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Electric Motor Controls 1
ELECTRIC MOTOR CONTROLS
Once the proper motor is selected, understanding the many various control devices available and
their uses and limitations becomes an important part related to reliable operation and protection of
the motor and the personnel using the motor.
Motor Control Topics
There are four major motor control topics or categories to consider. Each of these has several
subcategories and sometimes the subcategories overlap to some extent. Certain pieces of motor
control equipment can accomplish multiple functions from each of the topics or categories.
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The four categories include:
1) Starting the Motor
Disconnecting Means
Across the Line Starting
Reduced Voltage Starting
2) Motor Protection
Overcurrent Protection
Overload Protection
Other Protection (voltage, phase, etc)
Environment
3) Stopping the Motor
Coasting
Electrical Braking
Mechanical Braking
4) Motor Operational Control
Speed Control
Reversing
Jogging
Sequence Control An understanding of each of these areas is necessary to effectively apply motor control
principles and equipment to effectively operate and protect a motor. 2
MOTOR STARTING
All motors must have a control device to start and stop the motor called a motor controller.
Motor Controller
A motor controller is the actual device that energizes and de-energizes the circuit to the motor so
that it can start and stop. Motor controllers may include some or all of the following motor control functions:
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starting, stopping, over-current protection, overload protection, reversing, speed
changing, jogging, plugging, sequence control, and pilot light indication.
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Controllers range from simple to complex and can provide control for one motor,
groups of motors, or auxiliary equipment such as brakes, clutches, solenoids, heaters,
or other signals.
Motor Starter
The starting mechanism that energizes the circuit to an induction motor is called the starter and
must supply the motor with sufficient current to provide adequate starting torque under worst case
line voltage and load conditions when the motor is energized. There are several different types of equipment suitable for use as motor starters but only
two types of starting methods for induction motors:
1.
Across the Line Starting
2.
Reduced Voltage Starting
Across the Line Starting of Motors
Across the Line starting connects the motor windings/terminals directly to the circuit voltage across
the line for a full voltage start. This is the simplest method of starting a
motor. (And usually the least expensive). Motors connected across the line are capable
of drawing full in-rush current and
developing maximum starting torque to
accelerate the load to speed in the shortest
possible time. All NEMA induction motors up to 200
horsepower, and many larger ones, can withstand full voltage starts. (The electric
distribution system or processing operation may not though, even if the motor will).
Across the Line Starters 3
Figure 26. Manual Starter
There are two different types of common across the line starters including
1. Manual Motor Starters
2. Magnetic Motor Starters
Manual Motor Starters
A manual motor starter is package consisting of a horsepower rated switch with one set of contacts
for each phase and corresponding thermal overload devices to provide motor overload protection. The main advantage of a manual motor starter is lower cost than a magnetic motor starter
with equivalent motor protection but less motor control capability. Manual motor starters are often used for smaller motors - typically fractional horsepower
motors but the National Electrical Code allows their use up to 10 Horsepower. Since the switch contacts remain closed if power is removed from the circuit without
operating the switch, the motor restarts when power is reapplied which can be a safety
concern. They do not allow the use of remote control or auxiliary control equipment like a magnetic
starter does.
Magnetic Motor Starters
A magnetic motor starter is a package consisting of a contactor capable of opening and closing a set 4
Figure 27. Magnetic Starter
of contacts that energize and de-energize the circuit to the motor along with additional motor
overload protection equipment.
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Magnetic starters are used with larger motors (required above 10 horsepower) or where
greater motor control is desired. The main element of the magnetic motor starter is the contactor, a set of contacts operated by
an electromagnetic coil.
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Energizing the coil causes the contacts (A) to close allowing large currents to be
initiated and interrupted by a smaller voltage control signal.
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The control voltage need not be the same as the motor supply voltage and is often low
voltage allowing start/stop controls to be located remotely from the power circuit. Closing the Start button contact energizes the contactor coil. An auxiliary contact on the
contactor is wired to seal in the coil circuit. The contactor de-energizes if the control circuit
is interrupted, the Stop button is operated, or if power is lost. The overload contacts are arranged so an overload trip on any phase will cause the contactor
to open and de-energize all phases.
Reduced Voltage Starting of Motors
Reduced Voltage Starting connects the motor windings/terminals at lower than normal line voltage
during the initial starting period to reduce the inrush current when the motor starts. 5 Reduced voltage starting may be required when:
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The current in-rush form the motor starting adversely affects the voltage drop on the
electrical system.
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needed to reduce the mechanical starting shock on drive-lines and equipment when
the motor starts. Reducing the voltage reduces the current in-rush to the motor and also reduces the starting
torque available when the motor starts. All NEMA induction motors can will accept reduced voltage starting however it may not
provide enough starting torque in some situations to drive certain specific loads.
If the driven load or the power distribution system cannot accept a full voltage start, some type of
reduced voltage or "soft" starting scheme must be used. Typical reduced voltage starter types include:
1.
Solid State (Electronic) Starters
2.
Primary Resistance Starters
3.
Autotransformer Starters
4.
Part Winding Starters
5.
Wye-Delta Starters
Reduced voltage starters can only be used where low starting torque is acceptable or a means exists
to remove the load from the motor or application before it is stopped. 6
MOTOR PROTECTION
Motor protection safeguards the motor, the supply system and personnel from various operating
conditions of the driven load, the supply system or the motor itself.
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Motor protection categories include
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Overcurrent Protection
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Overload Protection
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Other Types of Protection. The National Electrical Code requires that
motors and their conductors be protected
from both overcurrent and overload
conditions.
Overcurrent Protection
Overcurrent protection interrupts the electrical circuit to the motor upon excessive current demand
on the supply system from either short circuits or ground faults. Overcurrent protection is required to protect personnel, the motor branch circuit conductors,
control equipment, and motor from these high currents. Overcurrent protection is usually provided in the form of fuses or circuit breakers. These
devices operate when a short circuit, ground fault or an extremely heavy overload occurs.
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Most overcurrent sources produce extremely large currents very quickly. 7
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100
200
300
400
500
600
Full Load Amps (%)
0
1
2
3
4
5
6
7
8
9 10 11 12
Time (Minutes)
Motor Heating Curve
Motor Damage
Allowable Operation Area
Amperage
Time
Motor Current Draw
Motor Running Current
Starting In-Rush Current
Overload Protection
Overload protection is installed in the motor circuit and/or motor to protect the motor from damage
from mechanical overload conditions when it is operating/running. The effect of an overload is an excessive rise in temperature in the motor windings due to
current higher than full load current.
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Properly sized overload
protection disconnects the
motor from the power supply
when the heat generated in the
motor circuit or windings
approaches a damaging level
for any reason.
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The larger the overload, the
more quickly the temperature
will increase