I have a need to control the speed of a number of different fans, and find all solid state solutions quite noisy. I have several Variacs in locations where fan noise should be kept to a minimum, but in others I have solid state, as Variacs aren't that cheap.

I have this specialized motor speed control.
https://www.greenheck.com/shop/accessories/speed-controls/385031

It is much nosier compared to a Variac. So I decided to compare this Greenheck to a 1980's Lutron incandescent light dimmer that I had in the parts bin. Put a clamp on the output of both and looked at the output on an o-scope. Identical! There was no difference in the wave whatsoever.

So the question is, is there really such a thing as a purpose built solid state motor speed controller? Or are they basically just a light bulb dimmer in a different box?

 

the simple answer is - it depends...

while the products may use same basic principle, there are differences too. for example if they can handle different type of load (inductive/resistive), what the limits are etc. for example light is usually controlled in the range 0-100%. motors normally do not like that. at low speed they may lack cooling so one may want to limit the adjustment range to something else (30-100% for example).

 

Motor speed controllers come in many varieties, depending on the motor type. Incandescent light dimmers are all pretty much the same thing, using a triac to switch the power on late in each cycle to get the dimming effect. These might and might not work to control a motor depending on the type of motor. They certainly won't work for a DC motor of any kind.

 

Fans generally fall into AC induction motor variety, either shaded pole, or capacitor start/run typically controlled with a Triac controller.
Which tends to be noisiest at low RPM's.
The difference between the fan and lamp-dimmer, is the former usually has choke suppression.

 

Many AC motor speed controllers are a bit more complex, and some of them provide quite good speed control. I have used one from Grainger, installed it in a client's house and they were quite pleased.
I am assuming that the noise complaint is about electrical noise, but that is not clear if it is electrical or acoustic noise that is the issue. So more information will be helpful.
The obvious difference that I saw was the price.

 

There is very few options out there for 1ph AC induction motors, other that the triac based versions.
Even the 1ph VFD's that have seen the market, have had very much less than stellar success.
The Triac provides some noise, so the degree of suppression is given more effort in some better versions.

 

I am assuming that the noise complaint is about electrical noise, but that is not clear if it is electrical or acoustic noise that is the issue.

All noise is acoustic, no?If it's between electrical and mechanical noise, I think the answer to that is: mechanical. The "unsmooth" delivery of power causes mechanical noise which is coming from the motor itself.

 

All noise is acoustic, no?If it's between electrical and mechanical noise, I think the answer to that is: mechanical. The "unsmooth" delivery of power causes mechanical noise which is coming from the motor itself.

That has not been my experience at all, except for PWM speed controls on cheap DC motors.

 

All noise is acoustic, no?If it's between electrical and mechanical noise, I think the answer to that is: mechanical. The "unsmooth" delivery of power causes mechanical noise which is coming from the motor itself.

The motor acts as a Sound Transducer, using electrical energy to create the mechanical vibrations to disturb the surrounding air producing a sound audible in nature.

 

NO, not by the standard definition!! Electrical noise is very real, although clearly unknown to many.
Electrical noise is mentioned in most extended technical training courses.

 

Regardless of whether or not the "Noise" is Electrical, or Mechanical,
the only solutions are ........
either a custom designed Electrical-Filter-Network,
( matched to the Electrical-characteristics of each different Motor model ),
or, mechanical Isolation-Mounts, ( usually Rubber-Motor-Mounts ),
or both, depending on how quiet the Fan operation must be.

Sometimes it's worthwhile to apply an Asphalt-based/Adhesive-backed sound deadening material
such as "Kill-Mat" ( normally used in automotive applications ),
this stuff is great for damping high-frequency-noise radiated from large sheet-metal surfaces.
( do not apply this type of material to the exterior of any Motor, overheating will be guaranteed )
.
.
.

 

NO, not by the standard definition!! Electrical noise is very real, although clearly unknown to many.
Electrical noise is mentioned in most extended technical training courses.

So what I'm describing is electrical noise? How about a transformer humming? Any other examples?

 

Have You ever listened to an AM-Radio in a Car ?
Have You ever listened to a CB-Radio ?
Some Electrical-Noise is random, and some repetitively follows the 60Hz-Mains-Frequency.
Many Electrical-Noise-Sources are at Frequencies that You can not hear with your Ears,
but can be visually displayed on an Oscilloscope.
Digital-Data-Transmissions may also create Electrical-Noise-problems.
Turning On or Off a Light-Switch in your House generates Electrical-Noise.
Sometimes Electrical-Noise is entirely inconsequential, and goes completely un-noticed,
in other situations it can destroy near-by Electronic-Equipment instantly.
.
.
.

 

Have You ever listened to an AM-Radio in a Car ?
Have You ever listened to a CB-Radio ?

Yes, I would say that's electrical noise. Perhaps even lightning or any other arching. As there is no intermediary between electricity and air being disturbed to create audible sound.

Whereas for example a transformer humming is mechanical noise.

 

Certainly the simpler "fan speed control"devices as shown in the link will indeed produce a fair amount of electrical hash noise, as well as a mains frequency modulated RF noise. It is possible to greatly reduce the electrical noise with adequate filtering.
Adding adequate electrical noise filters to an existing controller installation is quite possible but it will be a physical challenge because of dealing with mains power voltage and current. There are much quieter controls available but not from the source in that attached link. THAT PRODUCT IS VASTLY OVERPRICED.

 

The bottom line is there are very few options for the RPM control of a 1ph induction motor.
Hence the fact that most are based around Triac control.
To answer the initial question, the main difference is the use of extra suppression (capacitive/inductive) components used in the motor versions

 

Please excuse my unwillingness to join the philosophical debate on the nature of noise.

Put a clamp on the output of both and looked at the output on an o-scope. Identical! There was no difference in the wave whatsoever.

So the question is, is there really such a thing as a purpose built solid state motor speed controller? Or are they basically just a light bulb dimmer in a different box?

Take a triac light dimmer and add a few components to suppress voltage spikes from an inductive load, and you have a simple motor speed controller like the one you linked to. You can get away with running a motor on a light dimmer switch, until you can't. Depending on the motor, the switch, the load, eventually voltage spikes will kill your dimmer switch. Or they won't. For peace of mind, use a device designed for motors. You don't need to pay $115 though, check out the "router speed controller" from harbor freight.

 

You don't need to pay $115 though, check out the "router speed controller" from harbor freight.

I do own that Greenheck, except I got it for maybe $5 on Ebay in NOS condition. I got it precisely because it was a device designed explicitly for motor speed control.

But when comparing it to an incandescent light dimmer, that's decades old, they wave form out of both devices looks identical. That's the part that does not make sense to me. I did not see any spikes or noise on the dimmer.

 

I do own that Greenheck, except I got it for maybe $5 on Ebay in NOS condition. I got it precisely because it was a device designed explicitly for motor speed control.

But when comparing it to an incandescent light dimmer, that's decades old, they wave form out of both devices looks identical. That's the part that does not make sense to me. I did not see any spikes or noise on the dimmer.

The differences are in power ratings, filtering, and physical arrangement, including packaging.. Also heat management. The functions are indeed rather similar.
Some motor speed controls feature curret sampling to help regulate the speed, most do not.

 

But when comparing it to an incandescent light dimmer, that's decades old, they wave form out of both devices looks identical. That's the part that does not make sense to me. I did not see any spikes or noise on the dimmer.

At least one of the suppression devices seen in the Triac motor control is this type of choke.