Speed Control of DC machines

(You should know the basic theory of DC machines before reading this)
It is essential to vary the speed of the DC motor as the load demands. In case when the load is alternator (i.e. DC motor is prime mover), as the speed of motor varies so does the output voltage of the alternator. It is necessary to keep the output voltage of the alternator constant inspite of the varying load on the alternator. This is achieved via speed control of prime mover via AVR loop. Speed control of DC machine is a part of AVR loop.

We know that

Substituting for

in terms of torque and flux, the relationship between

and

is given by:

If the armature terminal voltage

and airgap flux

are held constant, the above equation can be written as:

The Y axis intercept represents the no-load speed which depends only on the terminal voltage and air gap flux. The variation of speed with torque is shown in figure

Generally, the drop in speed with increase in torque is small (it is desirable that the speed of rotation is independent of load coupled to the motor shaft). A point to be noted is that in an actual machine, armature reaction helps in maintaining the speed almost constant. Therefore if

and

are held constant, the speed of a separately excited dc motor will remain almost constant and it is independent of torque applied to the shaft. Hence in order to vary the speed of over a wide range, the no-load speed (magnitude of

) should be varied. This can be achieved by the following methods:

By controlling the voltage applied to the armature terminals of the machine
By controlling the flux produced by the field winding

Armature Voltage control

The schematic diagram for this control technique is shown in figure.

In this method, the field current is held constant at its rated value and

is varied. The speed-torque characteristics for this method are shown in figure at the bottom of the page. These characteristics are drawn for various values of

and fixed value of flux. This method of speed control is used for speed below the rated value. It is worth noting that since

is held constant, the speed of rotation changes linearly with

. Motor will draw a constant armature current

from the source if it is driving a constant torque load. Under this working condition, the power (P) drawn by the motor varies linearly with the speed. This mode of operation is known as constant flux or constant torque mode

Field control

The schematic diagram for this control technique is shown in figure.

In this method, the armature voltage is held constant at its rated value and the field current is reduced. The speed of the motor changes in inverse proportion to

. The speed torque characteristics for this method are shown in figure at the bottom of the page. This portion of speed cpntrol is called as field weakening zone. These characteristics are drawn for various values of

and fixed value of

. It should be noted that the reduction in speed with torque is higher compared to that in the previous method. This method of speed control is used for speed above the rated value. It is worth noting that this is a inverse non-linear control of motor speed. This method also changes the value of developed torque for a given armature current. If the armature current is held constant at the rated value, the input power and therefore output power remains approximately constant (assuming that frictional and windage losses remain constant). Hence, this operating zone is also known as constant hp (horse-power) mode. Generally the maximum speed of is kept within 150 % of the rated value.