A DC motor is an essential device that converts direct current (DC) electrical energy into mechanical motion. It plays a crucial role in various applications by transforming electrical energy into useful mechanical work, making it a key component in many machines and systems. This article explores the working principles, types, losses, and speed control methods of DC motors, providing a comprehensive understanding of their operation and significance.
Quick Answer
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A DC motor works by putting current-carrying conductors in a magnetic field. The armature conductors feel a force (Fleming’s left-hand rule), which produces torque and turns the shaft. Every half turn, the commutator and brushes reverse the current in each coil so the torque keeps pushing the same way and the motor keeps rotating.
As it speeds up, the armature generates a back EMF that opposes the supply and limits the current. Speed is set by the armature voltage and the field flux.
Table of Contents
What is a DC Motor?
A DC motor is a device that converts direct current (DC) electrical energy into mechanical energy. The structure of a DC generator and a DC motor is the same.
How Does a DC Motor Work?
The field winding (or permanent magnets) sets up a magnetic field from the north pole to the south pole. When DC current flows through the armature conductors inside that field, each conductor feels a force F = BIL. By Fleming’s left-hand rule, conductors on opposite sides of the armature are pushed in opposite directions, so together they make a turning force (torque) on the shaft.
Without help, that torque would reverse as each coil passes the neutral axis. The commutator, a split copper ring fed by carbon brushes, reverses the current in each coil at exactly that point, so the torque always acts in the same direction and the armature keeps turning. Reversing either the armature current or the field reverses the direction of rotation.
Torque is proportional to flux and armature current (T ∝ ΦIa). Speed is proportional to back EMF divided by flux: N ∝ (V − IaRa) / Φ.
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Types of DC Motors
DC motors can be categorized into three types based on the connection between the field and the armature:
- Shunt Motor, Eb = V – IaRa
- Series Motor, Eb = V – Ia(Ra+Rse)
- Compound Motor
Compound motors can further be divided into two types (Rse = series field resistance, IL = line current):
- Long Shunt Compound Motor, Eb = V – Ia(Ra+Rse)
- Short Shunt Compound Motor, Eb = V – IaRa – ILRse
Losses in DC Motors
- Copper Loss
- Core Loss
- Friction Loss
- Windage (Air Friction) Loss
Description of Losses
Copper Loss
- Armature Copper Loss = I2aRa
Field Copper Loss
- Shunt Field Loss
- Series Field Loss
Brush Contact Resistance Loss
- Magnetic Loss or Core Loss
- Hysteresis Loss
- Eddy Current Loss
Mechanical Loss
- Friction Loss
- Air Friction Loss
Other types of losses include:
- Stray Loss = Magnetic + Mechanical
- Constant Loss = Shunt Field + Stray
- Variable Loss = I2aRa
- Total Loss = Variable + Constant
What is Shaft Torque in DC Motors?
The torque obtained at the armature of the motor is not considered as shaft torque because some torque is lost in the iron and bearing parts of the motor. Therefore, the torque available for doing work is called shaft torque.
What is Back EMF in DC Motors?
When the armature of a DC motor rotates within the magnetic field of the field windings, it cuts the magnetic lines of force, inducing a voltage in the armature conductors. This induced voltage is opposite in direction to the supply voltage and is known as Back EMF. It is given by Eb = PΦZN / (60A), where P is poles, Φ is flux per pole, Z is armature conductors, N is speed in rpm and A is parallel paths. Back EMF is why a motor draws a large current at startup and much less at full speed.
Power Stages of DC Motors
⇒ Motor Power Input ⇒ Copper Loss ⇒ Mechanical Power Developed ⇒ Iron and Friction Loss ⇒ Motor Output Power
How to Control the Speed of DC Motors?
- By varying the voltage
- By series-parallel control
- By changing the magnetic flux
- By adjusting the resistance in the armature circuit
What is Speed Regulation in DC Motors?
Speed regulation is defined as the percentage difference between the no-load speed and the full-load speed, divided by the full-load speed.
Speed Regulation = (No Load Speed – Full Load Speed) * 100 / Full Load Speed
Frequently Asked Questions
What is a DC Motor?
A DC motor is a device that converts direct current (DC) electrical energy into mechanical energy. It shares the same construction as a DC generator.
How Does a DC Motor Work?
Current through the armature conductors sits in the magnetic field of the poles, so each conductor feels a force (Fleming’s left-hand rule). The forces make a torque that turns the shaft, and the commutator reverses the coil current every half turn so the motor keeps rotating in one direction.
What Are the Main Types of DC Motors?
DC motors are classified into three types based on how the field winding is connected to the armature: Shunt Motor, Series Motor, and Compound Motor (which is further divided into Long Shunt and Short Shunt Compound Motors).
What Are the Different Losses in a DC Motor?
The losses in a DC motor are copper losses (armature and field I²R), core losses (hysteresis and eddy current), mechanical losses (bearing friction and windage) and brush contact loss.
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How is the Speed of a DC Motor Controlled?
The speed of a DC motor can be controlled by varying the voltage, using series-parallel control, changing the magnetic flux, or adjusting the resistance in the armature circuit.
What is the function of a DC motor?
A DC motor’s function is to convert direct-current electrical energy into rotating mechanical energy, which drives fans, pumps, conveyors, power tools, toys and vehicle accessories.
Why does a DC motor need a commutator?
The commutator reverses the current in each armature coil every half turn. Without it the torque would reverse direction each half turn and the armature would just rock back and forth instead of rotating.
Why should a DC series motor never be started without a load?
A series motor’s field current is the armature current. With no load the current and flux fall, and since speed varies inversely with flux, the motor can overspeed to a dangerous level.
Related: AC induction motors, synchronous motors and how to read a motor nameplate.
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