Are start caps the same as run caps

[VTP99]

The day I was diagnosing the fan .. I didn't have that oddball cap , so yes I made what looks like a dynamite pack strapped to the inside of hood exhaust fan housing

Fired right up

Does it magically connect to the motor ?

[Snapperhead]

After you finish strapping them in place wit that black strapping and connect them red extension wires

[thatguy]

After you finish strapping them in place wit that black strapping and connect them red extension wires

You should connect it through wifi. That’s how all the kids are doing it these days. It’s the latest rage.


Sent from my iPhone using Tapatalk

[508gm1]

240V motors : Start winding is centre tapped between the 2 run windings; it is only subjected to 120V. See attached picture.

[Core_d]

240V motors : Start winding is centre tapped between the 2 run windings; it is only subjected to 120V. See attached picture.

Doesnt look to be the case from your drawings, looks parallel to both run winding circuits.


Edit: atleast on the lower picture. Not shure how i missed that lol

Sent from my iPhone using Tapatalk

[Core_d]

That is interesting, dont believe ive ever seen a dual voltage psc motor before.


Sent from my iPhone using Tapatalk

[VTP99]

Isn't the high voltage in series and the low voltage in parallel in his illustration ?

[Poodle Head Mikey]

Maybe the back EMF is less than 165 volts?

When you measured it - what was it?

So how are manufacturers getting away with adding a 165v start cap into 220v supplied motors

[Galt]

Its a capacitor start motor with a center tapped start winding that only sees 120 volts.

[Snapperhead]

Maybe the back EMF is less than 165 volts?

When you measured it - what was it?

My meter isn't quick enough to test that during start up.

[Snapperhead]

Little trivia for ya'll

Did you know you can use a Run cap in place of a Start Cap ?

Yup , the only problem is you'd have to hook a bunch of them together to get the mfd needed

BUT you cannot use a start cap in place of run cap

[Poodle Head Mikey]

My Flukes have a peak-hold setting. Although I virtually never use them and I'm not sure if my Fieldpiece's have that feature.

I do not Know - but I very much suspect that either the back EMF is less than the capacitor rating - or the motor maker has determined that the spike is too short to have bad effect.

Run capacitors still last a long-ish time even with back EMF in excess of their rating. They certainly don't fail immediately. So maybe the voltage rating is in reference to 'continuous' back EMF - and not a momentary spike.

Hey! Where are all the resident electrical geniuses? Why is a dummy like me having to attempt the heavy lifting here?

PHM
-----------

My meter isn't quick enough to test that during start up.

[timebuilder]

I don't know about the genius part, but I was there in posts 6, 8, and 10.......

[508gm1]

That is interesting, dont believe ive ever seen a dual voltage psc motor before.


Sent from my iPhone using Tapatalk

The picture just happened to be on my work phone; different application but same idea. Two winding's connected in series, start winding centre tapped between both winding's. Assuming the same inductor values for both winding's, both winding's would be subjected to half of the supply voltage.

I assume in a PSC motor the start winding protects the capacitor from over voltage as they are in series together (half the voltage just as above)

I am electrician with limited HVAC knowledge - just so everyone knows my back round.

[itsiceman]

Just looked at 2 door cooler compressor not starting. Had one silver cap screwed next to the peckerhead. Had to do a double take on the mfds

[BBeerme]

Actually, I think it's just the opposite. It has to do with the same principles as a transformer. The start windings are greater than the run windings, which means the voltage measured on the start winding will be greater. And in the case of a potential relay, the windings in the potential relay are greater than the start winding, so that creates the forces for the potential relay to pull in [opening the normally closed contacts], once the voltage increases after the motor is up to speed.

With greater voltage, comes less current. Which, and I could easily be wrong on this point, the start cap is used to shift the phase and deliver more current to the start winding to get it going [the proper direction?].

Think I remember most of this from trade school. I found it fascinating.

On a bit of a tangent, you don't need this information to be an awesome service tech. There are specific skills/procedures for troubleshooting these things. But since I had a very good and well rounded electrical background before I went to HVAC trade school, learning these things really did fascinate me.

From what I’ve seen start capacitors are electrolytic while run capacitors are oil filled.

I think pecmsg is on the right track with saying the voltage just isn’t there while the motor is starting up. Because the voltage it operates at is influenced by the back EMF, voltage is very low during the time it is in operation. Motor not spinning means no back EMF. Motor spins slow means low back EMF. Motor spins fast means high back EMF.


Sent from my iPhone using Tapatalk

[R600a]

I think someone with a peak reading meter should test the start cap voltage and report back. I would volunteer but I am pretty sure my fieldpiece doesn't read peak.

Also I am not sure the original question was ever answered.
Can you use a higher voltage start cap than originally specified?
I would say yes because voltage rating is just what it can handle. The problem would likely be size.

I agree with the OP I always use 440v run caps also. Our supply house doesn't sell 370v.

Sent from mars using Tapatalk

[timebuilder]

You would probably need to measure short duration potentials with a recording oscilloscope.

[thatguy]

Actually, I think it's just the opposite. It has to do with the same principles as a transformer. The start windings are greater than the run windings, which means the voltage measured on the start winding will be greater. And in the case of a potential relay, the windings in the potential relay are greater than the start winding, so that creates the forces for the potential relay to pull in [opening the normally closed contacts], once the voltage increases after the motor is up to speed.

With greater voltage, comes less current. Which, and I could easily be wrong on this point, the start cap is used to shift the phase and deliver more current to the start winding to get it going [the proper direction?].

Think I remember most of this from trade school. I found it fascinating.

On a bit of a tangent, you don't need this information to be an awesome service tech. There are specific skills/procedures for troubleshooting these things. But since I had a very good and well rounded electrical background before I went to HVAC trade school, learning these things really did fascinate me.

I think we are saying the same thing just using different words. Because the start capacitor is removed from circuit once the motor speed is sufficient it won’t see the high voltage created by the back emf while its at full operating soeed. It’s only in circuit when the motor is stationary or low RPM so the generated voltage is still quite low at that moment in time.


Sent from my iPhone using Tapatalk

[BBeerme]

Could be, I'm not very literate.

I think we are saying the same thing just using different words. Because the start capacitor is removed from circuit once the motor speed is sufficient it won’t see the high voltage created by the back emf while its at full operating soeed. It’s only in circuit when the motor is stationary or low RPM so the generated voltage is still quite low at that moment in time.


Sent from my iPhone using Tapatalk