3ph motor started with 1ph motor

[donselectric]

hi all.
anyone ever use a single phase motor to start a 3ph motor and run ?
i have a guy that wants to do that for a metal brake. i remember motor- gen
sets in school but dont know how this would work or how to wire it .
i would think it would single phase cause he said it was just to start
the 3ph motor ....thanks

 

[Jraef]

What's the purpose? Is he wanting to make a Rotary Phase Converter of run a machine from the 3 phase motor with giving it only 1 phase power?

Some people seem to think they can run a machine with a 3 phase motor from 1 phase power if they can just get it started, so they think that using a pony motor setup like this will suffice. But he also has to consider that the 3 phase motor will end up severely unbalanced. You basically have to de-rate the HP load on the 3 phase motor to 1/3 of the motor's nameplate rating to be safe, and that makes it extremely inefficient compared to just buying a 1 phase motor of the same net HP.

So for example, if he has a 30HP 3 phase motor and he spins it up with a small 1 phase motor before applying the 1 phase power to it, the load he can safely connect to that 3 phase motor will be no more than 10HP (maybe less). If the load only requires 10HP, he would be far far better off from an efficiency standpoint just using a 10HP 1 phase motor.

 

[donselectric]

its a 3ph piece of equip and he has a 1ph service ... i tried telling him it wont be rite
but some guys you cant convince so i figured i could use it as a learning experience.
but i dont know how to hook it up...

 

[Jraef]

So it sounds like he wants to create a "pony start RPC", Rotary Phase Converter. There's more to it than just the motors by the way. This is a pretty good design, you may be able to find others on the web, but they are generally "self starting" which means more capacitors and no 1 phase motor. But notice that this 3 phase motor is labeled as an "idler", which means it runs all the time, separately from the 3 phase motor on the machine. That's how this system works; you can't hook up your load to the idler. And the idler needs to be larger than the motor you connect to it. Opinions vary on that and it depends a lot on the nature of the tool, but generally its by about 50% (but I've hear everything from 20% to 100%).

 

[eric9822]

I have never seen that. Thanks for posting Jraef.

 

[Rick Christopherson]

All he is doing is creating a "mechanical" static converter. That's because a static converter does nothing more that gets a motor turning, and then cuts out to leave the motor to single-phase. This in no way creates a rotary phase converter, not even if the motor is treated as an idler motor that is separate from the final drive motor.

 

[broadgage]

A 3 phase motor will run on a single phase supply if started by some external means. To prevent overheating, the current input and therefore the HP output must be limited to about one third of the figures when used as intended on a 3 phase supply.

It is usually simpler and more economical to either replace the motor with a single phase one, or to produce 3 phase from single phase.
This may be done either by a static phase converter, or by a single phase motor mechanicly driving a 3 phase alternator.
Alternatively, many but NOT ALL variable speed drives will operate a 3 phase motor from a single phase input.

 

[massfd]

A 3ph motor can be made to start and run by it's self on single phase by using a potental relay, a start cap and a run cap.

The start cap and run cap bridge 1 incoming phase to the motors 3rd phase lead, after the motor starts the potental relay disconnects the start cap but leaves the run cap in the circuit.

It basicly makes a 3 phase motor a single phase cap start/cap run motor except the potental relay replaces the centrifical switch.

Most 3 wire well pumps (in the well type) use a control box that switches the caps in for starting as does most single phase commercial refrigeration equipment.

There is some loss on horsepower but with the proper size run cap it's not that much. I have an old ingersoll rand air compressor (5 horse direct drive) that I have been running this way for years. It starts unloaded and when it loads up it runs just like it did on 3ph.