Plugging
Plugging is a method of motor braking in which the motor connections are reversed so that the motor develops a counter torque which acts as
a braking force.The counter torque is accomplished by reversing the motor with the reversed motor torque opposing the foward inertia torque of the motor and it`s mechanical load.
Plugging a motor allows for a very rapid stop.
Although manual and electromechanical controls can be used to reverse the direction of a motor, a plugging switch is normally used in plugging applications.
A plugging switch is connected mechanically to the shaft
of the motor or driven machinery.The rotating motion of the motor is transmitted to the plugging switch contacts either by a centrifugal mechanism or by a magnetic induction arrangement.
The contacts on a plugging swith are NO. or NC. or both,and
acuated at a given speed. The primary function of a plugging switch is to prevent the reversal of the load once the counter torque action of plugging has brought the load to a stand still. The motor and the load would continue to run in the opposite direction without stopping
if the plugging switch were not present.
Plugging Switch OPERATION. Plugging switches are designed to open and close sets
of contacts as the shaft speed on the switch varies.As the shaft speed increases,
the contacts are set to change at a given rpm. AS the shaft speed decreases , the contacts return to there normal condition. AS the shaft speed increases, the contact setpoint
(point at which the contacts operate) reaches a higher rpm than the point at which the contacts reset(return to there normal position) on decreasing speed.
The defrence in these contact operating values is the differential speed or rpm.
PlUGGING USING A TIMMING RELAYS.
Plugging can also be done by using a timming relay. The advantange of this is cost.
The disadvantage is that the timming relay can not compensate for a change in load
condition which effects stop time, unlike a plugging switch.
PRECAUTIONS IN PLUGGING APPLICATIONS - There are many
advantages in plugging a motor to a stop, however, care must
be taken that these advantages are not lost through misapplications.
TYPICAL ANTIPLUGGING CIRCUIT - This circuit is used in cases
where damage would result from a sudden reversal of power.
The speed switch contacts are designed to keep the reverse
circuit open until the motor has coasted to a preset speed.
1. When plugging a motor to a stop, full load motor current
may be greatly exceeded. The electrical supply must be
able to handle the excess load.
2. The resultant forces of a plugged stop may be too great for
the driven machine to withstand. For example, a twisted or
broken shaft may result. . -
3. The motor may not withstand braking duty. The duty cycle
and load must be considered and the motor manufacturer
should be consulted to determine if a particular motor is
adequate for plugging duty.
PLUGGING - on a speed switch hook up - Turn adjusting screw(s) in or out to achieve
stopping at zero speed. Adjust both screws when circuit requires
switch to be wired for plug stopping in both directions.
Adjustment depends upon the speed of the machine. Improper
adjustment will cause the machine to either:
1. Coast to a stop, caused by not providing enough countertorque;
2. Permit a back-spin (shaft rotating in opposite direction),
caused by providing too much counter-torque.
Adjustment of the speed switch must be made between these
two conditions to achieve zero shaft speed.
Limitations of Plugging
Plugging is the one method of braking that may not be applied to all motors and or apllications. Braking a motor to a stop using plugging
requires that the motor be a reversible motor and that the motor can be reversed .
Plugging is a control function that provides braking by
reversing the motor. The phase sequence is reversed so that
the motor develops a counter-torque that exerts a retarding
force. Antiplugging protection is the effect of a control device
that operates to prevent the application of counter-torque by
the motor until the motor speed has been reduced to an
acceptable value.
