Help with slowing down and reducing the noise on a shaded pole 230VAC fan motor

Firstly, you are playing with fire if you use this fan in an open environment.
It is only single insulated and no earthing into the bargain.
Shaded pole motor is also a synchronous motor meaning it's speed is dependant on mains frequency.

Would be more sense to use a low voltage fan and a plug pack.

 

A shaded pole motor is not a synchronous motor, it will run slower than a synchronous motor would. The shaded pole motors are very inefficient and so produce a lot of heat. This is not a problem if they are driving a fan but it may be a problem if running slow.

 

Shaded-pole synchronous motors ..... From Wikipedia

https://en.wikipedia.org/wiki/Shaded-pole_synchronous_motor

 

I assume you are describing acoustic noise as the problem as opposed to electrical noise. A fan (because of its motions) can create acoustical noise. The faster the fan goes the more noise it will create. The two main sources of noise would be the blades cutting the air and the friction of the rotor against the bearings. I would suspect that the internal construction of the motor may be the biggest source of noise. An experiment with a good lubricant might determine if there was room for improvement in this area.

 

I have seen many shaded pole motors, non of them synchronous. I doubt if the motor shown could be made to click into synchronism with the large inertia of the fan. Also it does not need to be synchronous.
The book (remember them?) "Alternating current machines" by M.G.Say has a large section on synchronous motors but these are big three phase machines.

If the motor runs cool on 110V then it should be all right to use a resistor or capacitor to drop the voltage. You will need to measure the current taken by the motor to size the resistor. Have you got a lamp limiter to test various resistances? The resistor will get hot so should be of adequate capability

A capacitor has the advantage that it will not dissipate any power but there may be some effects due to resonance. Any capacitor should be of the "motor run" type for reliability and of high voltage rating.

It seems as if the switch changes the taps on the coil, effectively changing the voltage supplied.

 

It is an induction motor and as such is dependent on the Mains frequency, but does not actually run in synchronism with the supply as there has to be a few cycles for the slip frequency to occur.
As mentioned, a shaded pole motor is not very efficient and especially if means such as Triac control is used in order for it to run slower can cause quite a bit of heat.
The starting torque, efficiency and power factor are very low, so these motors are only suitable for low power applications
Using a PSC (permanent start cap) motor can improve performance and allow cooler running with speed control.
M.

 

I seem to recall, from a power electronics course I once took (and passed) sometime in the 1970s that induction motors work on the principle of a spatially rotating magnetic field, produced by excitation of poles on the stator, interacting with the poles of a magnetic field in the rotor. From there it gets a little complicated, what with computing magnetic flux paths, induced voltages and currents, phasors, torque and whatnot. I think it was the whatnot that made me realize I should stick with integrated circuit electronics and leave motor design to more capable hands.

However, one takeaway I got from the course was a graphical explanation of a rotating magnetic field, presented to me by my instructor, a really old codger who had in a previous life, before becoming a university professor, practiced electrical power engineering. This guy really "knew" motors... all kinds of motors... but his math skills appeared to me to be a little rusty. So, a lot of his teaching involved graphics instead of mathics (if that is even a word). This was quite a contrast to my other EE courses that emphasized using calculus, differential equations and Laplace transforms for circuit analysis. I was taking these course on a part-time basis, over a period of ten years, so it was quite refreshing to find a course that didn't expect me to acquire and use additional math skills..

One day my instructor made me sit down with pencil and paper and plot three sine waves, one above the other, with identical frequency and amplitude but separated in time from each other by one hundred twenty degrees. Then he told me to add them together, using a pair of dividers to add and subtract amplitudes algebraically at arbitrary increments in phase from zero to three hundred sixty. The result was astounding. If you consider the three sine waves to represent the magnetic fields generated by three sets of stator windings whose poles are equally spaced around the circumference of the motor the result is... a constant amplitude magnetic field that rotates in space!

Motors and generators using commutators were extremely successful commercially by the time Tesla (and others) discovered the principle of using the rotating magnetic field to create an induction motor. The construction of electrically-excited stator magnetic poles and rotating armatures was an established part of the budding field of electrical engineering. All anyone had to do to make a working induction motor that did not require a mechanical commutator was to change how the stator poles were excited, and use the transformer principle (also newly discovered at the time) to excite the rotor from the alternating current in the stator windings. It didn't take long (as far as historical things go) for engineers to accept and build on the concept.

From the beginning, induction motors had one wee bit of a problem: the speed of the rotating magnetic field depended on the frequency of the alternating current excitation. That meant that, unlike DC motors, the speed of an AC motor was not easily varied. Another problem that occurred immediately was the general non-availability of three-phase power from public utilities. It costs a lot more money to distribute low-voltage three-phase power than it does to distribute low-voltage single-phase power. Three "pole pig" transformers, instead of just one, for every ten or so houses on a street meant that homes did not receive three-phase power. Only factories, with large motor horsepower requirements, could afford the "new" three-phase power distribution. Still, there was (and is) a huge consumer market for fractional horsepower electric motors. Attempts to satisfy this market led to a range of design concepts, all efforts to create a rotating magnetic field from a single-phase alternating current source.

The constant speed problem has never been efficiently addressed with mass-produced fractional horsepower motors. Spatially rotating magnetic fields are simple to produce with three-phase power sources, but rotating magnetic fields can also be produced from two-phase power sources. The magnetic fields from a two-phase source do rotate in space, but they also vary in amplitude and this produces unwanted variations in motor torque. Not a big problem with fractional horsepower motors used for fans: the inertia of the fan "smooths" out the torque variations.

There are only a few simple ways to generate a two-phase power source, with the requisite difference in phase to create a rotating magnetic field, from a single-phase source. For fan motors, the shaded pole technique works well enough, although it is horribly inefficient. Capacitors can be used to provide a phase shift necessary to generate two-phase power, and hence a rotating magnetic field. They are typically found in larger motor applications, up to about one horsepower, for such things as refrigerator compressors, water pumps, and saws. Sometimes two capacitors are needed: one to provide starting torque and another to provide running torque with a centrifugal switch "cutting out" the starter capacitor when operating speed is reached. However, none of these techniques addresses the speed control problem.

There is a short thread on Stack Exchange that addresses speed control of shaded-pole motors only. And here is a link with some "simple" math that explains how shaded-pole motors work. Your best bet might be to experiment with an ordinary wall-mounted "light dimmer" to see if it will provide a satisfactory range of control.

Or maybe turn instead to modern 21st Century technology. It appears we have come full circle and are back to DC motors, but without mechanical commutators. Electronics to the rescue! Pulse-width modulation! Variable frequency excitation! Hall-effect and current sensors for electrical commutation with MOSFETs and IGFETs and all manner of other semiconductor devices! It is a Brave New World for the current generation of engineers!

For a "solution" to your problem, I would recommend that you try powering the fan from a cheap wall-mounted light dimmer first because it could get very expensive trying to lower the voltage sufficiently, while still maintaining enough torque to turn the fan blades, using non-polarized AC motor capacitors. Those puppies are expensive!

 

The capacitor should be installed in series with the supply. The switch will still affect the speed but may work in reverse since the motor will be fed with a current source rather than a voltage source.
A 400V capacitor may well work but I would go to 600V or even 1000V. Try 1μF and then add another in parallel with it if the fan is too slow. Motor run capacitors are quite large but would be the best type, you would not want to buy a selection to find the correct value.

 

These motors generally have 3 windings in series, the switch just switches power to each 3 in turn for the different speeds.
M.

 

That is right.

If the motor takes very little current, then the capacitor will drop very little voltage. If the motor takes a lot of current, then the capacitance will drop a lot of voltage. You have no alternative to trying a capacitor and seeing its effect. Previously I suggested 1μF, this was just a guess. Motor run capacitors are used with bigger motors and are often about 4μF.
The capacitor should be capable of high voltage and be non-polarised. Motor run capacitors are designed for this purpose.

 

No, not a resistor. It functions like a very fast switch that is turned on and off 120 times per second, on then off once each half-cycle of the AC line voltage, repeating during the next half-cycle and every cycle thereafter.

A light dimmer is based on a semiconductor device that behaves like an electrically-controlled on/off switch, either a silicon-controlled-rectifier (SCR) or the bi-polar, bi-directional, version called a triac. Years ago, before triacs became commercially available at affordable prices, I responded to a challenge my father made: instead of me spending all my time "tinkering" with electronics, why don't I build something useful? I was in my late teens and had been "tinkering" with electrical and electronic "stuff" since about age five or six, following the lead of my grandfather who had retired as an electrician in the coal mines of West (by God) Virginia. He brought a lot of motors, selenium rectifiers and other fancy "stuff" home when he retired. For a few years he was my first mentor.

Dad couldn't see any benefit from my "tinkering" since all he had learned about electricity was the practical business of how to safely wire up a house... which he taught me how to do. Anyway, I decided to use an SCR to make a light dimmer, mainly because an SCR was all I could afford to buy.

As mentioned above, an SCR behaves like an electrically controlled on/off switch, but with one "strange" property: once it is turned "on" it stays on until the current through it drops below a certain "holding current" level. The SCR, as the name implies, is also a rectifier: it conducts current in only one direction. A triac, on the other hand, also behaves like an on/off switch but it conducts in both directions. And like the SCR, once a triac is turned on it stays on until the current drops below a certain "holding current" level.

Either device, SCR or triac, can be used to construct a light dimmer by controlling exactly when they turn on during each cycle of the alternating current line voltage. Since the AC line goes through zero voltage twice each cycle, the current also goes through zero twice each cycle, effectively commutating (turning off) both an SCR or a TRIAC.

There is one common way to use either of these devices as light dimmers: control when, during an AC line cycle, that the device turns on. It then conducts until the next zero-crossing of the AC line cycle, whereupon the circuit waits a little bit and then turns the device on again. Wash, rinse, and repeat. The resulting AC current waveform looks like a sine wave, but with the front end cut off of each positive and negative half-cycle. This is called phase-controlled switching. It is very efficient but electrically very noisy because both the SCR and the triac turn on very fast, within microseconds of being electrically triggered to do so.

The AC line voltage has already begun to increase from zero when triggering occurs, so there is a large current transient each time the SCR or triac turns on. This is somewhat reduced when the "wait" period is extended past the peak value of the AC line to almost the next zero-crossing of the AC line voltage. When the line voltage is past its peak value and beginning to decrease, it is in this conduction realm... past the peak but before the next zero-crossing... that most of the dimming occurs. Thus the AC current is interrupted one hundred twenty times per second (in North America) or one hundred times per second (in some other countries) and stays off for a variable period of time. This is a form of pulse-width modulation (PWM) and has been used since nearly the beginning of the 20th Century with mercury-vapor rectifiers and thyratrons instead of SCRs and triacs.

To get an SCR to work as an AC lamp dimmer requires full-wave rectification of the AC line voltage applied to the SCR. The easiest way to do this is to connect the SCR anode to the positive terminal and the SCR cathode to the negative terminal of a four-diode bridge rectifier. The AC line and the AC load are then wired in series with the AC input to the four-diode bridge rectifier. This allows the SCR to act as an on/off switch in series with two diodes during each half-cycle of the AC line voltage. The disadvantage of such a circuit is the three-diode voltage drop across two of the diodes and the SCR when the SCR is turned on. Plus it is more complicated and expensive (today) than just a triac circuit with identical purpose.

So, with some considerable effort on my part, I built an SCR light dimmer and gifted it to my father. He seemed suitably impressed, or at least satisfied, that I wasn't totally wasting my time "tinkering" with electronic "stuff". He never complained to me about that afterward.

A few weeks after my SCR light dimmer project was completed, commercial versions started appearing, compactly constructed to replace a wall-mounted toggle switch for dimming lights in an entire room. They were not inexpensively priced at first, but over the years the price kept getting lower and lower until finally (fifty years later) I purchased one to dim the eight 60W globe lights on either side of the bathroom vanity mirror of my house. Six hundred watts of dimming capacity for around ten bux, IIRC. So, this is why I suggested trying this "solution" first. It's very inexpensive to purchase a lamp dimmer, and if it works you are done. If it doesn't work, you aren't out a lot of money, and you may find another use for the lamp dimmer.

One reason a dimmer it might NOT work is the inductive reactance of the motor winding introducing a phase lag between the AC line voltage and the AC line current. This may cause the zero-crossing of the current waveform to be shifted in phase with respect to the voltage across the device, causing continued conduction even in the absence of a triggering waveform. Only practical way to find out is to try it and see.

 

Another common use of SCR's is in some of the Treadmill controllers to control a DC brushed motor with a SCR bridge connected across the incoming AC supply.
M.

 

The polarity won't matter on the cap.
M.

 

More, accurately, the capacitor MUST be non-polarized. Do not try to use a polarized electrolytic capacitor.

Bob

 

He mentions using a cap from an existing PCM .
If it has dual coloured wires, usually the only reason for this on a non-polarized cap is to indicate the connection (blk) to the outer foil when one conductor is to be connected to earth GND in a different application..
M.