Which tap to use on multi-tap motor to slow it down?

Thread Starter

MikeA

Joined Jan 20, 2013
187
I have a 120v AC induction motor with 4 taps: high, medium-high, medium-low and low.

Even the low tap is much too fast for my application, so I'll use a variac to bring the voltage down even further.

The question is: does it matter which tap I use with the variac?

Intuition suggests to use low, but I can't formulate a scientific explanation.
 

Reloadron

Joined Jan 15, 2015
4,595
What is the HP (Horse Power) rating of the motor? My reason for asking is that induction AC motors come in several types and their speed can be controlled in several ways. A variac will vary the voltage to the motor and that is all it will do. You are sure the motor speed taps are not using internal capacitors (much like some ceiling fans) to change motor speed?

Small AC Induction motors will change speed using a simple and inexpensive SCR Type speed controller similar to this example which can be bought in any home improvement store. Larger motors like 1/4 HP and above normally use a VFD (Variable Frequency Drive) similar to this example. In most cases the use of a variac is not always a good idea. As to the variac the taps are on the input side and you choose a tap based on your line voltage. The output will usually be 0 to Something and if you want a low voltage out it really won't matter which input taps you use. It's a little difficult to answer your question without knowing more about the motor.

Ron
 

Reloadron

Joined Jan 15, 2015
4,595
With a 1/2 HP motor I would be looking at VFD. Also with a 1/2 HP motor I doubt capacitors are used as in the ceiling fan example I mentioned. You can try a variac but I see a VFD as a better alternative than dropping the voltage. Hopefully more people will chime in, Max always has very good suggestions for motor speed control.

Ron
 

Thread Starter

MikeA

Joined Jan 20, 2013
187
VFD is a bit expensive. And I have several variacs already.

What I'm trying to figure out if there will be any thermal difference/efficiency difference between different taps.

That is, is there a difference between running it at say 80V on the low tap, or running it at 40V on the high tap (made up numbers).
 

shortbus

Joined Sep 30, 2009
7,091
I wouldn't do this using a variac. The taps you're talking about are usually just changing the number of poles used in the stator, combining them in different configurations to get the different speeds. When you just change voltage in an AC motor they will run hot and burn up.

What is this motor from and what are you using it for? The usual way of changing speed on a given drive system is belts and pulleys to change drive ratios.
 

MaxHeadRoom

Joined Jul 18, 2013
19,199
What is your application?
As a general rule 1ph AC motors do not control well, hence most use the 3phase version with a VFD 1ph supply.
You could use a gear box, or alternatively adapt a powered hand tool, drill etc that has a Triac controller built in.
Depends what job it is for.
Max.
 

Thread Starter

MikeA

Joined Jan 20, 2013
187
This motor drives a fan, directly. So it is self cooling in a sense, as the fan is pulling air over the motor.

At full voltage, the motor stays as cool as ambient. When I drop it down to half voltage, the motor warms up to about 110F. I would assume this is well within the design spec. At lower voltages there is less air flow so it gets even warmer.

The taps you're talking about are usually just changing the number of poles used in the stator, combining them in different configurations to get the different speeds.
My goal here is to keep the motor as cool as possible. It sounds like using the high wire would distribute the heat more evenly?
 

MaxHeadRoom

Joined Jul 18, 2013
19,199
Really small fans are shaded pole motors which are Very inefficient, hence the heating at low rpm.
The PSC cap variety are a little better but still not very efficient at low rpm's.
If it is simply air flow you are trying to control, can a venturi or shutter be used?
Max.
 

Thread Starter

MikeA

Joined Jan 20, 2013
187
Is this a small fan? :D

f77cc5edada6edd71f9d7d67c81804ce.jpg
I'm just trying to quiet it down. Around 40v it moves enough air for my needs and is pretty quiet.
 

MaxHeadRoom

Joined Jul 18, 2013
19,199
I would peg that for a shaded pole fan.
The quietest is to use the variac, a triac 'dimmer' with increase the noise (buzz).
Use the lowest tap when using the Variac.
Max.
 

Thread Starter

MikeA

Joined Jan 20, 2013
187
It would make more sense to reduce a low rpm by say 10% than the high range by 80% in order to obtain the same rpm.
But is there a reason from electrical theory for why it makes more sense?

I'm not debating, I'm just trying to understand why.
 

MaxHeadRoom

Joined Jul 18, 2013
19,199
It depends somewhat on the motor technology, but if you consider a typical fan like that that has a start winding and cap and 4 speed main winding.
The start winding and cap do not change, but the motor windings are switched in as needed.
In the normal order of things, the motor current stays fairly constant over the RPM range.
If you drop the voltage significantly by using the hi rpm winding the fan current is going to drop much more than if using the low winding at with a minimum drop in voltage.
Hence the motor is running at far less efficiently.
Max.
 
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