adding speed control to a blower motor?

Thread Starter

Firstname Lastname

Joined Nov 2, 2017
20
I tried an AC light dimmer (rated for 800W) on a 250W air blower used for inflating medium sized inflatables. It seems to do the job of controlling the speed.
But I've been told by some that doing that to a brushless AC motor will burn it. Then some say otherwise so I'm not sure what to think.

There are similar "dimmers" on eBay that claim they can control fan motors speeds as well but I'm not sure whether to trust these chinese item descriptions or not. https://www.ebay.com/sch/i.html?_fr...rol.TRS0&_nkw=ac+motor+speed+control&_sacat=0

I'm disassembled the motor case and there are three cables coming out of it, two are connected to each other and to a large capacitor (maybe for kickstarting the motor?) and there's one neutral wire.

So what's the case? Can I use these off-the-shelf dimmers for control the intensity of my blower or am I at risk of damaging it?

A side question, mine is meant for 220V AC, if we use it in the US will it just blow slower or is again at risk of getting damaged?
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
You should be using it on 240v in the US.
It is most likely a PSC (permanent start cap) fan motor and in most cases these can be ran with a Triac dimmer style control, just monitor it for overheat and any drop out of run at low rpm's.
Otherwise it should be OK, the majority of fans these dimmers are made for are for ones known as having a shaded pole in place of the phase shift capacitor..
Max.
 

Thread Starter

Firstname Lastname

Joined Nov 2, 2017
20
Thank you. I thought these dimmers for the lights were merely bulky rheostats but it seems I was wrong.

Any ideas where this fear of burning a brushless motor with a dimmer comes from? 3 out of 5 people I asked were sure it will burn the motor

You should be using it on 240v in the US.
Sure. Am I correct to assume otherwise it will run slower, around half the speed?
 

Thread Starter

Firstname Lastname

Joined Nov 2, 2017
20
this is the reasoning I was given:
To sum it up they say: 1) to slow down an induction motor you need to lower the AC frequency not the voltage, 2) triac based dimmers dont change the frequency but the voltage 3) it might appear to work if there is some load 4) but its not meant to be used for this and might damage the motor over time, use a frequency controller instead. I have yet to hear the "why", though.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
That is true in the video for start/run motors over 1/2HP, there are ceiling fans and HVAC air exchange motors all over the world that are running with simple Triac control.
The 3ph induction motor is ran with a VFD, the 1ph version > 1/2hp is rarely ran this way due to dropping out of run.
The term Brushless Motor is more used to describe a P.M. type motor, not an induction type.

I would be Leary of running it on 120v 1ph.
The handtool/appliance/vacuum motors are often Universal motors (brushed AC/DC).
Max,
 

Thread Starter

Firstname Lastname

Joined Nov 2, 2017
20
I see, thank you.

Are the speed controllers consisting of a triac and a rheostat and some resistors universal? Seller label them 120V or 240V but that may be due to how the max Watts should be calculated rather than only working with certain AC Voltage.
https://www.ebay.com/itm/Newest-For...938587&hash=item4670dc23bf:g:GzQAAOSwmudaF5IZ

(also has the warning "Attention: Will not work with brushless type motor ")
 

Thread Starter

Firstname Lastname

Joined Nov 2, 2017
20
Im still curious what would happen if Id run this brushless AC motor designed for EU and 240V in US at 120V.
I think it should just run at half the speed but thats just an assumption.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
It will attempt to run at the frequency of the supply, this is the basic method of how a induction motor works, if due to insufficient current for the rotor to maintain the normal RPM, the rotor will lag dependent on friction and inertia of the rotor. etc.
This rpm will be arbitrary and depend on the latter conditions.
Max.
 
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