Ac series motor
Ac series motor or the universal motor is an electric motor which 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 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 the way, the voltage drop across the field windings reduces and the power factor improved.
- Also, a low reluctance magnetic circuit is used to obtain a high field flux.
- And we use a high resistance lead that connects the coils and the commutator segments to eliminate the possible sparking 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 characteristics and operation of Ac motor it’s 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.
- Kitchen applications.
- Table fans.
- Portable drills.
- Food Mixers.
- Hand tools.
- Grinding mills.
- Vacuum cleaner.
- And electric shavers.
Working principle of Ac series motor:
Of course, the working principle of the Ac motor doesn’t differ a lot than the Dc motor, but it’s able to run on both Ac and Dc current; We have a wound armature and field that are interconnected in series.
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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 produced flux is only alternating not rotating) this flux reacts with the armature current to produce torque.
Characteristics of Ac series motor:
There are some characteristics or features of the Ac motor such as:
- 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 noise (acoustically and electromagnetically) because of the commutator.
- The power factor is about 90% at full load.
Types of Ac series motor:
That’s very well, but we have a big problem with the universal motor; The series motor has a fewer number of turns as we said to reduce the reluctance of the field winding but when we reduce the turns the field mmf decreases and by the way the air gap flux decrease and that makes the speed of the motor increases and the torque decreases and it’s desired.
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 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.