Page 132 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INDUCTION MOTORS

INDUCTION MOTORS 6.21

The devices labeled OL provide the overload protection for the motor. They consist of
two parts: the overload heater element and overload contacts. During normal operation, the
overload contacts are closed. When the temperature of the heater elements increases to a
set limit, the OL contacts open. This deenergizes the M relay opening the M contacts and
isolating the motor from the power supply. If the motor is overloaded for an extended
period of time, damage will occur due to the excessive heating caused by the high currents.
The inrush current would not affect the heater elements because of its short duration. The
heater elements will operate when there is a high current for an extended period, isolating
the motor from the power supply.
The controller provides the undervoltage protection. The control power for the M relay
comes from directly across the incoming lines to the motor. If the voltage level drops
beyond a certain level, the voltage applied to the M relay will also fall, resulting in the deen-
ergization of the relay.
Figure 6.25 illustrates an induction motor starting circuit with resistors to reduce the
starting current. Relays 1TD, 2TD, and 3TD are on-time delay relays. When these relays
are energized, there is a set time delay before their contacts close.
When the start button is pushed, the M relay gets energized. Its contacts close, allowing
power to reach the motor. Since the contacts for the 1TD, 2TD, and 3TD relays are all open,
the current is forced to flow through the full starting resistor, reducing the starting current.
The 1TD relay gets energized as soon as the contacts of the M relay close. However, there
is a set time delay before the contacts of the 1TD relay close. During this time, the motor
would have gained some speed, and the starting current decreased.
After the specified time delay, the contacts of the 1TD relay close, cutting out the first
part of the starting resistance and simultaneously causing relay 2TD to energize. After
another delay, the contacts for relay 2TD close, cutting out the second part of the starting
resistor and causing relay 3TD to energize. Finally, the contacts of relay 3TD close,
causing the resistor to be bypassed.
The values of the resistor and time delays can be selected to prevent the starting current
from becoming dangerously large while allowing enough current to ensure adequate accel-
eration to the normal operating speed.

REFERENCE

1. S. J. Chapman, Electric Machinery Fundamentals, 2d ed., McGraw-Hill, New York, 1991.

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