480V 50A 3P needed. 240V 200A 1P available.

[scsmith42]

There are such things as soft starters, which will mitigate the inrush. Most power companies require them on anything over 5 or 10 HP.

That's a great concept, but isn't a soft starter suitable for the current inrush of a 50hp RPC going to cost in the thousands of dollars?

What I'm wondering for the OP is that by the time he purchases a 50hp RPC, and a soft start suitable for the huge current inrush on a 50hp motor, would he spend less by buying a good, used 3 phase generator?

I haven't found the specs for current inrush on a 50hp single phase motor, but am estimating it to be well over 250A

 

[JST]

Not necessarily. Another contactor of the same rating as the one to turn on the motor, a time delay, and some reasonably high power resistors.

Only "reasonably" high power because they are not in-circuit long enough to heat up much.

And the point is to NOT have that high current... so the resistor need only handle what it allows to flow in terms of current, and that for a short time. Same idea as VFD braking resistors, which theoretically dissipate kilowatts, but are often rated in the 200-300 watt area..

Then also, some soft starts are SCR, so their dissipation is very low and their cost is very reasonable (depends on current level)

 

[johansen]

you just run the 50hp up with a pony motor run from a vfd. this is like.. 400$, 250 for a good vfd and 150 for a 1hp 3 phase motor.

two contactors may be warranted, connect the motor (and its hundreds of uf of run caps) to the grid through a 2 ohm resistor, to limit the inrush current to 100 amps. wait 1 second and short out the resistor(s)

 

[snowman]

I'll find a motor for $800. A transformer will cost about the same. I can start off a 2 hp pony motor on a VFD. Once it's up to speed, turning the power on will keep it going. I think I've already got a 200 amp knife and some rated contactors that I bought for the silver content but never tore down.

I'd love a 3 phase 480 generator. Cost would probably be at least 10 grand. I can buy a 208 15 kw induction melter new for 17 grand, it'll be chinese, but it'll work. As it is, this induction melter needs work, and I don't know what it's going to need.

 

[idacal]

Gov planet has 100 kw gensets low hour going for 2-5 grand with 3306 cats on them. I run quite a bit of 480 stuff off of A gen set. its painless besides the fuel up. But then you know how much your costs are no suprise charges on your power bill. No burning uptransformers. They actually dont think thats very funny. My shop has 3 phase 200 amps 240 volt and i am not permited to start more than 25 horse or run more than 40. The few times i tried it costed way more than the 5 gallons of diesel in the generator per hour.

 

[Garwood]

Why can't you just spin up a 50hp rpc motor with a small vfd?

There was a guy on here that showed spinning up a 100hp motor using a tiny vfd powered by like a 16awg extension cord.

 

[scsmith42]

I'll find a motor for $800. A transformer will cost about the same. I can start off a 2 hp pony motor on a VFD. Once it's up to speed, turning the power on will keep it going. I think I've already got a 200 amp knife and some rated contactors that I bought for the silver content but never tore down.

I'd love a 3 phase 480 generator. Cost would probably be at least 10 grand. I can buy a 208 15 kw induction melter new for 17 grand, it'll be chinese, but it'll work. As it is, this induction melter needs work, and I don't know what it's going to need.

In addition to the GovPlanet gensets, your local utility company may have good used take outs from their facilities. I have a 400hp 480V gen set that I bought used for $3,600 about 12 years ago. I had to put around 3K into it, but it only had 307 hours on it so the investment has paid off.

Two years ago I bought a surplus natural gas 35KW genset from the local utility for 3K. It can be wired for single or 3 phase and only has 435 hours on it.

 

[snowman]

Gov planet has 100 kw gensets low hour going for 2-5 grand with 3306 cats on them. I run quite a bit of 480 stuff off of A gen set. its painless besides the fuel up. But then you know how much your costs are no suprise charges on your power bill. No burning uptransformers. They actually dont think thats very funny. My shop has 3 phase 200 amps 240 volt and i am not permited to start more than 25 horse or run more than 40. The few times i tried it costed way more than the 5 gallons of diesel in the generator per hour.

Seriously? That's dirt cheap.

I had no clue as I just did a basic google search. My experience with generators has been limited to the natural gas one I bought at the vet hospital, and it sure as heck set us back more than 5 grand, and still doesn't have nearly that capacity.

 

[scsmith42]

My wife is a vet with two mobile hospitals. They each have 12KW EPS diesel generators that we’ve been real pleased with.

I can see where a new NG generator rated for continuous power would be expensive. The key thing is to find a good, used takeout from a commercial operation that outgrew it.

 

[SomeoneSomewhere]

So, it's actually possible to build a voltage doubler & rectifier using nothing more than diodes and capacitors, though you'll want some extra inductance and snubbing and some pre-charge, especially at this power level.

Simulator example.

A capacitor array to deliver 15mF+ at 400VDC (rated) with ~100Arms of ripple is going to be fairly expensive and sizeable (and you need two) , but likely far better than an RPC or genset unless you're getting them at scrap prices. More efficient too, although the power factor on this still isn't going to be great. This is delivering 27kW at ~520VDC, with an input current of 150Arms - 36kVA.

Rough estimates show spending $3-400 all up on the cap bank, and another $1-200 on the rectifiers and auxiliaries.

 

[snowman]

I like it, but I'm afraid it's beyond me a bit.

 

[JST]

So, it's actually possible to build a voltage doubler & rectifier using nothing more than diodes and capacitors, though you'll want some extra inductance and snubbing and some pre-charge, especially at this power level.
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Output is DC...... I thought the OP wanted to run a motor......

 

[snowman]

Output is DC...... I thought the OP wanted to run a motor......

No, I want to run an induction heater. The first thing the induction power supply does is run the power through a 3 phase rectifier. He's saying to rectify the 240 with a voltage doubler circuit and bypass the rectifier I already have.

 

[johansen]

No, I want to run an induction heater. The first thing the induction power supply does is run the power through a 3 phase rectifier. He's saying to rectify the 240 with a voltage doubler circuit and bypass the rectifier I already have.

If the caps in the induction heater are 2x 400v in series, you already have a doubler. Connect one line to the mid point of the capacitor string, the other goes to all 3 line input terminals of the rectifier.


You will need a soft start circuit (or a variac and a relay to bypass it) and a lot more 400v capacitors to take up the current. But it would be cheaper than a transformer and a motor.