New(to me) tools

[ODdave]

My mill is 1.5hp, the 3phase motor thats been sitting in my shop for 3yrs is 3hp. Will that work?

It should be fine, Just keep an "eye" on the mills motor. let it run for a while and keep checking it to see if it starts to get abnormally hot.

There is a chart somewhere for the correct size, but I dont remeber where.

 

[m16ty]

I've built several phase converters for other people. I've always been told to go at least a few hp over the motor you're trying to run.

I've recently added a 3 phase lathe to my home shop and have all the stuff to build a rotary converter. I'm thinking I may just go with a static unit though for simplicity. I've even thought about just swapping the motor out to a single phase motor. It's just a 3/4hp motor and it's the only 3 phase tool in my home shop.

 

[crazywelder72]

i wouldn't change to a single phase. if you do and need torque at low rpm you wont have it.

 

[m16ty]

i wouldn't change to a single phase. if you do and need torque at low rpm you wont have it.

Could you explain this statement more? I'm not understanding why you would loose torque at low RPMs. The motor always runs a constant speed on the lathe.

 

[crazywelder72]

It was explained to me and i will do my best to explain it to you but i am no electrical engineer. (but i know a few so i may ask) Yes the motors spin the same rpm wise but the lower the speed the easier it is to bog down, even stop a single phase motor (if taking a heavy cut or boring a large hole). Somehow a 3 phase motor is able to escape from the cause that would bog down the single phase. No matter how slow the rpm, it has it's full torque from the motor to keep rotating.

I guess it's like saying if you trying to pull out a stuck vehicle and you are going 5mph in 4WD High, nothing will happen. but the same speed in 4wd Low will get it out. it's the best i can think of right now. but i will ask my friend.

 

[ODdave]

It has to do with the capacitor starter on the single phase motor. There is a cetrifugal switch in the it that opens when the motor reaches speed. When you bogg a single phase motor the switch will chatter and cause irattic pulses through the windings therefor loosing power.

A 3 phase motor dosnt need capacitors to start so it dosnt have this problem. Its still not good to bogg it down but it will handle it better.

Think of it as comparing a gas engine to a diesel

 

[crazywelder72]

Thanks for helping me try to explain it. I hope my information was not to far off target. I am curious to what my electrical engineer friend sends me for a description... This should be good. As soon as i get it, I will post it.

 

[gimpyrobb]

- Power in a 3 phase power supply never drops to zero but in single phase the power falls to zero three times. Therefore for a 3 phase power supply the power delivered is the same at all times.
- Smaller conductors can be used in 3 phase power supplies and therefore it makes it 3 phase more efficient at supplying power to large electrical machines.

Can you tell I've been reading up?

 

[Speddmon]

you can read?????????

 

[crazywelder72]

Yeah, what he said.... lol seriously that makes perfect sense (at least to me now)

Gimp, you got to change that avatar. that is just disturbing.

 

[gimpyrobb]

I've been trying, but I can't get a picture that was texted to me onto my pc! Wait till you see the new one.

 

[crazywelder72]

you can read?????????

I can read fine, just not type to good. lol just ask DH

 

[crazywelder72]

I've been trying, but I can't get a picture that was texted to me onto my pc! Wait till you see the new one.

try to email it to your self or pull the card and use a card reader. or bluetooth it to the pc...

I need a geek smiley, or maybe a Darth Vader smiley. (r2d2 would be cool as a smiley also)


Did R2D2 have a single phase or 3phase motor

 

[m16ty]

- Smaller conductors can be used in 3 phase power supplies and therefore it makes it 3 phase more efficient at supplying power to large electrical machines.

I know 3 phase is more efficient but is efficiency lost when you use a phase converter of some sort? I would think a 3 phase motor ran by a converter would actually be more inefficient than if you just used a single phase motor but I'm not sure.

 

[gimpyrobb]

I understand your question. The answer I posted was dealing with 3ph vs single phase, no converters accounted for, just power vs power.

 

[crazywelder72]

Ok Here IS the answer... Straight from one of the best electrical engineers i have ever met, and i have met a few over the years.

Hi Dave,
Any AC motor turns when there is a phase shift. But, with a single phase motor the phase shift is developed by some clever phase shifting electrical arrangements. You will see a lot of different types of phase shifting designs (capacitors, shaded pole, split phase, etc.) All of these clever methods are designed to work at certain speeds and will not give a constant torque over the entire speed range. However, with a three-phase motor, the phase shift is a constant 120 degrees and the frequency of the phases is determined by the source (60 Hz from the power company or a variable Hz from a motor speed controller). The three phases produce a rotating magnetic field in the motor stator. The rotating field induces a current in the rotor which cause the motor to turn. As long as the rotating stator field is a little faster than the rpm of the motor there will be a torque and the motor will turn. Even if a 3-phase motor is completely stalled, the rotating magnetic field will still !
produce full torque. With a single phase motor, the designer has to select some speed range to optimize and that gives very bad performance at other speeds.
I hope that helps.
Gary

Thats the best i can give you for an answer.
Dave A.K.A. Crazywelder72