Question for Electronics Experts

[The_Doctor]

I sent an email anyway with zero crossing switch detail etc, but a second mail stating something worth adding here too, for general info.

A small fan motor is likely a shaded pole induction motor. In which case use a standard lamp dimmer (400W version to get better ratings), or a drill speed controller, which is similar, but will stand to regulate speed of high speed brushed motors too.

If you get this triac-based controller cheap, try it, it will work, or it will burn out. Not much to lose either way, and if it works, it works.

[Genesis]

the zero crossing system (with a good snubber network to protect the triac) will not work for brushless application. or not as much of the variable frequency converter. because it depends of the inertial mass of the rotor. the motor use a rotating magnetic field so if you want to modify the speed you must modify the frequency.
if your application is for a little brushless fan the zero crossing will work to adjust the spedd over 100/50%. but the motor will make more noise, vibrations and heat.

[The_Doctor]

I mentioned the zero crossing circuit only for some of the things it used, like SCR switching on rectified side of a bridge, or the ease of extracting enough DC supply for a logic circuit's supply.

Also, technically the simple diac and triac dimmers use zero crossings as a starting reference for timing (voltage threshold, but that IS how zero crossing detection works, it's just that diacs use higher thresholds). Despite this, a shaded pole induction motor can be controlled with a basic lamp dimmer or drill speed control circuit. (You can control them with a simple resistance in series if power dissipation losses aren't a concern).

Synchronous motors need frequency control but shaded pole motors don't. They rely on the voltage and frequency being fixed for their speed consistency, but they are not synchronous. It's a standard method to use a triac based controller for shaded pole motors up to around 1/3 horsepower. I doubt the Wikipedia reference to it would stand if this was easily proved wrong. This page describes some more detail: http://www.act-remote.com/PCC/kingery02.htm

Protecting a triac with a snubber generously rated is enough, plus a 100R resistor in series with the triac gate. (I corrected a Maplin Electronics design that was destroying Siemens touch dimmer IC's at 6 quid a time, using this resistor to protect it and the triac, because without it both would usually blow up together.)

[Genesis]

this is what I've said the circuit will depends of the type of motor. the shaded pole motor with a resistor in serie like you've said will have less torque (due to the resistor) and the speed will depend of the rotor inertia (mass). If the starting torque is strong sometimes it can't start to spin.this is why the practical test is better in that case. my system is using zero crossing (atmel AVR with zero crossing detector as counter interupt) for light dimming and variable frequency for my motor because the mine (big fan) is not working with a zero crossing circuit. the variable frequency works great and I can adjust between really 0 to 100% with high torque

[shrad]

VFDs are the industrial standard, and now you see thyristor controller banks only for motors of at least 30kW which need high starting currents and more ruggedized hardware (I have several old ones with bussbars inside, which handled 100kW of power inside a volume of 1/4m³... if someone needs big SCRs they can ask )

[The_Doctor]

Shaded pole motors will start with low torque with triac control (and may hum a bit, or warm up a bit more), but if it works, and is acceptable, then it's almost certainly cheaper than getting a VFD controller. It's powering a fan. Unless there is something wrong with it like friction in bearings, or the motor really isn't a shaded pole or other type known to work ok with triacs, it will work. The one thing that really could overrule this as first effort is if the flue has a draught that is strong enough to resist the starting torque.

Btw, what's that 'speed will depend of the rotor inertia (mass)' bit? Acceleration will be reduced by increased inertia, but not final speed Speed would be changed by friction or air resistance in a fan, but not by its mass.

[buffo]

Thanks for all the replies folks. Sorry I'm late to reply, but I was in Portland for the NWLEM over the weekend.

The motor is actually rather heavily loaded. The problem is that the motor is under-powered for the size of the squirrel-cage blower rotor. That's why I'm running the voltage higher; I need more speed than I can get when running it on 110V.

The idea is to step the voltage up to 220 and then knock it down to between 140 and 160 to operate the fan. But I won't ever go below 110. The fan doesn't move enough air at slower speeds to be useful.

I'm not sure exactly what type of motor it is, apart from the fact that I know it's an induction motor, and thus it does not operate at synchronous speed. However, given that I don't remember seeing a start capacitor, I strongly suspect that it's a shaded-pole.

Adam

[CountFunkula]

1/10 HP single-phase AC induction motor with no start capacitor SCREAMS shaded-pole. I would be very surprised if it were anything else.

[The_Doctor]

1/10 HP single-phase AC induction motor with no start capacitor SCREAMS shaded-pole. I would be very surprised if it were anything else.

Agreed. And in this case the simple triac based circuit will almost certainly work because if the speed will always be moderate to high, the low starting torque isn't an issue (especially if you set it high on starting), and the wave will likely never need to be cut short by as much as half its duration. To get the range of drive strength wanted in this one-off situation the wave may still be close enough to complete at lowest drive, that RFI, heating of motor, and humming, will all be minimal. It may be enough to try a basic 400W dimmer, set the pot to the lowest speed wanted, then carefully avoid shifting its rotational position while isolating it and measuring the resistance on both sides of its wiper to find a ratio. You can then create a dead-range with a fixed resistor giving the pot full of the limited range needed based on the ratio of the original divider. This will prevent the dubious operation you want to avoid.

[Genesis]

yes it depends of the final application