AC Series Motor / Universal Motor

October 27, 2020

AC Series Motor / Universal Motor

We call AC series motors Universal Motors because they are electric motors that operate in either AC or DC power at the same speed and output, and they can also work as generators. Besides, the universal motor is a modified DC series motor. What does this mean; it means that we modify the DC series motor to work in an AC single-phase supply. This modification includes the following:

  • 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 field winding’s reluctance 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 an AC supply.

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

Applications of AC Series Motors

Before we search for AC motors’ characteristics and operation, it is important to know where we use the series motor. AC motor has a high starting torque, which is cheap so that 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 an AC Series Motor

The working principle of the AC motor doesn’t differ a lot from the DC motor. Moreover, it can run on both AC and DC. There are a wound armature and field that are interconnected in series. When we apply an alternating E.M.F to the terminals, as a consequence 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 Motors

The series motor has fewer turns to reduce the reluctance of the field winding. Consequently, when we reduce the turns of the field, the M.M.F decreases. As a result, there might be a problem with the universal motor. By the way, the air gap flux decreases, making the motor’s speed increase and the torque decrease. To solve this problem, we use a compensating winding which may be connected to give us:

  1. Conductively compensated motor: In a 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 90° with the main field axis.
  2.  Inductively compensated motor: In an 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 compensating winding current.

Characteristics of the AC Series Motor

Some characteristics or features of the AC motor are:

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