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Electric Drives - DC Motors
(Description and Applications)
Brushed (Field Energised) Motors (Motors using wound rotors)
The traditional DC motor needs two current supplies, one through the stator windings to provide the magnetic field and the other through the rotor windings to interact with the magnetic field to generate the motive force. There are three ways of accomplishing this, each one resulting in unique characteristic motor performance. Because they all use wound rotors, they all need a commutator to feed the current into the rotor windings.
Speed is controlled by varying the rotor voltage and hence the rotor current, or by varying the magnetic flux in the air gap by changing the current in the field windings.
With access to both the field and rotor windings, all DC motors offer the facility of simple speed and torque control.
- Series Wound
The series wound motor has only one voltage supply to the motor and the field winding is connected in series with the rotor winding.
The series motor has poor speed regulation. It delivers increasing torque with increased motor current but this is at the expense of speed which falls with increasing torque demands.
This motor has a very high starting torque because there is zero back EMF at zero speed however as the speed builds up so does the back EMF causing a reduction in torque.
Increasing the load on the motor tends to slow it down, but this in turn lowers back EMF and increases the torque to accommodate the load.
Speed control is possible by varying the supply voltage.
Under no load conditions the speed will accelerate to dangerous levels possibly causing destruction of the motor.
The motor can be reversed by reversing the connections on either the field or the rotor windings but not both.
Regenerative braking is not possible since the field current needs to be maintained but it collapses when the rotor current passes through zero and reverses.
The series DC motor is an industry workhorse for both high and low power, fixed and variable speed electric drives.
Applications range from cheap toys to automotive applications.
They are inexpensive to manufacture and are used in variable speed household appliances such as sewing machines and power tools.
Its high starting torque makes it particularly suitable for a wide range of traction applications.
- Shunt Wound
The shunt wound motor also has only one voltage supply to the motor but in this case the field winding is connected in parallel with the rotor winding.
- Field Weakening
The speed of a shunt wound motor can be controlled to a limited extent without affecting the supply voltage, by "field weakening". A rheostat in series with the field winding can be used to reduce the field current. This in turn reduces the flux in the air gap and since the speed is inversely proportional to the flux, the motor will speed up. However the torque is directly proportional to the flux in the air gap so that the speed increase will be accompanied by a reduction in torque.
The shunt wound motor turns at almost constant speed if the voltage is fixed. The motor can deliver increasing torque, without an appreciable reduction in speed, by increasing the motor current.
As with the series wound motor, the shunt wound motor can be reversed by reversing the connections on either the field or the rotor windings.
Regenerative braking is possible. Self excitation maintains the field when the rotor current reverses.
Fixed speed applications such as automotive windscreen wipers and fans.
- Separately Excited
The separately excited motor has independent voltage supplies to the field and rotor windings allowing more control over the motor performance.
The voltage on either the field or the rotor windings can be used to control the speed and torque of a separately excited motor.
Train and automotive traction applications.
Permanent Magnet Motors
As the name implies, these motors use permanent magnets rather than electromagnets to provide either the rotor or the stator field. They are used extensively in small DC motors and to an increasing extent in traction applications.
- Rotor Magnets
These are by far the most common types of permanent magnet motors. They have no rotor windings but use permanent magnets to supply the rotor field and they behave like shunt wound DC motors with a fixed shunt current.
Their major advantage is the elimination of the commutator.
- Field Magnets
These motors have no field winding but use permanent magnets to provide the magnetic field. Current is still supplied to the rotor via a commutator as in other brushed motors and the speed can be controlled by varying the voltage on the rotor windings. In this way their behaviour is similar to a series wound DC motor.
Permanent magnet motors are explored in more depth in the section on Brushless DC and AC Motors
In a series wound DC motor, reversing either the field winding leads or the rotor winding leads will reverse the direction of the motor. However, simply reversing the leads from the power supply will have no effect on the direction of rotation since it is equivalent to reversing the current through both the individual windings - in effect a double reversal. In other words the motor will turn in the same direction even though the current through the series windings is reversed. This means that the motor can run on alternating current as well as direct current since the direction of rotation is independent of the direction of the current through the series windings.
Universal motors are often used in power tools and household appliances such as vacuum cleaners and food mixers. See more about universal motors.
Electric Machine Fundamentals
Brushless DC and AC Motors