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Old 04-04-2009, 03:15 AM   #1
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Split 3-phase


I have recently moved into a shop that has only 3-phase power available. The electrician has split the 3-phase so I am able to run a table saw with a 230v, 13amp, single phase motor. The electrician is adamant that the motor should run, with no issues, on the 208V that the split 3-phase is providing. I am not convinced that this is true. The saw seems to run fine with only a single blade attached, but if a dado blade is used, the saw will not start. It turns slowly at startup and then trips the 20amp breaker. I am guessing that this is because of the extra mass of the dado set as compaired to a single blade. The motor is also getting very warm after running. I don't beleive the extention cord is part of the problem; it is 12 guage and 25' long.

Right now, getting single phase is not an option and I really don't want to have to buy a new 3-phase motor. My question to all you wise folks out there is: Can a 230V single phase motor run, happily, on split 3-phase at 208V's?

As a added note, the electrician believes that the problem is the motor itself

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Old 04-04-2009, 03:51 AM   #2
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Split 3-phase


It should but when its running close to its maximum load as youve seen problems arise to be sure theyreshould be a rating plate on the motor that states the allowable max and min voltages. Have someone with a meter doublecheck the output voltage sometimes on three phase one leg can have a slightly higher voltage perhaps that will be the case and it can be switched to that leg

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Old 04-04-2009, 09:15 AM   #3
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Split 3-phase


Using two wires of a three phase system is used like you describe is done everyday. I would say a bad capacitor coupled with an extension cord and heavy dado cutter is your culprit.

Most 230 V motors run happily on 208. Get a new start capacitor and a bigger extension cord.
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Old 04-04-2009, 10:51 AM   #4
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Split 3-phase


Quote:
Originally Posted by InPhase277 View Post
Using two wires of a three phase system is used like you describe is done everyday. I would say a bad capacitor coupled with an extension cord and heavy dado cutter is your culprit.

Most 230 V motors run happily on 208. Get a new start capacitor and a bigger extension cord.
what he said
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Old 04-04-2009, 06:16 PM   #5
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Split 3-phase


I don't know how many hundreds of 230 volt single and 3 phase motors I've connected to 208 volt systems without problems of any sort.

Generally speaking, a 230 volt motor operated on 208 volts will perform as follows.

1) Full-load current will be about 10% higher.

2) Locked-rotor current will be about 15% lower.

3) Locked-rotor torque will be about 20% lower.

4) Breakdown torque (push a board into a saw until the motor loses speed and almost stalls) will also be about 20% lower.

5) Full-load speed will be about 5% lower.

6) Operating temperature will be about the same, maybe slightly lower.

As stated above, if the motor won't start the saw with a dado blade installed, I'd suspect the start capacitor. Figure around $10-15. Match MFD pretty close, voltage of the new one can be higher, not lower. When you get to it, short the terminals out with a screwdriver before you grab them.

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Old 04-04-2009, 07:14 PM   #6
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Split 3-phase


Also, search

dado

on this site. We just had one of these problems.

For such a short length of #12 copper, the contact impedance at each of the two plug/socket interfaces may be more than the 80 milliohms of wire resistance, depending on how much the socket contact spring tension has been reduced over time and the cleanliness of the contact surfaces.
The default fix for this is to clean, and then bolt or wirenut all four extension cord connections.

Last edited by Yoyizit; 04-04-2009 at 07:48 PM.
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Old 04-04-2009, 08:57 PM   #7
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Split 3-phase


Per usual, I agree with 277. The motor will run on 208, though it is not an optimal condition. Sounds like the capacitor could be getting flaky.
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Old 04-05-2009, 12:46 PM   #8
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Split 3-phase


Some/Most motor manufacturers today are name plating at 120/208/240. Some advise not to run on 208 unless the motor nameplate indicates 208. Your motor is proprietary to the saw manufacturer. Try calling them and see what they say about the 208.
The advice above is excellent as usual from the usual people.
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Old 04-06-2009, 06:00 PM   #9
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Split 3-phase


Quote:
Originally Posted by mdwkailua View Post
I have recently moved into a shop that has only 3-phase power available. The electrician has split the 3-phase so I am able to run a table saw with a 230v, 13amp, single phase motor.
Right now, getting single phase is not an option and I really don't want to have to buy a new 3-phase motor. My question to all you wise folks out there is: Can a 230V single phase motor run, happily, on split 3-phase at 208V's?

Why don't you use 2 legs, A and B, A and C, or C and B so you can have single phase 230? If you have a high leg Delta you have all the necessary voltages, you have single phase already.
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Old 04-06-2009, 11:03 PM   #10
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Split 3-phase


Quote:
Originally Posted by Silk View Post
Why don't you use 2 legs, A and B, A and C, or C and B so you can have single phase 230? If you have a high leg Delta you have all the necessary voltages, you have single phase already.
?? That's what the OP is talking about. He has a 208/120 V service. He has used two legs to send single phase 208 to his saw and thinks the lower voltage is causing his problem.
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Old 04-06-2009, 11:05 PM   #11
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Split 3-phase


Quote:
Originally Posted by InPhase277 View Post
Most 230 V motors run happily on 208. Get a new start capacitor and a bigger extension cord.
To a 220-240 volt motor, 208 volts is a brownout.

The motor will probably draw more amperes but may work OK. Or maybe it might overheat under heavy load.

For a high leg system with 240 volts phase to phase, any two legs can be used for a "plain" 240 volt single phase circuit. But don't use the high leg together with the neutral.
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Last edited by AllanJ; 04-06-2009 at 11:11 PM.
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Old 04-06-2009, 11:11 PM   #12
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Split 3-phase


Quote:
Originally Posted by AllanJ View Post
To a 220-240 volt motor, 208 volts is a brownout.

The motor will probably draw more amperes but may work OK. Or maybe it might overheat.
Do you see many 230 V motors that aren't rated to run on 208? I don't/haven't. They exist, but most of the motors I have installed have a rating of something like "115, 208/230" or "208/230, 460", or something like that.
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Old 04-06-2009, 11:15 PM   #13
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Split 3-phase


What is the extension cord rated for?
I've seen a lot of 12g cords only rated for 15a
Usually just the plugs causes the 15a rating....
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Old 04-06-2009, 11:16 PM   #14
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Split 3-phase


There is a slight loss of power and speed because of the phase shift.
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Old 04-07-2009, 12:00 AM   #15
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Split 3-phase


I want to thank you all for the insights. I will look into the possible solutions; change capacitor,extension cord, and will certainly talk to Powermatic again. The electrician did meantion that he was surprized to see no frame number on the motor, leading him to think it is a proprietary motor. I'll let you know how things work out.

Thanks again,
Mark

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