Appendix E, Motor Control, Rosemount 848L

Reference Manual

00809-0100-4696, Rev AA September 2004

Rosemount 848L

Appendix E

Motor Control

INTRODUCTION TO MOTOR CONTROL

Introduction to Motor Control . . . . . . . . . . . . . . . . . . . . . . page E-1 Variations on Motor Control . . . . . . . . . . . . . . . . . . . . . . . page E-2 Writing 848L Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . page E-4

Industrial motors require about a kilowatt per horsepower, usually delivered as three phase AC at 440 volts or higher. This requires a special switch to turn them on and off. The switch is called a contactor, in which a solenoid is energized to pull a set of three power contacts to close the circuit to turn the motor on. The contacts are large enough to carry the starting current without welding. They are separated by insulation suitable for the supply voltage. The solenoid is de-energized to turn the motor off. Springs quickly separate the contacts to prevent arc damage, which can be severe at higher voltages. A contactor for a 400 HP 2500 VAC motor may be housed in a steel box that is two feet square and five feet high.

The three phase wires to the motor go through three overload heaters. There are no contacts in this wiring, just heaters that mount on screw terminals. The same contactor may be used for different motor sizes by changing the heater overload rating. When an overload occurs, the heaters cause a contact to open that is in series with the contactor’s solenoid, which removes power from the motor. (This action is called a “trip” because it is mechanically like tripping an alarm mechanism. Alarms are said to trip because the early electric bank alarms used a trip wire to detect a robber.) The trip is supposed to happen before the motor windings overheat and destroy their insulation. After things have cooled off and someone has removed the cause of the overload, a reset button must be pressed to close the heat triggered mechanical latch for the overload contact. This allows power to flow in the solenoid circuit again.

The solenoid runs at a lower voltage than the motor, called the control voltage. This voltage is taken from a transformer within the contactor enclosure that is connected to two of the supply wires. The circuit breaker for the contactor may be in another box somewhere. When the breaker is turned off (or trips) the contactor enclosure is electrically dead, even for the control voltage. The contactor’s solenoid may run at a higher voltage to get enough power to move the contact assembly against its springs. A pilot relay is used to switch that voltage within the enclosure. The control voltage seldom exceeds 120 VAC or is less than 24 VAC. One side of the control voltage is always grounded. Both the overload mechanism and the pilot relays are now available in solid state form.

The enclosure containing the contactor, overload mechanism and control power supply may be called a motor starter. The contactor’s solenoid or that of the pilot relay carrying control voltage may be called a coil, as in relay coil. A group of motor starters may be called a Motor Control Center (MCC).