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Fan Speed Control 101

Basic electrical methods for stealth fan oporation

Fan Speed Control 101

I. Know your fan
In order to control fan speed, you have to know what type of motor your fan uses. Motors can be divided into two categories based on power source AC, and DC. AC fans are those you plug directly into a wall receptacle (Vortex, Dayton, Can-Fans), for more information about AC motors go to numeral II. DC motors are generally found in small fans like computer fans and rely on a DC power source like a rectifier (wall wart) or battery. For more information about DC fans, see numeral V.

AC fan:

DC fan:

II. Know More About Your Fan
AC fans use many types of motors but three types are important to us cabinet-bedroom size growers, shaded pole induction motors, permanent-split capacitor (PSC) motors, and AC-DC Universal motors (Brushed Motors). For a list of motor type based on manufacturers, see numeral III. Shaded pole induction motors are simple single phase motors known for low starting torque and long duty cycles. PSC motors are also single phase motors but unlike shaded pole motors, PSC motors use a capacitor to help them start. AC-DC Universal motors are general purpose motors found in many household appliances like power drills and vacuum cleaners; their short service life makes them an unattractive option for fans. There are other types of AC motor that have been used for ventilation purposes, but aside from shaded pole, PSC, brushed motors, and three phase motors (which are beyond the scope of this course), ac motors are unable to be speed controlled without serious modification and/or risk.
There are primarily two ways to control the speed of these motors, voltage control and frequency control; see numeral IV for more information.

III. Fan Manufacturer List
This information is relevant as of march 2009 but always be sure to double check by contacting your retailer or fan manufacturer! Using the wrong type of control with your equipment puts the fan and your controller at risk. This section is always in need of additions and corrections so if you have anything to add, post it or send me a P.M. and I'll be sure to include it.

Permanent-Split Capacitor
-Dayton squirrel cage blowers (Generally PSC motors, but they also make a shaded pole version)
-S&P Mix-Vent TD line
-Grainger squirrel cage fans

Shaded Pole Motor
-Active Air

IV. AC Speed Control Methods

-Rheostat (Old dimmer switches): Poor choice, excess power converted to heat.

-Triac (New dimmer switches): Poor choice, inherent problems with triac controls risk fan lifespan.

-Triac w/ Snubber circuit (Fan speed controllers, Solid state controllers): Good choice, snubber circuit removes most of the risk to the fan. Still not the best because it can cause some motors to hum.

-Capacitance level control (No hum fan speed controllers, 3 speed controllers): Good choice, only limited by the discrete speed choices, no infinite control.

-Variable autotransformer (Variac): Better choice, continuous sine wave of a lower voltage (unlike a triac which chops up the sine wave to acheive a decreased voltage) allows fan operation with no hum.

-Varable Frequency Drives (VFD, AFD): Best choice, complex circuitry senses changes in the motor allowing it to vary frequency of the AC source as well as the voltage which maintains a constant torque, unlike voltage regulation.

V. DC Speed Control Methods
DC speed control is much simpler than AC speed control. In order to lower the speed of a DC motor we must lower the voltage, this is accomplished in various ways. Linear voltage regulation relies on resistors, and diodes to remove the excess voltage as heat. Pulse Width Modulation sends bursts of energy to the fan and the averaging effect results in a lower net voltage and reduced energy use.

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