Question for Electronics Experts

[krazer]

I am not 100% sure what you are planning to do, but your comment about using a variac to overvolt the motor, and then hoping to use this to do the same thing makes me worry. The variac is capable of going up to 140v because of the way the windings are tapped (it acts like a bit of a step-up transformer), but this controller will only go up to your normal 120v (ie, when it is set to full power the motor will act as if it were plugged directly into the wall). You could always run it off 220v though

[SaltyRobot]

Some of the VFDs I use in industry can well exceed 60Hz and can often double to 120Hz so that would be twice as fast. I do not have a recommendation for one though because typically we are using 3 phase 480VAC and 220VAC with the motors we use. Although the solution you have now will probably be much cheaper than a VFD.

[kecked]

PWM control is much more reliable. 1/10hp is about 2amps at 115v. You can control this with a single pass transistor. Set the base frequency to 60 or 120 hz and pwm the square wave. This would work better with a dc motor and a cap across the pwm input to the motor. 230w is not much to control. you can even generate the pwn on a pic and use feedback from temp or such to control the pulse width.

PS: you can generate the higher voltage with a voltage doubler

send me email if you want some designs.

[The_Doctor]

Seconded! I stayed out so far because nearly everyone posting knows more than I do about the things they posted about. But the big thing with PWM is that there is no locked-on state implied by the high-power semiconductors used, so the whole timing and power relation can be calculated in software.

Also, less power is lost in the switching devices if you can use MOSFET's, saving losses that otherwise risk burning out an SCR or triac. I won't try to get into the implications for inductive loads though, other than suggesting that watching for zero crossings might give a basis for timing switching events at appropriate times. MOSFETs can handle some brutal currents anyway, if you choose them with low RDSon values.

[buffo]

Um, correct me if I'm wrong, but PWM on an AC circuit requires a triac, not a single bipolar transistor. (Otherwise you won't have any conduction on the negative half of the cycle.) OK, I suppose you could use a pair of transistors in a push-pull arrangement, but at that point it would be simpler to use a triac. Besides, the controller I referenced above is essentially a PWM circuit.

As for over-volting the motor, it's rated for 110/220 operation, so I should be OK there. And yes, I may need to step the voltage up prior to the controller with a small transformer.

Marc - a 1/10th HP motor should be around 75 watts, not 230. (746 watts = 1 horsepower)

As I said, my concern is whether a 75 watt purely inductive load will cause problems with the controller latching in the on state due to the phase shift. But given that the controller is rated for 4KW, I'm thinking it should be OK. Worst case, I'll add a resistive load in parallel with the motor, but I'd prefer to avoid that.

Adam

[The_Doctor]

I made a zero crossing switch for AC using an SCR instead of a triac by fullwave rectifying first. NOTE: The AC load is in series with the bridge rectifier, ONLY the switching circuit sees rectified current... Given that either of those have about two silicon diode drops you could do it with a FET at low RDSon, because rectifier diodes are cheaper than triacs for a similar current, have no worse losses, and if anything burns it is likely a rectifier diode, which protects the rest like a fuse would have. Power to run the switching logic is fed by pulses into a capacitor, aimed at keeping enough steady low-volt supply to the logical stuff.

Where my notion might come unstuck is the upper voltage limit of a MOSFET these days. I can't remember what those limits are.

I don't know who mentioned bipolar transistors but it wasn't me. I wouldn't use one for this.

As far as I know the implications of inductive loads are high current pulses or high voltage pulses greater than the semiconductor's rating, so uprating the switching device should be good enough, for a small inductive load. I think the more formal and correct AC-wave-maths methods are used when the cheap simple ones aren't cheap anymore. (Or when they're not safe, won't last long, or need expensive repeat maintenance).

[buffo]

Hey Doc;

Since the original controller I ordered was actually listed as an SCR and not a triac, I'm wondering if it isn't operating exactly as your design did. Would make a lot of sense that way, and it should also eliminate the problems with sticking on due to phase shift.

It should be here soon, and I'll give it a shot and see how it runs. If it has problems with the inductive load, I'll probably just go back to a variac.

Adam

[The_Doctor]

Interesting, I hadn't thought of what rectifying for the switch might do for the triac/SCR behaviour. My zero-volt thinger was just for small resistive loads..
I could send you a circuit of it in case it offers useful ideas. It might, if you want to watch efficiently for zero crossings for time basing, or to use that pumped-capacitor PSU idea.

The aim of my circuit was to reduce standby consumption to extremes with a very small count of discrete parts. I don't know how well it can be cannibalised for your purposes but you're welcome to try.

[kecked]

I had looked up a 1/10hp motor and calc the EI for power dissipation 2.2A*115V=230W Which is wrong cuz it isn't DC. Wasn't thunking.

Yes a mosfet would work better. What I had in mind was a pic to make the pwm and a plain jane power amp to amplify it and then rectify the pwm with a cap to run a DC motor.

[Genesis]

the best way is to find a second hand "altivar" or variable frequency converter.
because AC moteur can't really be dimmed.
you must rectify the AC line and create a 3 Phases control UVW

this is what i'm using to control my fog fan with DMX. i've found this one for 30 bucks!


http://www.industrial.omron.com.br/u...ser_Manual.pdf

find a one phase model.
you set the motor voltage, max current, frequency, max speed and that's all! a 0-10v voltage will modify the moror speed.
I've connected it on a big fan. the original motor is a one phase but if you open the switch control you have a dephasing cap.
you remove this one and you get the 3 phases to connnect on the variable frequency controller