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Reference Manual
00809-0100-4696, Rev AA
September 2004
Rosemount 848L
E-2
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 | |
+--O|O-----+--O O--+------------( )---O|O--+ Contactor M
| | | |
| | M | |
| +--| |--+ |
VARIATIONS ON MOTOR
CONTROL
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.