The synchronous and universal motors are families of motors designed for needs different from the classic induction motor, such as speed precision or high revolutions.
We already know the asynchronous motor. Now we will compare other families with different characteristics and applications.
The Synchronous Motor
In the asynchronous motor, the rotor always rotated slightly slower than the magnetic field (slip). In the synchronous motor, the rotor creates a field using permanent magnets, electrical excitation, or a reluctance design.
By having its own fixed magnetic field, the rotor remains magnetically locked with the rotating field of the stator.
- Behavior: In steady state, its average speed matches the synchronous speed (
). - If the grid is at 50.00 Hz: The speed also depends on the number of poles.
- If you increase the load: The angle between the stator and rotor fields increases. If the maximum torque is exceeded, the motor may lose synchronism.
Where is it used?
- Generators: Almost all power plants use synchronous generators.
- Electric cars: Many use permanent magnet synchronous motors for their power density and good efficiency.
- Industry: Where speed must be constant regardless of conditions.
The Universal Motor
If you open a blender, a drill, or an angle grinder, you’ll see a motor inside that makes a lot of noise, has sparks and brushes. It’s a Universal Motor.
It’s called that because it has a clever trick: It works the same on Alternating Current (AC) and Direct Current (DC).
The Physical Principle
It is basically a DC Series motor. The current passes through the stator windings and then goes to the rotor through the brushes.
- When AC changes polarity (positive to negative), it changes the direction of the current in the stator AND ALSO in the rotor simultaneously.
- Since both fields reverse at the same time, the torque maintains the same direction.
Advantages and Disadvantages
- Advantage: It can rotate extremely fast (10,000, 20,000, 30,000 rpm). An induction motor cannot exceed 3000.
- Advantage: It has a brutal starting torque. That’s why drills have so much power when starting.
- Disadvantage: It has brushes. They wear out, create sparks and noise. They need to be replaced.
Danger: A universal motor should never be started “unloaded” (without a connected load) at full power. Having no speed limit by frequency, it can overspeed and self-destruct due to centrifugal force. (Hand tools have internal friction that prevents this).
Which Motor Should I Choose?
To close the module, let’s keep a quick selection table based on the project:
| Motor Type | Principle | Speed | Maintenance | Typical Application |
|---|---|---|---|---|
| Asynchronous (Squirrel Cage) | Induction (Slip) | Close to synchronous; adjustable with VFD | Low | Fans, pumps, conveyor belts. |
| Synchronous | Magnets, excitation, or reluctance | Equal to synchronous in steady state | Low or medium | Generators, servomotors, traction. |
| Universal | Brushes (Series) | Very high and variable (>10000 rpm) | High (Replace brushes) | Drills, blenders, (old) washing machines. |
| DC (Magnet/Coil) | Direct Current | Easily variable | Medium (Brushes) | Toys, automotive (windshield wipers). |
| Brushless (BLDC) | Electronic commutation | Variable and high | Low | Drones, computers, RC cars. |
