Phase Convertor Conductor Sizing.

[frankft2000]

I have a cut sheet here for a 29 KW phase converter by TEMCO. It states that the 1-phase fuse recommended is 175 amp, fine. Then it states the recommended 1-phase wire size is 1AWG (I'm assuming copper). I am going through articles 430 and 455 but I'm having trouble justifying the #1 on a 175 amp fuse, even with start up. Can any body shed any expertise on this subject?

 

[frankft2000]

I take it no! lol

 

[ericsherman37]

I don't have any expertise on the subject (never installed one), but 455.6(A) and 455.7 seem pretty black-and-white to me.

Looks like the wire and OCPD sizing depend on the load (fixed or variable).. does the manufacturer's recommended sizing take that into account? Or does it give just a "one size fits all" deal?

 

[iwire]

It is a motor / generator so it does not surprise me that the fusing would be in line with motors.

1 AWG is more then large enough for 29 KW @ 240 single phase

I assume the primary single phase voltage is 240?

 

[VT@VC]

here is what most phase converters do....it electroniclly "fakes" a third 120 degree (out of phase) leg for your 3 phase motor (?). This is for starting only. Once the motor is up to speed and the CEMF is high enough the "fake" leg drops out. The motor(?) will continue to run with single phase power. What needs to be understood is the 240v supply voltage will become "fake"240v 3 ph for only a short period. The single phase running current (FLC) will be much higher and the motor will run warm. For short duration jobs on a 3 ph. mill or lathe (20 minutes or less) you should be OK. NEC overcurrent protection values will be different depending on the type of OPCD you use. Assuming this is a "fixed load" take 29,000 watts/240 volts or 121 amps and multiply by 125% {455.7 (B)}. This gives you 151 amps. The next standard size OCPD will be a 175 amp time delay fuse {240.6(A)}. These converters are very power sensitive. I wouldn't oversize or undersize but keep it real close to your power needs. Staying within listed nameplate values will reduce the hazards of manipulating power systems into doing something other than what they are designed to do.

 

[iwire]

here is what most phase converters do....it electroniclly "fakes" a third 120 degree (out of phase) leg for your 3 phase motor (?).

Lets say some phase converters work that way. :smile:

This is for starting only. Once the motor is up to speed and the CEMF is high enough the "fake" leg drops out.

I disagree with that entirely.

What needs to be understood is the 240v supply voltage will become "fake"240v 3 ph for only a short period.

Where did you get this information?

 

[VT@VC]

I hooked one up for a neighbor. It was an electronic 240 volt 1 phase to 3 phase converter. He had an old Mazak (sp) 3 phase mill. I read the instructions and listing info that came with the unit. There was a wiring diagram with it. You could actuall hear (click) the starting leg drop out. It was rated in specific horsepower, 3.5 - 5 HP. No lower and no higher. This was quite a few years ago. I'm sure it was old square wave or modified sine wave technology. I know these things may work differently today with pure sine technology. i beleive he got this one at Graingers (sp ?). Initially he tried to hook it up himself and "single phased" the mill motor when it tried to start. It took out the service main several times. He also smoked the first unit. As far as I know its still working ? Sorry you disagree. Whats your take on these things ?

 

[iwire]

I You could actuall hear (click) the starting leg drop out.

That would be the starting winding for the single phase motor of the rotary converter, not the derived third phase output.

I would be pretty upset if I bought a 3 phase converter that only made 3 phase for a short time.

 

[VT@VC]

The M/G type would give a continuous 3 phase output. My neighbor just wanted a cheap way to run (start) his machine. The power company loved it.

 

[iwire]

The M/G type would give a continuous 3 phase output.

Yes, and the rotary type will as well.


You can actually just take a standard three phase motor feed it single phase and connect a three phase load to to that. The only issue with this is the three phase motor you using for the converter might need a push to get it rolling.

 

[iwire]

How does a rotary phase converter work?

 

[VT@VC]

the unit I installed was the solid state electronic type about the size of a cigar box (not the motor generator type) 2 wires w/ground in 3 wires w/ground out I think Graingers still sells them.

 

[iwire]

the unit I installed was the solid state electronic type about the size of a cigar box (not the motor generator type) 2 wires w/ground in 3 wires w/ground out I think Graingers still sells them.

That sounds like a 'static' phase converter. The type the person is asking about is not a solid state type.

As I understand it, and I may be wrong there are 3 basic designs of phase converters.

1) If you have a solid state unit it is a static type phase converter.

2) If you have a AC motor directly coupled to a AC generator you have a motor generator type phase converter.

3) If you have one that just looks like a single electric motor without any shaft sticking out you have a rotary phase converter.

It is this last one that is what the person is asking about.

 

[frankft2000]

here is what most phase converters do....it electroniclly "fakes" a third 120 degree (out of phase) leg for your 3 phase motor (?). This is for starting only. Once the motor is up to speed and the CEMF is high enough the "fake" leg drops out. The motor(?) will continue to run with single phase power. What needs to be understood is the 240v supply voltage will become "fake"240v 3 ph for only a short period. The single phase running current (FLC) will be much higher and the motor will run warm. For short duration jobs on a 3 ph. mill or lathe (20 minutes or less) you should be OK. NEC overcurrent protection values will be different depending on the type of OPCD you use. Assuming this is a "fixed load" take 29,000 watts/240 volts or 121 amps and multiply by 125% {455.7 (B)}. This gives you 151 amps. The next standard size OCPD will be a 175 amp time delay fuse {240.6(A)}. These converters are very power sensitive. I wouldn't oversize or undersize but keep it real close to your power needs. Staying within listed nameplate values will reduce the hazards of manipulating power systems into doing something other than what they are designed to do.

Thanks to all of those who replied. This unit is a rotary type.

 

[ericsherman37]

I would be pretty upset if I bought a 3 phase converter that only made 3 phase for a short time.

lol.. "Quick, you have about 30 seconds to get that timber through the planer before it single-phases and burns up!"

or..

"How much milk can your milking machine get per cow in 30 seconds time?"

 

[VT@VC]

check out the Grainger website. The 3-5 HP static converters are still there at about 20% of the cost of the M/G type. Looks about the same size to.

 

[hurk27]

As I understand it, and I may be wrong there are 3 basic designs of phase converters.

There is one more that manufactures use to run three phase motors on single phase, but are not available as a package to be sold as they are engineered for a particular motor. Wascomat washers use this system, which comprises of two banks of capacitors used to shift the third phase, if I remember right one is startup, and one bank is run.

Most static converters will only allow a motor to produce 60-75% of it's original HP, but you can slave another 3-phase motor to it to flywheel the main motor up to full power, most roto-phase units are nothing but a 3-phase motor without an output shaft, with the capacitors to start them as in the above Wascomat washers, or have the static converters driving the motor.

 

[acrwc10]

How does a rotary phase converter work?

Interesting link. I have never connected a phase converter ( when ever someone has asked me about one the price scared them off )In the link, it looks as if you are just connecting A & B phase of the motor to both the line and the output and then C phase to the output, is that correct ? or is there a seperation between the input and output on A & B phase ?

 

[iwire]

In the link, it looks as if you are just connecting A & B phase of the motor to both the line and the output and then C phase to the output, is that correct ?

With the rotary type it is just that simple and as I mentioned you can make one out of a standard three phase motor.

 

[stew]

there is no separation between the a/b . the clicking noise is of course the start caps dropping out. this design wiil of course still give you the third phase at all times. ARCO produces a unit which I sold and installed for many years that does not use any relays. There is actually a modification made to the rotor also which allows more slip and lets the unit start smoothly. Arco uses a connection like a capacitor start/capacitor run motor without the need of a relay. The arco rotary as well as most other rotary units will give very nearly full hp and torque as opposed to the electronic or all cap type which will only give 65 to 70 %. Thats why I only sold the Arco unit as we NEVER had a connected load be a problem as long as the unit was sized correctly.