DC motors are electrical devices that draw direct current and convert it into mechanical energy. Direct current is especially useful for machines that need to be driven at variable speeds. These motors are easy to miniaturise, thus making them suitable for compact devices. They come in handy during low power and low voltage applications.
Author: Jeson Pitt, D&F Liquidators
You can even use DC motor for high-power applications that require several megawatts. They are useful in places where speed variation is needed at high performance levels without complicated motor control systems.
How it works
An armature carrying a current, which is connected through brushes and commutator segments to the supply end, is placed between two magnets. They can either be an electromagnet or a permanent one. The North and South poles will be perpendicular to the armature.
When the current flows through this armature (using a motor starter), it will produce rotational movement.
Construction of DC motor
DC motor is made of the following parts:
Types of DC motor
There are three types of DC motors, which are based on the types of winding:
If both amperes turns add their effect, it works as an additional flux motor, and if they work against each other, it functions as a negative flux motor. This mounting method is rarely used since it can be unstable with high loads.
The aim of circuit
The objective of the circuit of DC motor is to keep the permanent magnet DC motor running at a constant speed that is set externally. You can achieve this by monitoring the current and voltage across the brushes of the motor.
The voltage consists of two components:
The current through the motor armature passes through a resistance (rm/10), which means, approximately 0.1 as large as the ohmic resistance of the motor. Then, the voltage across this resistance is amplified by a factor of approximately 10, and the resulting voltage is added to a second voltage in a differential amplifier, which is the voltage as measured across the two brushes of the motor.
The output of this amplifier is compared to the reference voltage (which is provided externally to the circuit and it determines the speed of rotation of the motor) in another differential amplifier. You can control the output of a power outage stage to drive the motor.
The reference voltage is compared to the back-EMF and the motor is caused to run at a constant speed set by the reference voltage. You have to superimpose a sawtooth waveform on the reference voltage to soften the switch from driving to not driving.