Emerson manual Variations On Motor Control, Rosemount 848L, Reference Manual

Rosemount 848L

Reference Manual

00809-0100-4696, Rev AA September 2004

VARIATIONS ON MOTOR CONTROL

The contactor is never controlled with a toggle switch, because that would leave one side of the solenoid electrically hot when the motor overload trips. Standard procedure calls for start and stop push buttons in combination with an auxiliary contact on the contactor. This contact closes when the solenoid is energized and the motor contacts close. The auxiliary contact is rated for the control voltage and current, and is far away from the high voltage motor contacts. The stop button is normally closed and is in series with the control power. The start button is normally open and is also in series with the control power. The auxiliary contact is normally open and in parallel with the start button. When the start button is pushed, the solenoid is powered and the motor and auxiliary contacts eventually close. The start button can be released and control power will continue to flow in the auxiliary contact. Two things can stop the motor. Pressing the stop button removes power to the solenoid, causing the auxiliary contact to eventually open along with the motor power contacts. The stop button can then be released because there is no complete circuit to the solenoid. The same thing happens if an overload trip opens the circuit to the solenoid. When the overload is reset, no power will be applied to the solenoid until the start button is pushed.

	___		___			Over
	\_/STOP		\_/START			load
					M
+	--OO-----	+--	O O--	+------------	( )	---OO--	+ Contactor M
			M
		+--	--	+

Interlock

There may be a process condition where it is not safe to run the motor. If this condition can be detected and transformed into the change of state of a contact, then the normally closed contact may be inserted in series with the control voltage. If the interlocked condition occurs then the motor will not run or start. An example is a low level condition in a tank feeding the suction of a pump. The pump will be damaged if the suction goes dry, so a low level switch is put in series with the control voltage for the pump’s motor starter.

Permissive

There may be a process condition that is required to be present when a motor is started, but is not required once the motor is running. A contact that is closed when the permissive condition is true is placed in series with the start button. An example is auxiliary lubrication for a large motor that is required to flood the bearing housings to prevent contact between the motor shaft and the bearing material (not ball bearings). Once the motor is turning, lubrication is maintained by shaft rotation and the auxiliary pump can be shut off.

Emergency Shutdown

A process may have an emergency shutdown requirement for all motors. This requires a contact or logic input for all affected motor controls. For example, there is an emergency stop button for a natural gas processing plant located near the exit, so that the operator can hit it while running away.

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