5 replies to this topic

[stangbat]

How picky is a PSC motor when it comes to the start-run capacitor's value. The motor I have requires a 5 uF capacitor. I have two 3 uF capacitors that can be paralleled to get 6 uF. Will this work? Or should I just buy a 5 uF capacitor? This is a low load/torque situation for my HLT stir motor.I have one of the Molon gear motors that I've been using for my HLT stirrer. It starts and runs without the capacitor and I've been using it without it. But I have since realized that it is a PSC motor and it needs the capacitor while running. No wonder it was running hot. I had attributed it getting hot to the fact that it sat above 160+ degree water for several hours but I now realize this isn't the case. I can be such a bonehead sometimes. So I figure I'd better get a capacitor on it so that I don't burn it up. Surprisingly it hasn't burnt up yet as it is.

[stellarbrew]

Although I'm far from an expert on this, I'm pretty sure that if the PSC motor circuit is designed for a 5 microfarad run capacitor, you should have more favorable run and start phase angles with a 6 microfarad capacitor in place than you had when running without any capacitor. If it was me, I'd give it a try with the 3 microfarad capicitors paralleled, or even with just a single 3 microfarad capacitor, and see if it runs any cooler. I believe (but am not 100% sure) that those capacitors may have as much as 20% variance from their rated capacitance anyway. I know that you would prefer a definitive answer from someone who really knows this stuff, but I figured I'd chime in until that person comes along.

[stangbat]

Although I'm far from an expert on this, I'm pretty sure that if the PSC motor circuit is designed for a 5 microfarad run capacitor, you should have more favorable run and start phase angles with a 6 microfarad capacitor in place than you had when running without any capacitor. If it was me, I'd give it a try with the 3 microfarad capicitors paralleled, or even with just a single 3 microfarad capacitor, and see if it runs any cooler. I believe (but am not 100% sure) that those capacitors may have as much as 20% variance from their rated capacitance anyway. I know that you would prefer a definitive answer from someone who really knows this stuff, but I figured I'd chime in until that person comes along.

Thanks for the reply. I'm going to give it at try with the two 3 uF caps in parallel. It certainly can't be worse than running it with no caps as I was.

[Stout_fan]

... those capacitors may have as much as 20% variance from their rated capacitance anyway. ...

My point, you might be OK with the two threes in parallel.

[stangbat]

Update: Saturday was my first brew session since installing the capacitors on the motor. It got just as hot as it did without the capacitors. I'm sure that having the capacitors installed is better for the motor, but I also think that these motors run hot regardless. They are sealed and non-ventilated, so even under ideal conditions it would be warm compared to other motors. I also think that the stir bar convects some heat up to the motor and some heat also probably goes into the motor from the lid and gearbox. Anyway, it works and hopefully it will continue to work.

[zymot]

As far as the value of the cap, the motor will be very forgiving. Motor starting caps are like +/- 20% tolerance. So two 3 mfd caps in parallel should close enough.Perhaps you know this already, but it is worth mentioning.It has been a few years since I have been behind the electronics parts counter. I recall that motor starting caps are rated XX mfd, YYY Volts AC. If you are using a cap that is rated as XX mfd, YYY Volts DC, that is a "regular" cap and it won't work properly.To be honest I forget what the technical difference was, but you had to replace a motor starting cap with one rated as "AC Volts." And it has nothing to do with polarized/non-polarized electrolytic caps.zymot