Speed Control of DC Shunt Motor

If we want to speed control of DC shunt motor by reducing the speed as per our work requirement it’s easy to control the speed of dc motor because it can provide a broad variation in the speed by using different methods. In some applications, we require slow speed whereas in other it’s moderate or at other fast speed as per our requirement, we can control the speed of the dc shunt motor.

Speed of shunt motor is given by

n=\frac { V_a-I_ar_a }{ k\phi }

where Va is the voltage applied through the armature and \phi is the flux per pole which proportional to the field current If. However, Ia is the armature current present on the shaft of the motor. Therefore, by varying Va and If we can vary n as per the formula of speed. For fixed supply voltage to the motor and connected as a shunt to the motor, we can vary Va by controlling an external resistance connected in series with the armature.

Thus for dc shunt motor we have essentially two methods for speed control of dc shunt motor by

  • Varying Armature Resistance
  • Varying Field Resistance

Speed Control Of Dc Shunt Motor By Varying Armature Resistance

The internal armature resistance ra of the motor is small so that speed n versus armature current Ia characteristic will be a straight line with a small negative slope as shown in the figure below given.

The internal armature resistance ra is small so speed n versus armature current Ia characteristic will be a straight line with a small negative slope

At no load i.e., Ia = 0 speed is high and n_0=\frac { V_o }{ k\phi } =\frac { V }{ k\phi }. For shunt motor voltage applied to the field circuit and armature circuit are the same and equal to the supply voltage V.

As the motor is loaded, Iara drop increases which decrease the speed a little less than the no-load speed n0. The drop in speeds is about 3 to 5% with respect to no-load speed in dc shunt motor. Speed regulation of the motor is defined as the drop-in speed from no load to full load condition expressed as a percentage of no-load speed.

Speed Regulation = \frac { n-n_0 }{ n_0 } \times 100

As T_e=k\phi I_a for constant \phi operation, Te proportional to Ia. Therefore, speed vs torque characteristic is also same as the speed vs armature current characteristic as shown in the figure below given.

speed vs torque characteristic of dc shunt motor

The slope of the n vs Ia or n vs Te characteristic can be modified by connecting external resistance rext in the armature circuit of the motor.

The slope of the n vs Ia or n vs Te characteristic can be modified by deliberately connecting external resistance rext in the armature circuit.

One can get many curves of speed vs armature curves as shown in figures for various values of rext. From these characteristics, it can be explained how speed control is achieved in the dc motor.

n vs Te characteristic can be modified by deliberately connecting external resistance rext in the armature circuit.

Speed Control By Varying Field current

Speed of shunt motor is given by

n=\frac { V_a-I_ar_a }{ k\phi }

If we change in any value of If then flux \phi will also change, hence speed will change as per the formula given above. To change the value of If an external resistance is required which is connected in series with the field windings. When no external resistance is connected then the field coil produces a rated flux and the rated voltage is applied across the field coil.

We can only decrease flux from its rated value By adding external resistance we can decrease the flux from its rated value. Thus the speed of the motor will increases as we decrease the field current and speed control above the base speed will be achieved. Speed versus armature current characteristic is shown in the figure.

We can only decrease flux from its rated value by adding external resistance. Thus the speed of the motor will rise as we decrease the field current and speed control above the base speed will be achieved. Speed versus armature current characteristic

For two flux values \phi and \phi_1. Since \phi_1<\phi, the no-load speed n0  for flux value \phi_1 is more than the no-load speed n0 corresponding to \phi. However, this method will not be suitable for constant load torque.

When rated armature current is less then the new armature current and the motor will be overloaded. This method will be suitable for a load whose torque demand decreases with the rise in speed by maintaining the output power constant as shown in the figure given below. This method is based on the flux weakening of the main field.

When new armature current will be greater than the rated armature current and the motor will be overloaded. This method will be suitable for a load whose torque demand decreases with the rise in speed keeping the output power constant as shown in the figure given below. This method is based on the flux weakening of the main field.

Speed Control By Armature Voltage Variation

Speed Control By Armature Voltage Variation

In this method, the armature current is supplied from a separate variable d.c voltage source, while the field is separately excited with fixed rated voltage as shown in the above fig. Here the armature resistance and field current are not changed. Since the no-load speed n_0=\frac { V_o }{ k\phi }, the speed versus Icharacteristic will shift parallelly as shown in the below-given figure for different values of Va.

Armature voltage control method is adopted for controlling speed from base speed down to very small speed as one should not apply across the armature a voltage which is higher than the rated voltage.

The armature voltage control method is chosen for controlling speed from base speed down to very small speed as one should not apply across the armature a voltage which is higher than the rated voltage.

Speed Control of DC Shunt Motor By Ward Leonard method (Va and If control)

In the war Leonard method, both field and armature control are integrated as shown in the figure. Arrangement for field control is simple. The rheostat is connected to the field circuit for this purpose. A prime mover is required to run the generator. A 3-phase induction motor is used as the prime mover that is supplied from a 3-phase supply as we usually do. The generated emf Va can be varied by controlling the field current of the generator. The potential divider connection is used by two rheostats in parallel to make easily the reversal of the generator field current.

Speed Control of DC Shunt Motor By Ward Leonard method (Va and If control)

Firstly the induction motor is started with the generator field current zero with the help of rheostats start. The field supply of the motor is switched on with motor field rheostat set to zero. The applied voltage to the motor Va can now be slowly increased to the rated value by increasing the generator field current.

In the Ward Leonard method, no starter is required for the d.c motor as the applied voltage to the armature is gradually increased. Speed of the d.c motor below base speed can be controlled by armature voltage, excitation of the d.c generator is increased or decreased, while to control the speed above base speed field current of the d.c motor is increased or decreased by maintaining constant Va. We can reverse the direction of rotation of the motor can be obtained by adjusting jockeys of the generator field rheostats.

However, the wide range of smooth speed control is achieved, the cost involved is rather high as we require one additional d.c generator and a 3-phase induction motor of similar rating as that of the d.c motor whose speed is deliberated to be controlled.

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