GE ZIC36ON Series Technical Service Manual page 18

The evaporator fan utilizes a permanent magnet, 4-pole, DC motor that can operate at three different
speeds: high, medium, and low (medium and low are the same speed, controlled by the main control
board). The speed of the fan is controlled by the voltage output from the main control board. Voltage
output from the control board to the fan is 12.6 VDC; however, in order to regulate the speed of the fan,
the control board uses Pulse Width Modulation (PWM) during low speed and medium speed operation.
When operating in low and medium speed, voltage is sent in pulses (much like a duty cycle) as
opposed to an uninterrupted flow. This pulsing of 12.6 VDC produces effective voltage being received at
the motor, which is the equivalent to a reduction in voltage. Fan speed will be selected and maintained
by the control board regulating the length and frequency of the 12.6 VDC pulse.
One complete revolution of the motor is comprised of all 4 poles. To determine the rpm of the fan:
Measure the frequency being applied to the motor. Multiply this number by 15 (60 seconds divided by 4
poles). For example, a frequency measurement of 200 Hz multiplied by 15 would show a fan speed of
3000 rpm (15 x 200 = 3000). Temperature may cause some fan speed variation. Fan speed may vary
+/- 5%, depending on the temperature, with higher temperatures causing slightly higher speeds.
The evaporator fan motor uses a 4-wire connection, utilizing a common wire (white), feedback/rpm
wire (blue), supply wire (red), and a signal wire (yellow).
12 VDC
12 VDC
0 VDC
12VDC
8 VDC
0 VDC
12 VDC
4 VDC
0 VDC
High Speed (12 VDC measured)
High Speed (12 VDC measured)
Medium and Low Speed (8 VDC measured)
Medium Speed (8 VDC measured)
Low Speed (4 VDC measured)
EVAPORATOR FAN SPEEDS
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GEA01139