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?