5 HP Motor/ Capacitor question

single phase start and run capacitor?

what are the capacitor values?

should be a centrifugal clutch switch somewhere.

Are both start type capacitors?

If both are start type, then they are likely to be parallelled, assuming the voltage is correct.

If one is a start and the other is a run type, then they again should be paralleled, BUT in such a way that when the start switch is only in series with the start cap. The run cap should stay in-circuit.

The use of a run cap usually reduces the running current, and improves the power factor. The size is normally chosen to minimize the current at full load, or close to it.

Both Cap's are start. 125VAC and 413-500 mfd. 20 amps no load is seems pretty high. 22amps when both are connected together.

PWP said:

Both Cap's are start. 125VAC and 413-500 mfd. 20 amps no load is seems pretty high. 22amps when both are connected together.

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125 vac? often the voltage across the start cap exceeds the supply voltage.

try a 40uF at least 370 vac motor run capacitor, and a 600uF start capacitor. there must be a switch somewhere to disconnect the start cap.

I can't promise you this will work but you can remove the PTC resistor off of a fridge compressor and put it in series with the start capacitor. if that works, buy one sized for a 5 hp motor. or use a potential relay, ptc resistors take a few minutes to cool off before you can start the motor again.

usually, with a regular 6 lead motor (8 lead with protector), the start circuit is connected across only ONE of the two primary sections. Therefore it sees a nominal 120V in either 120V or 240V connection. So the 120V nominal capacitor is OK.

The two parts are no doubt used parallel to reduce the current per capacitor to a workable value. I have a motor of that sort, IIRC it is a 3 HP or so motor that came with a machine that I converted to 3 phase.

26A is about right for full load current. I would expect it to pull around 10 to 12 A no load.

If it is pulling a higher current, are you certain you have it wired correctly?

Is the start circuit dropping out? With a clamp-on meter, you should see the current spike up on start, then come down as the motor speeds up, and finally there should be an abrupt drop as the start circuit drops out at something like 70% of full speed..

Since you had it apart, it is possible there is something stopping the start switch from working. A washer in the wrong place, that sort of thing, maybe. Or, possibly a wiring issue.

I am sure that these caps are original and there would no need to change(at this time). Lead #8 going to both cap 1 and 2 is the start winding. Power on, the current on lead #8 is approx 6 amps. Running it drops to 0 amps. Switch is working. I am going to assume that this is a cap start/run motor. The question is, where does the vacant lead on cap 2 connect to. O amps on running show either a cap start or a missing run cap. The connections are correct, like most motors, you reverse rotation swapping leads 5 and 8. However I am confused. If lead 8 goes to 0 amps, why does the no load current to the motor increase when connected to the other cap? I need to reconnect cap 2 and check lead 8 current. However, I need a second pair of eyes.

Why do you think there is a run capacitor needed? With a motor of this size, two start capacitors are fairly normal.

You ask "why does load current increase when the other capacitor is connected?" What exactly do you mean? Load current when starting? Load current when running?

If the capacitor current drops to zero abruptly, then the switch is working. Connecting either one, or two, capacitors in parallel should make no difference when the motor is running. It would only make a difference when starting. If "connecting" it increases current, WHERE did you "connect" it?

If the motor starts with one, does it start slowly? Does it start better with the two connected in parallel? If so, it is likely intended to have two.

If there is supposed to be a run capacitor, there will generally be another wire for it, since it must go to a point that is NOT disconnected by the start switch. But you show no such wire or connection.

How about a picture of the capacitor box? One showing the whole setup and box, and one closer if that will show anything more clearaly

Is 20 amps Normal for no load?. The measurements are while it is running. The caps were connected in parallel with a jumper. The motor starts instantly, no lag. I agree another wire should be present. Only 4 wires per my drawing. This all looks opriginal except for the missing lead on the second cap.

Update. I rewired the power to the motor and checked currents again. No change wit the other cap connected, Still right at 20 amps no load.

That is about double what I would expect. Not correct, especially given the relatively good (0.8) power factor under load, and the 78% efficiency.

So, what did you change to get the non-changing current?

It seems difficult to foul this up, 4 wires (single voltage motor) and all numbered. Possibly the motor has a coil problem. A shorted turn can cause this sort of thing. You might check to see if there is any particular area that gets hotter, although that is hard to do.

With 26A FLA, I would expect no more than 10 to 13 amps unloaded. Is the motor turning at it's rated speed? Does it spin freely?

Obtained motor as wired in diagram. I change nothing. I just have a habit of checking currents out before I go through the trouble of installing. Running as is for 30 minutes motor was warm bu not hot. Crrent never chnaged during this period. I have a clamp on digital meter at work. I will bring it home and check readings. My AMP probe has a few miles on it. Motor spins good. New bearing installed. Original were full of rust. Imagine that sound!

If it runs 30 min and does not stink, it likely does not have a shorted turn or other similar problem.

The 20A, if a real current, is also within the allowable current, so it should not overheat even of that is real. See what a true RMS meter shows. Sometimes averaging type meters can be messed up and not read right.

You also can check, if you have a standard old 100W bulb, measure it's current. Should be about 0.833A. If your meter reads that OK, then it may be correct. See if it looks right on the same range you used for the motor. It will be low on the scale, and may not be very accurate there..

What I think

That's what I think also. Pretty common for a motor that size to have two start caps, if not more.

Bump. I'm hoping to get some answers for a very similar question. I have an American made (Wisconsin) "Doerr" brand 5hp / 29 amp rated single phase motor I'm cleaning up for an air compressor build. It has three large capacitors inside a metal box that is mounted directly on top of the motor. Two of the caps are the exact same, while one is different (but are all Dayton brand).

Unfortunately the labeling on the largest / different one is degraded to a point that I don't know what to look for to replace it.

They are;

Two black (round canister shape) Dayton brand - 1000-1200 MFD // 110/125 VAC // 1A571 // 235-8510-04 (they each have 2 poles + / -)

One silver (rectangular/or oval shaped) Dayton brand - 20 MFD // 370 VAC // 50-60Hz // 4X45QA or 4X450A or AX45QA or AX450A?? (has 2 pole sets for + / - / and gnd)

I don't know the age of the motor but it looks fine and was not too dirty on internal inspection and blowout. The bearings also move smoothly but don't know if they were ever replaced. I assume it's from the 1980's so I was just going to replace the capacitors for now and for good measure and or need a good step-by-step checklist on how to verify if they are ok to leave as is. Regardless, I think I'd feel safer replacing them unless they are overly expensive. I've attached a diagram of how they are wired. I guess my main question is does this look correct?

Good luck with your motor. Mine lost it's magic smoke and has been sold for scrap. I did just put together a rusted 7.5 HP that was under water for some time. I have it running but no application so far. Waiting for another large compressor. It required all new caps and bearings.

TheGreyGoose said:

Bump. I'm hoping to get some answers for a very similar question. I have an American made (Wisconsin) "Doerr" brand 5hp / 29 amp rated single phase motor I'm cleaning up for an air compressor build. It has three large capacitors inside a metal box that is mounted directly on top of the motor. Two of the caps are the exact same, while one is different (but are all Dayton brand).

Unfortunately the labeling on the largest / different one is degraded to a point that I don't know what to look for to replace it.

They are;

Two black (round canister shape) Dayton brand - 1000-1200 MFD // 110/125 VAC // 1A571 // 235-8510-04 (they each have 2 poles + / -)

One silver (rectangular/or oval shaped) Dayton brand - 20 MFD // 370 VAC // 50-60Hz // 4X45QA or 4X450A or AX45QA or AX450A?? (has 2 pole sets for + / - / and gnd)

Click to expand...

The numbers for the 20uf capacitor are correct, and if that capacitor is good there is no reason to replace it. As for the start capacitors they dry out, but if the motor has no trouble starting the air compressor then there is also little reason to replace them.. but this also depends on how often they will be used.


If you are certain those start capacitors are wired in series, then you can replace them with a 250-300uF 220v start capacitor.

johansen said:

If you are certain those start capacitors are wired in series, then you can replace them with a 250-300uF 220v start capacitor.

Click to expand...

My motor diagram is exactly how everything was wired up when I found it. The 5hp Doerr is part of an old Saylor-Beall 705 compressor that I bought for $300 but when I went to pick it up they had already disconnected everything from the service panel so I had no way of testing it. I don't have doubts that it all works (-"ok"-) but upon motor inspection one connector on one of the two start caps looked really fried so I just decided to replace all three caps and the the two motor bearings (even though they seem ok)..all for piece of mind since this is going in my home garage shop which is attached to the house so I'm just wanting to be as safe as I can. And again, even if they are all ok, they are still likely very old. I figured to replace the 2 bearings and 3 caps for ~$70 total is really not bad.

I sourced all three caps, American made, for ~$15 each. I got an AmRad (20uf for run cap) and two BMI brand 1000-1200uf for the two start caps. (The AmRad off eBay and BMI's off Amazon) I assume since they are in series as shown is why they are both rated at 110/125 VAC? Lastly, I'm glad you pointed out I could switch to a single 220v start cap as I was definitely wondering about that but is there any particular reason to do so aside from just simplifying the connections? Is one method particularly better than the other?

I did a mod on my Horror Fright 170amp welder last year by adding in a large cap and recall some discussions of how using a few smaller ones in series was somehow better or may have also been due to just physical space constraints. The cap I have for my welder is about the size of a 12oz beer can lol. I think using two 6oz would have been better! (space-wize at least)

maybe it would help.... i have a run capacitor ( I AM DYING TO KNOW THE VALUE OF IF ANYONE CAN HELP) and a pair of starting capacitors, heres how they were connected.... realize it is not the same motor model, but it may have relative information. and unfortunately, some of the pictures are too big to upload