1, put S=1 in r₂^’ then r₂ becomes zero. At this time we can say that the secondary winding is short-circuited.

Schrage Motor Theory

Three-phase AC commutator motors are a special type of three-phase induction motor. The commutators are used to convert AC to DC or DC to AC in the DC generator. Here the commutators are not using to convert AC to DC or DC to AC, but they are used only to supply current in one circuit to another circuit.

The commutator is needed because they give some special properties like constant speed drive like a shunt machine, a wide range of speed with uniform accelerations, power factor (PF), and overall operational efficiency is high. The speed control mechanism and power factor mechanism are the two construction aspects. The power factor control mechanism is basically obtained by brush shifting and the speed control mechanism is obtained by injection of EMF (Electro-Magnetic Field) at a proper frequency. There will be a rotor EMF injection in the speed control mechanism. The rotor circuit is shown below.

In the circuit above, SE2 is the input voltage to the rotor. The rotor has its own impedance like ‘ Z2’. The current in the rotor can be given by

I2=SE2/Z2

We know that torque in the induction motor is directly proportional to I₂²*R₂/S. If we increase the current, the torque will be increased. If the torque increases, the speed will be decreased. Another name of Schrage motor is the rotor fed three-phase AC commutator. This motor is a special type of an inverted induction motor that has a three-phase supply on the rotor and the stator.

Construction

The Schrage motor has stator and rotor, where the rotor is the input and it has two winding components like primary winding and regulating winding. The primary winding receives a three-phase supply, and the main flux required for the machine is produced by primary winding which is present on the rotor.

The regulating winding is also called as tertiary winding. The main purpose of this winding is to support commutation. The stator has only a single winding that is secondary winding, this winding is a 3-phase short circuit winding. This motor has six brushes like A1, A2, B1, B2, C1, and C2 which are made up of phosphor bronze. The commutator is basically circular in shape, the three-phase Schrage motor is shown in the below figure.

Suppose, if we want to move or shift the ‘A1’ terminal at an angle then the terminals B1 & C1 are also shifted along with ‘A1’ terminal. The terminals A2, B2 & C2 are aligned in the same mechanism. The brushes like A1, B1, C1 moves in one direction and the brushes A2, B2, and C2 moves in another direction that is opposite to the terminal A1, B1, and C1.

The angle maintained between A1, B1, and C1 is 120⁰ similarly, the angle maintained between A2, B2, and C2 is also 120⁰. The angle maintained between A1 & A2, B1, and B2, C1 and C2 are a point of consideration is called as beta (β) angle which is called a brush shift angle. By changing this beta (β) only we can obtain the power factor control. The entire operation is dependent on how many angles you shift or how many angles do you maintain at the starting and finishing end of one phase winding. This is the explanation of the Schrage motor construction.

Working

The working of the Schrage motor is simple, when you give three-phase supply to the rotor, then it will produce a Rotating Magnetic Field (RMF). This rotating magnetic field rotates at synchronous speed (Ns), initially the speed of the rotor at ‘Nr’ will be equal to zero. The stator is always zero because it is a stationary point that is not going to rotate. If the rotating magnetic field rotates in the clockwise direction, the EMF will be induced at two places at secondary winding and at regulating winding or tertiary winding.

The regulating windings are induced by transformer action and the secondary windings are induced by dynamically induced EMF. Compare to a normal induction motor, the rotor RMF is at SN_S with respect to the rotor and at N_S with respect to the stator. The N_s– N_r is the air gap speed with respect to the stator. In the below characteristics, we can observe that when the load increases, the power factor increases, speed decreases, and the efficiency increases.

Power Control Factor

The ‘ρ’ angular displacement is introduced between the secondary and tertiary winding axis in order to improve the power factor. The flux cuts the tertiary winding axis when it covers ‘ρ’ angular displacement. In between primary and regulating windings, the transformer action will be occured and in between secondary and primary windings, the induction motor action will be occured.

Speed Control of Schrage Motor

The Schrage motor speed can be controlled by varying the injected electromagnetic field (EMF) into the motor. The brushes are connected to commutators, the below figure shows the connection of brushes to the commutator.

In figure(a), both the brushes A and B are connected to a single commutator or same commutator. The injected electromagnetic field is zero and the n_r is equal to n_s (n_r=n_s) in this case.

In figure(b), the brush ‘A’ is connected to ‘a’ terminal and the brush ‘B’ is connected to a terminal ‘b’. In this case, the n_r is less than n_s (n_r<n_s).

In figure(c), the positions of the brushes are interchanged in this case and the n_r is greater than n_s (n_r>n_s).

The injected EMF for any brush separation ‘θ’ is given by

E_j=E_jmax sin (θ/2)

When θ=0, the injected EMF E_j=0 and when θ=90⁰, the injected EMF E_j= E_jmax.

Advantages

The advantages of the Schrage motor are

• Speed is good
•  Power factor (PF) is high for high speed
•  Easy to control the speed

Disadvantages

The disadvantages of the Schrage motor are

•  Losses are more
•  Structure is complicated
•  Low efficiency

Applications

The applications of the Schrage motor are

• Cranes
• Hoist fans
•  Centrifugal pumps
•  Printing and packing machinery
•  Conveyors
•  Knitting and ring spinning
•  Paper mills
•  Stokers
•  Feed and separator drives
•  Frequency changing
•  Miscellaneous

FAQs

1). What is the most efficient motor?

The most efficient motor is a brushless motor.

2). What is the wound rotor motor?

The wound is an alternating current electric motor.

3). What is a single induction motor?

The single inductor motor is one type of alternating current motor, which is used to perform physical tasks.

4). Which motor has the highest starting torque?

The direct current motors have the highest starting torque.

5). What is a self-starting motor?

The self-starting motors are the motors that run automatically without any additional force or external force.

In this article, the overview of the Schrage motor working, circuit diagram of Schrage motor, power factor control, and speed control, advantages, disadvantages, and applications are discussed. Here is a question for you what are the types of an induction motor?