3 phase motor on 220 volts single phase- % horsepower decrease?

[KYErik]

I have an old white westinghouse 1150 rpm 7.5 hp 3 phase electric motor that I am running on 220v with an factory made static phase converter (simply a large bank of capacitors with a magnetic switch controller that starts the motor turning and then kicks out).

I am running a home made bandsaw mill with this motor. The motor starts fine, but I am needing more power from it when I cut 20+ inch logs.


Here are a few questions for the electricians/electrical engineers:

1. About how many HP is this 7.5 hp 3 phase motor really developing since I am running it on 220v single phase?

My dad has a 5 hp single phase low rpm motor that I was thinking of swapping onto the mill- but would this yield any more power over what I have now?


2. I also have 2 more 7.5 hp low rpm 3 phase motors (I don't recall the brand names right now- but they are identical to each other). Could I run one or both of them as rotary converters and supply enough power to the motor on the bandmill's 3rd leg to get it up near the 7.5 hp nameplate rating?


So, my goal here is to determine how much hp I am getting from the 7.5 hp 3 ph motor on the bandmill and try to figure out how to get more from it or if I should just switch over to a 5 hp single phase motor.



3. Related question- is this 3 phase motor running at nameplate rpm when running on single phase or is it actually turning slower since the waveform is different?



.

 

[Cat_Driver]

http://www.lmpforum.com/forum/topic/1586-3-phase-motor-on-single-phase/

 

[Craig Clayton]

I understand what you are doing but I can not determine you HP.
Here is another thought .

Stay with the 3 phase 7.5 HP
use your 240 V single phase supply to supply a VFD .

My knowledge of VFD,s is on 600 VAC 3 phase supply to 600 VAC 3 phase loads.
I program them at the plant in Canadian electricity.

VFD means variable frequency drive. They take 60HZ convert it to DC then pulse it back as a square wave at 60 HZ 3 phase.
The advantage of a VFD is they will control the inrush on start up and then the HZ of the motor .
The only slight problem is they might strain the insulation of the older motors. New motors are rated as invertor duty.
You might need a matching tranformer from 240 V to VFD for 3 phase.

If you want more info I will post back.

Craig Clayton

 

[KYErik]

Craig- thanks for the info

I am on a tight budget- are you saying that I can easily and cheaply make a VFD out of the 2 extra 3 phase motors that I already own?

 

[Kyle_in_Tex]

I believe those statics reduce your HP by 1/3

Find an old big 3ph motor and build your own rotary.

 

[BertZegers]

The HP of the motor is about 50% if you run a 3 phase motor on single phase with use of capacitors.
The starting torque is 1/3 of the 3 phase starting torque, which doesnt matter to mach because you start without load.
A VFD would be ideal but expensive.
You cannot make a device out of your other motors, but you could put a second motor on the machine.(The 5 HP if the RPMs are the same)

Your motor runs very close to the rated RPMs

 

[KYErik]

The HP of the motor is about 50% if you run a 3 phase motor on single phase with use of capacitors.

A VFD would be ideal but expensive.
You cannot make a device out of your other motors, but you could put a second motor on the machine.

Thanks Bert- so with 50% decrease in power, then it is really only supplying about 3.75 hp...

If there is no way to use the other two 7.5hp 3 phase motors as rotary converters, then it looks like my best choice is to put the 5 hp single phase motor on the mill.

I'm not sure that the matched pair of 7.5 hp 3 ph motors would fit on the sawmill's cutter platform that raises and lowers- and if they did, I fear all of the extra weight would stress the adjusting system.

 

[KYErik]

I believe those statics reduce your HP by 1/3

Find an old big 3ph motor and build your own rotary.

Thanks Kyle- how much bigger of a 3 phase do you think I'd need to find in order to use it as a rotary converter?

 

[Tractor Seabee]

Thanks Kyle- how much bigger of a 3 phase do you think I'd need to find in order to use it as a rotary converter?

If your voltage on all three legs are the same and your wire is sized for the HP the HP output should be close to the same. Any of these solutions have a tendency to feed a lot of transients onto the power system that can screw up electronic stuff. The older the equipment is, the less effecient it will be. Your motor is dumb hunk of metal and is trying to develop the HP it was built for. The effeciency factor applies to the incoming power to the motor and phase converter. Back in the fifties when I was doing HVAC work in Phoenix AZ folks were ramping up on AC systems and all the fast growing resedential areas only had single phase power distribution. The power company solution was to build open-delta three phase transfromer banks. AC with 3 phase motors were a lot cheaper and readily available than expensive single phase motors. Open delta only uses 2 transformers on the poles. They had to add a lot of capicitor banks into their system also as the motor loads screwed up the power factor. On that system only the 3rd leg was used for motors as it was called the wild leg and had around 200 volts to ground, not good for lights and toasters. Any way a 7.5HP compressor still delivered 7.5 HP. Check this out with a clamp on amp meter, each leg at the motor should be close to the same and at full load draws around 22 amps with volts a 220. As the load goes up the amps go up, over the full load and the motor and wiring will get hot quick, hence overload protection. HP is a function of the amps and volts in simplicity. With your system or an open delta transformer system motors will run somewhat hotter due to the imbalances. I tried to make this simple but it a complex issue that is primarily of an engineering nature and I left a lot of details out. Lots of good info on the web though.

Ron

 

[KYErik]

If your voltage on all three legs are the same and your wire is sized for the HP the HP output should be close to the same. Any of these solutions have a tendency to feed a lot of transients onto the power system that can screw up electronic stuff.

Ron

Ron- are you saying that when I have this 3 phase motor turned on, I am altering the wave form for all of the circuits supplied by this transformer?

So, while I am running this motor off a circuit in the garage, electronic equipment in the house might also be affected?

 

[slowzuki]

I bit the bullet and got a single phase 10 hp for my mill. If I did it again, I would have put 2x 5 hp motors instead. Cheaper, easier to start (lower peak inrush). The 2.5 hp carriage advance motor I have is 3 phase but I put an ebay vfd on it. Don't need a single phase rated inverter, you can just derate them.

 

[LD1]

I am not sure, but I think he only means screwing up stuff you are feeding with the phase converter???

I agree that the best "low cost" solution would be a rotary.

A 3 phase motor will "actually" run on 240v, it just wont start.

A rotary converter uses capacitors to "start" the motor, and then it runs on 240 single phase through two of the three sets of windings in the 3PH motor. Induction causes power to be created and sent out that 3rd leg.

Due to the inefficiency of that process, the converter is usually upsized one. So if you want to power a 7.5HP motor with a rotary, the base motor for the rotart should be a 10HP.

And while they are close, the voltage on the 3 legs (two from your 240 supply, and the one created) are not exactally equal. This will screw with electronics like lathes and mills that have certain features.

But for what you want, a rotary should work just fine:thumbsup:

 

[LD1]

I bit the bullet and got a single phase 10 hp for my mill. If I did it again, I would have put 2x 5 hp motors instead. Cheaper, easier to start (lower peak inrush). The 2.5 hp carriage advance motor I have is 3 phase but I put an ebay vfd on it. Don't need a single phase rated inverter, you can just derate them.

I was replying while you must have been typing that:confused2:

That should work just fine:thumbsup:

 

[zzvyb6]

My 50kw Winpower pto generator has 3 phase taps on it. It was only $900. You have a tractor, eh?

What about converting the bandsaw to tractor powered by rigging up a belt or chain drive to the pto directly? There is a sawmill that I go to a few miles up the road that uses a JD 4430 to drive a huge circular sawmill blade with its pto output. That baby cuts thru 30" oak and hickory logs without a single smoke puff from the governor (or the Attorney General).

 

[Craig Clayton]

I wanted to give you a quick wiki about VFD,s. This teck is newish.

Variable-frequency drive - Wikipedia, the free encyclopedia

It will control the inrush on motors.

Craig Clayton

 

[Tractor Seabee]

Ron- are you saying that when I have this 3 phase motor turned on, I am altering the wave form for all of the circuits supplied by this transformer?

So, while I am running this motor off a circuit in the garage, electronic equipment in the house might also be affected?

Yes, everything on the output side of the utility company transformer on the pole, could be affected, depends on how good the items filter system in its power supply. Depending on the vintage of your phase converter the tgransients fed back could vary a lot. That is one reason why commercial and industrial buildings have seperate transformers for lighting/plug and motor/power loads other than different voltages.

Bottom line, if it all works don't worry about it. If there is an interference problem this info can give you a clue why and where it comes from. 3 phase motors cost a lot less than single phase and are more effecient. We always specify motors and other power loads 1 HP and larger as three phase when 3 phase is available.

My brother-in-law has a big farm in Oregon where there is no three phase utility power; nhe tried to get them to do a seperate open-delta system for pumps and stationary machines and they refused. No reason except they never do anymore, they bwanted $15K to run 10 miles of wire to give him regular 3 phase. He declined and uses all single phase motors. I offered to do the seperate transformers on his side of the meter which would have included a isolation transformer ahead of the open delta ones but he did not want to bother at that time. That would have been around $3K for the loads he planned.

No, I am not an engineer or an electrician, just a guy with a lot of practical experience working around motors.

Ron

 

[5030]

30% useable power reduction with static conversion. That's 30% nameplate horsepower reduction, and starting torque (because an electrical motor produces maximum torque at stall).

What occurs is the capacitor bank provides the T3 current and electromechanically excites the T3 windings when starting.

All 3 phase motors regardless of winding style will self excite the T3 leg after reaching rated rpm. (Wye or Delta wound)

Rotary conversion is much better (and more expensive) because a slave motor provides the necessary T3 excitation and running current. It's quite possible to spin an old 3 phase motor with a single phase motor (induction-repulshon 3 phase motor) and pull the generated T3 leg to run your 3 phase motor efficiently at rated nameplate power. Just keep in mind that the T3 motor you use a slave motor must have a higher nameplate power rating than the motor(s) you wish to run, however, 3 phase motors are stackable, that is, multiple motors at running RPM will generate the T3 leg in enough amperage to run additional motors (so long as your wiring is of sufficient gage to carry the amperage).

I use dedicated rotary conversion in my shop because, one, it creates less internal heat in the motor, two, no loss in starting/running power and three, I need true 60hz power to run my frequency controlled machines.

 

[KYErik]

What about converting the bandsaw to tractor powered by rigging up a belt or chain drive to the pto directly?

My bandmill has a moving carriage/blade assembly (log is stationary), so designing some sort of 13 foot sliding driveline would be tough.

Here is the thread that I posted (with pics) when I finished building the mill about 6 yrs ago: http://www.tractorbynet.com/forums/build-yourself/74963-home-built-bandsaw-mill.html

 

[KYErik]

Yes, everything on the output side of the utility company transformer on the pole, could be affected, depends on how good the items filter system in its power supply. Depending on the vintage of your phase converter the tgransients fed back could vary a lot. That is one reason why commercial and industrial buildings have seperate transformers for lighting/plug and motor/power loads other than different voltages.

Bottom line, if it all works don't worry about it. If there is an interference problem this info can give you a clue why and where it comes from.

The static phase converter is at least 40 years old. The capacitors are stacked inside a grounded metal box. This static converter starts up the motor nicely, but sparks literally fly/arc between the caps when the power to the motor is shut down.

Is it possible to detect any interference/transients using a digital voltmeter? (I don't have an oscilliscope).

 

[KYErik]

30% useable power reduction with static conversion. That's 30% nameplate horsepower reduction, and starting torque (because an electrical motor produces maximum torque at stall).


Rotary conversion is much better (and more expensive) because a slave motor provides the necessary T3 excitation and running current. It's quite possible to spin an old 3 phase motor with a single phase motor (induction-repulshon 3 phase motor) and pull the generated T3 leg to run your 3 phase motor efficiently at rated nameplate power. Just keep in mind that the T3 motor you use a slave motor must have a higher nameplate power rating than the motor(s) you wish to run, however, 3 phase motors are stackable, that is, multiple motors at running RPM will generate the T3 leg in enough amperage to run additional motors (so long as your wiring is of sufficient gage to carry the amperage).

a home made rotary converter- This is what I was considering using the 2 extra 7.5 hp 3 phase motors for (as rotary converters). So if I was to spin up the slave motors (one at a time), then I could gain back the 30% reduction and could run the 7.5 hp bandmill motor at its full rated capacity?

Do you have a guess as to how many amps of single phase your rotary converter uses by itself?

I was just considering how much I'd have to upsize the breaker and wire for this circuit if I was also running the two other 7.5hp motors as slave motors for the one on the bandmill and knowing the slaves' consumption would help me to calculate the increase in electrical load.