Electric Motor Controls
• Electric motors present some special
problems from the standpoint of
control. An electric motor will try to
provide the power required by a load,
even if it results in self-destruction.
Therefore a motor must be protected
• A motor draws up to six times as much
current when the rotor is not turning as
it does when operating at full speed.
• If a motor should stall the control
device and the disconnect switch must
be capable of handling the high
Types of Motor Controllers
• A controller is simply a means of
closing the circuit supplying power to
an electrical motor.
• The most simple control device is a
cord and plug. (1/8hp or less)
• A branch-circuit protective device, ie
breaker, may serve as the controller
for stationary motors of 1/8 HP or less
that normally operate continuously
• A time delay circuit breaker is also
allowed as a control device.
• A fusible knife switch may act as a
motor controller provided it is rated for
the horsepower rating of the motor
Magnetic Motor Starter
• The main contacts are located inside
• Repair kits are available
• When current is supplied to the
internal solenoid the motor-starter
contacts are closed.
• An overload relay section is added to
the motor starter to sense motor
• NEMA has developed a standard
numbering system for motor starter
• All manufactures use this system
• The larger the rating number the
greater the horsepower rating of the
• Are rated according to the current they
will be expected to carry continuously,
and interrupt the if the motor should
• Motor starters come in 2 or 3 poles. 2
pole is for single phase and 3 pole for
• A 3 pole motor starter will handle a
single phase motor with a horsepower
rating half as large as 3 phase motor.
• The motor-starter housing is selected
for the environment in the area.
• NEMA has developed a standard
numbering system for enclosures and
require they be marked by the
– NEMA 1 - dry and free from dust and
– NEMA 3 – weather resistant suitable
for outdoors but not dusty conditions.
– NEMA 4 – water and dust tight.
– NEMA 4x – water and dust tight but
also corrosion resistant.
– NEMA 7 – Explosion proof.
– NEMA 9 – Dust Ignition proof
Motor-Full load current
• The motor full-load current is used to
determine the minimum size of circuit
• The name plate current rating must be
used for selecting the maximum size
motor running overload protection.
• Wires supplying a single motor shall
have an amperage rating of not less
that 1.25 times the full-load current of
• Motor circuits must be protected from
excessive current, ground faults, and
• Over current protection can be fused
or circuit breaker a circuit beginning.
• Short circuit and ground fault must be
capable of carrying the starting current
of the motor.
• A single-phase, 5-hp, 230v motor with
a nameplate of 28amps drives a
normal load. Determine the overload
– Full load current = 28A
– Fuse multiplier = 125%
– Use a 50-A fuse.
Service supplying several motors
• The feeder wires are required to have
a minimum rating equal to the sum of
the full-load current of all motors
supplied, plus 25%.
– Motors of 5-hp 28a , 3-hp 17a, 1-hp 8a,
and 2-hp 12amp.=72amps.
– No. 4 AWG minimum (from wire charts)
• The disconnect must be selected for a
horsepower rating of not less than the
sum of all HP ratings.
– The disconnect closest is likely 15 hp
– If a breaker is used it must be a
minimum of 115% of the sum of the
full-load current for all motors served.
• A common practice is to tap motor
branch circuits from a motor feeder.
• This is permitted as long as the
branch-circuit wire is terminated at an
over current of proper size.
Motor Control Circuit
• A magnetic motor starter is operated
with an electric solenoid coil.
• Here we have a typical wiring diagram
for a magnetic motor control. The
thick lines are the power flow to the
motor, the thin lines belong to the
control circuit and generally are pre-
wired by the manufacture.
• Common wiring schematics use basic
symbols and terminology.
• Switches and contacts are the basic
control components, and they take
several different forms.
• They can be operated mechanically,
by timer, change in condition like
temperature, pressure, flow, liquid
level, or humidity.
• Switches and contacts are described
by the number of pole and number of
– Poles – the number of paths into a
switch or contact.
– Throws – the number of paths leaving
• It must also be known if the switch is
normally open or normally closed
• The common types of switches and
contacts are incorporated into various
types of control devices.
Reversing Motor Starters
• Electric motors may be called upon to
drive a machine which must be
• A reversible motor starter is required
to select the desired rotation direction.
• 3-phase – only two of the three leads
need to be reversed.
• Single phase the start windings must
be reversed with respect to the run
• Drum switch has forward, reverse, and
stop. Note the phase rotation at the
motor for forward and reverse.