Question

Question 1 (a) Briefly explain the construction and operation of a DC Shunt motor.

Answer 1

The shunt motors are used where constant speed is required and starting conditions are not severe. The various applications of DC shunt motor are in Lathe Machines, Centrifugal Pumps, Fans, Blowers, Conveyors, Lifts, Weaving Machine, Spinning machines, etc

Whenever the DC shunt motor’s armature winding rotates within the magnetic field which is generated by the field winding. Thus an e.m.f can be stimulated within the armature winding based on the Faradays law (electromagnetic induction). Although, according to Lenz’s law, the induced e.m.f can act within reverse direction toward the armature voltage supply.

Thus, this e.m.f is named as the back e.m.f, & it is represented with Eb. Mathematically, this can be expressed as,

Eb = (PφNZ) / 60A V

Where P = no. of poles

Φ = Flux for each pole within Wb

N = Motor’s speed in revolutions per minute

Z = No. of armature conductors

A = No. of parallel lanes

DC Shunt Motor Speed Control

The speed characteristic of a shunt motor is different compared with a series motor. As a DC Shunt motor attains its complete speed, then the armature current can be directly connected to the motor load. When the load is extremely low within a shunt motor, then the armature current can also be low. When the DC motor attains its complete speed, then it remains stable.

The speed characteristic of a shunt motor is different compared with a series motor. As a DC Shunt motor attains its complete speed, then the armature current can be directly connected to the motor load. When the load is extremely low within a shunt motor, then the armature current can also be low. When the DC motor attains its complete speed, then it remains stable.

The DC shunt motor speed can be controlled very easily. The speed can be maintained constant until the load changes. Once the load changes, then the armature tends to delay, which will result in the less back e.m.f. Thus, the DC motor will draw extra current, this will consequence in enhance within torque to gain speed.

So, whenever the load enhances, the net result of load on speed in a motor is approximately nil. Similarly, once the load decreases, then the armature achieves speed & produces extra back e.m.f.

The DC shunt motor speed can be controlled in two ways

• By altering the sum of current flowing through the shunt windings
• By altering the sum of current flowing through the armature

In general, DC motors appear with a particular rated voltage & speed in (revolutions per minute. Once this motor functions under its complete voltage, then the torque will be reduced.

Brake Test on DC Shunt Motor

The brake test is the one kind of load test on dc shunt motor. Generally, this test can be done for the low rated DC machines. The main reason for doing this test is to identify the efficiency and also by using this test, the output of mechanical power can be calculated & separate the same by using electrical input. So this is the reason to calculate the efficiency of the DC motor, this test is used. Therefore, this type of test cannot be employed on the superior rated machines.

Characteristics of DC Shunt Motor

The characteristics of shunt DC motor include the following.

• This DC motor works at a fixed speed once the voltage supply is set.
• This DC motor is upturned by the turn around the motor connections like a series motor.
• In this type of DC motor, by a rising motor current, torque can be improved without reducing in speed
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