- DC Motor Definition: A DC motor is defined as a machine that converts direct electrical energy into mechanical energy, using various configurations of winding and magnetic fields to achieve desired operation characteristics.
- Applications of DC Motors: These motors are integral in countless devices and systems, highlighting their versatility across different sectors including automotive, industrial machinery, and consumer electronics.
- Shunt and Series Characteristics: Shunt wound motors maintain constant speed, whereas series wound motors offer high starting torque with speed varying according to the load.
- DC Compound Motor: The dc compound motor is a versatile type, combining characteristics of both shunt and series wound motors to provide reliable operation under varying load conditions.
- Understanding Torque Control: In DC motors, torque can be controlled by adjusting the current through the armature or the magnetic flux in the field windings, which directly affects the motor’s speed and power output.
The types of DC motor are grouped by how the field is made and how it is connected to the armature: permanent-magnet, separately excited and self-excited shunt, series and compound machines. DC motors drive loads from shavers to vehicles.
The types of DC motor include:
- Permanent Magnet DC Motor (PMDC Motor)
- Separately Excited DC Motor
- Self Excited DC Motor
- Shunt Wound DC Motor
- Series Wound DC Motor
- Compound Wound DC Motor
- Short shunt DC Motor
- Long shunt DC Motor
- Differential Compound DC Motor
Each type is taken in turn below. For more DC motor study, see the list of basic electrical questions.
Separately Excited DC Motor
In a separately excited DC motor the field and armature windings have their own supplies. Armature current does not flow in the field winding. Field current can then be set independently of armature current.
From the torque equation of DC motor we know Tg = Ka φ Ia so torque can be changed by changing field flux φ without having to change armature current Ia.
Permanent Magnet DC Motor
The permanent magnet DC motor (also known as a PMDC motor) still has an armature winding, but it has no field winding. Radially magnetized permanent magnets on the inner stator bore supply the field flux.
The rotor is a conventional DC armature with commutator segments and brushes. The diagram below shows that layout.
The DC motor torque equation then gives
Here φ stays at the magnet value chosen at build and is not varied in service.
For a permanent magnet DC motor,
where Ka1 = Ka.φ is then a constant. Torque is changed by changing the armature supply.
Self Excited DC Motor
In a self-excited DC motor the field winding may be series with the armature, parallel with it or a mix of both. Those connections give three machines:
- Shunt wound DC motor
- Series wound DC motor
- Compound wound DC motor
The three self-excited types are described next.
Shunt Wound DC Motor
In a shunt wound DC motor, or shunt wound self excited DC motor, the field winding sits in parallel with the armature so it sees the full terminal voltage, as in the figure below.
The voltage equation for this connection is
[Where, E, Eb, Ia, Ra are the supply voltage, back emf, armature current and armature resistance respectively]
[since back emf increases with flux φ and angular speed ωω]
Substitute Eb from (2) into (1):
The DC motor torque equation is of the form
That is a straight-line torque-speed relation. The shunt wound self-excited DC motor characteristic is then
A shunt wound DC motor is treated as nearly constant-speed. Speed does fall a little with load because of armature resistance drop, but the drop is small compared with a series motor.
Series Wound DC Motor
In a series wound self-excited DC motor, or simply a series wound DC motor, the same armature current also flows in the field winding because the field is in series with the armature. The connection is shown below.
The torque-speed relation for this type follows from the voltage equation.
From the circuit, the voltage equation becomes
Back emf is still Eb = kaφω
If saturation is neglected,
[since field current = armature current]
From (5) and (6)
That equation gives the torque-speed curve
In a series wound DC motor, speed changes strongly with load. That is the main operating difference from a shunt wound DC motor. A series motor is not run without load, because speed then rises without a useful limit.
Compound Wound DC Motor
Compound excitation mixes the shunt and series excited DC motor field connections. A compound wound self-excited DC motor, or compound wound DC motor, has field winding both in series and in parallel with the armature, as below:
Compound wound DC motors are cumulative or differential, according to how the two fields add.
Cumulative Compound DC Motor
If the shunt field flux adds to the series-field flux, the machine is a cumulative compound DC motor.
Differential Compound DC Motor
In a differential compound DC motor the shunt field opposes the series field, so net flux and torque fall as series current rises.
Net flux is then lower than the series flux alone, so this connection is rarely used.
The compounding curves of the self-excited DC motor are shown below.
Cumulative and differential compound DC motors may each be short-shunt or long-shunt, according to the shunt-field connection.
Short Shunt DC Motor
If the shunt field is paralleled with the armature alone, leaving the series field outside that parallel pair, the machine is a short-shunt compound wound DC motor.
The short-shunt DC motor circuit is shown below.
Long Shunt DC Motor
If the shunt field is in parallel with the armature and the series field together, the machine is a long-shunt compound wound DC motor, or simply a long-shunt DC motor.
The long-shunt DC motor circuit is shown below.
