Newcastle Engineering
studies
Engineered Products – The Electric Motor
June 2015
Basic Principles
From Michael Faraday’s notebook September 3 1821:
•“....continually turn round. Arranged a magnet needle in a glass
tube with mercury about it and by a cork, water, etc. supported a
connecting wire so that the upper end should go into the silver cup
and its mercury and the lower move in the channel of mercury
round the pole of the needle. The battery arranged with the wire as
before. In this way got the revolution of the wire round the pole of
the magnet. The direction was as follows, looking from the above
down.
Very satisfactory, but make more sensible apparatus.”
Basic Principles
Basic Principles
• When a wire carrying an electric current is placed in a magnetic
field it is subjected to a force
F = Il x B
Where:
F is the force vector in N
I is the current in A
l is the length vector in m
B is the magnetic field vector in T
Basic Principles
Basic Principles
• The DC motor:
– In modern DC machines we replace the permanent magnets
with electromagnets – the field coils
– Practical machines have more than two commutator
segments (and therefore sets of coils on the rotor, or
armature) – this gives more uniform DC voltage in the case
of a DC generator
– Practical machines also have more than one set of stator
poles (dependent on the size and speed of the machine)
– Field coils can be connected in series (as in traction motors)
or shunt (as in rolling mill motors)
Basic Principles
• The DC motor:
Car Starter Motor
DC Traction Motor
Basic Principles
• The DC motor:
Motor Armature
Motor Field Coils
Basic Principles
• The DC motor:
Rolling Mill DC Motors
The Rotating Magnetic Field
• Developed by Nicola Tesla in 1882
• The result of adding three 120-degrees delayed sine
waves on the axis of the motor is a single rotating vector
which remains always constant in magnitude. The rotor
has a constant magnetic field. The N pole of the rotor will
move toward the S pole of the magnetic field of the stator,
and vice versa.
• A permanent magnet in such a field will rotate so as
to maintain its alignment with the external field.
The Rotating Magnetic Field
Demonstration
The Synchronous Motor
• The synchronous speed of the rotating magnetic
field in rpm is:
Where:
f = supply frequency in Hz
p = number of pole pairs in motor
The Synchronous Motor
• Stator produces a rotating magnetic field
• Rotor is excited by DC current producing
electromagnets
• Synchronous machines operate at synchronous
speed
• They must have a pilot motor or a squirrel cage
in the rotor for starting
• DC field current is applied once machine is close
to synchronous speed
Synchronous Motor
The Synchronous Motor
• Used for constant
loads such as:
– Compressors
– Pumps
– Motor/ generator set
drive
• Also useful for power
factor correction
Large Synchronous Motor
The Three Phase Induction Motor
• The three phase induction motor:
– Uses the same stator as the synchronous motor
– Current is induced in the rotor to set up the rotor
magnetic field
– Stator and rotor magnetic fields interact to
produce rotational torque
– Speed must be less than synchronous speed to
allow induction of current in rotor
– This slip speed is stated in rpm or per cent
The Three Phase Induction Motor
• The three phase induction motor:
The Single Phase Induction Motor
• Has a normal squirrel cage rotor but a single
stator winding
• The magnetic flux produced by the stator
winding is fixed in space and varies in
magnitude with time
• At standstill the net torque developed is,
therefore, zero
• At speed, however, the rotor flux interacts with
the stator flux to produce torque
The Single Phase Induction Motor
• This means that the single phase induction
motor requires a mechanism for starting:
• Three methods are available:
– Shaded pole
– Split phase
– Capacitor motor
The Single Phase Induction Motor
• Shaded pole:
– Restricted to very small motors
– Part of the pole is shaded by
conducting ring which acts as
the secondary of a short
circuited transformer
– Flux in the shaded portion of the
stator lags the main flux in time
producing a rotating field
The Single Phase Induction Motor
• Split phase:
– Auxiliary high resistance winding
produces a rotational torque
– Auxiliary winding is
disconnected at running speed
– Represents a poorly balanced
two phase motor
The Single Phase Induction Motor
• Capacitor motor:
– Phase shift for the auxiliary
winding is obtained by a
capacitor in series with the
winding
– This produces a time phase
difference between main &
auxiliary windings of about 90°
– Some capacitance can remain in
circuit when up to speed to
improve performance and power
factor
The Linear Motor
• If the stator is cut and rolled out we produce the
stator of a linear motor