DC motor types
There are more than 5 basic configurations of direct current motors, classified according to the connection of their field windings, with powers ranging from fractions of a watt to several megawatts and typical efficiencies between 70 % and 95 %. These machines convert direct current electrical energy into rotary mechanical energy, being the first electric motors for mass use due to their ease of speed control and high starting torque.
The speed of a DC motor is governed by the equation ( V = E_b + I_a R_a ), where ( E_b ) (counter-electromotive force) is proportional to the magnetic flux and angular speed, with nominal speeds that can vary from 1000 rpm / 16.7 rps to 3000 rpm / 50 rps. The magnetic field of the stator, generated by permanent magnets or electromagnets, interacts with the current flowing through the armature to produce an electromagnetic torque that drives the rotor. The collector (commutator) and brushes reverse the current in the armature coils every half turn, maintaining the torque in the same direction.
Classification of direct current motors
Section titled “Classification of direct current motors”DC motors are divided into two large groups: permanent magnet and wound field; the latter are subdivided into 3 categories according to the connection of the field winding: separate excitation, self-excited (shunt, series, and compound). Each configuration offers particular speed‑torque characteristics that determine its industrial application.
Permanent magnet motor (PMDC)
Section titled “Permanent magnet motor (PMDC)”The magnetic field of the stator is produced by permanent magnets with high flux density. Since it does not require a field winding, the armature current is the only control variable. Speed is practically linear with applied voltage and torque is directly proportional to armature current, since ( \phi = ) constant. It is used in low power applications such as toys, cordless tools, and actuators.
Separately excited motor
Section titled “Separately excited motor”The field winding and the armature winding are supplied from separate direct current sources. The field flux ( \phi ) can be adjusted independently of the armature current ( I_a ), allowing precise control of torque and speed. The field current does not depend on the load, which facilitates closed-loop control for precision drives.
Shunt motor (parallel)
Section titled “Shunt motor (parallel)”The field winding is connected in parallel with the armature, so that the entire line voltage ( V ) is applied to the field. The field current ( I_f = V / R_f ) is constant if the voltage is stable, keeping the flux practically fixed. Speed varies little with load (constant speed characteristic). Torque is proportional to armature current, giving a slightly decreasing speed‑torque curve. It is used in centrifugal pumps, fans, and machine tools where regulated speed is required.
Series motor
Section titled “Series motor”The field winding is in series with the armature, so the field current is equal to the armature current. At low speeds, the high current produces a strong magnetic flux, generating a very high starting torque (up to 5 times the rated torque). Speed drops drastically as load increases, and conversely, without load the motor can run away. Typical applications: electric traction, cranes, internal combustion engine starters, and high-power portable tools.
Compound motor (combined)
Section titled “Compound motor (combined)”It combines a series winding and a shunt winding on the same field core. Depending on the arrangement it can be:
- Cumulative compound: the series and shunt fluxes add, improving speed regulation compared to the series motor and increasing starting torque compared to the shunt.
- Differential compound: the fluxes oppose; offers almost constant speed regulation but can become unstable under overloads.
Furthermore, the physical connection determines:
- Long shunt: the series winding is in series with the armature and the shunt winding is in parallel with the armature+series assembly.
- Short shunt: the shunt winding is in parallel with the armature and the series winding is in series with the line.
Applications: elevators, presses, rolling mills, and systems requiring high starting torque with controlled speed.
Fundamental equations
Section titled “Fundamental equations”η_m = P_{salida} / P_{entrada}
Where ( P_{salida} ) is the mechanical power on the shaft (W) and ( P_{entrada} ) is the electrical input power (W). If the output power is measured in horsepower (hp), the equivalent expression is:
η_m = P_{salida} × 746 / P_{entrada}
| Variable | Meaning |
|---|---|
| η_m | Motor efficiency (dimensionless) |
| P_{salida} | Shaft power (W or hp) |
| P_{entrada} | Electrical input power (W) |
| 746 | Conversion factor: 1 hp = 746 W |
The electromagnetic torque equation for any DC motor is:
T = K_a φ I_a
| Variable | Meaning | Unit |
|---|---|---|
| T | Electromagnetic torque | N·m / lb·ft |
| K_a | Armature construction constant | — |
| φ | Magnetic flux per pole | Wb |
| I_a | Armature current | A |
The steady-state angular speed is obtained from:
ω = \frac{V - I_a R_a}{K_a φ}
where ( V ) is the line voltage, ( R_a ) the armature resistance, and ( ω ) the angular speed (rad/s). Converted to revolutions per minute (rpm), it results:
n = \frac{60}{2π} ω
Comparative table of DC motor types
Section titled “Comparative table of DC motor types”| Type | Field connection | Speed‑torque characteristic | Starting torque (relative to rated) | Typical applications |
|---|---|---|---|---|
| Permanent magnet | No winding required (magnets) | Linear, speed proportional to voltage | 1.5 – 2.5 times | Cordless tools, robotics, household appliances |
| Separate excitation | Separate supply | Widely adjustable | 1.5 – 2.5 times | Precision drives, CNC machine tools |
| Shunt (parallel) | Field in parallel with armature | Almost constant speed, slightly decreasing with load | 1.3 – 1.8 times | Centrifugal pumps, fans, compressors |
| Series | Field in series with armature | High speed at no load, strong drop with load; high torque at low speed | 4 – 5 times | Electric traction, cranes, starters |
| Cumulative compound | Series + shunt (fluxes added) | Intermediate regulation, good starting torque | 2 – 3 times | Elevators, rolling mills, presses |
| Differential compound | Series + shunt (fluxes opposed) | Very stable speed (almost constant) | 1.5 – 2 times | Special applications where speed must be invariant with load |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the typical efficiency of a small DC motor?
Section titled “What is the typical efficiency of a small DC motor?”Small DC motors up to 1 hp show nominal efficiencies between 70 % and 85 %, depending on the design and quality of materials. In higher power industrial motors, the efficiency can exceed 90 % at full load.
What speed range does a 4-pole shunt motor supplied at 60 Hz reach?
Section titled “What speed range does a 4-pole shunt motor supplied at 60 Hz reach?”A 4-pole shunt motor designed for 1800 rpm / 30 rps synchronous speed typically runs at 1725 rpm / 28.75 rps at full load, with a speed regulation of less than 5 % between no load and full load.
What is the typical armature current of a 10 hp compound motor at 500 V?
Section titled “What is the typical armature current of a 10 hp compound motor at 500 V?”A 10 hp (7.46 kW) compound motor at 500 V draws about 15 A of armature current at full load, while the shunt field current is around 0.5 A, resulting in a total line current of approximately 15.5 A.
What armature resistance does a 2 hp DC motor have?
Section titled “What armature resistance does a 2 hp DC motor have?”Armature resistance in 2 hp, 180 V DC motors ranges from 0.2 Ω to 1.0 Ω, causing an internal voltage drop of 2 V to 10 V at full load and affecting the slope of the speed‑current characteristic.
What is the safe speed limit for a shunt motor without load?
Section titled “What is the safe speed limit for a shunt motor without load?”A shunt motor can run away up to 3000 rpm / 50 rps if the field is interrupted while the armature receives full voltage; therefore, protection systems limit the maximum speed to 120 % of the rated speed, for example, 2160 rpm for a 1800 rpm motor.
What relative starting torque does a series motor develop compared to a shunt motor?
Section titled “What relative starting torque does a series motor develop compared to a shunt motor?”Series motors develop up to 5 times the rated torque, while a shunt motor typically delivers 1.3 to 1.8 times the rated torque. This difference makes the series motor preferred in applications requiring starting with heavy load.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html
- electrical4u.com: https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/