##
Voltage Flicker

Starting motors with large loads causes voltage drop which is often evidenced
by flickering lights. This flicker is objectionable only when the magnitude
and frequency of the voltage drop exceed certain thresholds.

This threshold of objection is shown on a Voltage-Flicker curve. If the
magnitude of the voltage drop and the frequency of occurrence lie below the
threshold of perception, people generally do not notice any flicker.

Example: A five horsepower three phase motor is supplied by a 208 Volt feeder
which also supplies 120 Volt lighting circuits. Assume that the feeder
resistance = 0.06 ohms. For a 5 horsepower motor, the Full Load Amps (FLA) = 16
amps. The motor starting current = 16 amps x 6 = 96 amps. V drop = starting
current x feeder resistance = 96 amps x 0.06 ohms = 6 Volts.

The 6 Volt drop along the feeder is equal to 5% of the voltage on the 120 V
lighting circuit, and it causes a noticeable flicker. If the motor is started
once every hour, then the representative point, on the flicker curve is in the
objectionable range. To correct this problem, supply the lighting circuits from
a separate feeder, or reduce the voltage drop along the feeder. A reduced
voltage starter often presents a very cost effective solution.

**
Top of Motors
| Index
| FAQs
**