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 Post subject: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 4:43 am 
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Our beloved room HEPA filter motor won't run. It's not seized. It's well lubricated and the squirrel cage rotates freely.

I measured the 4.5 uF cap in the photo to be 2.38 uF. Two questions:

Is that a start cap or a run cap?
How far off can the uF be before the motor won't work?

THANKS for your gift of knowledge.

(BTW, before anyone suggests it would be cheaper just to buy a new HEPA, just know I stocked up on ten years of filters that will fit only this unit.)


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 5:12 am 
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I don't know which cap that is but they're cheap, so replace both.

As a general comment...
RUN caps are generally of decent quality since they're in service all the time.
START caps are the cheapest things that can be called caps since they have a low duty cycle.

Replace both with decent quality parts and don't worry about it.

- Leigh

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 5:26 am 
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to be a start cap there must be some way to switch it out of the circuit. Either a centrifugal switch or a relay. I would suspect the switch has failed before i would suspect the cap. there is also a Permanent Split Capacitor motor type that is becoming more popular. These capacitors are always in circuit and at lower capacity will cause issues at startup. If the capacitor isn't labeled as Start or Run, I would suspect the motor is PSC type.


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 5:36 am 
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That motor is too small for a start cap, so it is a run cap.

Start+run caps are used on motors over about 1/2HP.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 5:57 am 
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There is also a thermal fuse inside against the the motor winding's that can pop and keep it from running.

If the motor turns out to be bad I would think you cold find a new motor online.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 6:43 am 
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All good info, thanks. I bridged the cap with a 2 uF 250 volt film cap to get it well within 10% of design value and the motor is still dead as a doornail. Now I guess I have to tear into the thing. The film type panel has me press the same pad 3 times for low-medium-high with beep and LED indicators still functioning. So it thinks it's working. I need to see if I can apply power directly to the motor unless someone has a better idea.


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 1:32 pm 
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I posted this before, maybe you missed it!

Run and Start Capacitors. What they do?
viewtopic.php?p=2892739#p2892739

Greg.


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 5:22 pm 
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Measure the windings to be sure there is still continuity. If a one-time thermal cutout is involve a winding will be open. If good and "new" cap motor should run directly. If it is a multiple speed winding there "may" be a color code for the windings. Coils unforgiving if powered across one of the "speed" windings. Possible controller has failed too.

I use a HEPA in my bedroom, runs all night, too lazy to bend down and futz with rotary control so it is set, plugged in/out of outlet in easy reach...

I have been cleaning the BIG round HEPA filter for many years, never, replaced. Take it outdoors, soap it up, hose it off, air dry, done for another year...

If a no-name motor try FASCO for direct replacement, may have to cut shaft or deal with twin shaft. Opt for a motor that specifies for vertical use if possible, has a low friction thrust bearing. Shop online once the correct model number is found prices vary WIDELY :shock:

Last FASCO I bought for my heating system, from an ebay outfit but great reputation. Seems FASCO built a defective motor, windings would not start. Sent another one in 3 days, FREE...

I could not wait. Made up a motor from my windings and new rotor and end plates from defective one. Now have a spare motor...

Chas

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Wed 09, 2019 6:02 pm 
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This looks to be an induction type of motor if it's a single speed .you should see 3 or 4 wires going to the motor.
Check for continuity between the wires. Since you have already tried replacing the cap, it look as if one of the winding may have failed. One winding will be connected to the power directly, and the other via the cap in order to provide a phase shift of 90 degs.
Don't apply power to the winding's directly unless you know which one is which.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Fri 11, 2019 6:08 am 
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This is all good. The control board is burned, so I guessed the motor was still good. I studied up on PSC motors and found this diagram that seems to correlate with mine. I got the motor to run on the three speeds by bypassing the control board and connecting the motor windings to 120 VAC (using variac!) as shown in the diagram. There are six bundled wires going into the motor, green, red, yellow, white, gray and black. White and gray come from the capacitor. The only thing I can't figure out is that the black (not shown) is connected to the AC line but appears not to be connected to anything in the motor. There's no continuity to the black from any of the other wires.

Note that the selector switch in the diagram is drawn as manual switch. The board uses variacs, which I tested as good. However, they didn't work when I drove the variac gate resistors with six volts (that's a guess, but the ribbon cable shows a 5 volt source.) I'll deal with that later after I try to figure out what burned.

Mainly now I need help understanding what that black wire connected to AC is for and where it goes inside the motor. THANKS.


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Fri 11, 2019 6:50 am 
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I was starting that the black wire could be connected to the case of the motor. But then it would be connected to the safety ground, not the AC line.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Fri 11, 2019 5:21 pm 
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Tom Schulz wrote:
I was starting that the black wire could be connected to the case of the motor. But then it would be connected to the safety ground, not the AC line.

Indeed. I hoped that would be the case (pun intended!). But it's not. Even though the black wire has no apparent continuity with anything else, with it disconnected from the line, I can measure on it various AC voltages on it when probing other parts of the motor. For example, it's about 5 volts from black wire to case, 12 -24 volts to the various colored wires, and 50 volts to the gray wire. Where is that voltage coming from?

I don't want to tear the motor apart while I know it's working. Right now I can install a four position switch and bypass the electronic one. But if I disassemble the motor there may be unintended consequences. Is the only way to check that black wire is to tear the motor apart?


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Fri 11, 2019 7:03 pm 
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Macrohenry wrote:
Where is that voltage coming from?

I don't want to tear the motor apart while I know it's working.

It could be stray capacitive coupling. Any two conductors form a capacitor. So the black wire will form a small capacitor with all the other wires and possibly the windings. A small capacitor but big enough for your meter to read a voltage. I would hope that there is no intentional capacitor built into the motor.

Can you measure the current drawn by the motor? If you can, you could monitor the current while connecting and disconnecting the black wire and see if anything happens. In any case, if the motor does not overheat while running then everything is probably OK.

If the motor is held together by screws you can most likely safely take it apart. If you would have to bend metal tabs then that is a different story.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Fri 11, 2019 9:17 pm 
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The black wire goes to the neutral connection. The filter uses a polarized plug, so would that be some sort of intent to ground the motor? Given that it's not connected to the motor casing, the black wire would have to be connected to something internal to the motor. I don't see much benefit to grounding it there.


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Fri 11, 2019 11:09 pm 
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Start capacitors aren't poor quality because they have a low duty cycle.
In a fractional horsepower single phase motor, the way torque for starting
is obtained is by making the start winding a higher resistance than the
run winding.

The start torque is a function of the phase difference between the start
and run winding. This design makes the start winding have a low duty cycle.

A capacitor can provide this phase differential. Size makes an electrolytic
capacitor practical to mount on the motor. Some capacitors are hundreds
of micro farads, non polarized and carry high amperages.

The way some HVAC technicians test the capacitors is to connect them
across a 120 volt AC 60 Hz line and measure the current with a clamp type
ammeter.

22.5 uFd per amp is the number I use.

Not a recommended method for those who don't work in the trade.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Sun 13, 2019 2:40 am 
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After drawing out the whole circuit, the stumbling block kept coming back to that mystery black wire going into the motor. I took apart the motor today to find a blown thermal fuse connecting the black wire to one of the capacitor wires. That's the same cap terminal I used to connect the AC Line to, so that makes sense. I can get a new fuse for $3.95.

I tested the motor by temporarily shorting the fuse, and all works as normal, although I don't know if it's normal for the Zener resistor to get so hot.

In the meantime I've developed some questions. The power supply that drives the triacs and control panel doesn't make sense to me. It uses a 5V Zener diode to put the AC Neutral line at +5V DC potential. Why on earth (pun intended) would they do that?

Also the 330 Ohm resistor gets too hot to touch, even burned the board. What's that about? The 5 volts is used only to drive the touch pad control panel and send power to 1/4 watt 330 Ohm resistors that drive the triacs that connect the motor windings to neutral.

Below is the power supply portion. How does it work? Why does that 2 watt 330 Ohm resistor get so hot?

Why do those two 0.155 caps in parallel read 1.42 uF? What are they for?


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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Sun 13, 2019 12:13 pm 
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The capacitors in parallel drop the 120 volt AC voltage through ~ 1900
ohms of reactance which causes about 65 mA of bygoshandbygum current
through the 330 ohm resistor making it throw off about a watt and a half.

If you opened the motor and replaced the thermal fuse, throw the motor
out and buy a new one.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Sun 13, 2019 7:25 pm 
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The motor could have been running hot because that start/run cap is low in value. I would put in a new thermal fuse and then run the motor and see how hot it gets. And then change the cap and see if the motor runs cooler.

For that resistor, a resistor running anywhere near It's rating runs quite hot. Manufactures will often mount such a resistor so that it stands 1/4 of an inch away from the board.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Sun 13, 2019 9:48 pm 
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radiotechnician wrote:
The capacitors in parallel drop the 120 volt AC voltage through ~ 1900
ohms of reactance which causes about 65 mA of bygoshandbygum current
through the 330 ohm resistor making it throw off about a watt and a half.

If you opened the motor and replaced the thermal fuse, throw the motor
out and buy a new one.

Excellent info. Before I took the motor out, I removed the squirrel cage and noted it took the motor shaft 1.3 seconds to spin down from low speed. Before reassembling the motor, I polished the scored shaft with 1000 and 1500 grit, then crocus cloth. I cleaned the bronze bearings and relubricated them with turbine oil. Now the motor takes 4+ seconds to spin down. Mounted horizontally as intended It draws .32 A on low speed, .47 A with the shaft vertical. So it loads itself when it's rubbing against the bearing.

I've already ordered the thermal cutoffs. Since I've refurbished the motor and can't get a replacement from Hunter, I'd have to get a box fan motor (which are wired very much the same). Is that preferable to seeing how long this one will run?


Last edited by Macrohenry on Jan Mon 14, 2019 12:33 am, edited 1 time in total.

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 Post subject: Re: Start capacitor vs run capacitor
PostPosted: Jan Sun 13, 2019 10:11 pm 
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Tom Schulz wrote:
The motor could have been running hot because that start/run cap is low in value. I would put in a new thermal fuse and then run the motor and see how hot it gets. And then change the cap and see if the motor runs cooler.

The existing cap reads 4.5 uF, did measure 2.38 uF, now measures 1.92 uF. Motor runs at .32 A on low. When I parallel a 2 uF cap to get 3.85 uF, motor runs at .48 A, same speed. So I expect it's just creating heat. It's perplexing that the cap value is less than half the rated value, yet the motor runs at almost half again the current.

Tom Schulz wrote:
For that resistor, a resistor running anywhere near It's rating runs quite hot. Manufactures will often mount such a resistor so that it stands 1/4 of an inch away from the board.

This resistor is on the board board and has burned it. The resistor's markings are burned off, but the value is good according to the silk screened spec. There's room to mount a 5 watt resistor. Or I could heat sink this one and move it off the board where it burned it. If this really is a a design flaw, Hunter should know better. Planned obsolescence using something that gets hot shouldn't be allowed.


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