Motor Control Ciircuits

CHAPTER 9

  • Relays, contactors, and starters pass power to the motor by closing sets of contacts

–Contacts controlled by coils in the control circuit

–Starting relays are only in the active circuit for a short period of time

–Determined by the size and application of the motor used

  • Typical motor wiring diagram utilizing a contactor and a potential magnetic relay (PMR) as a motor-starting relay
    Typical motor wiring diagram utilizing a contactor and a potential magnetic relay (PMR) as a motor-starting relay

    Run-Load and Locked-Rotor Amperage

  • Running load amperage (RLA)

–Similar to full load amperage (FLA); amperage drawn while operating

  • Locked rotor amperage (LRA)

–Five to seven times greater than RLA or FLA; amperage drawn on startup

  • Both RLA and LRA must be considered when choosing a control device

The Relay

  • Uses a magnetic coil to open or close one or more sets of electric contacts

–Relays are not repaired; replace on failure

–Used for light duty applications

–Can be used as a pilot-duty relay

–The relay contacts must be able to handle the amperage draw of the load being controlled

A double-pole–single-throw relay with two sets of contacts that close when the coil is energized
A double-pole–single-throw relay with two sets of contacts that close when the coil is energized
A relay with two sets of contacts that close and one set of contacts that opens when the coil is energized. It has two normally open (NO) sets and one normally closed (NC) set
A relay with two sets of contacts that close and one set of contacts that opens when the coil is energized. It has two normally open (NO) sets and one normally closed (NC) set

The Contactor

  • Larger version of the relay

–Has movable and stationary contacts

–Holding coils are rated at different voltages

–Can have one or more sets of contacts

–Some are equipped with auxiliary contacts

–Contacts and coils can be replaced

–Use the exact replacement whenever possible

Modern-day contactors for larger motors. There are circuit breakers and current sensing coils before each contactor. The current sensors monitor current flow through the motors for feedback to a microprocessor providing running status, overload protection, and remote monitoring (B) Courtesy Meijers Corporation. Photo by John Tomczyk
Modern-day contactors for larger motors. There are circuit breakers and current sensing coils before each contactor. The current sensors monitor current flow through the motors for feedback to a microprocessor providing running status, overload protection, and remote monitoring
(B) Courtesy Meijers Corporation. Photo by John Tomczyk
This contactor (a single-pole contactor) has only one set of contacts. Only one line of power needs to be broken to stop a motor like the one in the diagram. This is a method commonly used to supply crankcase heat to the compressor during the off cycle. When the contacts close, current will bypass the heater
This contactor (a single-pole contactor) has only one set of contacts. Only one line of power needs to be broken to stop a motor like the one in the diagram. This is a method commonly used to supply crankcase heat to the compressor during the off cycle. When the contacts close, current will bypass the heater
A contactor with auxiliary contacts for switching other circuits
A contactor with auxiliary contacts for switching other circuits

Motor Starters

  • Contactor equipped with overload protection
  • Coils, contacts, and heaters replaceable
  • Contacts become pitted over time

–Pitting increases resistance across the contacts

–The voltage across contacts will increase

  • Should be about zero volts across a good set of contacts

Motor Protection

  • Motors are expensive and must be protected
  • Fuses and circuit breakers protect the entire circuit, not the individual circuit components
  • Motors can operate under an overcurrent condition for a short period of time
  • Most small motors have no overload protection
  • The larger the motor, the more elaborate the method of motor protection should be
  • Motor protection can be inherent (internal) or external
Two motors in the same circuit are served by the same conductors and have different overload requirements. The main circuit fuses protect the entire circuit and not the individual circuit components
Two motors in the same circuit are served by the same conductors and have different overload requirements. The main circuit fuses protect the entire circuit and not the individual circuit components

Inherent Motor Protection

  • Internal thermal overloads

–Usually embedded in the motor windings

–Open on a rise in temperature

  • External thermal overloads

–Thermally activated bimetal snap discs

–Located on the compressor’s exterior shell

External Motor Protection

  • Devices that pass power to the holding coil of the starter or contactor

–Devices open when an overcurrent condition exists

–The trip point and type of overload protector are determined by the manufacturer

–The overload device takes the service factor of the motor into consideration

National Electric Code (NEC) Standards

  • Sets standards for electrical installations

–Conductor sizes and ampacities

–Cable materials and applications

–Electrical devices

  • Sets standards for overload protection
  • Consult published code book if questions/concerns are encountered on the job

Temperature-Sensing Devices

  • Bimetal elements

–Heaters wired in series with load

  • Exposed to the current draw of load

–Bimetal warps/opens when it gets too hot

  • Then de-energizes the starter holding coil
  • Solder pot:

–Uses solder with a low melting point

  • Solder melts with excessive temperature resulting from the overcurrent condition

Magnetic Overload Devices

  • Accurate means to provide overload protection
  • Device is not attached to the starter
  • Device is not affected by increased ambient temperatures
  • The contacts within the device will open to de-energize the motor at the desired amperage level

Restarting The Motor

  • Do not restart immediately

–Cause for the overload condition must first be located and repaired

–Motor must be given ample time to cool

–Many control devices are manually reset

–Some controls reset automatically after a predetermined time delay

–Time delay feature prevents short cycling

Source: 1

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