Types of DC Machines — Classification, Circuits & Applications
Introduction to DC Machine Types
A DC machine can operate as either a generator or a motor. The fundamental difference between the two is the direction of current flow — the construction remains identical. DC machines are classified based on how the field winding is connected relative to the armature winding.
The principal types of DC machines are:
- Separately Excited DC Machine — field winding energized by an external DC source
- Self-Excited DC Machine — field winding supplied by the machine itself
- Shunt Wound DC Machine
- Series Wound DC Machine
- Compound Wound DC Machine
In self-excited machines, the current flowing through the field winding is supplied by the machine itself, eliminating the need for a separate power supply. The type of excitation determines the machine's torque-speed characteristics, voltage regulation, and suitability for specific applications.
Where: V = terminal voltage, E = back EMF (motor) or generated EMF (generator), Ia = armature current, Ra = armature resistance
1. Separately Excited DC Machine
As the name suggests, the field coils are energized by a separate DC source that is independent of the armature circuit. This gives precise control over the field current and hence the magnetic flux, making these machines ideal for applications requiring accurate speed or voltage control.
Key characteristics:
- Field current is independent of armature current
- Better voltage regulation compared to self-excited types
- Requires an external DC supply for field excitation
- Used in Ward-Leonard speed control systems and laboratory setups
For Motor: V = Eb + Ia × Ra
Field current: If = Vf / Rf (independent of load)
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| Separately Excited DC Generator |
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| Separately Excited DC Motor |
2. Shunt Wound DC Machine
A machine in which the field coils are connected in parallel (shunt) with the armature is called a shunt wound DC machine. Since the shunt field winding receives the full terminal voltage, it is wound with a large number of turns of fine wire and carries a relatively small field current.
Key characteristics:
- Field winding connected in parallel with armature
- Approximately constant speed motor (speed drops slightly with load)
- Good voltage regulation as generator
- Field current: If = V / Rf (nearly constant)
- Most widely used DC motor type for constant-speed applications
Supply current: IL = Ia − If (motor, where IL is line current)
Speed: N ∝ (V − Ia×Ra) / Φ
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| Shunt Wound DC Generator |
Shunt Wound DC Motor
3. Series Wound DC Machine
A DC machine in which the field winding is connected in series with the armature is called a series wound DC machine. Since the field winding carries the full armature current (which is high), it is made of thick wire with a small number of turns to keep resistance low.
Key characteristics:
- Field winding in series with armature — carries full load current
- High starting torque (torque ∝ Ia² at low flux saturation)
- Speed varies widely with load — dangerous at no-load (runaway)
- Never start without mechanical load connected
- Used in traction, cranes, hoists, and electric vehicles
Torque: T ∝ Φ × Ia ∝ Ia² (before saturation)
Speed: N ∝ 1/Ia (approximately, before saturation)
DC Series Generator
DC Series Motor
4. Compound Wound DC Machine
A DC machine having both shunt and series field windings is called a compound wound DC machine. The shunt winding has a large number of turns of thin wire, while the series winding has few turns of thick wire. This combination provides characteristics between those of shunt and series machines.
Types of compound winding:
- Short-shunt compound: Shunt field connected in parallel with armature only
- Long-shunt compound: Shunt field connected in parallel with both armature and series field
- Cumulative compound: Series field flux aids shunt field flux — better voltage regulation
- Differential compound: Series field flux opposes shunt field flux — used in special applications
Long-shunt: V = Eg − Ia×(Ra + Rse)
Net flux (cumulative): Φ_total = Φ_shunt + Φ_series
Net flux (differential): Φ_total = Φ_shunt − Φ_series
Short-Shunt Compound Generator
Short-Shunt Compound Motor
Long-Shunt Compound Generator
Long-Shunt Compound Motor
Comparison of DC Machine Types
Applications of DC Machines
Frequently Asked Questions
Q1: What is the difference between self-excited and separately excited DC machines?
In a separately excited DC machine, the field winding is powered by an external DC source independent of the armature. In self-excited machines (shunt, series, compound), the field winding derives its current from the machine's own armature output, eliminating the need for a separate supply.
Q2: Why should a DC series motor never be started without load?
At no-load, the armature current is very small, producing weak field flux. Since speed is inversely proportional to flux (N ∝ Eb/Φ), the motor accelerates to dangerously high speeds that can mechanically destroy the armature. This is called "runaway" condition.
Q3: What is the advantage of compound wound DC machines over shunt or series types?
Compound machines combine the constant-speed characteristic of shunt machines with the high starting torque of series machines. Cumulative compounding provides better voltage regulation in generators and improved torque characteristics in motors for varying loads.
Q4: Which type of DC motor is used in electric vehicles?
Series wound DC motors are traditionally used in electric traction and older EVs due to their high starting torque and ability to handle overloads. Modern EVs increasingly use BLDC and PMSM motors, but the series DC motor principle remains relevant in understanding EV drivetrain fundamentals.
Q5: How does a cumulative compound generator maintain constant terminal voltage?
In a cumulative compound generator, as load increases, armature current rises, strengthening the series field. This additional flux compensates for the voltage drop due to armature resistance and armature reaction, maintaining nearly constant terminal voltage across varying loads (flat-compounding).