project maker
07-08-2014, 10:57 AM
MOTOR CONTROLS
[attachment=66686]
INTRODUCTION TO MOTOR CONTROL DEVICES
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
The type of motor control used is determined by the size and application of the motor used
THE CONTACTOR
Larger version of the relay
Has movable and stationary contacts
Often times only one set of contacts opens which opens only one side of the power to condensing units. This provides power for off-cycle heat to the compressor.
Holding coils are rated at different voltages
Can have one or more sets of contacts
Some are equipped with auxiliary contacts, which are usually rated at a lower amperage than the primary contacts.
Contacts and coils can be replaced, the material most used on the contacts is silver.
Use the exact replacement whenever possible
MOTOR STARTERS
Contactor equipped with overload protection, which actually are designed to protect the motor where fuses or circuit breakers protect the entire circuit.
High amp draw will cause the starter to open.
Coils, contacts and heaters can be replaced
Contacts become pitted over time
Pitting increases the resistance across the contacts, if the pitting is bad enough it can cause motor burn out.
The voltage across the contacts will increase
The voltage across a good set of contacts should be about zero volts
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
Motor service factor is determined by the motor’s reserve capacity
RESTARTING THE MOTOR
Motors should not be restarted 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, this is to ensure that technicians are aware of a possible problem which caused the starter to trip in the first place.
Some controls reset automatically after a predetermined time delay
Time delay feature prevents short cycling
[attachment=66686]
INTRODUCTION TO MOTOR CONTROL DEVICES
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
The type of motor control used is determined by the size and application of the motor used
THE CONTACTOR
Larger version of the relay
Has movable and stationary contacts
Often times only one set of contacts opens which opens only one side of the power to condensing units. This provides power for off-cycle heat to the compressor.
Holding coils are rated at different voltages
Can have one or more sets of contacts
Some are equipped with auxiliary contacts, which are usually rated at a lower amperage than the primary contacts.
Contacts and coils can be replaced, the material most used on the contacts is silver.
Use the exact replacement whenever possible
MOTOR STARTERS
Contactor equipped with overload protection, which actually are designed to protect the motor where fuses or circuit breakers protect the entire circuit.
High amp draw will cause the starter to open.
Coils, contacts and heaters can be replaced
Contacts become pitted over time
Pitting increases the resistance across the contacts, if the pitting is bad enough it can cause motor burn out.
The voltage across the contacts will increase
The voltage across a good set of contacts should be about zero volts
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
Motor service factor is determined by the motor’s reserve capacity
RESTARTING THE MOTOR
Motors should not be restarted 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, this is to ensure that technicians are aware of a possible problem which caused the starter to trip in the first place.
Some controls reset automatically after a predetermined time delay
Time delay feature prevents short cycling