Speed Control of DC Motor

Speed control is an important term for DC motor operation. DC Motors are used in various applications, starting from household appliances to industrial applications. Somewhere constant speed is required, but in most of the cases, the motor speed is required to vary in a wide range of speed. Imagine that, you are turning ON a fan. The speed of the fan must be controllable by adjusting the regulator. In the same way, other industrial motors should be controllable by the operator. Because, most of the industrial applications requires a wide range of motor speed. On extension with the torque and working principle of D.C. motor, we must now deal with the most essential topic regarding the Speed Control of a D.C. Motor. The back e.m.f in a motor can be expressed as Eb= φZNP / 60A = kφN Volt ,
the symbols denoting:
Z= no. of conductors
P=no. of poles
A= no. of parallel paths in the armature
φ = flux per pole
k= constant for particular machine.

This back e.m.f in the motor is always less than the applied e.m.f by IaRa the armature voltage drop, so that Eb= E – IaRa,
or, k φ N= E- IaRa
or, Speed in r.p.m, N = (E – IaRa) / kφ

The above expression reveals that the speed may be controlled by varying the applied voltage E, the armature current Ia, the armature circuit resistance, Ra, the flux per pole φ. So now we can easily say that according to the principle of speed control as we mentioned it above the following are the methods used in practice:
A. Rheostatic control.
B. Field control.
C. Series-Parallel control.
D. Special control method is Ward Leonard system.

Why proper speed control of DC Motor is neccesary

The rotary motion of DC motor is used in various industrial applications, like conveyor belts, production machinaries, automated robots etc. In cae of large DC motors, the motion of motor is controlled from a distance, more specifically from a control cabin. The required speed is not constant for all the time, sometime variation of motor speed is required. But the vatiation of supply voltage is quite difficult. Therefore some special speed control techniques are used for controlling the motor speed easily. In general, variable resistors or rheostats are used widely, as they provide the oppertunity of controlling the speed at a wide range. Suppose a mill is rotating with the help of DC motor.The rate of production is increased directly by increasing the motor speed. So, the opeartor controlls the speed and bound the speed under the rated speed by regulating the rheostat. So, by proper speed control, DC Motors can be used for a wide range of applications and also gives the ease of use for the operating personnel.

Rheostatic control or Armature resistance control:

In DC motor speed N is depended upon the armature voltage and field current. So, it is possible to vary the speed by changing the armature voltage and field current. We know that, the supply voltage is constant for DC Motors and changing the supply voltage of Motor is quite difficult. So, the armature voltage is controlled instaed of directly controlling the supply voltage. A variable resistance is provided with the armature in series. The armature voltage is controlleed by varying the variable resistance or rhrostat. This method of speed control is used for both DC series motor and DC shunt motor.

armature-voltage-control-of-dc-motorLet us take a resistance Re be the resistance ( variable resistor or rheostat) placed in series with the armature circuit. Then, speed, N= {E-Ia(Ra+Rse)}/kφ. This shows that the speed of the motor decreases as the series resistance Rse is being increased. More simply, if the resistance of the rheostat is increased, less amount of voltage will applied to the armature. Similarly, if the resistance o the theostat is decreased, more amount of voltage will applied to the armature circuit, and automatically yhe speed will increase. The field curent is kept constant in this technique. The chief disadvantage of this method is that there is considerable power dissipitation in the series resistance. This method is useful where reduction in speed is required for a short period. This difficulty can be avoided to some extent by the use of Armature Diverter resistance. For a given load torque, if Ia is reduced due to the diverter connected in parallel to the armature, φ must increase, which is T=ktφIa. Thus this increase in flux φ is associated with a reduction in speed as N ∝ (1/φ).

Field Current Control

The main field fluxes are produced bt the field current. So, the speed of DC Motor is easily controlled by varying the field current. They are of two types:

  • Field Rheostat: This method is mainly used to control the speed of DC Shunt motor. Here an additional resistance is connected in series with the shunt field circuit. The field current is controlled by varying the rheostat. This is the most satisfactory and economical way to control the speed of D.C. motors. Since the field current is usually very small, the losses in the resistance may be considered negligible. By increasing the resistance, the field is weakened and thereby a considerable variation in speed of shunt motor above normal may be obtained.

dc-motor-field-current-control

  • Field Diverter: In this case of a series motor, a diverter i.e. a variable resistance is connected in parallel with the series field. This will divert a part of the load current and thus the field is weakened with a result that the speed of the motor is increased. The parallel resistor is termed as diverter.
  • Tapped field method: In this method, tapped-field-control-of-dc-motorthe ampere turns are varied by changing the field turns The motor runs at its minimum speed when the full winding is effective. Cutting of the field turns in steps increases the speed of the motor. In this method, it is specially employed for the speed control of traction motors. By this method full and half speed may be obtained without any rheostatic loss. Other speeds may be obtained when an auxiliary series rheostatic control is used. The resistance is gradually cut out as the train speeds up. Then the motors are connected in parallel with the resistance in series which is again cut out gradually.

Special control method is Ward Leonard system

A very special and delicate method of speed control is this Ward Leonard System which is used for controlling the speed of a D.C. motor over a wide range from crawling speed to full speed particularly for those drives where rapid reversibility is an additional requirement. A brief discussion on this topic particularly has been done separately, kindly move to it for more information.

These are the conventional methods of speed control of DC Motor.