Classification of DC Machines: Types of DC Generators and DC Motors
A DC machine can operate as either as a dc generator or a dc motor, therefore their classification is applicable to both forms. DC machines are primarily classified based on their field excitation method, which leads to two major categories:
- Separately Excited DC Machines
- Self-Excited DC Machines
This classification is valid for both: DC Generators and DC Motors.
1. Separately Excited DC Machines
In this type, the field winding is powered by an external source, making it electrically independent from the armature.
Key Features:
- Precise control of field current
- Common in labs, research, and Ward-Leonard systems for speed control of DC motors
- Higher cost due to need for an additional power supply
Example: PMDC Motors
Permanent Magnet DC Motors use magnets instead of a field winding to provide constant flux. Commonly used in toys and small appliances.
2. Self-Excited DC Machines
Here, the field winding is energized by the current generated by the machine itself. In this type, field winding and armature winding are interconnected in various ways to achieve a wide range of performance characteristics (for example, field winding in series or parallel with the armature winding).
A small amount of flux is always present in the poles due to the residual magnetism. So, initially, current induces in the armature conductors of a dc generator only due to the residual magnetism. The field flux gradually increases as the induced current starts flowing through the field winding.
Subtypes of Self-Excited DC Machines:
2.1 Series Wound DC Machines
- Field winding is connected in series with the armature
- The field winding carries whole of the load current (as in searies with armature winding).
- That is why series winding is designed with few turns of thick wire and the resistance is kept very low (about 0.5 Ohm).
2.2 Shunt Wound DC Machines
- Field winding is connected in parallel with the armature
- Hence, the full voltage is applied across the field winding.
- Shunt winding is made with a large number of turns and the resistance is kept very high (about 100 Ohm).
- Consumes low current (less than 5% of armature current)
2.3 Compound Wound DC Machines
In this type, there are two sets of field winding. One is connected in series and the other is connected in parallel with the armature winding. Compound wound machines are further divided as:
- Short Shunt: Shunt field is parallel to armature only
- Long Shunt: Shunt field is parallel to both armature and series field
Classification Chart
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| Classification of DC Machines |
Why Classification Matters
Choosing the right type of DC machine affects performance, efficiency, and cost. The classification helps engineers select machines for:
- Torque and speed control
- Load adaptability
- Voltage regulation and stability
Explore more:
Summary Table
| Type | Field Connection | Use Case | Field Resistance |
|---|---|---|---|
| Separately Excited | Independent Power Source | Lab, Speed Control | Variable |
| Series Wound | Series with Armature | High Torque: Cranes, Hoists | Very Low (~0.5 Ω) |
| Shunt Wound | Parallel with Armature | Constant Speed: Fans, Blowers | High (~100 Ω) |
| Compound Wound | Series + Parallel | Elevators, Presses | Moderate |
Frequently Asked Questions
- What are the main types of DC machines?
DC machines are mainly classified as separately excited and self-excited types, including series, shunt, and compound wound machines. - What is the difference between a shunt and series DC machine?
In a shunt DC machine, the field winding is in parallel with the armature, while in a series DC machine, the field winding is in series with the armature. - Where are separately excited DC machines used?
They are used in labs, research, and Ward-Leonard systems where precise speed control is required.