Unisaw restoration - 3ph motor not running correctly...What could be the cause?

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Forum topic by klw21 posted 03-20-2012 09:22 PM 7301 views 2 times favorited 32 replies Add to Favorites Watch
View klw21's profile


3 posts in 3595 days

03-20-2012 09:22 PM

Calling in the experts…Please help troubleshoot my used Unisaw I recently found on CL…

I’ve been restoring a Unisaw model# 34-806 (3ph 5hp Marathon Motor – 3480rpm) The saw has a LVC magnetic starter and is being operated with single phase power using a Phase-A-Matic PAM-900HD static converter- Which is sized correctly 4-8hp (and proper operation has been recently verified/tested by the Phase-A-Matic company)

I have been restoring this saw from the ground up…I have replaced all the bearings both arbor and motor assemblies, taken everything completely apart, cleaned, and repainted the Chassis etc. I am now to the point of trying to get this 3ph motor to run correctly on single Phase power and am running into problems. I am able to get the motor to start fine but it only runs freely for appoximetly 1.5-2mins. (with no load or pulley installed) then the saw’s overload protection trips and the motor shuts down. (The 30-amp circuit breaker does not trip.) I have replaced the original overload relay and associated “heaters” in the saw’s starter panel (thinking the relay may have gone bad) with a new one of the exact same specification (model and make). Then I verified the “Heaters/fuse elements” are sized correctly to the Motor’s full load amp rating (12amps @ 240v) listed on the motor nameplate. After the motor shuts down about 30 seconds later the outside of the motor case and rotor is very hot to the touch and remains so for about 15 mins.

Other relevant info:
All associated power wiring is 10-gauge wire and all connections have been tightened and verified. The power panel is about 130 feet away from were the dedicated table saw outlet is located. Then there is an additional 15 feet of wire length and phase converter to the saw itself.

I did replace the motor bearings (NTN bearings- original spec)…I made sure to do this carefully to ensure proper seating on the rotor- so I do not suspect it’s a misaligned bearing problem. I also made sure to apply Exxon motor bearing grease to the bearings and bearing inlets prior to reassembling the motor case. The rotor spins beautifully smooth if your rotate it by hand. The motor windings appear to be intact with no overt signs of damage such as discolorations or fowl odors. Also I successfully performed a series of electrical tests to confirm the integrity of the motor’s windings as outlined at the bottom of this website (very useful tests I might add):
When the motor runs you can hear a very faint pulsing oscillation sound, if that makes sense, it’s not a constant humming sound (i.e. as the motor rotates it makes a slight pulsing sound). I have also verified the cooling fan of the motor is pointing in the correct direction – in order to cool the motor.

1. Does anyone have ideas what could be causing this problem or had similar experiences?
2. Does anyone know the effects of using a static phase converter in relation to the proper sizing of magnetic starter heaters? Meaning since the motor, when started with a static phase converter, is unbalanced electrically speaking. (The motor is only running with 2 of the 3 winding series and not with 3 phases) – does this fact change or alter the “full load amperage rating” of the motor? Stated differently: Does running a 3ph motor on 1ph power increase current draw (FLA) of the motor?

My last option, if no one has any other ideas, will be to purchase a replacement 1ph motor. I am leaning towards a 3hp Baldor motor, but it has 3450rpm rating and the original Marathon motor had 3480rpm rating so I may have to purchase a new pulley to balance with the new motor?

Thank you in advance for your insight!!!

See attached pictures for reference:

32 replies so far

View rkober's profile


137 posts in 3629 days

#1 posted 03-20-2012 11:09 PM

Maybe you’ve done it and I missed it but you need to check the basics:
1) voltage to motor – should be 230 volts (plus or minus 10% at the extreme). Check wire to wire (ie 1 to 2, 2 to 3, 1 to 3).
2) amperage – should be below nameplate (12A) and certainly at no load
3) check your motor wiring to make sure it’s wired 230 volt (follow nameplate diagram). Wired 460v would do what you’re describing.

From what you’ve described it seems a though you are indeed pulling high amperage and building heat in the motor. The overloads are tripping as they should. It doesn’t seem to be a mechanical issue (ie bearing, etc), but an electrical issue. It sounds like you are using an excessive amount of energy and in turn building heat.

You asked if it could be running on 1 or 2 phases. No. Furthermore your voltage and amp check should verify. You should have balanced voltage and amperage on each leg. I suspect a wiring issue and or phase converter issue.

Going to a 3 hp motor as you described would not require changing any pulley. Nominal speed is considered the same in this case (2 pole motors). You would not want to go to 1750 rpm though.

Let us know what you learn. Good luck.

-- Ray - Spokane, WA - “Most people don’t recognize opportunity because it’s usually disguised as hard work.” - Unknown

View SCOTSMAN's profile


5849 posts in 4922 days

#2 posted 03-20-2012 11:17 PM

Sometimes static convertors don’t like being used with an electric built in braking system as a general rule.
I would strongly advise dust extraction round the motor for future useage . I know you don’t need telling this but let it be a lesson to all others I have two large extractors on my big wadkin sliding table saw.One at the port on the back and the other , over the blade with a guards I made with extractor point.So now no dust is allowed or very very little is allowed to escape.A difficult lesson but your a capable man and got it sorted Have fun.Kindest regards Alistair

-- excuse my typing as I have a form of parkinsons disease

View RandyM68's profile


693 posts in 3655 days

#3 posted 03-20-2012 11:49 PM

I can tell you that the rpm difference in the Baldor won’t make any difference at all. As long as you are over 3000 rpm’s you are fine. I can replace a motor and wire it in. I can also tear one apart fine, I am just a little fuzzy on putting it back together. I can put in a breaker box and wire the whole house, but that higher electrical theory is over my head. Good luck. Nice saw, by the way.

-- I can explain it to you, but I can't understand it for you. I'm sorry,thanks.

View bigkev's profile


198 posts in 3965 days

#4 posted 03-21-2012 12:03 AM

I don’t know much about electricity, but I know you did a helluva job on that restoration.

-- Kevin, South Carolina

View BlankMan's profile


1491 posts in 4690 days

#5 posted 03-21-2012 12:20 AM

I second the check how the motor is wired.

Is it the contactor heaters that are opening the circuit or is there one in the motor itself that is over temping and opening?

I poked around Phase-A-Matic site and couldn’t find anything that stated whether it generated a pure sine wave or a modified sine wave. My guess would be modified sine wave which is a stepped sine wave or one could say made up of square waves. The leading edge of these square waves, the fast rise time of the voltage, increases the current flow rather quickly and can lead to heating in the windings. It’s possible that particular motor can’t handle that. I’d try contacting Marathon (a great Wisconsin company!), they might be able to tell you if the motor is compatible or maybe not a good idea. I might have an email address for support people there around someplace if you need it and if it still works. But if I remember correctly for the last time I needed to contact them they were rather slow to respond. Took a while to route my issue to the person that could help because it wasn’t a “Is it plugged in?...”

-- -Curt, Milwaukee, WI

View hhhopks's profile


663 posts in 3714 days

#6 posted 03-21-2012 12:27 AM

I sounds like your OL heaters are doing it’s job and is protecting the motor.

The picture of the motor shows 230/460 3-phase 12/6 A.
Which voltage is the motor current wire up as?
230 or 460 ? You should check the motor jb wiring.

I am not familar with these static phase converters but it needs to output a voltage to match your motor as wired.

You may be single phase. Is your static phase converter provide power on all three phase (voltage & current).
Perform the checks that rkober had suggested?

-- I'll be a woodworker when I grow up. HHHOPKS

View Tom148's profile


39 posts in 3601 days

#7 posted 03-21-2012 01:16 AM

Sounds like one phase is overheating and tripping the thermal overload causing a locked roter condition and the other phase is heating up until it trips which is why you have a hot shaft. It sounds like a winding problem.

I have sone experience with this failure mode from aircraft motor work I have done.

I would take the motor to a rebuild shop and have the winding checked. It is not something you can do in the wood shop.

-- Tom

View BlankMan's profile


1491 posts in 4690 days

#8 posted 03-21-2012 01:55 AM

Hey Tom you sure about that? I used to work at Allen-Bradley Drives Division, now Rockwell Automation, in engineering and we used contactors and heater overloads in just about everything we made. If I remember correctly, if one heater overloaded the whole overload tripped. Because if you lose one of the three wires/circuits you lose 2 phases leaving 1 phase powering a 3 phase motor. Not good. I am remembering this now, went to MSOE for Power Technology too. Waking up the dormant neurons I guess.

-- -Curt, Milwaukee, WI

View MedicKen's profile


1615 posts in 4799 days

#9 posted 03-21-2012 02:05 AM

PM me your address and I will send ya the bullets for the handwheels. I have an extra set here

-- My job is to give my kids things to discuss with their [email protected]

View Tom148's profile


39 posts in 3601 days

#10 posted 03-21-2012 02:40 AM

I don’t know much about the phase inverter setup being used but I do know that when I was dealing with three phase motors for a pump application (several thousand units) we had a continuing problem with locked rotors that was traced back to winding issues from the supplier. As a result we would smoke a fair number of motors. Nothing irritates the pilot like smoke in the cabin…not to mention the passengers!

So I based on my experience (assuming the wiring is correct) I would look at the windings. The discretion of the failure might indicate a mechanical binding due to heat build up but that may also point back to windings. I have never heard of dropping a phase on a 3 phase motor and having it work. But as I say I don’t have experience with this system.

-- Tom

View barecycles's profile


257 posts in 3665 days

#11 posted 03-21-2012 02:45 AM

Maybe you’ve already done this but have you tried posting this to the guys over at In particular Richard?

-- Sweeping up sawdust in Texas

View BlankMan's profile


1491 posts in 4690 days

#12 posted 03-21-2012 02:49 AM

Tom, you can’t drop “a” phase, if you lose 1 of the 3 wires, say it’s connected through a fused disconnect and one of the 3 fuses blows you lose connectivity through 1 of the 3 wires but you lose 2 of the 3 phases then. That’s why I’m pretty sure that if 1 of the 3 heaters overloads the whole overload trips because you don’t want to remove 2 of the 3 phases powering the 3 phase motor.

-- -Curt, Milwaukee, WI

View Tom148's profile


39 posts in 3601 days

#13 posted 03-21-2012 03:03 AM

I did some checking and this may be either a winding issue with one of the legs (not a complete failure) or a phase balance problem. The heat build up in three phase units is usually related to an imbalance on one of the legs which generates excessive heat. This condition would not be present at startup or the motor would just be a locked rotor and overheat causing the heaters to trip. However once the unit is running it will stay running until the heat builds and it trips.

At least that’s what I have been told. So my info points to either power balance or a winding problem. The things are always interesting (and frustrating) to run down. I’ll be interested to see what the root cause is.

-- Tom

View TopamaxSurvivor's profile


23308 posts in 5012 days

#14 posted 03-21-2012 04:18 AM

#1, you need to check the motor’s insulation with a high voltage megger. It should read at least 10 mega ohms, but preferably near infinity. Any motor shop can do that.

If the insulation is good and assuming you are wired correctly and do not have excessive voltage drop; that motor’s name plate says it has a Service Factor of 1.0. That means it will not stand any overload at all. A static converter is not really 3 phase power. It is basically a straight line (Edit: single phase) with a 90 degree leg kicked off to fool the motor instead of 3 legs 120 degrees apart. If it were a 1.15 or 1.25 SF motor, it would probably work without tripping the overloads.

You might try putting an idler motor on line with it. That should more or less fix your 3 phase power.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View BlankMan's profile


1491 posts in 4690 days

#15 posted 03-21-2012 04:41 AM

Topa, thanks, I just saw the light so I did some checking. So these static converters aren’t like inverters creating a true 3 phase like I thought and are mechanical. Using a capacitor to phase shift one leg to get the motor spinning then drop it out but maybe connect a run capacitor then. So only one of the three windings in the motor has full current in it. Whoa. So the other two windings being in series only get half the current they should get. Double whoa.

No wonder they only get 2/3 the nameplate HP. That also explains why you have to match it to the motor and shouldn’t go bigger so the cap is right. And they charge hundreds of dollar to do that? What a scam in my mind…

-- -Curt, Milwaukee, WI

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