Motor guy question

[ptonsparky]

Problem: Blown start cap. As in paper everywhere. Don’t see that often.
Replaced all caps being sure to not screw up. One of new blew. It will be going to the motor shop but when I started removal this is what I found in the fused disconnect. The small red that comes from the left lug hits all three of the line side of the contactor plus it jumps back to one side of the coil. Bottom lugs go to #7 of the motor lead. Single phase 15 hp. White going to other side of coil wire comes from #2 motor lead.

I suspect this a home made start switch replacement by a motor shop. Any SEWAG?

No wiring diagram on the motor and IDK the MFG although i do have picture of nameplate.

 

[ActionDave]

An exploded start cap is usually because it is left in the circuit too long usually because of a bad starting switch.

I don't know what #7 means. Standard leads for a single phase motor would be 1,2 and 3,4 for the running winding and 5 and 8 for the starting winding. 6 and 7 are almost made up internally.

 

[Jraef]

Looks as though the contacts are jumpered on both sides, meaning they are paralleling the contacts to get more capacity from them, which leads me to believe it is indeed a makeshift Start Relay. But what would take it off line? If there is no centrifugal switch, you would need a Potential Relay looking at the back EMF of the motor to know when the rotor is approaching the right value to where the cap needs to be turned off. Without it, yep, you blow up the cap.


If they were thinking they were making a Potential Relay using a standard relay like that, it shows they didn't understand the concept. The relay coils are not the same.

 

[retirede]

Makes me wonder how long it operated before the first blown cap? Or has something in the control system changed recently?

 

[ptonsparky]

Makes me wonder how long it operated before the first blown cap? Or has something in the control system changed recently?

The motor shop pulled up the wiring diagram. It is a Baldor and yes, it should drop out the start caps. It has been running at least a couple years but as Jraef stated, how did they know what the drop out voltage is? They most likely didn’t. I suspect at least one of those contacts has welded closed. I will look tomorrow.

This was new used equipment to the customer and he stated the electrician blew several capacitors before he got it running.

 

[ptonsparky]

Now that I look closer, he didn’t need to know the drop out voltage. The relay has failed, or the contact(s) are stuck.

 

[kwired]

View attachment 18944

Now that I look closer, he didn’t need to know the drop out voltage. The relay has failed, or the contact(s) are stuck.

Seen that more then once on Baldor 10, 15 and 16 Hp motors. Relay is still driven by a centrifugal switch on ones I have worked on, and is in motor terminal box along with capacitors. Looks like yours uses a potential relay to drive the start capacitor switching contactor.

And yes they do parallel through a double pole contactor, but is a cheap definite purpose contactor and contacts seem to weld closed easily - then it takes out start caps.

 

[Jraef]

Paralleling contacts only works for resistive loads. Anything capacitive or inductive and the micro-level of closing and opening delay in the contacts between each other puts an extreme stress on the others, leading to burning and welding.

 

[ptonsparky]

Paralleling contacts only works for resistive loads. Anything capacitive or inductive and the micro-level of closing and opening delay in the contacts between each other puts an extreme stress on the others, leading to burning and welding.

One of the few formulas I remember and used, prior to meters that actually told me the mfd of capacitors, was (2650xI)/E=mfd. Parallel start caps of 259 give me 518 mfd. 250v max.

Using some backwards math I get about 49 amps of current. Can I use a SEWAG that a DP contactor with a 60 amp Inductive rating would be sufficient? Dependent on OEM part availability of course.

 

[kwired]

Paralleling contacts only works for resistive loads. Anything capacitive or inductive and the micro-level of closing and opening delay in the contacts between each other puts an extreme stress on the others, leading to burning and welding.

Baldor has been putting these contactors in 10-16 HP motors for past few years now. Older motors used to switch the starting caps directly through the centrifugal switch, now they drive the contactor coil with the centrifugal switch. The contactor is a definite purpose contactor, and though it is usually a square D contactor it must be sort of an OEM thing as it is on older version of what they now have as today's version of the same contactor. The newer version doesn't readily swap out because it don't have the same style mounting foot I have found Grainger does still have apparently same thing with the Dayton name on it and it readily swaps out with what Baldor originally used.

One of the few formulas I remember and used, prior to meters that actually told me the mfd of capacitors, was (2650xI)/E=mfd. Parallel start caps of 259 give me 518 mfd. 250v max.

Using some backwards math I get about 49 amps of current. Can I use a SEWAG that a DP contactor with a 60 amp Inductive rating would be sufficient? Dependent on OEM part availability of course.

You have the aux winding of the motor in parallel with your capacitors which will vary how much current you have depending on load conditions

 

[steve66]

The schematic says Electrolytic Caps? Aren't they DC only?

 

[ptonsparky]

The schematic says Electrolytic Caps? Aren't they DC only?

Must not be...

 

[kwired]

The schematic says Electrolytic Caps? Aren't they DC only?

that is typically what they say on a lot of motor schematics. The electrolytic caps are the higher mfd starting caps and in a plastic case. The oil caps are lower mfd run caps and in a metal case.

Maybe capacitors on PC boards are commonly "electrolytic" and operate on DC only for a reason I am not all that aware of.

Normal operation these electrolytic "start capacitors" are only in the circuit for a second or two at the most when the motor is starting. If left in the circuit too long, it is common for them to spit their guts all over inside whatever is housing them.

My guess is it is the most economical way to produce a high mfd capacitor especially when it is only needed for short duration at a time. Otherwise they likely would be oil capacitors if they remained in the circuit longer then they typically do.


Add: Even static phase converters use them for short periods to get motors started, but run mode is on oil caps.

 

[steve66]

that is typically what they say on a lot of motor schematics. The electrolytic caps are the higher mfd starting caps and in a plastic case. The oil caps are lower mfd run caps and in a metal case.

Maybe capacitors on PC boards are commonly "electrolytic" and operate on DC only for a reason I am not all that aware of.

Normal operation these electrolytic "start capacitors" are only in the circuit for a second or two at the most when the motor is starting. If left in the circuit too long, it is common for them to spit their guts all over inside whatever is housing them.

My guess is it is the most economical way to produce a high mfd capacitor especially when it is only needed for short duration at a time. Otherwise they likely would be oil capacitors if they remained in the circuit longer then they typically do.


Add: Even static phase converters use them for short periods to get motors started, but run mode is on oil caps.

Sounds like they do make AC electrolytic caps with 2 dielectrics in series. From Wikipedia:

Standard electrolytic capacitors are polarized components due to their asymmetrical construction, and may only be operated with a higher voltage (ie, more positive) on the anode than on the cathode at all times. Voltages with reverse polarity, or voltage or ripple current higher than specified (as little as 1 or 1.5 volts may suffice), can destroy the dielectric and thus the capacitor. The destruction of electrolytic capacitors can have catastrophic consequences (explosion, fire).

Bipolar electrolytic (aka non-polarized) capacitors which may be operated with either polarity are special constructions with two anodes connected in series.

 

[GeorgeB]

Bipolar electrolytic (aka non-polarized) capacitors which may be operated with either polarity are special constructions with two anodes connected in series.

When I did this as a hobbyist 40 years go, I would put a pair of 1N4006 diodes to prevent reverse capacitor voltage on 120VAC work.

 

[ATSman]

Looks as though the contacts are jumpered on both sides, meaning they are paralleling the contacts to get more capacity from them, which leads me to believe it is indeed a makeshift Start Relay. But what would take it off line? If there is no centrifugal switch, you would need a Potential Relay looking at the back EMF of the motor to know when the rotor is approaching the right value to where the cap needs to be turned off. Without it, yep, you blow up the cap.
View attachment 18937

If they were thinking they were making a Potential Relay using a standard relay like that, it shows they didn't understand the concept. The relay coils are not the same.

Jraef,
Typically I see wiring the contacts in Series when they want to double the current rating. Wouldn't that be a better design in this application?

 

[retirede]

When I did this as a hobbyist 40 years go, I would put a pair of 1N4006 diodes to prevent reverse capacitor voltage on 120VAC work.

Yep - I've done that, too.

 

[GoldDigger]

Jraef,
Typically I see wiring the contacts in Series when they want to double the current rating. Wouldn't that be a better design in this application?

You usually wire the contacts in series to increase the voltage interrupting or withstanding capacity. It will have no effect on running current capacity and a minimal effect on fault interrupting capacity if the voltage is already within the capacity of a single contact.

 

[Phil Corso]

ATSman & Jraef...

I believe both of you have misinterpreted how the sketch (starting a CSIR w/o a centrifugal switch) you address should work! Would you like to hear my explanation?

Phil Corso

 

[ptonsparky]

The contacts aren’t stuck or welded although we did not use a BFH to visually access the points. Leaves the start relay.