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# Back to the Basics in DC Motor Speed Control

Back to the basics in DC motor speed control

Ever since British scientist William Sturgeon invented the first commutator DC motor in 1832, there have been significant developments in this technology. It is now among the most widely used motor type in a broad range of industries.

DC motors offer several advantages, including high starting torque, quick starting and stopping, ease of maintenance, and high efficiency. But one of the key advantages contributing to the high demand for DC motors is their adjustable speed, which allows engineers to set up their motors to meet different operational requirements.

But there are several DC motor speed control methods, with each method having its unique setup, advantages, and suitability for certain applications. This article will get back to the basics of DC motor speed control. It will explain the different speed control methods of DC motors and their suitability for a particular application.

There are several methods of DC motor speed control

What is a DC motor, and how to control DC motor speed?

DC stands for direct current. As the name implies, a DC motor is operated using “direct current” to convert electrical energy into mechanical energy (in the form of shaft rotation). DC motors work on the principle of Lorentz law, which states that a conductor (that is carrying current) experiences a mechanical force when placed in a magnetic field. This force causes the motor to rotate.

This rotating action of the conductor produces an electromotive force (EMF), which tends to oppose the supplied voltage. A DC motor adjusts its torque in case of varying load due to the back EMF. The rotational speed of a DC motor can be calculated using:

Where:
N = rotational speed of the motor
K= constant
Eb = Back EMF
Ø = flux per pole

From this set of equations, one can see that the speed of a DC motor can be controlled by varying the supplied voltage, armature current, resistance, or flux. The next section will explain methods of DC motor speed control.

Speed control methods of DC motors

To explain the speed control methods of DC motors, we will be considering two types of self-excited DC motors:
1. DC shunt motor
2. DC series motor

Speed control methods for DC shunt motor#1 Flux control method

Since the flux per pole (or magnetic flux) is inversely proportional to the motor speed, engineers can increase the motor speed by decreasing the flux and vice versa. To control the flux, add a variable resistor (also called rheostat) in series with the field winding.

Increasing the electrical resistance of the rheostat causes lesser current to flow the field winding, which causes the flux produced to decrease and the speed of the motor to increase.

#2 Armature control method

Recall that the DC motor speed directly correlates with the back EMF (Eb). Therefore, engineers can increase the motor speed by increasing the back EMF. Back EMF is related to the supply voltage, armature current, and armature resistance according to:

Where:
V = supplied voltage
Ia = armature current
Ra = armature resistance

Therefore, the motor speed equation becomes:

As such, the motor speed can be controlled by varying any of these parameters. For example, adding resistance in series to the armature resistance causes the voltage across the armature and motor speed to decrease. This method is particularly useful when engineers are looking to achieve a motor speed below its normal operating range.

#3 Voltage control method using Ward Leonard system

The voltage control method using the Ward Leonard system is a popular DC motor speed control method. This method was introduced in 1891 and is achieved by varying the applied voltage to the armature.

Consider the figure below, which shows the typical Ward Leonard system circuit of DC motor speed control.

Ward Leonard system of DC motor speed control
Source: Spaceel/CC [SA] [3.0]

Subscript “m” represents the main DC motor to which speed control is required, while subscript “g” represents a generator driven by another motor. By varying the voltage (Vfg) fed into the generator field by this motor, engineers can control the generator's output voltage. This output voltage is then fed into the second motor's armature (where speed control is needed), causing it to rotate.

Speed Control Methods of DC Series Motor #1 Armature controlled resistance techniqueThis speed control technique is achieved by introducing resistance in series with the armature, which allows engineers to reduce the voltage across the armature, and subsequently, the speed of the motor.

#2 Field diverter technique

As its name implies, the field diverter technique uses a field diverter (in the form of a variable resistor) to control the speed of the motor. The variable resistance is connected parallel to the series field, allowing the desired electric current to be diverted through it. This, in turn, allows engineers to control the flux and, subsequently, the speed of the motor.