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AC Motors

July 12, 2020

AC series motor or the universal motor is an electric motor that operates in either Ac or Dc power at the same speed and output. Also, Ac series motor is a modified Dc series motor.

What does this mean; it means that we modify the DC series motor to be able to work in AC single-phase supply. This modification includes the following as:

  • We laminate the entire magnetic circuit of the DC series motor so the eddy current loss reduces. But this means the construction of the Ac motor will be more expensive.
  • We use a few turns series field windings so the reluctance of the field winding reduces; By that way, the voltage drop across the field windings and the power factor improved.
  • Also, a low reluctance magnetic circuit is used to obtain a high field flux.
  • A high resistance lead that connects the coils and the commutator segments are used to eliminate possible sparking. The spark is produced between the brushes and the commutator when the motor works on Ac supply.

It’s important to know that the relative efficiency of small universal motors is about 30% and of the largest motors is about 70-75%.

Applications of universal motor

Before we search for the characteristics and operation of Ac motor, it is important to know where we use the series motor. Ac motor has a high starting torque, it’s cheap so we can find it in many industrial and domestic applications as:

  • Sewing machines.
  • Electric traction.
  • Hoists.
  • Kitchen applications.
  • Hairdryers.
  • Table fans.
  • Portable drills.
  • Food Mixers.
  • Locomotives.
  • Hand tools.
  • Grinding mills.
  • Vacuum cleaner.
  • And electric shavers.

Working principle of AC series motor

The working principle of the Ac motor doesn’t differ a lot from the Dc motor. However, it is able to run on both Ac and Dc current. There are a wound armature and field that are interconnected in series.

When we apply an alternating E.M.F to the terminals, alternating current flows through both the field and the armature windings. So, the field winding produces an alternating flux (this flux is only alternating not rotating). This flux reacts with the armature current to produce torque.

Types of Ac series motor

There might be a problem with the universal motor. This is because the series motor has a fewer number of turns to reduce the reluctance of the field winding. But when we reduce the turns the field, M.M.F decreases.

By the way, the air gap flux decrease, and that makes the speed of the motor increases and the torque decreases.

To solve this problem we use a compensating winding which may be connected to give us:

  1. Conductively compensated motor:
    In conductively motor, we connect the compensating winding in series with the armature circuit and we put it in the stator slots. The electrical axis of this compensating winding is 90o with the main field axis.
  2.  Inductively compensated motor:
    In inductively motor, there is no interconnection between the compensating winding and the armature circuit as there is a transformer action. The armature winding acts as a primary winding of the transformer and the compensating winding acts as the secondary of the transformer. The current flows in the armature winding will be in phase opposition to the current in the compensating winding.

Characteristics of Ac series motor

Some characteristics or features of the Ac motor are:

  • It’s self-starting and it has a high starting torque so we don’t require a starting device.
  • Has a lightweight besides it’s compact.
  • It’s easy to control it.
  • It operates at high speed (1500-15000 r.p.m).
  • We can use tapped coils which makes the motor electromechanically or electronically.
  • And unfortunately, it’s typically very noisy (acoustically and electromagnetically) because of the commutator.
  • The power factor is about 90% at full load.