Universal Motors Operational Principle and Application in Human life

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Universal Motors – Working, Characteristics, and Applications

What is a Universal Motor?

A universal motor is a type of series-wound commutated motor that can operate on both AC and DC supply. It is essentially a DC series motor modified for AC operation. These motors are compact, inexpensive, and capable of running at very high speeds, making them ideal for household appliances and portable power tools.





Applications of Universal Motors

Universal motors are widely used in small and medium appliances where variable speed and high starting torque are essential:

  1. Power tools (drills, grinders, saws)
  2. Food processors & mixers
  3. Vacuum cleaners
  4. Sewing machines
  5. Washing machines (agitation + spin cycles)
  6. Hair dryers & small domestic appliances


Construction & Operating Principle

  • Series-wound design: Armature and field winding connected in series through a commutator.
  • On AC supply: Current reverses in both armature and field simultaneously, keeping torque direction constant → continuous rotation.
  • On DC supply: Works like a normal DC series motor.

Thus, the motor can run on either AC or DC, hence the name universal motor.

Speed–Torque Characteristics

  • Similar to DC series motors:
    • High starting torque at low speeds
    • Speed decreases as load increases
    • Very high no-load speed (controlled using centrifugal switch & resistors)
  • Speed can be controlled by adding a series resistance.


Why Not All DC Series Motors Work on AC?

Drawbacks of running a DC series motor on AC supply:

  1. High hysteresis and eddy current losses → low efficiency
  2. High reactance of windings → poor power factor
  3. Excessive brush sparking

Design Modifications in Universal Motors

To overcome these drawbacks, the following changes are made:

  • Laminated steel rotor to reduce eddy currents
  • Fewer field turns with larger pole area to reduce flux density & iron loss
  • More armature conductors to maintain torque at low flux
  • Compensating winding for improved commutation and reduced reactance

Advantages

  • Compact and inexpensive
  • High starting torque
  • Speed can reach 20,000 rpm+ (ideal for tools/appliances)
  • Easy electronic speed control

Limitations

  • Noisy operation
  • Brush wear & sparking
  • Shorter lifespan compared to induction motors
  • Lower efficiency

Maintenance

  • Most are sealed and permanently lubricated → no maintenance.
  • Some types have oil ports → lubricate every 6 months (avoid over-lubrication).
  • Brushes need replacement periodically due to wear.

Universal Motor vs Induction Motor

Feature

Universal Motor

Induction Motor

Supply

AC & DC

AC only

Speed

Very high (20k rpm)

Moderate

Torque

High starting torque

Medium

Lifespan

Short (brush wear)

Long

Cost

Cheaper

Costlier

Noise

High

Low

✅ Key takeaway: Universal motors are best suited for short-duration, high-speed applications (like washing machines, mixers, drills), but for long continuous use, induction motors are more reliable.

 

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