Motor Control Circuit
Mrs. Meera Ajit Khandekar
Department of Instrumentation & Control
College of Engineering, Pune
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NEC requirement for Motor Installation
• The basic components required for
motor installation are shown. The
motor controller is the device that
does the actual ON/OFF switching
of the motor.
• The controller must have a
horsepower rating not lower than
the horse power rating of the motor
it is controlling.
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• A full voltage starter is
designed to apply full line
voltage to the motor upon
starting.
• Full voltage starters may be
manual or magnetic. Manual
starters are often used for
small motors, those upto about
10 HP.
• A wiring diagram of a remote
stop-start station with 20-A fuse
added to properly protect the
motor
control
circuit
conductors.
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Reduced Voltage Starters:
• It limits line disturbances
• It reduces excessive torque to the driven equipment
When starting a motor at full voltage, the current drawn from the power
line is typically 600 percent of normal full-load current. The large
starting in-rush current of a big motor could cause line voltage dips and
brownouts.
Reduced voltage starters include primary-resistance starters, auto
transformers, wye-delta starters, part-winding starter
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Autotransformer starter
• An Autotransformer is a single-winding
transformer on a laminated core with taps
at various points on the winding.
• The taps are usually expressed as a
percentage of the total number of turns,
and thus a percentage of applied of
applied voltage output.
• Three autotransformers are connected in
wye configuration, with taps selected to
provide adequate starting current.
• The motor is first energized at a reduced
voltage by closing contact A.
• After a short time, the autotransformers
are Switched out of the circuit by opening
contact A and closing contact B.
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Primary-Resistance Starter
• The primary-resistance starters
adds resistance to the stator circuit
during the starting period, thus
reducing the current drawn from
the line.
• After a set period of time, contact A
open and the resistors are shorted
out by contact B, applying full
voltage to the motor.
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Wye-delta starter
• By first closing contacts A and B,
the winding are connected in a
wye
configuration,
which
presents only 58 percent of rated
voltage of the motor.
• Full voltage is applied by
reconnecting the motor in delta
configuration by closing contact C
and Opening A.
• The starting current and torque
are 33 percent of their full voltage
ratings.
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Two-start Part-winding Starter
• Part-winding starters are sometimes
used on motor wound for dual
voltage operation such as 230/460 V
motor
• These motors have two sets of
windings connected in parallel for
Low voltage and connected in series
for high voltage operation.
• When used on the lower voltage,
they can be started by first energizing
only one winding, limiting starting
current and torque to approximately
one-half of the full voltage values
• The second winding is then
connected normally once the motor
nears operating speed about 2 Sec
later.
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Direct Current across-the-line Starter
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Constant Speed DC Starter
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• When the M power contacts closes, full line voltages is applied
to the shunt field while the resistor is connected is series with
the armature.
After the first time delay A1 contact closes, by passing section of
resistance.
A2 contact closes after the second time delay, bypassing a
section of the resistance and allowing motor to operate at base
speed.
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Three phase reversing
• Mechanical Interlock
• Electrical Auxiliary Contact Interlock
• Electrical Pushbutton Interlock
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Mechanical Interlock and Electrical Auxiliary Contact Interlock
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Electrical Pushbutton Interlocks
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• The reversal of a dc motor can be accomplished in two ways
Reversing the direction of the armature current and leaving the
field current the same.
Reversing the direction of the field current and leaving the
armature current the same.
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Jogging is the momentary operation of a motor for the purpose of
accomplishing small movement of the driven machine.
• JOG/START/STOP Pushbutton control
• JOG/START/STOP Pushbutton control using a jog relay
• Selector switch jog control circuit
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JOG/START/STOP Pushbutton control using a jog relay
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Selector switch jog control circuit
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Motor Stopping
The most common method of stopping a motor is to remove the supply voltage
and allow the motor and load to coast to a stop. In some application the motor
must be stopped more quickly or held in position by some sort of braking
device. There are two different means of electric braking
• Plugging: Polyphase motor stops quickly, by momentarily connecting the
motor for reverse rotation while the motor is still running in the forward
direction.
• Dynamic Braking: braking uses the motor as a generator during the braking
period immediately after the motor is turned off. Connecting the motor in this
way makes the motor acts like a loaded generator that develops a retarding
torque, which rapidly stops the motor.
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Plugging a motor to STOP
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Antiplugging Protection Circuit
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Electric Braking applied to a three-phase Motor
• Electric braking can be achieved with a
three-phase induction motor by removing
the power supply and applying direct
current to one of the stator phases
• An OFF –delay timer is connected in
parallel with the motor starter coil.
• OFF-delay timer controls an NO contact
that is used to apply power to the braking
contactor for a short period of time after
the STOP button is pressed.
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DYNAMIC BRAKING APPLIED TO A DC MOTOR:
• When the STOP button is depressed,
The shunt field winding of the DC motor
left connected to the power supply.
• The armature generates a cemf voltage.
This cemf causes current to flow through
the resistor and armature.
• The smaller the ohmic value of the
braking resistor, the greater the rate at
which energy is dissipated and the faster
the motor come to rest.
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THANK YOU!!!
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