MOTORS AND DIMMERS

[dmxtothemax]

Heres a question for the electrical engineers,

Why exactily is it that when you put inductive loads like transformers ( PAR 46 ) or small personal fans ( shaded pole induction motors )

on to a dimmer that get quite hot very quickly and usually burn out.

Is it because they dont have snubber networks on the dimmers,

Or could it be spikes or hash, perhaps using a power filter.

I am looking for a way to power these things from dimmers if possible?

Does any one know for sure?

[PH_Zone]

It is all to do with inductive loads drawing current when there is no voltage,

which creates heat in the power control components.

All dimmers have snubber networks, these limit the instantaneous change in voltage that can cause large currents changes.

it is more to do with the ratings of the components and the design of the dimmer.

[Bobbsy]

To go a bit farther (though I certainly don't claim to be an electrical engineer):

 

Inductive loads such as motores or the transformer used with low voltage lamps, results in the current and voltage being out of phase; this causes problems where the zero-crossing circuit is triggered by the current, instead of when the voltage passes zero. The result of the reactive component presented by the transformer primary is erratic behavior of the control. The solution is to extend the pulse to last beyond the zero crossings of the current or issue a series of pulses instead of just one.

 

Bob

[powersolve]

I'm not sure it's really a current phase-angle issue, as has been suggested.

 

Induction motors are sometimes very easy to burn out if you run them at reduced voltage (which is what you're doing by dimming it). The reason is that a simple induction motor is not a variable speed device - it always wants to run at or near its rated speed (normally 3000rpm or some integer divisor) regardless of the magnitude of the voltage waveform. (It's the frequency of the voltage - 50Hz - that determines the speed). If you change the mechanical load on the motor shaft then its speed will change a bit, but that's ok - the motor doesn't mind.

 

If you reduce the voltage a bit, the motor starts pulling more current in an attempt to keep running at or near its rated speed. If you reduce the voltage far enough, the motor will slow down, but in doing so it can draw a lot of inductive current - much more than its windings are rated to carry for any significant period of time.

 

Result - hot motor, and maybe a hot dimmer, too.

 

If you want to run an induction motor at variable speeds, you really need a variable frequency drive, unless the motor is specifically designed for it. (E.g. central heating circulator pumps often have three switchable speeds - but I'm not sure how that's implemented.)

 

As for transformers, I'm not aware of any particular difficulties for the transformer with running it off a dimmer, although sometimes the dimmer might not like the very inductive load, especially if its quite small relative to the dimmer rated load. For example, I've run pinspots for hours on dimmer-packs without any problems.

 

Steve E.

[ianl]

more relevant to older dimmers but heres two more sugested reasons (in a simplified form):

 

Hysteresis (yes I know I can't spell): the turn on curve of the dimmer is different to the turn off curve, for small currents ie small fans or pinspots and the dimmer gets stuck between on and off. to solve this parralel the small load with a big load (ie a light) or pair up lots of pinspots

 

induced back voltage: dimmers dim by switching off part way through a sine wave. a transformer works by changing voltage in a priomary coil inducing magnetic fields inducing voltage in a secondary coil. the collapsing magnetic field caused by the dimmer switchng off part way through the cycle induces a voltage back into the primary winding ie back into the dimmer circuit which damages it. to solve this you can use a light in parralel to disipate this back voltage, however on some dimmers this may still cause damage

[Brian]

1) Motors - the other problem with motors is that some of them need to run at their rated speed for cooling. Some of them have fans; other rely on air moving past their various components.

 

2) Transformers - although people think of transformers as inductive that isn't always the case. It depends very much on the type of load.

[dbuckley]

If you use a dimmer rated for the job, then PAR46s with a tranny wont be a problem.

[dmxtothemax]

the par 46s seem happy enough when run on full power, thru the dimmers,

But at reduced power, they get rather hot, hotter then they should be,

 

But the small personel fan,even with a load of lights on the channel, get too hot and burn out

in quick time.

 

So there must be a reason, they are both inductive loads, but the motor seems

to be not able to cope with it, whilst the par 46s cope just!

 

I was thinking maybe the high frequency hash was causing the problems,

But why do the par 46s see to handle it better than the motor does?

 

 

If you use a dimmer rated for the job, then PAR46s with a tranny wont be a problem.