Can a defective DRC on a heat pump cause the fan motor to go bad?

[fourthindiana]

I noticed that the outdoor fan on my residential, split-system heat pump stopped working in mid-October. I don't know for sure exactly when my outdoor fan on my heat pump stopped working. It might be that my outdoor fan did not work for months before I noticed that the outdoor fan did not work in mid-October.

My outdoor fan motor on my heat pump is a PSC motor. My outdoor unit of my heat pump has a Dual Run Capacitor (DRC). My DRC is a 40/5 DRC. When I checked the capacitance on my DRC on my outdoor unit several days ago, the capacitance from common to fan was only 743 nanofarads, which is .743 microfarads. I replaced the old, defective DRC with a new 40/5 DRC. The capacitance on my new DRC from common to fan was 5.03 microfarads. When I put a new DRC in my outdoor unit, the outdoor fan worked for about 24 hours. Then, after about 24 hours, the outdoor fan stopped running in heat mode.

I Ohm'ed out the fan motor and found that the resistance from the start wire of the outdoor fan to the common wire of outdoor fan is 2.1 Ohms. Start wire of outdoor fan to run wire of outdoor fan reads OL (infinite resistance) on my multimeter. Run wire of outdoor fan to common wire of outdoor fan is also OL. This proves that the fan motor (or the fan motor's wires, anyway) is defective.

Furthermore, I used an amp clamp to check the amperage draw on the common wire of my outdoor fan motor. The Full Load Amps on the nameplate of my outdoor unit is .77 amps. This means that the LRA should be around 3.85 amps. When I first turned the setting on the thermostat from "Off" to "Heat Mode", the amperage stayed at zero amps for a few minutes. Then, after a few minutes, the amperage on the common wire of the outdoor fan motor went up to 2.7 amps for about a second. Then the amperage gradually dropped down to zero amps over the next 30 seconds or so. Then the amperage just stayed at zero amps for the next 5 minutes or so until I removed the amp clamp from the common wire.

I think that both the resistance readings I got on the C-S-R wires of the fan motor and the amperage draws (or non-existent amperage draws) on the common wire of the outdoor fan motor conclusively prove that the outdoor fan motor is defective.

From the information I have given, could my original, defective Dual Run Capacitor (that only had .743 microfarads) have caused the outdoor fan motor on my heat pump to go bad?

If so, how does a defective Dual Run Capacitor, with insufficient capacitance from common to fan, cause a fan motor to go bad?

 

[russ_watters]

I'm an HVAC guy, not an electrical guy, but I'd certainly bet one failure caused the other, but which caused which I wouldn't know.

Fortunately on my system when the capacitor failed (spectacularly), it didn't damage anything else.

 

[Averagesupernova]

@jim hardy . He is our resident go to guy on motors and such. A diagram or a link to a diagram would be most helpful. It's possible the capacitor could have failed in the short circuit mode at some point. The last outdoor unit I worked on with a bad capacitor tripped an overload relay within seconds, but this was on the compressor, not the fan.

 

[fourthindiana]

This is a photograph of the schematic diagram for this unit.

 

[Tom.G]

From the schematic the Fan motor is a Capacitor Run type, that is, it's a 2 phase motor. Most likely the capacitor failed as Open, as most electrolytic do. (ball-park lifetime roughly 10yrs.) This happens when the liquid electrolyte in them dries up with age and temperature. Without the capacitor there is no rotating magnetic field in the motor and the motor does not turn. This gives the same effect as if the motor shaft was locked so it could not rotate.

• simple version:
  • The motor then draws much current trying turn.
• technical version:
  • Without the rotor turning and generating a back EMF in the field winding, the field current rises to its Locked Rotor value which is several times the normal running value.

This extra current overheats the motor and destroys the insulation on its windings. The windings then short circuit and draw even more current. Eventually the motor gets hot enough to either burn thru a wire or melt some solder to a connection.

Cheers,
Tom

 

[jim hardy]

If so, how does a defective Dual Run Capacitor, with insufficient capacitance from common to fan, cause a fan motor to go bad?

hmmm you'd expect the fan motor to have a thermal overload , but maybe not.

Anyhow
when the start capacitor loses enough capacitance the fan motor will no longer be able to start
because there's not enough current through its start winding .
Current from L1 goes through contactor and DR relay, arrives at fan motor COMMON and splits between the two windings.
RUN winding current exits straight to L2
START winding current has to go through start cap to get to L2
and if cap won't let it out the motor can't start

When that happens the run winding will draw very nearly LRA because the motor can't get up to speed.
So one of two things happens -
EITHER
the motor's internal thermal overload cutout (if there is one) will cycle open-closed-open...etc because the run winding heats up until thermal cutout opens and motor cools back down , and cycle repeats ...
OR
the run winding overheats and burns up.

You can sniff the motor to smell for burnt windings, and maybe look in the vent holes on end to see if they're bright copper or charred ,,
With your readings i'd guess that the run winding is charcoal.

Anyhow - that's how a failed start cap can wreck a motor.
What is resistance reading between each wire and motor frame ?

 

[jim hardy]

Maybe this belongs in DIY thread ?

And i see @Tom.G posted while i was typing
and we agree

old jim

 

[fourthindiana]

hmmm you'd expect the fan motor to have a thermal overload , but maybe not.

Anyhow
when the start capacitor loses enough capacitance the fan motor will no longer be able to start
because there's not enough current through its start winding .

I don't know if the outdoor fan motor has a thermal overload or not. Isn't it possible that the fan motor has an internal thermal overload, but the internal overload eventually cycled so many times that it wore out and failed?

There is no start capacitor in this outdoor fan motor. Remember I said this is a PSC outdoor fan motor. I know that the schematic drawing has both a start capacitor and a start relay. However, you might not be able to see it on the photograph on your computer screen, but there is an asterisk next to both the start capacitor and the start relay. In a part of the sticker not included in the attached photograph of the schematic diagram, it says something to the effect that the units don't come with a start capacitor or a start relay, and it says that they included the start capacitor and the start relay in the schematic diagram to show where they should be if a technician adds a start relay and start capacitor to this unit.

However, I think that much or all of what you say about the start capacitor in this unit might apply to the dual run capacitor.

Current from L1 goes through contactor and DR relay, arrives at fan motor COMMON and splits between the two windings.
RUN winding current exits straight to L2
START winding current has to go through start cap to get to L2
and if cap won't let it out the motor can't start
View attachment 235961

Understood.

When that happens the run winding will draw very nearly LRA because the motor can't get up to speed.
So one of two things happens -
EITHER
the motor's internal thermal overload cutout (if there is one) will cycle open-closed-open...etc because the run winding heats up until thermal cutout opens and motor cools back down , and cycle repeats ...
OR
the run winding overheats and burns up.

You can sniff the motor to smell for burnt windings, and maybe look in the vent holes on end to see if they're bright copper or charred ,,
With your readings i'd guess that the run winding is charcoal.

I am planning on buying a new outdoor fan motor today. I plan on keeping the old, defective outdoor fan motor. I will probably open the old, defective fan motor one day to inspect it and see what it looks like when a motor shorts out.

Anyhow - that's how a failed start cap can wreck a motor.

Also, that's how a failed run capacitor can wreck a PSC motor; correct?

What is resistance reading between each wire and motor frame ?

I am going to take the outdoor fan motor out later today. I will check the resistance between each wire and the motor frame when I take the old fan motor out. I will probably let you know what the resistance reading between each wire and the motor frame is tonight.

 

[russ_watters]

Fortunately on my system when the capacitor failed (spectacularly), it didn't damage anything else.

Wait a minute, then why is there a condenser fan impeller sitting on a shelf in my basement? Did I replace the fan motor too?
[searches memory]
Error: record not found.

 

[Averagesupernova]

Wait a minute, then why is there a condenser fan impeller sitting on a shelf in my basement? Did I replace the fan motor too?
[searches memory]
Error: record not found.

I didn't think you were old enough for those kind of memory lapses.

 

[jim hardy]

I don't know if the outdoor fan motor has a thermal overload or not. Isn't it possible that the fan motor has an internal thermal overload, but the internal overload eventually cycled so many times that it wore out and failed?

Yes that is quite possible and I've seen it happen.

There is no start capacitor in this outdoor fan motor. Remember I said this is a PSC outdoor fan motor. I know that the schematic drawing has both a start capacitor and a start relay.

Yes and that's a big Ten-Mea-Culpa on my part
as much as i beat the drum for preciseness in communication
there i go getting sloppy with terminology.
A PSC motor won't start unless there's current through both windings
but after it's up to speed you could disconnect one winding and it'd continue running
and it's a holdover from my younger days that i still call the two windings "Run" and "Start".
A PSC run capacitor connects the two windings exactly the same as would a start capacitor - just they don't switch it out after start. That saves the cost of a centrifugal switch to do that job making the motor cheaper to build.

So yes i was sloppy in wording..

However, I think that much or all of what you say about the start capacitor in this unit might apply to the dual run capacitor.

Yes it does.
Start capacitors are not rated for continuous use , they are i believe electrolytic construction and can't take the heat.
Run capacitors I've opened were oil filled . There's not much dielectric heating in oil - think of the oil molecules in the dielectric as slippery so they can turn to align their dipoles without much friction.

I will probably open the old, defective fan motor one day to inspect it and see what it looks like when a motor shorts out.

Great ! You learn a lot doing post-mortems. Post a photo ? Pay attention to orientation of the two windings - is one offset from the other or are they aligned ?
They need angular offset between the tw magnetic fields.
The capacitor can provide that by shifting phase of current in the smaller capacitor fed winding
or they can wind it on a different axis

Also, that's how a failed run capacitor can wreck a PSC motor; correct?

Yep.

It's very rewarding for me to see your interest and your hands-on approach.
Thanks for your feedback

Have Fun - the world is chock full of interesting machines and every one of them has something to teach us.
This IS the golden age

old jim

ps

I am going to take the outdoor fan motor out later today.

squirt some penetrating oil around the fan hub - they sit out in the rain and rust in place. I've wrecked blades with the "Get A Bigger Hammer" approach.

 

[jim hardy]

There is no start capacitor in this outdoor fan motor. Remember I said this is a PSC outdoor fan motor. I know that the schematic drawing has both a start capacitor and a start relay. However, you might not be able to see it on the photograph on your computer screen, but there is an asterisk next to both the start capacitor and the start relay. In a part of the sticker not included in the attached photograph of the schematic diagram, it says something to the effect that the units don't come with a start capacitor or a start relay, and it says that they included the start capacitor and the start relay in the schematic diagram to show where they should be if a technician adds a start relay and start capacitor to this unit.

Ahh yes
that'd be this one and it's for the compressor

i've seen them called "Hard Start Kit". My local plumbing/electrical parts store carries them.
Capacitor SC is probably a few hundred microfarads to get the compressor started
after startup relay SR disconnects capacitor SC and the compressor continues running as a PSC on the 40 uf side of the dual run capacitor
fan both starts and runs on the 5 uf side of it.

Go ahead and cut open that dual capacitor too ...
Did it lose its fill ? Break an internal wire ?

 

[russ_watters]

Go ahead and cut open that dual capacitor too ...

Are we completely certain it is shorted/discharged?

 

[gneill]

Go ahead and cut open that dual capacitor too ...
Did it lose its fill ? Break an internal wire ?

If the unit is fairly new then it's probably safe to do so. In the past the oil used often contained polychlorinated biphenyls (PCBs) so not so safe to spill on your skin. If you decide to go ahead and slice it open, be sure to wear protective gloves, etc. Treat it as though it's hazardous from the start, not by surprise!

 

[fourthindiana]

A PSC motor won't start unless there's current through both windings

I'm a little bit confused. When you explained how having a defective run capacitor prevents a PSC motor from starting, you said that the current goes through the start winding, but the current that went through the start winding gets stoppped at the defective run capacitor. For a PSC motor to start, does the current have to both go through both windings AND the current from both windings has to make it to L2? If the only current requirement for a PSC motor to start is to have there be current through both windings, then a PSC motor should start if the current went through both windings but the start winding current got stopped at the run capacitor because the current through the start windings went all the way through the start winding.

Great ! You learn a lot doing post-mortems. Post a photo ? Pay attention to orientation of the two windings - is one offset from the other or are they aligned ?

post-mortems lol---I like that word in this context.

I cannot really tell if one winding is offset from the other winding because they are covered with a plastic material. The only part of the windings that's not covered by the plastic is the part of the run winding that got burnt. Here is a photograph of the motor with the end bell off:

There is plastic covering most of the windings, but the plastic was burned off the part of the run winding that was burned. It's at about 01:00 in the photograph. Or one could call it at the top and slightly to the right of center in the photograph.

It's very rewarding for me to see your interest and your hands-on approach.
Thanks for your feedback

I'm glad you find it rewarding. You're welcome.

 

[fourthindiana]

Go ahead and cut open that dual capacitor too ...
Did it lose its fill ? Break an internal wire ?

Unfortunately I already threw the defective dual run capacitor away. It's been hauled away by the garbage truck.

 

[dlgoff]

It's very rewarding for me to see your interest and your hands-on approach.
Thanks for your feedback

I'm glad you find it rewarding.

This is the kind of EE thread I like ...

... as an interested reader.

 

[jim hardy]

then a PSC motor should start if the current went through both windings but the start winding current got stopped at the run capacitor because the current through the start windings went all the way through the start winding.

remember your Kirchoff
in a series circuit current is the same everywhere
what goes in must come back out , and if it can't get out it won't go in. .

pink current is zero edit - or almost zero...
When motor tries to start red current will be LRA and that's why your run winding is burnt up.

 

[jim hardy]

I'm a little bit confused. When you explained how having a defective run capacitor prevents a PSC motor from starting, you said that the current goes through the start winding, but the current that went through the start winding gets stoppped at the defective run capacitor.

i see now the source of confusion

Anyhow
when the start capacitor loses enough capacitance the fan motor will no longer be able to start
because there's not enough current through its start winding .
Current from L1 goes through contactor and DR relay, arrives at fan motor COMMON and splits between the two windings.
RUN winding current exits straight to L2
START winding current has to go through start cap to get to L2
and if cap won't let it out the motor can't start

For a PSC motor to start, does the current have to both go through both windings AND the current from both windings has to make it to L2?

Yes.

If the only current requirement for a PSC motor to start is to have there be current through both windings, then a PSC motor should start if the current went through both windings but the start winding current got stopped at the run capacitor because the current through the start windings went all the way through the start winding.

i see where my wording confused you.
Start winding and that 5 uf capacitor make a Series circuit so current is the same in both of them.
My thinking was as follows:
Since you found the capacitir not completely open , current was more than zero but not by enough to let the motor start -
You said the capacitor read something like 0.7 microfarads not zero microfarads
so your 'almost open' capacitor choked off current through that winding so much that the motor couldn't start , and it died trying.""""If the only current requirement for a PSC motor to start is to have there be current through both windings, ...""
Well, each winding needs to have enough current to create its share of the magnetic fields inside the motor..

This is how one's thought process evolves, in little steps.
First we grasp the mechanics of the principle, that's the qualitative part.
Then we refine our thinking by adding numbers to it - the quantitative part.

Keep up the good work.

 

[fourthindiana]

remember your Kirchoff
in a series circuit current is the same everywhere
what goes in must come back out , and if it can't get out it won't go in. .

View attachment 236003

pink current is zero edit - or almost zero...
When motor tries to start red current will be LRA and that's why your run winding is burnt up.

If the capacitor was completely dead with 0 microfarads, then the pink current would be exactly zero; correct?

If the capacitor was only .7 microfarads, then the pink current would be almost zero; correct?

 

[jim hardy]

If the capacitor was completely dead with 0 microfarads, then the pink current would be exactly zero; correct?

If the capacitor was only .7 microfarads, then the pink current would be almost zero; correct?

You got it !

Have you covered calculating ohms for a capacitor yet ?

Ω = $\frac{1}{2πfC}$ which at 60 cycles $= \frac{1}{377C}$

5 microfarads gives 530.5 ohms
0,7 microfarads gives 3789 ohms
0 microfarads gives division by zero error
so take your calculator and figure ohms for just a few nf instead

 

[fourthindiana]

You got it !

Have you covered calculating ohms for a capacitor yet ?

No. We have not covered calculating ohms for a capacitor yet. But I don't think that HVAC technicians use that in the field.

 

[jim hardy]

But I don't think that HVAC technicians use that in the field.

you'll find it useful in your troubleshooting.

In this example,
how much current can 230 volts push through 5 microfarads?

we already figured it's 530 phms
take your calculator and do the following keystrokes to figure that
Ω = $\frac{1}{2πfC}$

2
X
π
= (should show 6.28+++)
X
60
= (should show 376,99+++)
X
5
exp
±
6 ( 6 is for microfarads)
= (should show 0.00188495+++)
1/x (should show 530.518+++)

that's the ohms
divide 230volts by 530.5 ohms and you get 0,43++ amps

if current through the fan motor winding isn't at least half that it's worth checking the capacitor
because if you find a capacitor that's on its way out (maybe lost half its capacitance) and save a customer from burning up a $100 fan motor by replacing a $6 capacitor,
well that's good for repeat business.

As a maintenance man you'll need every trick you can muster. So start building your "bag of tricks" now..

Good luck in your studies, and in your career.

 

[fourthindiana]

you'll find it useful in your troubleshooting.

In this example,
how much current can 230 volts push through 5 microfarads?

we already figured it's 530 phms
take your calculator and do the following keystrokes to figure that
Ω = $\frac{1}{2πfC}$

2
X
π
= (should show 6.28+++)
X
60
= (should show 376,99+++)
X
5
exp
±
6 ( 6 is for microfarads)
= (should show 0.00188495+++)
1/x (should show 530.518+++)

that's the ohms
divide 230volts by 530.5 ohms and you get 0,43++ amps

if current through the fan motor winding isn't at least half that it's worth checking the capacitor
because if you find a capacitor that's on its way out (maybe lost half its capacitance) and save a customer from burning up a $100 fan motor by replacing a $6 capacitor,
well that's good for repeat business.

As a maintenance man you'll need every trick you can muster. So start building your "bag of tricks" now..

Yes, I can see how knowing how much current will flow through the capacitor will give me an idea of how long the capacitor will be able to continue keeping a PSC motor going. However, I thought just checking the capacitance of the capacitor would give me the same knowledge to estimate how long the capacitor will last. Will checking the current that will flow through the capacitor give me any type of knowledge about how long the capacitor will last that I could not get from just checking the capacitance of the capacitor?

Good luck in your studies, and in your career.

Thank you.

 

[jim hardy]

Will checking the current that will flow through the capacitor give me any type of knowledge about how long the capacitor will last that I could not get from just checking the capacitance of the capacitor?

No it won't.
My thinking was when you are about to wrap up a routine seasonal clean and lubricate service call , hang your clamp-around on each fan motor lead and look for expected current.. Takes only seconds and you don't have to disconnect the capacitor..
It's just the sort of thing i do, quietly, to assess the overall health of any machine .
I love nonintrusive measurements. Sometimes you spot something that makes you look real smart.

As you gain experience you'll know how much current to expect.
Myself I am not to that point , fixing my own and my neighbor's airconditioners is all the experience i have.
So i have less than a half dozen under my belt and i'd have to guesstimate as in above example..

 

[fourthindiana]

No it won't.
My thinking was when you are about to wrap up a routine seasonal clean and lubricate service call , hang your clamp-around on each fan motor lead and look for expected current.. Takes only seconds and you don't have to disconnect the capacitor..
It's just the sort of thing i do, quietly, to assess the overall health of any machine .
I love nonintrusive measurements. Sometimes you spot something that makes you look real smart.

As you gain experience you'll know how much current to expect.
Myself I am not to that point , fixing my own and my neighbor's airconditioners is all the experience i have.
So i have less than a half dozen under my belt and i'd have to guesstimate as in above example..

There is a formula that one can use to calculate the capacitance of a capacitor by the current going to the capacitor. Resistance is not factored into this formula, if I recall correct. I don't have the formula memorized, but that formula is how HVAC technicians do "live tests" of capacitors so the HVAC technicians don't have to disconnect any wires. Perhaps you are familiar with the formula that I'm writing about. In this formula, I believe one of the numbers used as a constant in this formula is 2652.

 

[jim hardy]

no, i don't know that one.

Don't shy away from math. It'll help in your analytical troubleshooting
and it gives you something to think about while raking leaves.

 

[Tom.G]

In this formula, I believe one of the numbers used as a constant in this formula is 2652.

When you find that formula please post it here. That's something many of us would like to know.

Thanks,
Tom

 

[fourthindiana]

When you find that formula please post it here. That's something many of us would like to know.

Thanks,
Tom

I'm surprised I got a chance to teach you people something.

First, you measure start winding amps with an amp clamp on the wire between the capacitor and the start terminal of the outdoor fan motor or the start terminal of the compressor motor. Second, you use your multimeter to measure the voltage applied across the capacitor (one lead of multimeter on Fan Terminal of Capacitor or Herm Terminal of Capacitor and the other lead on Common terminal of capacitor). Third, you multiply the amps times 2652 and then divide the whole thing by the voltage (the voltage across the capacitor).

The formula gives you the actual capacitance of the capacitor. Then you compare the actual capacitance to what the rated capacitance on the label of the capacitor.
Here is the formula below:
Capacitance (MFD)= (Start winding amps X 2652)/Volts from start to run

 

[Averagesupernova]

Capacitive reactance is calculated using: Xc = 1/(2*pi*F*C)

Xc is capacative reactance which is the ratio of voltage/current across and through the capacitor.
F is 60 Hz in the case of your work with HVAC. (Line voltage frequency)
C is capacitance in FARADS, not micro farads.
-
Compare the formulas and you will find the one you gave just made some shortcuts.

 

[Tom.G]

Sneaky!
C= Capacitance
A= Amps
V= Volts
k= 2652

C= kA / V
C= k⋅(A/V) - - - (V/A)= R but it's a capacitor so (V/A)= X_c (capacitive reactance)
but it's not (V/A), it is (A/V) . so (A/V)≡ 1/X_c

C= k⋅(1/X_c)
Now (1/2652) = 0.000377
The formula @jim hardy gave for X_c has 377 in the denominator. Your formula has moved the reciprocal of 377 to the numerator, which has the same mathematical effect. The factor of 1 million difference between the two is because your formula gives the result directly in microfarads, whereas jims formula answers in farads.

Nice shortcut.

I see @Averagesupernova types faster than I do.

Cheers,
Tom

 

[jim hardy]

Aha !

2652 X 2π X 60 = almost a million

copying from Windows calculator

X 2π X 60 = exactly a million

to get from farads to microfarads

If i learn something every day i might know something someday !

old jim

 

[jim hardy]

Capacitance (MFD)= (Start winding amps X 2652)/Volts from start to run

Nice shortcut.

Indeed !
So nice i just had to figure out how a way that i can remember it.
With my scattered brain i'll roll digits

so i wound up back at conductance, the old Mhos trick

$Ohms = \frac{1}{2πfC}$
so $Mhos = 2πfC$
which means $C = Mhos X \frac{1}{2πf}$
but that's in Farads
in Microfarads it's $C = Mhos X \frac{1,000,000}{2πf}$

and since Mhos = Amps/Volts

$C= \frac{Amps}{Volts} X \frac{1.000.000}{2πf}$

and at 60hz, $\frac{1.000.000}{2πf} = 2652.58...$

hence your $C = \frac{Amps}{Volts} X 2652$ , good to four digits which is probably better than the ammeter.

it's easier for me to just remember C = Mhos/2πf
and that's what i'll do

Thanks, Guys,
for showing this old dog a new trick !

old jim

 

[fourthindiana]

Thanks, Guys,
for showing this old dog a new trick !

old jim

You're welcome.