What is the speed control of a motor?

Speed control by DC motor-speed controller is required to realize variable speed drive. Whenever a motor drives a particular load, it is called drive, and variable speed drive refers to a load requiring variable speed. All this is obtained by a DC motor speed controller.

What is the principle of speed control of motor?

Speed depends on armature voltage supervisor and flux. Though the speed depends on armature current, armature current cannot be controlled by the DC motor speed controller as per requirement, so speed can’t be controlled by using armature current.

  • Base speed- It is the rated speed of a motor and the rating of the nameplate of the DC motor.
  • Speed regulation- It is the proportion of the rated speed or base speed to the change in motor speed from no-load condition to full load condition. It is always desirable that s DC motor runs at a constant speed irrespective of the load connected at a shaft of the motor. So, the speed regulation should be as least as possible.
  • Speed range- It is the ratio of maximum allowable speed to the minimum allowable speed of the DC motor speed controller.
  • Constant power/energy drive- If a motor drives load that requires constant power over a DC motor speed range, it is said to be constant power/energy drive by DC motor speed controller.
  • Constant torque/energy drive- If a motor drives load that requires constant torque over a motor speed range, it is said to be constant torque/energy drive by DC motor speed controller.

Types of speed control

Armature resistance speed control

For shunt motor

A variable resistor Re is used in the armature winding in this manner. Because the field energy is strongly linked to the power supply, variations in the resistive load have no impact on the flux.

The figure shows the armature resistance control method's connection schematic for a shunt motor with the help of a DC motor speed controller.
Shunt motor
The figure shows the shunt motor's current speed characteristic. It can be noted that the speed decreases more prominently with a series resistor than the normal condition where no external resistor is present.
Speed Vs. Current

For series motor

The figure shows the connection diagram of speed control of the DC Series system by the armature resistance control.
Shunt motor in series

The current/power flow and flux leakage are altered by changing the DC motor contact resistance. As a result of the supply voltage monitor loss in the resistive load, the voltage monitor output to the actuator is reduced, and the DC motor speed is reduced.

The figure shows speed–current characteristic of a series motor. It can be noted that the speed-current curve decreases with a series resistor than the normal condition.
Speed Vs. Current for series motor
  • The DC motor rotates at a slow rpm when the resistive load Re is enhanced. Because the variable resistance carries the complete armature current, it must be designed to do so continually.
  • This approach is employed to adjust the frequency or DC motor speed of a series motor drive train, a hoist, and other devices.
  • Overall, this strategy can be employed to reduce the speed below the rated speed because the speed decreases as the external resistance increases.

Field control method

In this method, field flux should not increase above the rated value. Otherwise, there is a risk of saturation, so we can only reduce the flux below the rated value.

This method is also called the field weakening method because magnetic field energy is being reduced.

So, this process is used for obtaining a speed higher than the base speed. Disadvantages of field control method:

  • High speed is obtained for a very weak field.
  • For a particular load, if the field current is very low, then the armature current is very high.
  • The high motor shaft armature current results in more copper losses, which heats the armature and reduces the efficiency of DC motor.
  • At low field current, armature current and armature reaction are very high, and high armature reaction may reduce low field flux leading to dangerously high speed. So, DC motor operation becomes unstable.

Resistance can’t be added directly in the field winding in a series motor. In that case, additional resistance will be in series with a field, and the method then becomes armature resistance control, giving speed lower than base speed.

Various kinds of field control methods are discussed below:

  • Diverter control

In this method, field current and armature become unequal using diverter resistance. In this case, we can vary field current without varying armature current.

  • Tapped field control

In this method, taps are provided in the field coil winding by which we can change the number of turns in the field winding, and by changing the number of turns, we can alter the magnetomotive force (MMF) and flux produced by field coil winding. As flux decreases, speed increases.

  • Reconnection of a field coil

If all the field winding turns are not connected in series but turn divided into multiple parallel branches, we can change an effective number of turns to impact MMF by changing the flux and speed of the DC motor.

Armature voltage control

The terminal voltage supervisor, armature voltage supervisor, or voltage detectors cannot increase above the rated value; otherwise, the insulation of the machine can get damaged.

So, we use the terminal voltage supervisor below rated voltage supervisor to reduce speed in this method.

Ward Leonard method

This method can be employed for above and below base speed, i.e., variable speed technology. The design of this variable speed system is simple, wide range smooth speed control. Reversing the generator's orientation can change the direction of the DC motor. The efficiency at low speed is higher as compared to other methods.

Uses of the speed control method

Below base speed, we use the armature method in which the flux is taken as constant, and due to the constant armature current, the torque is constant. Above base speed, we use field control. So, the terminal voltage supervisor and armature are kept constant. Hence, the product of voltage supervisor and current is a power that is also constant.

Context and Applications

In each of the expert exams for undergraduate and graduate publications, this topic is huge and is mainly used for

  • Bachelor of Technology in Electrical and Electronics Department
  • Bachelor of Science in Physics
  • Master of Science in Physics
  • Parallel operation of DC motor
  • Braking methods of DC motor
  • Starting methods of DC motor
  • Optocoupler and voltage supervisor

Practice Problems

Q1. In small DC motors, the speed control is fulfilled by _________.

(a) armature speed control

(b) field control

(c) Both of the above-mentioned

(d) None

Correct option: (a)

Explanation: It is difficult to vary the speed of small DC motors by the field control method.

Q2. The armature voltage supervisor method is performed on a ____________.

(a) shunt motor

(b) series motor

(c) separately excited motor

(d) None of these

Correct option: (c)

Explanation: The voltage control method is performed on a separately excited motor because the effect of the field on the armature is not desired.

Q3. Which method provides speed above the base speed?

(a) Flux control

(b) Armature control

(c) Armature diverter

(d) None

Correct option: (a)

Explanation: The flux control method helps achieve speed above base speed with the help of weakening of flux.

Q4. Which method provides speed below the base speed?

(a)  Field diverter

(b) Tapped field control

(c) Armature diverter

(d) None of these

Correct option: (c)

Explanation: An armature diverter provides a decrease in current to an armature, thus decreasing the motor speed.

Q5. The Ward Leonard method helps achieve the motor speed _______ the rated speed.

(a) above

(b) below

(c) remains same

(d) above and below

Correct option: (d)

Explanation: The Ward Leonard method of speed control helps the motor to achieve complete speed control. This method is employed where perfect speed control is required, like in escalators.

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