LinkLine: Spring 2016

An Overview of Motor Controls

By Joe I. Moreland, PE, CSI (GA PE#26321)

Motor Controllers/Motor Starters and Their Various Forms

Motor controllers are defined in NFPA 70 (NEC) as “any switch or device that is normally used to start and stop a motor by making and breaking the motor circuit current.” The controller might utilize manual or automatic means to start and stop the motor and might also perform other supplementary functions including:

  • Providing motor overload protection and/or motor branch-circuit short-circuit and ground-fault protection.
  • Allowing for selection of forward or reverse motor rotation.
  • Allowing for selection of multiple motor speeds.
  • Allowing for motor jogging or inching, which provide for momentary motor operation for the purposes of performing small movements.
  • Allowing for motor plugging, which provides for momentary reversal of motor rotation for the purposes of braking.

Motor starters are a form of motor controller that include the switching means necessary for starting and stopping the motor in combination with overload protection.

Manual motor starters utilize contacts that are directly operated by mechanical means when a toggle switch or pushbutton is actuated by hand. They are available in non-reversing, reversing, and two-speed configurations. Motor-starting switches are manually-operated horsepower-rated switches that differ from manual motor starters in that they do not include overload protection.

Magnetic motor starters utilize a contactor with an electromagnetic coil, which may be electrically operated by control devices actuated by hand (either locally or remotely) or by automatic means. Combination motor starters combine a magnetic motor starter with an externally operable disconnecting means in a common enclosure.

Magnetic motor starters are classified by their starting methods. Full-voltage magnetic starters are often called "across-the-line" starters because they apply full line voltage to start the motor, which results in a high starting current and high starting torque. Reduced-voltage magnetic starters use various methods to limit starting current and/or torque. Reduced-voltage starting is commonly applied to larger motors, often to comply with utility company requirements for limiting line voltage dip. Multi-speed magnetic starters reconnect motor windings to achieve selected speeds. Reversing magnetic starters reconnect motor windings for rotation in either direction. Certain starting methods can only be applied to compatible motors with special winding arrangements.

Solid-state reduced-voltage starters are programmable to provide greater control flexibility as compared to magnetic reduced-voltage starters.

Variable frequency motor controllers utilize solid-state power electronics to adjust motor speed and torque by varying motor input frequency and voltage. Other terms frequently used to describe this type of controller are variable frequency drive (VFD), adjustable frequency drive, variable speed drive, and AC drive.

Motor Protective Devices

Overcurrent protection for motors consists of protection against faults, such as short circuits and ground-faults, and protection against overloads. This protection is usually provided by a combination of fuses or circuit breakers for fault protection and overload relays for overload protection. Overloads are defined in NFPA 70 (NEC) as “the operation of equipment in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating.” Causes of motor overloads include:

  • Locked rotor condition.
  • Load exceeds motor torque rating.
  • Bad bearings.
  • Low line voltage.
  • Loss of phase on polyphase motor.

Overload relays are reusable and resettable and are of either the thermal or solid-state electronic type. Thermal overload relays utilize interchangeable current elements called “heaters” to open contacts and remove power from the motor upon sensing excessive heat from an overload condition. Solid-state electronic overload relays do not sense heat but rather use a specific measured current to determine overload conditions.

Disconnects for combination motor starters are of either the fusible switch or circuit breaker type. Circuit breaker disconnects use either motor circuit protectors or thermal magnetic circuit breakers. Motor circuit protectors are instantaneous-trip (magnetic-only) circuit breakers furnished with magnetic instantaneous tripping elements for short circuit protection, but not thermal inverse time tripping elements for overload protection (thermal magnetic circuit breakers are furnished with both types of tripping elements).

NEMA vs IEC Motor Controllers

Motor controllers are designed to meet standards published by either NEMA (National Electrical Manufacturers Association) or IEC (International Electrotechnical Commission). NEMA motor controllers are most commonly specified in North America and IEC motor controllers are predominant in Europe, Asia, and most other global markets. IEC controller use in North America is primarily for installation in manufactured products and/or control panel assemblies by original equipment manufacturers (OEMs). The differences in NEMA and IEC controllers are significant, but beyond the scope of this article. For more information, refer to NEMA ICS 2.4 – NEMA and IEC Devices for Motor Service-A Guide for Understanding the Differences.

Motor Control Centers

Motor control centers are defined in NFPA 70 (NEC) as “an assembly of one or more enclosed sections having a common power bus and principally containing motor control units.” They provide an alternative to grouping individual enclosed motor controllers in a common location. Motor control centers may contain a combination of equipment including:

  • Main overcurrent protective device.
  • Feeder units.
  • Combination magnetic motor starter units.
  • Solid-state reduced voltage motor starter units.
  • Variable-frequency AC drive units.
  • Programmable logic control (PLC) units.
  • Power factor correction capacitor units.
  • Distribution transformers and/or panelboards.
  • Metering.
  • Surge protective devices.

Motor Control Circuits

Most motor control circuits utilize either two-wire control or three-wire control. Two-wire control circuits provide low-voltage release, which is the automatic restarting of the motor after a power interruption and subsequent power restoration. Three-wire control circuits provide low-voltage protection, which requires the motor to be manually restarted after a power interruption and subsequent power restoration. Two-wire control is applied where automatic restarting of the motor after power interruption is desirable, and three-wire control is applied where automatic restarting of the motor after power interruption could create a hazardous condition for the operator.

Auxiliary contacts are often provided on magnetic motor starters. They operate in unison with the main power contacts and are used to perform various control and/or status indication functions.

Pilot devices provide local control and/or status indication for motor controllers and may include pushbuttons, selector switches, and indicating (pilot) lights.

Control and timing relays are used in motor control circuits to perform various functions. Timing relays may be pneumatic or solid-state, with timed contacts applied for either on or off delay.

Control power transformers are used to deliver control power to the motor controller at the appropriate voltage. Control power may also be derived at line voltage or from a separate control power source.

Sequencing control involves the interconnection of magnetic starter control circuits to start and stop a number of separate motors in a definite sequence. Sequencing control is often applied to prevent large inrush currents resulting from multiple motors starting at the same time.

Specifying Motor Controls in SpecLink

Motor control centers and enclosed motor controllers may be specified in BSD’s Section 26 2419 – Motor Control Centers and Section 26 2913 – Enclosed Controllers respectively, which have both recently been completely revised and updated. A more in-depth guide to these products and how to specify them in SpecLink can be found on the Supporting Documents pages of BSD’s website.

Upcoming SpecLink Updates

Upcoming SpecLink updates include major revisions to Section 26 2923 – Variable Frequency Motor Controllers, and development of new Section 26 2913.16 – Reduced-Voltage Motor Controllers for solid-state reduced-voltage starters.