Motor Control

Operating motors require matching motor controls that run well and have adequate service. Damaged motor controls prevent a motor from running or safely moving its load. Consequently, identifying common problems and repairing or replacing problematic controls will ensure safety, reduce downtime and preserve productivity.

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Types of Motor Controls

Four types of motor controls exist, based on their power input, which are:

  • Alternating current (AC) motor control
  • Direct current (DC) motor control
  • Servo motor control
  • Stepper motor control

When researching motor controls and replacement parts, the terms drive and control may appear interchanged. Due to the close operation of controls and drives, the two tend to come integrated into one package. When selecting replacement motor controls, keep this naming convention in mind.

1. AC Motor Control

Alternating current motors typically have applications in changing the rotational speed of devices such as fans, pumps and blowers. They modulate the standard AC electrical input current to ensure the ideal torque and power required for controlling rotating machines. Controls for these motors are parts of the drive and regulate signals sent to the drive to turn the device that the motor operates.

2. DC Motor Control

DC motor controls change the incoming alternating current into a direct current. These motors usually drive the torque and motor speed in power tools, pumps and electric-powered vehicles. As with AC motor controls, DC controls and drives typically have an integrated construction.

3. Servo Motor Control

Servo motors most often regulate motion controls, motor speed or torque. These motors control conveyor belts, robotics, spindle drive tools and machine tools. The hallmark of a servo motor control is its ability to allow smooth movement within tightly controlled parameters. A servo motor may also go by the name servomotor amplifier.

4. Stepper Control

Stepper controls turn AC or DC power input into a stepped or pulsed output. Stepper motors frequently appear in many similar systems that also use servo motors, such as conveyors, machine tools and robotics. Like servo motors, stepper motors also regulate torque, motor speed and device position. However, unlike servo motor controls, stepper controls can hold a part in a specific orientation. Stepper controls turn the motor a set number of times to achieve their precision in motion.



Both industrial motor control parts and the drives come as one unit. However, each of these sections of the unit has distinctive functions. The drive components change the input power to the necessary output for the motor. The control components use the electricity from the drive to move parts of the motor in desired directions.

Common parts of industrial motor controls include:

1. Switches and Buttons

Switches found in motor control circuits include disconnecting switches, limit switches and cam switches. Disconnecting switches sever the power supply from the control unit. These switches have fuses, and their operation protects the motor against brief short-circuits that may occur at startup. In some devices, a manual circuit breaker replaces the task of the disconnecting switches.

Limit switches can open or close contacts and require low power to do so. Some move in response to the position of a part or time of rotation while others change their status based on environmental factors, such as temperature, liquid levels or pressure.

Cam switches control rotational motion through a series of interconnected contacts. Using a rotating control or handle, the operator can open or close a group of contacts.

Push buttons act like switches but require manual input from an operator to trigger the switch.

2. Relays and Contactors

Relays function the same as switches in their ability to open and close contacts. However, the mechanism used differs from that of switches. Control relays activate when the relay coil receives power.

Thermal relays work as fail-safe devices to shut off contacts when the temperature of the motor exceeds a specific level. Without working thermal relays, motors could sustain severe heat damage by continuing to run when too hot.

Magnetic contactors use a magnetic plunger coupled with a relay coil to open or close a set of fixed and moveable contacts. When contactors have their relay coils powered, their plungers lift to shift the moving contacts to touch the fixed contacts.

3. Transformers, Capacitors, Resistors and Reactors

Transformers, capacitors, resistors and reactors are standard components of changing the input power to output power. Additionally, these parts work to send power through the control and drive system in the amounts needed. For example, a series of resistors may step down incoming power to allow a motor to work with the electrical requirements of the line. Some motors may use transformers instead of resistors to reduce incoming voltage.

4. Circuit Breakers

Circuit breakers, like thermal relays, work to protect the system from power surges. Excessive current entering the motor will cause the circuit breaker to trip, stopping operation and protecting the system. Some systems use circuit breakers instead of disconnecting switches that use fuses.

5. Pilot Light

Though small, the pilot light on a motor control indicates whether the system is in operation. Therefore, this small light acts as a safety device, letting the operator know if the motor is on or off.

Signs of Common Problems

Most mechanical devices will wear out over time. Some motors and their controls will experience premature failure if they experience excessive loads, heavy operation or a lack of regular servicing. Even if used correctly and well-maintained, the motor or its controls can fail. Signs of a problem while running the motor may include the following:

  • Abnormal smells coming from the motor or controls
  • Smoke coming from the motor
  • A circuit breaker trips or a fuse blows
  • A thermal relay activates
  • Unusual sounds that don’t typically happen during operation
  • Obstructions in the motor
  • Reduced efficiency of operation

Any of the above issues requires a thorough examination and repair by a professional. Only allow a trained professional to handle industrial motor control repairs to avoid causing additional problems with the device. Expert repairs will reduce the downtime needed because the motor control has the proper repairs done the first time.

After repairs, schedule regular servicing of the motor and its controls to ensure the longevity of the system. Additionally, preventative maintenance can spot problems with the motor or controls before the system shuts down and causes productivity loss and downtime.

Contact Global Electronic Services for Industrial Motor Controls and Parts

To find out more about obtaining the parts required for replacing the missing motor control components or for the motor controls themselves, contact Global Electronic Services at 877-249-1701 or request a quote online.


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