DC Shunt Motors: Characteristics & Performance | Electrical Engineering

In this article we will discuss about the characteristics and performance curves of dc shunt motors.

Characteristics of a DC Shunt Motor:

1. Speed-Armature Current Characteristic:

If applied voltage V is kept constant, the field current will remain constant, hence flux will have maximum value on no load but will slightly decrease due to armature reaction as the load increases but for most purposes the flux is considered to be constant, neglecting armature reaction effect.

From speed equation, speed N is directly proportional to back emf E_b or (V – I_a R_a) and inversely proportional to the flux φ. Since flux is considered to be constant, so with the increase in armature current the speed slightly falls due to increase in voltage drop in armature and the speed-armature current curve coincides with the back emf-armature current curve. Since voltage drop in armature at full load is very small as compared to applied voltage so drop in speed from no load to full load is very small.

If demagnetising effect of armature reaction is considered, the drop in speed due to voltage drop in armature is compen­sated for up to some extent due to decrease in flux with the increase in armature current and speed-armature current characteristic is less drooping, as shown dotted in Fig. 1.15 or even be rising if demagnetisation is high. Thus the drop in speed from no load to full load is very small and for all practical purposes the dc shunt motor is taken as a constant speed motor.

Since there is a slight variation in speed of the shunt motor from no load to full load and this slight variation in speed can be made up by inserting resistance in the shunt field and so reducing the flux. Therefore, shunt motors can be used for the loads which are totally and suddenly thrown off without resulting in excessive speed.

Shunt motors being constant speed motors are best suited for driving of line shafts, machine lathes, milling machines, conveyors, fans and for all purposes where constant speed is required. It is not suitable for use with flywheel or with fluctuating loads or for parallel operation due to its constant speed characteristic. It is also useful where a moderate degree of speed control is required.

2. Torque-Armature Current Characteristic:

From the expression for the torque of a dc motor, torque is directly proportional to the product of flux per pole φ and armature current, I_a. Since in case of a dc shunt motor the flux per pole φ is considered to be constant, therefore, torque in­creases with the increase in load current following linear law, i.e., torque-armature current characteristic is a straight line passing through origin O (Fig. 1.15). But the weakening of field due to armature reaction causes the torque line to droop slightly, and the iron and friction losses cause it to be slightly lower than the line representing the electromagnetic developed torque corresponding to T = 0.159φ ZP I_a/A Nm.

3. Speed-Torque Characteristic:

Substituting I_a = T/K₂φ from Eq. (1.27) in Eq. (1.26), we have-

For a dc shunt motor, φ remains more or less constant and, therefore speed of the motor is given as-

N= C – DT …(1.29)

Where, C and D are constants for the machine, i.e., the characteristic will be linear in nature, as illustrated in Fig. 1.16.

This type of motor is used in applications requiring me­dium starting torque such as centrifugal pumps, blowers, fans, conveyors, boring mills, shapers, woodworking ma­chines, spinning and weaving machines, printing presses, machine tools etc.

DC shunt motors should never be started on heavy loads because such loads need heavy starting current.

4. Break in Field Circuit of a DC Motor:

It is impera­tive that the field circuit of a dc motor never be opened while it is running. If the field circuit is opened, the flux will drop to practically zero and, therefore, the speed would in­crease to an extremely high value. The centrifugal force developed at the high speed may cause the windings to be pulled out of the slots or may cause the commutator seg­ments to buckle, resulting in damage to the motor.

Proper use of overload devices would protect the motor. Since back emf would be reduced due to reduction in flux and cause increase in armature current or current drawn from the supply mains by the motor. This excessive current would operate the overload device, disconnect the motor from the supply, and thus protect the motor.

Performance Curves of a DC Shunt Motor:

In Fig. 1.17, four important characteristics of a dc shunt motor, namely, torque, speed, armature current, and efficiency, each plotted against the useful output power, are shown. These curves are also known as performance curves of a motor.

From the speed-useful output power curve it is seen that the shunt motor has a definite speed at no load. Hence it does not ‘run away’ when the load is suddenly thrown off provided that the field circuit remains closed. The fall in speed from no load to full load is small hence, these motors are used where a substan­tially constant speed is required as in machine shop drives.

From the efficiency-output power curve it is noted that the efficiency of a shunt motor increases rapidly in the be­ginning, reaches its maximum value and then falls. In shunt wound motors the field, friction and iron losses are approxi­mately constant but at light loads their percentage is high as compared with the load. The armature copper loss varies as the square of the current so this loss, termed as variable loss, increases rapidly with the increase in load. The efficiency of the shunt machine becomes maximum when variable loss is equal to the constant loss.

From armature current-output power curve, a certain value of current is required even when the output is zero. The motor input power under no-load condi­tions is consumed in meeting the various losses occurring within the machine.

A shunt wound motor is said to have a low starting torque as compared to other motors but it does not mean that a shunt motor is incapable of starting a heavy load. A shunt motor is capable of starting heavy load also but it would require more excess of current input over normal values than the series and compound motors. For instance, if twice full-load torque is required at start the shunt motor will draw twice full-load current (since T α I_a) but a series motor would draw only ap­proximately 1.414 times the full-load current (Since T α l_a²). Due to-this reason, shunt motors are always started at no load or light load and then the load is applied. When a motor is required to start heavy loads series or compound motor is used.