In a DC motor, torque is created by the interaction between armature current and the magnetic field. How that field is produced distinguishes the principal designs.
The four principal designs
| Design | Characteristic | Typical application |
|---|---|---|
| Permanent-magnet motor | Compact, no excitation current and good performance at low to medium power. | Mobile systems, pumps and compact actuators |
| Shunt motor | Field winding parallel to the armature; relatively constant speed under changing load. | Machine drives requiring stable speed |
| Separately excited motor | Field and armature supplied separately; wide control range and flexible characteristic. | Controlled industrial, test-bench and special-purpose drives |
| Series-wound motor | Very high starting torque; speed depends strongly on load. | Hoisting, traction and short-term high-load drives |
Why the characteristic curve matters
Permanent-magnet and separately excited motors have a largely constant excitation field. Their speed decreases predictably as load rises. A shunt motor behaves similarly, but its field and armature voltage cannot be adjusted independently. In a series motor the same current flows through field and armature: as load falls, the field weakens and speed rises disproportionately.
Which design is suitable?
Rated power and voltage are only the starting point. Load torque, speed range, duty, control method, installation space and environment must be considered together. Permanent magnets improve efficiency at low and medium power, but do not permit field-weakening control.
Data needed for sizing
- Supply voltage and required speed range
- Rated, starting and peak torque
- Duty type and cyclic duration factor
- Space, shaft, flange and ingress protection
- Controller, brake, encoder and other interfaces
Frequently asked questions
Which DC motor type offers the widest control range?
A separately excited motor generally offers the greatest flexibility because armature voltage and field current can be controlled independently.
Can a series-wound DC motor run without a load?
No. Without a suitable load its field weakens and its speed can rise to a dangerous level.
What information is needed to select a DC motor type?
Supply voltage, speed range, continuous and peak torque, duty type, mounting dimensions, environmental conditions and control interfaces are required.