# 3hp Unisaw start capacitor?



## BThomas (Jan 9, 2017)

Hi All!

I have a 1979 Unisaw 34-771 with a re-motored Delta/Baldor 3hp 115/230v, single phase, 32/16Amp, 3450RPM motor. The motor is Delta number 83-651.

It's currently wired to run on 230v, 1ph power and has a magnetic starter. I've only had the saw a short time, and yesterday when I turned it on; it only hummed (no blade turn). I did some research, blew out the starter and motor, then I checked the start capacitor which was completely blown--guess I found my problem (hopefully).

The capacitor that was blown was a Vanguard BC-800. It's a 110/125V, 50-60hz, 829-995 MFD.

I noticed the capacitor was rated for 110/125V, which kind of surprised me as I thought that Start capacitors voltage was supposed to be higher than the line voltage, but I'm no technician or electrician so I've been looking everywhere to make sure that the next one I buy and put in is the correct capacitor.

Does anyone have better knowledge on this than I do??? 

Thanks a bazillion! And thanks for all the help you have given me the last few months while I was lurking!

Brent


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## Steve Neul (Sep 2, 2011)

That capacitor I don't see how it's wired to 220v. Are you sure there is only one of them?


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## BThomas (Jan 9, 2017)

Steve Neul said:


> That capacitor I don't see how it's wired to 220v. Are you sure there is only one of them?


Yes Sir.... It makes me wonder if the previous owner was running it on 110v?

I bought it from a knowledgable third party who never used it. The rest of the saw/wiring looks correct. Here is a pic of it:


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## BThomas (Jan 9, 2017)

BThomas said:


> Yes Sir.... It makes me wonder if the previous owner was running it on 110v?
> 
> I bought it from a knowledgable third party who never used it. The rest of the saw/wiring looks correct. Here is a pic of it:


Here are more pics of what I have....


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## woodnthings (Jan 24, 2009)

*What's the plug look like?*

You had better start with the wiring diagram for that motor and see what's going on. 










The plug will definitely tell you how it's wired. You did run it on 220 V right? Why would you suspect the previous owner, the one who actually used it had it wired for 120V? Did you rewire the plug end and did you convert the internal wiring to 220V?

Too many unknowns to did much advice.


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## BThomas (Jan 9, 2017)

woodnthings said:


> You had better start with the wiring diagram for that motor and see what's going on.
> 
> The plug will definitely tell you how it's wired. You did run it on 220 V right? Why would you suspect the previous owner, the one who actually used it had it wired for 120V? Did you rewire the plug end and did you convert the internal wiring to 220V?
> 
> Too many unknowns to did much advice.


I am not an electrician, but I did receive it wired for 220 according to previous owner. I've checked the magnetic switches and it looks to me like it is wired properly when compared to diagrams I have showing 1 Ph, 220V. I also had it plugged into a 220v, 20amp circuit in my garage. I started it about 10 times, and ran a 6ft piece of 3/4" plywood through it before it stopped.

I thought it may have been wired for 120V because the start capacitor that blew was a 120V rated capacitor, not a 240V rated capacitor. I'm really trying to find out what start capacitor is supposed to be in this engine as I think the one I'm replacing may not be correct....

Thanks...


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## Brian(J) (Feb 22, 2016)

BThomas said:


> I am not an electrician, but I did receive it wired for 220 according to previous owner. I've checked the magnetic switches and it looks to me like it is wired properly when compared to diagrams I have showing 1 Ph, 220V. I also had it plugged into a 220v, 20amp circuit in my garage. I started it about 10 times, and ran a 6ft piece of 3/4" plywood through it before it stopped.
> 
> I thought it may have been wired for 120V because the start capacitor that blew was a 120V rated capacitor, not a 240V rated capacitor. I'm really trying to find out what start capacitor is supposed to be in this engine as I think the one I'm replacing may not be correct....
> 
> Thanks...


I'm not an expert with motors, but have worked a lot with them over the last 40 years.
Let's look at what exactly the capacitor is and what it is doing. At the dam, a rotor spins past magnets which wired up in three sets- A, B, and C. Each set is called a Phase and connected to a wire, so you have three wires coming from the dam, A phase, B phase, and C phase. When the wires get to your shop, you connect each wire to one of the three in your 3 Phase Unisaw motor and you are good to go ( I'm simplifying this just a tiny little bit). The direction the motor will spin is easy, it will spin A B C just like back at the dam. If you want it to spin the other way just change it to A C B. So simple.
Problem is, you've got Single Phase, which is just two of those wires. When you connect the motor to A and B, it has no way to know which way to go, so it just sits there and vibrates, while getting hot.
In a Capacitor equipped motor the capacitor is wired onto one of the phases, in your case both are 120v phases. The capacitor acts like a bucket for the electrons. The effect of that is to make the power from the capacitor phase 'late' getting to the motor, because it had to fill up with electrons and that took time, allowing the motor shaft to rotate more into the opposite phase's pole (magnet). Having filled up (charged) the capacitor with electrons, now the capacitor discharges but the motor shaft has rotated enough that this discharge is now 'pushing' the shaft in the direction you want.
This entire process is called 'lagging' and most likely takes place on only one of the two 120 volt phases. So a 120 volt capacitor is probably fine.


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## Brian(J) (Feb 22, 2016)

BThomas said:


> I am not an electrician, but I did receive it wired for 220 according to previous owner. I've checked the magnetic switches and it looks to me like it is wired properly when compared to diagrams I have showing 1 Ph, 220V. I also had it plugged into a 220v, 20amp circuit in my garage. I started it about 10 times, and ran a 6ft piece of 3/4" plywood through it before it stopped.
> 
> I thought it may have been wired for 120V because the start capacitor that blew was a 120V rated capacitor, not a 240V rated capacitor. I'm really trying to find out what start capacitor is supposed to be in this engine as I think the one I'm replacing may not be correct....
> 
> Thanks...


Also, and I am dreading getting into this because of the endless discussions of 120v vs 240v wiring for table saws, any motor that can be hooked up to 120 or 240 has ONLY 120 volt components. These are hooked up in series for 120v and parallel for 240v, the motor sees only 120v. You can run 240v to the wiring box on the motor but after that it is 120v all the way. It is possible to get 240v into the motor itself, it will burn out in a very short time, under a minute.


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## BThomas (Jan 9, 2017)

Thank you Brian. That really helped me understand this better. You did a better job explaining that than the motor repair guy did a few minutes ago. He also told me the 120 is correct and sold me another sc for $18. I'll go plug it in and try again.

Thanks to you and everyone who reached out to help me--really appreciate it!


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## Catpower (Jan 11, 2016)

Brian(J) said:


> Also, and I am dreading getting into this because of the endless discussions of 120v vs 240v wiring for table saws, any motor that can be hooked up to 120 or 240 has ONLY 120 volt components. These are hooked up in series for 120v and parallel for 240v, the motor sees only 120v. You can run 240v to the wiring box on the motor but after that it is 120v all the way. It is possible to get 240v into the motor itself, it will burn out in a very short time, under a minute.



Actually they are in series for 240 and parallel for 120, not a big deal, unless you put power to the windings in parallel with 240, then you will let out the magic smoke pretty quick


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## BThomas (Jan 9, 2017)

The saw runs again! Hopefully it keeps going! Thank you!!!


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## briall (Nov 20, 2017)

Thanks, Brent. Your mention of the capacitance of the start capacitor for your saw saved me! The old capacitor that came out of my saw was unreadable and NO ONE (not Baldor, not Delta, not any electric motor repair shop) could tell me what the capacitance would be. I installed the new capacitor, and it worked! Thanks again for being so complete in your discussion!


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## BThomas (Jan 9, 2017)

Your welcome! I learned so much from the folks on this forum! Glad it worked out!


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## Clarksound (Jun 22, 2014)

Brian, I have the exact same motor and the same problem. I cannot read anything on the start capacitor. Can you tell me a product number for your replacement?

Thanks Jack


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## Jig_saw (May 17, 2015)

A 110V capacitor will produce twice the starting current when run on 220V. That is why it was running your 220V motor, but probably blew a fuse somewhere due to the higher current. I suggest you replace it with a 220V rated capacitor.


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## briall (Nov 20, 2017)

I had the same "no readable info on the old start capacitor" problem. I used the microfarad range (829-995) and voltage (110-120) that BJ Thomas was kind enough to include in his original post last year. Got the capacitor on-line, installed it, and all is now well.


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## Catpower (Jan 11, 2016)

Capacitance is an engineered value, there are all sizes that are specifically sized to a motor for many different reasons, a start cap shifts the phase angle of the incoming waves between the start and run windings to give higher starting torque

Changing to a higher voltage is acceptable but you should never use a lower voltage than the original

As far as voltage ratings I have seen 480 volt motors use 370V run caps. The start caps we are talking about are only in the circuit for about 1/2-1 second, then taken out by a potential relay or centrifugal switch


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## gmyrick (Aug 13, 2018)

Thanks Brent! The description of your saw, and the pictures of your capacitor and motor plate were the best leads I found for my forty year old Unisaw, which suddenly started humming when turned on. I purchased mine at about the same time.


Info for others with the "humming" motor problem: In talking to some guys at a electrical motor shop, I found out that the capacitor could be 110V or 240V, depending on how the motor was wired by the manufacturer. On these particular motors, the start capacitor is easy to get at, on the outside of the motor. My only issue was that it was soldered in. I had to de-solder it. I then soldered spade clips onto the wires, so that in forty years when the capacitor goes out again, it will be an easy change out.


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## Mycrossover (May 18, 2018)

Steve Neul said:


> That capacitor I don't see how it's wired to 220v. Are you sure there is only one of them?


When there is more than one, the other one is a run capacitor, used in some designs but not all.

Sent from my SM-G900V using Tapatalk


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## Mycrossover (May 18, 2018)

It is correct There is a start winding that is in series with the capacitor and the centrifugal switch. There are two run windings . For 120 operation the two run windings and the start circuit (cap, switch, start winding) are all in parallel. For 240 operation the run windings are in series so there is 120 across each of them. The start circuit is wired across only one of the run windings so it still only sees 120.

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