07-12-2012, 06:42 PM
Types of Electric Motors
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DC Motors
1. Shunt DC motor: The rotor and stator windings are connected in parallel.
2. Sparately Excited motor: The rotor and stator are each connected from a different power
supply, this gives another degree of freedom for controlling the motor over the shunt.
3. Series motor: the stator and rotor windings are connected in series. Thus the torque is
proportional to I2 so it gives the highest torque per current ratio over all other dc motors.
It is therefore used in starter motors of cars and elevator motors (p. 563 Chapman, 4e,
2005 McGrow Hill).
4. Permanent Magnet (PMDC) motors: The stator is a permanent magnet, so the motor is
smaller in size.
Disadv: only used for low torque applications
Disadvantages of DC motors
1. Brush wear: Since they need brushes to connect the rotor
winding. Brush wear occurs, and it increases dramatically
in low‐pressure environmet. So they cannot be used in
artificial hearts. If used on aircraft, the brushes would
need replacement after one hour of operation.
2. Sparks from the brushes may cause explosion if the
environment contains explosive materials.
3. RF noise from the brushes may interfere with nearby t.v.
sets, or electronic devices,..etc
AC Machines
• Induction Motor: So called because voltage is induced in
the rotor (thus no need for brushes), but for this to happen,
the rotor must rotate at a lower speed than the magnetic
field to allow for the existance of an induced voltage.
Therefore a new term is needed to describe the induction
motor: the slip.
• Synchronous Motor: So called because rotor tries to line up
with the rotating magnetic field in the stator. It has the
stator of an induction motor, and the rotor of a dc motor.
Reluctance motors
1. Reluctance motor: A synchronous‐induction
motor. The rotor has salient poles and a cage
so that it starts like an induction motor, and
runs like a synchronous motor.
2. Uses soft magnet rotor that is magnetized by
the application of current in the stator coils
3. The magnet is spun by a 3 phase AC motor
with no rotor voltage
4. The gear teeth are designed to increase the
magnetic flux driving the rotor while
providing asymmetry that gives it the
rotational acceleration
5 Permeability of the rotor provides
6. Full solution of Faraday’s law with time and
velocity dependence.