I have purchased a 1 Gang Dimmer 2 Way Push On/Off 400w ~ style
I am using it, so far successfully, to control a fan.
I am aware that this dimmer is mainly used for mains voltage tungsten lighting.
In the dimmer's leaflet I read: "Standard tungsten dimmers are not suitable for the control of motors and fans".
Why is this? and, more importantly, what will happen longer term when used with the fan?
Thanks,
Antonio
most likely because the switch on load of a fan could exceed the rating for the dimmer. Long term this could lead to premature failure due to overheating or failure of the semi-conductor device.
The fan motor is an inductive load. It will repeatedly force the triac into conduction by hitting it with voltage spikes. The dimmer might behave ok, might not. It might last long term, might not.
NT
Not quite right.
A very old story... I was involved in developing a speed controller for fractional horsepower single phase fans. Mid 1960's when thermistors and triacs were young.
Triacs switch inductive loads happily and the square law load characteristics of a fan make the system stable.
Lots of these units were sold in the UK for purposes such as refrigeration/air con fans or where temperature could be controlled by varying air speed.
All went well until some salesman won an export order to France. The dept. was then deluged with complaints about radio interference. Apparently the French, at that time, used short wave radio bands for local broadcasts which picked up the triac switching spike.
A small ferrite cored choke was quickly added and we heard no more......
Indeed. Our old kenwood chef happily uses a triac to modulate the motor speed.
There are issues with switching inductive loads, but they are not the issues meow raised. Those don't exist.
The usual reason to diasallow MOTOR loads is that switch on surge will kill the triac.
I had an old dimmer which zapped the broadband. When the router was positioned close to the computer so used most of the house telephone wiring - unlike now where it's situated where the line comes into the house.
Well if the fan has an induction mote, it won't go slower just less well, if its a brushed fan it will slow down, but the back emf and inductiveness of the windings of the fan can wreck standard dimmers. they need to be designed for the type of load. Drill speed controllers are better. Of course you might be lucky and the circuit is staandard or at least tolerent of it. I tried it once with a drill and the dimmer lasted ten minutes before losing its ability to control anything! Brian
No, the LACK of back EMF and the LACK of inductance..can wreck dimmers.
Back EMF and inductance LIMIT current peaks.
I.e. if youu were to take the copper wire out of a motor winding and connect it across the mains what do you think would happen?
Its the fact its wound on an iron armature that gives it inductance and the fact that the motor spins generating back emf that limits the voltage that makes it possible for it not to go up in flames.
I didn't think the startup surge of a squirrel cage was any worse than with= filament lamps, 8x run current. A 20w motor on a 400w rated dimmer also gi= ves a 20x margin.
NT
filament lamps, 8x run current. A 20w motor on a 400w rated dimmer also gives a
20x margin.well yes.
I dunno what the startup surge is either, and its critically dependent on the resistance of the windings and their inductance.
Things which are NOT necerssarily predictable between different manufacturers .
And in thse case teh dimmer companies cover their arses by saying 'not suitable for' meaning 'could very well be suitable for but we are buggered if we are going to take your moans if it isn't'
Also there is a very nasty side effect of chopping AC to throttle a motor. Its not getting a nice sinusoidal waveform and that means that its back EMF no longer remotely matches the incoming waveform. That means that peak currents COULD be very high as they are limited then only by winding resistance and inductance.
In this situation inductance is the Triacs friend. And sometimes the better the motor is the less inductance and resistance there is.
Was that when they started to boast about "electronic" speed control, when all they had done was to retain the mechanical governor, but switch a triac instead of the motor directly?
Chris
No. It's a proper phase chopping triac.
Does a food mixer use an induction motor? Seems a strange choice given it could have high starting loads - and doesn't have to run for ages.
Speed control for AC motors ideally depends on type.
filament lamps, 8x run current. A 20w motor on a 400w rated dimmer also gives a
20x margin.Its in a predcitable limited range for a given squirrel cage motor size, and its nothing huge.
A motor's output torque responds mainly to mean current. Its only at low output with low mean current that the waveform gets quite peaky. Its a long way within the abilities of a 400w rated dimmer to run a 20w squirrel cage.
it reduces the peakiness
to a rather limited extent
NT
If it does pack up, you can get fans with remote speed control from TLC.
Fan motor may overheat if it's self-cooling reduces much faster than its heat output as the dimmer is turned down. This is a common scenario when reducing motor speeds, and something the motor's cooling needs to be designed to handle if it's designed for this type of control.
I learned this as a teenager playing with reducing motor speed using a dimmer circuit I'd built, where I overheated the windings at lower speed.
The motor almost certainly also becomes less efficient when controlled in this way generating even more heat in proportion to its speed.
Yes and no. It is very complicated. What is certainly true is that the triac gets MUCH less efficient.
With a low inductance motor you get very high peak currents and I squared R rockets upwards, on the other hand the total time when the I is flowing is less. Likewise the magnetic losses - core heating - will be high with high peak currents. But agin, the proportion of time the current is high is less.
In model aircraft motors where the limits are always close around 80-90% of full throttle is the usually worst case,.
The ideal may be simply put a lamp i parallel to the motor.; This is what a model maker did when a company I worked for ordered some equipment. A simple alternative may be to fit a barreta or a -ve tep coefficient resister in the supply that should remove in surge spikes
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