Universal motor

Design of Universal motor

Universal motors have two windings connected in series – the exciter winding and the armature winding. The rotor is designed as a laminated steel core. Several coils form the armature winding, whose winding turns are arranged in grooves located on the rotor surface. The ends of each coil are connected to the neighbouring collector segments. The stationary carbon brushes feed the coils with current one after the other as the rotor rotates. The stator is equipped with two salient poles which accommodate the exciter windings and is also built of laminated steel sheeting. To reduce power losses the windings can also be inserted into grooves.

How the motor operates

 

As the exciter winding and the armature winding are connected in series, the same current flows through them both, i.e. the magnetic fields in the stator and the armature are in phase. The magnetic fields situated perpendicular to each other cause torque to be exerted on the rotor of the motor – the north pole of the armature field is drawn by the south pole of the stator field and repelled by the north pole. The important thing here is that both fields do not rotate. The alternating current only causes the simultaneous direction reversal of the two magnetic fields. The direction of the torque exerted on the rotor remains unchanged.

Torque, speed and back emf

 

As long as the rotor has not been put into motion, the armature current is solely limited by the winding resistance. The torque is at its maximum and the motor's speed rapidly increases. Back emf is induced in the armature winding, which opposes the externally applied voltage in conformity with Lenz's law. The higher the rotor speed, the greater the back emf induced in the armature. As a result the current and thus the torque developed by the motor is diminished. An equilibrium comes about – the motor turns with a constant speed at which the torque being developed is adjusted to the load.

Operating response

 

In its design and the way it operates the universal motor is analogous to the series-wound motor. It has a very high starting torque. Its rotation speed can also be high and is variable over a wide range. Phase control allows for continuously adjustable speed/power control. The pole reversal of the stator or armature winding causes reversal of the rotation direction. Short-term overloads are not dangerous for these motors. However, when the load is so high that the motor comes to a standstill, there is a danger that the winding insulation may become damaged. At no-load the universal motor can end up "racing" which can lead to the destruction of the armature winding.

Applications