TE Technology TC-720 - Manual (Page 98)

Appendix E – Additional Notes on Fan Control
Fan Speed Control, 2-Wire Fans
It is possible, though expressly NOT recommended, to pulse-width modulate (PWM) standard two-wire fans by connecting
the ground wire to the Alarm 1 JP2-8 terminal and selecting “FAN CONTROL+ ”. However, this can be dangerous. The fan
may appear to work for a while, and the fan speed may vary based on the control, but there can be long term problems.
First the control may not be uniform. The speed response of the fan may not be uniform with the speed control output%.
The speed response can also vary depending on the frequency of the control.
Second, and most important, the PWM input can cause physical damage to the fan motor. Since the fan is not rated for
PWM speed control there is no specific PWM frequency that will be recommended. Higher frequencies may cause the
internal commutation circuitry to malfunction. The motor will almost always run hotter. This degrades the spindle bearings
more quickly, and can overheat the electrolytic capacitors within the fan. During PWM control, the voltage applied to the
fan is either its rated voltage or zero volts. However, because the fan is spinning at something less than its rated speed, its
back EMF is reduced. This causes higher-than-nominal current flow through the windings during the on period of the PWM
cycle. The motor windings can overheat, causing the winding insulation to break down. These negative effects may be
minimized by using a low PWM frequency, but they will never be eliminated. The end result is that the fan can fail, and the
user will not know when or how the fan will fail. Thus, the recommendation is do not PWM speed control a two-wire fan
unless you know it is designed for PWM speed control.
However, some two-wire fans are designed for PWM control. They often incorporate an input filter to change the pulsed
DC into a steady DC voltage. These fans are designed for a specific PWM frequency range.
Fan Speed Control, 3-Wire Fans
Typically the fan’s power leads are connected to the power source and the control is connected to the fan speed control
terminal. The most common control will be “FAN CONTROL - ” mode. Refer to the fan manufacturer’s data for proper
operation.
Issues to be mindful of are:
• Be sure that the speed control can be accomplished through a PWM input, and not an analog input.
• Many of these fans have a specific PWM frequency that is recommended, use the proper frequency.
• Sometimes 0 % duty cycle on the fan will cause the fan to operate at a minimum speed, other times it will stop the
fan blades from rotating. Be sure to know how the fan you are using will respond.
• Some fans have an internal pull-up resistor on the input terminal, other times you may need to add this pull –up
resistor externally.
Fan Speed Control, 4 Wire Fans
These are similar to three wire speed controllable fans, so refer to the notes regarding 3-wire fans. 4-wire fans often have
an additional output wire used as a speed monitor or an output alarm. The TC-720 cannot monitor these inputs. Be sure
not to connect the speed monitor or an output alarm wires to the PWM control terminal on the TC-720.
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