Minding your PSCs

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Knowing the right tests can eliminate confusion in maintaining Permanent Split Capacitors

By George Lanthier


In this article, I’m going to discuss the Permanent Split Capacitor, or PSC motor. About five years ago, these motors hit the United States oil-burner industry and have taken off like no device I’ve ever seen. What’s funny to me as a teacher and industry history buff is that everyone thinks that it’s all new technology. Boy, are they ever wrong on that one.

A person who services warm-air systems and does air-conditioning service knows what I’m saying is right. Commercial burner people will also feel this way. A few who have found out ‘the secret” about those small, water-lubricated circulators know I’m right. So, why is it everyone thinks it’s all new?

Well, basically it’s because the split-phase motor ruled the roost for a long time and most thought that was the only show in town. But consider this: one of the first oil burners ever produced was a Timken BL rotary burner and it had a PSC motor in 1912. Regarding gun-type burners, the first one I can find was the Williams Oil-O-Matic model Fifty-Ten that came out in 1948, and although it was a low-pressure burner, it was still a gun-type burner.

In recent history, Riello reintroduced the PSC in 1975 with its MEC Series and R.W. Beckett has had it on their AFII since 1989, so it just isn’t new. Right now, Carlin has one of the broadest lines of these motors, which includes a replacement to fit all of those 1725-rpm burners that are regretfully still out there.

So, I’m going to take you for another visit to my friends at B&B Mechanical and see what the guys are up to. It’s a Sunday afternoon and Bubba is on call.

‘Boy, am I glad I caught you at home,” announces Bubba. ‘I’m at the Sickandtired Nursing Home and the Humongous Oil Burner out here has a motor problem. I’m pretty sure that it’s dead, but I don’t know if we have one back at the shop.”

‘I’m almost sure that we don’t have one for that burner,” says Bruce, ‘but let me come out and give you a hand and check it out. If we do need one, I might be able to borrow one from our pals at Friendly Oil. I’ll be right over.”

The 3/4-horsepower motor in question is on a low-high-low burner firing at 17.00 gph, so let’s return to the job as Bruce arrives.

‘So what have you got?” asks Bruce.

‘Well, the motor won’t turn over. I took it off the burner and if I throw the switch and spin the shaft, the motor will start, but it won’t start on its own. The fan was not stuck and the pump is free and it’s not bound up. I’m pretty sure the starting switch is gone,” states Bubba.

Bruce answers Bubba with, ‘You’re right on the button, but you see that little hump on top of the burner motor? There’s a capacitor in there and there’s a better than 50/50 chance that it’s bad. Take the cover off, and I’ll go out to my van and get something I need.”

Bruce goes to his van and comes back with a couple of things, including a sleeve of light bulbs.
‘Hey, Uncle Brucie, what are you going to do with the light bulbs, we got enough light already?” wonders Bubba.

‘Well, we’re going to find out if that capacitor is bad. See, I have this regular pigtail light bulb holder (Figure 1) and I have some alligator clips on the leads. I put the holder leads onto the capacitor leads (Figure 2), screw in a 100-watt light bulb, turn on the power and see what happens,” explains Bruce.

‘What’s supposed to happen when you turn the power back on?” asks Bubba.

‘If the light bulb lights and the motor starts, we know for sure we have a bad capacitor. If the light bulb lights and the motor doesn’t start, we have a bad motor. If the light bulb doesn’t light, we either don’t have any power or we have a bad connection or we have a bad light bulb, that’s all there is to it,” answers Bruce. ‘Turn the switch on. OK, see what happened? The light bulb lights, the motor turns and we are in business. Now, we will just leave the light bulb in place and that will get them through the night. I’ll send you back with a new capacitor in the morning. Let’s get out of here, the game starts in 45 minutes,” concludes Bruce.

I have used this trick on just about every kind of capacitor motor, driving just about everything from an oil burner motor, to a furnace blower, air conditioning compressors and condensing fans and, yes, even those little circulator pumps. I originally wrote this back in 1992 and since then I’ve found out even more about these great motors. I also left something out back then and so we’ll bring you up-to-date and I’ll fix an error of omission.

Because of the popularity of these motors in HVAC work, many techs now use an inexpensive capacitor tester to check capacitors (Figure 3). If you want to get crazy and cover all of the bases, you can either carry a capacitor for every job or use a device called a capacitor substitution box (Figure 4). These can get pricey, but they also can be used to dial in the microfarad rating of just about any capacitor on the planet. Although I’ve carried one for many years, they may not be for everybody. For my money, you can’t beat the light bulb, it’s something everybody has and if you want to leave your customer with heat, you have to have a spare capacitor, a light bulb or that capacitor substitution box.

Another way to check out PSC motors is with a V-O-M-type meter that has a capacitor function that allows you to check the capacitor in microfarads like the one shown in Figure 5. I came across these this past summer while teaching some Carlin University classes and was very impressed with the meter and this procedure. So, with many thanks to all at Carlin U and Carlin Combustion Technology, here goes:

In Figure 5, we have a motor that has been opened up and the capacitor has been removed. By the way, and this is very important, do you know that this capacitor is in fact dead? Did you know that a capacitor, if it’s big enough, could carry enough of a charge to knock you out?
The very first thing to do in all capacitor troubleshooting is to expose the capacitor leads and short them to each other. Don’t worry about hurting the capacitor; it does that all the time when it’s working. Now, take out an ohmmeter and check the resistances between the leads as shown in this chart:

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