some asked for some advice and directed me here to deposit it on ya'll
heres some nuggets of fan motor wisdom for you:
The speed of an a.c. induction motor is inherently determined by the
number of poles wound on the stator and the
frequency of the applied power. These motors do not lend themselves to the sort of speed control that you desire. Most people do not realize how severe a duty that air-handling is for a motor. They are designed so as to be operating at full-tilt, so to speak. You won't find a fan motor that is only loaded at 2/3 or 3/4 of its load capacity. (unless it is dual spped motor with two separate inputs for hi/lo speed operation)
This has
always been an inherent problem with controlling the speed of an a/c motor. Due to the nature of a/c power itself, it is. On a d/c motor controlling the speed is as simple as limiting the available current to the motor, the less power, the less speed.
However with a/c it works differently.
'frequency of the applied power' is in italics for a reason... For almost 75 years the only way to control the speed of a motor was by limiting the amount of current applied to the motor. Early electricians found out the hard way about motor growl, because back in those days, big a/c motors were all capacitor starts. If you tried to limit the current to a shaded pole/capacitor start motor, the centrifugal switch on the capacitor would short out and burn. no good. So they simply started building motors with reduction gears to control the speed. And thats the way it was for 50 years untill the invention of the wound induction motor, aka squirrell cage. this is the motor most of us are familar with in our fans. A squirrel cage motor is different from a shaded pole motor in that it does not require a capacitor to start, so now, there was no longer a danger of catastrophic equipment failure. Solid state variable speed controls hit the market. and the motors still growled... but why??
Because an induction motor
needs straight line voltage to operate. think of the name...
induction motor
During the a/c power cycle, voltage starts at 0v, rises to 177v positive polarity, reverses direction, drops to 177v negative polarity. 60 times a second. 60hz. thats what the 60hz means on all your equipment in the us. europe likes 50hz cycle. during that cycle, there are two moments in time when the voltage is @ absolute zero. the period of time that this state occurs is almost neglible, 1/120th of a second, to be exact. During that time, there is also
zero resistance across the the motor to electrical current. electrical current loves zero resistance! because of this zero resistance ,
a massive amount of inrush current is inducted on the motor windings...It is this principle of induction that allows an a/c motor to maintain its speed during operation, and also supplies the large amount of current needed to accelerate a motor to its operating RPM from 0.
Now back to solid state motor controls... like the ones your having problems with. A solid state motor speed controller is a
current limiting device. It limits the amount of power (in this case amperage) that is allowed to be applied to the motor. By doing this, you severly impede the amount of inrush current available to the motor poles.... its like putting bad gas in a car... yeah, it'll run, but it runs like shit and its loud and it wont last very long.
So how do I control the speed of an a/c motor without making it growl?
Thank god for technology..... back in the mid to late 80's, with the advent of printed circuit boards, surface mount components, and digital memory chips, a new product hit the market.
Its called a VFD, or Variable Frequency Drive. A vfd operates on a very different principle than a motor speed controller. Instead of limiting the current, instead it
altars the frequency of the line voltage. Say your motor is running to fast, you want to slow it down. The VFD simply slows the cycle of the a/c current
without limiting the amount of amperage in the circuit. If your motor is too fast @ 60hz, you simply adjust the VFD to run @ 45hz, or 9 hz, or 1.8 hz, whatever works for you. Some VFD's can even accelerate a motor, tho this is not always a wise thing to do...lol.
So in a nutshell, having said all that, to stop your fan motors from growling, you need to get a VFD.
Shitty side to that is a VFD is expensive, imagine that, and must be sized to your motor accordingly. Good thing for you, i happen to know that the size rating for a fan motor is going to be NEMA SIZE 00....... unless your fan is turbine powered, lol. and trust me your fan is NOT turbine powered
so google a VFD and see what you come up with. the smaller ones usually start @ about 120USD.