Going for a dim spin
In this article, Harry Simidis, looks at how dimmers and motors work and then puts the two together to discuss the result.
The humble lighting dimmer is probably one of man’s more ingenious inventions – it alone forms the foundation of modern day lighting control systems.
But, as useful and purposeful as these units are, they are sometimes used inappropriately with serious repercussions. I refer to the alarming misconception that it’s ok to use any dimmer for the regulation of ceiling fan speed.
Before we dive into any sort of explanation, let’s recap on how a basic leading edge dimmer works. These units are typically comprised of a semi-conductor called a triode for alternating current (TRIAC), driven by a sophisticated triggering circuit. Imagine the TRIAC is like a switch that’s triggered by a positive or negative voltage. When this voltage is applied it switches on and allows current through. It switches off by itself when this current falls below a certain threshold. The firing circuit is where sophisticated electronics reside and is what separates the really good dimmers with longer ‘rise times’ from those that switch the TRIAC abruptly on and off. Longer rise times preserve lamp life and make for a much quieter dimmer.
Figure 1 is a typical oscilloscope view of the ‘chopped’ sine wave allowed through a leading edge dimmer, to the load. The reason we see the light dim is because the dimmer ‘chops’ some of the raw AC power going through to the lamp or transformer. The lamp still switches on and off 100 times every second, but our eyes cannot discern this activity and as such the light appears uniformly dim.
It’s also important to note that dimmers, like the ones described above, were originally designed for resistive loads, such as the now almost-extinct incandescent lamp. They can also work with magnetic and compatible electronic transformers for lamps, such as halogen downlights, among many others. However, as each manufacturer has their own performance standards, it’s always a good idea to ensure that the lamp and transformer you want to use in the project has been tested and approved for use by the manufacturer of the lighting control system you intend going with. Even if you decide to use a standalone wall switch rotary dimmer, it’s still necessary that you ascertain compatibility with the chosen lamp.
Okay, so now that we better understand how a dimmer works, let’s take a quick look at how a ceiling fan works. Permanent split capacitor-type AC motors are usually found in ceiling fans due to their low cost of manufacture, comparatively low energy use (and hence low torque) and compact size. As you may recall from basic AC motor theory (many, many, many moons ago – for me, at least!!) asynchronous motor design consists of a magnetic field set-up in the stator (the stationary part of the motor). An AC current is passed through the stator windings, which induces a current in the wire wound rotor (the moving part of the motor).
Thanks to some very smart dudes named Michael Faraday and Heinrich Lenz, we can explain what happens to the rotor, when the stator generates this changing magnetic field. The rotor experiences a turning force, or torque, to align itself with this moving field. This is because the induced current generated in the rotor will always try to oppose the change in magnetic flux that produces it (see Figure 2). This ‘reverse’ voltage is called Back EMF (electromotive force) and is what limits the rotor from spinning out of control. In fact, for asynchronous motors the rotor must always spin slightly slower than the magnetic field in the stator, to ensure that current is induced in the rotor and hence generate torque. This is often referred to as slip.
The speed that an asynchronous AC motor can reach is determined by the supply frequency and the number of poles. The item to the far left of Figure 3 is the stator and is the component of the motor that physically moves the fan blades, with reference to the stationary rotor, cast in the outer fan housing. It can be seen that the rotor has diagonal lines appearing along its surface which is in keeping with the conventional squirrel cage design. Each of these lines is actually a bar or rod embedded in the cast aluminium housing, and forms a coil with its mirror opposite and the top and bottom housing rim rings. The stator in this image appears to have 16 poles and two sets of windings (inner and outer), more than likely a start (or auxiliary) and run winding respectively.
Garden variety ceiling fan motors usually come with the ability to set 3 different speeds from High, Medium, Low to Off. This is often achieved through series capacitors housed in a potted module in what’s termed a ‘capacitor bank’. These module typically house between two to three different capacitors and are connected to the stator windings is accordance to the speed setting chosen.
Figure 4 is the wiring schematic for the ceiling fan motor discussed earlier. Without any capacitance, this motor would never start to turn unless it was manual pushed in a certain direction. This is the reason why the 1µF (micro Farad) capacitor is included in the first ‘low’ speed setting. Capacitance in this largely inductive circuit has the effect of introducing a phase shift between the two windings thus giving way to a moving magnetic field in the stator. As can be seen from the nature of the connections for different speeds, the highest speed setting incorporates the most capacitance as it combines the 1 and 2µF capacitors in series with the auxiliary winding of the motor. Again, this increases the phase difference between the magnetic fields generated by two windings, which has the effect of speeding the motor up. Lowering the capacitance in the auxiliary winding reduces the phase difference and slows the motor down.
Alright, now that we hopefully have a better understanding of the inner workings of both dimmers and ceiling fans, let’s see what happens when a dimmer is used for fan speed control. Firstly, the chopped AC sine wave will likely generate much vibration in the motor stator windings leading to a noticeable and very annoying hum. This would probably result from the TRIAC ‘chops’ on the supply current and the associated ‘shock’ on the stator windings.
What needs to be remembered is that the windings are comprised of coils and coils of low resistance cable which combine to make a large inductor. Inductance has the characteristic of generating reverse energy spikes to counteract any sudden changes in current, as described earlier. Hence, not only will this inevitably create a vibration effect on the windings but also has the capability of adversely affecting the dimmer itself. This is largely because the dimmer is called on to supply larger than designed for loads, frequently.
Another serious concern has to do with the fan motor heating up as a result of being fed a dimmed supply voltage from the dimmer. The supply to the motor would look similar to the one shown in Figure 5. This is no longer a smooth sine wave but rather a series of pulses that could potentially prevent the motor from gaining enough inertia to overcome its stationary state and actually start spinning. This would then cause overheating concerns in the rotor due to large induced currents from the stator windings and no back EMF generated to regulate this.
In general, although there are motors that may be successfully controlled with a dimmer, it’s best to check with the fan and dimmer manufacturers about compatibility before connecting them together.