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#129174 09/13/04 05:08 PM
Joined: Dec 2001
Posts: 300
How do they work? I'm talking about the simple two wire type with the knob like a dimmer switch used on residential ceiling fans.

Took one appart today and it's a rheostat with a transistor and that's not what I expected.

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#129175 09/13/04 09:15 PM
Joined: Nov 2003
Posts: 269
That "transistor" is probably a triac which is essentially two SCR's back to back. Simply put the reostat controls the conducting point of the triac and thus the voltage output.

#129176 10/07/04 11:08 AM
Joined: Jul 2002
Posts: 8,443
Likes: 3
A fan controller works in much the same way that a light dimmer does.
Varying the resistance of the rheostat, alters the charging time constant of a capacitor, that triggers the Triac, via it's Gate terminal.
The more pulses given to the Triac, the more current is "let through" to the load, in a given half cycle of AC current.
And hence, the faster the fan will turn.

#129177 10/07/04 05:48 PM
Joined: Dec 2001
Posts: 300
Okay, I guess I asked the wrong question.

I guess what I should have asked was how the motor speed is controlled by changing the voltage.

In an AC motor, the motor's "synchronous" speed is a function of the AC frequency. I've worked with VFD's quite a bit. But how do you vary an AC motor's speed by changing the voltage?

[This message has been edited by maintenanceguy (edited 10-07-2004).]

#129178 10/10/04 01:53 AM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and


I guess what I should have asked was how the motor speed is controlled by changing the voltage.

No problem. Hopefully, I can answer these questions concisely.
Let's take the normal stabs at the possibilities at hand.


In an AC motor, the motor's "synchronous" speed is a function of the AC frequency.

This is the common method of speed determination with an Induction Motor.
The Rotor (Secondary) will respond to the Frequency of AC Induced into it from the Stator Windings (Primary).
For example, a 2 Pole Motor connected to 60 Hz spins the Rotor at 3600 RPMs. A 4 Pole Motor at 60 Hz spins the Rotor at 1800 RPMs.


I've worked with VFD's quite a bit.

Same thing here, only the VFD alters the Hz delivered to the Motor to obtain a given Rotor Speed - across a Fixed Load.


But how do you vary an AC motor's speed by changing the voltage?

Normally, this will only limit the Volt-Amps reaching the Motor, and when applied to a Motor with a Fixed Load - such as a Surfacer, Lathe, Grinder, etc., the Rotor stalls under heavy Load and no real reduction in Speed is observed.
In the case of a Split Phase Single Phase Motor with a Start switch, the reduction in Speed results in Aux. Circuit being activated, with an end result of smoking the Aux. Winding or tripping the Overload Protection.

Now substitute a PSC (Permanent Split Capacitor) type Motor, or a Shaded Pole Motor, in place of the other types.
Along with this, make the Load type very Linear - meaning the higher the Speed, the higher Power requirement will be needed, with a corresponding higher amount of work done (or larger volume moved by the Motor).

In simpler terms, apply a PSC Motor for Fan Duty.
Attach a simple Axial Fan Blade assemblage to the Shaft of the PSC Motor, so the Motor moves Air.

This Linear type of Load (Environmental Air, or similar compressible Gases pushed by an Axial Blower), will respond to the Speed of the Blower.

For a given Fan size, if the Speed is fast, the CFM throughput is large. If the Speed is slow, the CFMs throughput is low.
The more volume - in CFM - that is required, the more True Power is required at the Rotor - delivered to the Motor from the Power Supply.

If the input restricts the level of True Power reaching the Rotor, then the Rotor's Speed is affected - and therefore spins slower (along with moving less Air).

This is achieved by reducing the input Voltage at the PSC Motor's Stator windings.
The Aux. Circuit is always in connection, so there's no Overload threat for it.

Basically, reducing the input Voltage results in a reduction of Rotor Speed and a corresponding reduction in Air flow.
The Motor draws "X" amount of True Power, and once the Air flow matches the drawn Power level, the Rotor's Speed becomes stable.

This is the same for a Shaded Pole Motor, and most Brush type Communtator Motors.

This would also apply to Cap Start, Resistance Start, and Cap. Start/Run type 1 Phase Motors, only that they will slip into the point where the Start Switch is activated.

The key reason this works is the Load Characteristics vs the Motor design.

Just wish I could explain this stuff better! I can think it, but not say it!
Hopefully another Member will come along and wrap this up in a simple, easy to understand answer.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#129179 10/10/04 03:26 PM
Joined: Dec 2001
Posts: 300
Thank's Scott

Am I right that what's actually happening is the motor is slipping under load. That we're just overloading the motor (at the reduced voltage) to the point that it will only run at a fraction of its synchronous speed?

#129180 11/20/04 11:57 PM
Joined: Nov 2004
Posts: 3
Junior Member
Yes, the speed decrease is accomplished by increased slip.

Operating speed is determined by the intersection of the load (fan) torque speed curve and the motor torque speed curve.

Fan curve has increasing torque for increasing speed.

NEMA B and D torque speed curves shown at Figure 1 here

Decreasing the voltage lowers the motor torque speed curve (decreases the torque at any speed).

For a standard NEMA design B industrial motor this is not an effective speed control method because speed can only decrease over a very narrow range below syncronous before breakdown torque is hit.

But, the fan acts more like a NEMA design D motor. It has a very gradually varying torque speed curve from sync down to zero speed. Adjuting the voltage is an effective means for contorlling speed of this type of motor.

#129181 11/21/04 08:18 AM
Joined: Sep 2003
Posts: 650
Small point of clarification:

An induction motor under load _never_ runs at synchronous speed. Slowing an induction motor down by producing larger slip is not necessarily an overload condition.

An induction motor _requires_ slip to induce current in the rotor conductors, and so will always operate below synchronous speed under load. (Or, if the load is overhauling, the machine will operate above synchronous speed, but will produce negative torque and act as a generator.)


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