220V 3 phase => 60 HP, how many amps?

[Rich Grise]

I've googled the formula for 3 phase power calcs, and sadly,
I'm stumped.

The PHB just bought a used machine with a 60 HP motor; he
says the motor is DC, but the machine comes with a 440V
3 phase motor-generator, to make the DC.

His shop has 220V three phase, and he wants to know if he
can run this thing off a 100A panel; he needs to buy a
220->440 3ph. transformer, and needs to know how many amps
of tranformer he needs. But transformers are rated in Volt-Amps,
which is another confusion factor.

I never learned the 3-phase fomulas in the first place, and
it's been so long since I've done any trig and stuff that
I've pretty much forgotten it all. )-;

In other words, he's got
220V 3 phase -> transformer -> 440V 3 phase -> MG -> 60 HP motor.

Can someone either point me to a simplified formula that I can
grasp, or want to do my homework for me?

I'm not getting paid for this, it's included in my "retainer".
(he lets me park my RV in his parking lot, and use the computer
after hours.)

Thanks,
Rich

 

[Ralph in NH]

I've googled the formula for 3 phase power calcs, and sadly,
I'm stumped.

The PHB just bought a used machine with a 60 HP motor; he
says the motor is DC, but the machine comes with a 440V
3 phase motor-generator, to make the DC.

His shop has 220V three phase, and he wants to know if he
can run this thing off a 100A panel; he needs to buy a
220->440 3ph. transformer, and needs to know how many amps
of tranformer he needs. But transformers are rated in Volt-Amps,
which is another confusion factor.

I never learned the 3-phase fomulas in the first place, and
it's been so long since I've done any trig and stuff that
I've pretty much forgotten it all. )-;

In other words, he's got
220V 3 phase -> transformer -> 440V 3 phase -> MG -> 60 HP motor.

Can someone either point me to a simplified formula that I can
grasp, or want to do my homework for me?

I'm not getting paid for this, it's included in my "retainer".
(he lets me park my RV in his parking lot, and use the computer
after hours.)

Thanks,
Rich

Hi Rich,

eBay usually has plenty of transformers, there are some good deals if you're
patient and have good timing - search on "75 kva transformer" and you should
get some hits. I'm guessing what you have is 120/208 VAC service, and you
want to go to a 480 VAC delta for the motor. 480 delta to 208 Y seem to be
the most common stepdown transformers offered there. Usual disclaimers,
YMMV, I never said any of this, not a PE although I'd be willing to play one
on TV. Be conservative and allow yourself plenty of headroom.

Best regards,

Ralph in NH

 

[Rich Grise]

eBay usually has plenty of transformers, there are some good deals if you're
patient and have good timing - search on "75 kva transformer" and you should
get some hits. I'm guessing what you have is 120/208 VAC service,

Eek. I've just gone and probed a panel, and it's 240 phase-to-phase,
and 208 to "neutral" on one phase, and about 120 to "neutral" on the
other two. Sounds like delta with one winding center-tapped, which
is why I say EEK!

Thanks,
Rich

 

[Jamie]

Eek. I've just gone and probed a panel, and it's 240 phase-to-phase,
and 208 to "neutral" on one phase, and about 120 to "neutral" on the
other two. Sounds like delta with one winding center-tapped, which
is why I say EEK!

Thanks,
Rich

33kv xformer is what you need.
60 HP at 480 is ~ 70 amps.
after step up, this will be 140Amps on the
primary side (220)/

The 100A panel won't do it.

Oh well.
Change the motor to a 220V system.
it's cheaper.

 

[John Larkin]

Eek. I've just gone and probed a panel, and it's 240 phase-to-phase,
and 208 to "neutral" on one phase, and about 120 to "neutral" on the
other two. Sounds like delta with one winding center-tapped, which
is why I say EEK!

That's the way our building is wired. The 208 line is called the
"stinger." Ignoring the neutral, it's a delta with 240 line-to-line.

Is the motor 440 l-l delta?

Hey, I spent most of the day pulling wires inside crawl spaces and
trying to to figure out what's connected to what. Why is it that
buildings don't come with schematics?

John

 

[Paul Burke]

The PHB just bought a used machine with a 60 HP motor; he
says the motor is DC, but the machine comes with a 440V
3 phase motor-generator, to make the DC.

His shop has 220V three phase, and he wants to know if he
can run this thing off a 100A panel; he needs to buy a
220->440 3ph. transformer, and needs to know how many amps
of tranformer he needs. But transformers are rated in Volt-Amps,
which is another confusion factor.

Power = sqrt(3)*V*I*cos(phi)

60HP = 45kW

V = phase to phase voltage
I = Power/(sqrt(3)*V*cos(phi))

Assume power factor = 0.8

So for a phase to phase voltage of 440V

I = 45e3/(1.73*440*0.8) = 74A

and VA rating of transformer = 75 * 440 * 3 = 100kVA.

 

[[email protected]]

I've googled the formula for 3 phase power calcs, and sadly,
I'm stumped.

The PHB just bought a used machine with a 60 HP motor; he
says the motor is DC, but the machine comes with a 440V
3 phase motor-generator, to make the DC.

His shop has 220V three phase, and he wants to know if he
can run this thing off a 100A panel; he needs to buy a
220->440 3ph. transformer, and needs to know how many amps
of tranformer he needs. But transformers are rated in Volt-Amps,
which is another confusion factor.

I never learned the 3-phase fomulas in the first place, and
it's been so long since I've done any trig and stuff that
I've pretty much forgotten it all. )-;

In other words, he's got
220V 3 phase -> transformer -> 440V 3 phase -> MG -> 60 HP motor.

Can someone either point me to a simplified formula that I can
grasp, or want to do my homework for me?

I'm not getting paid for this, it's included in my "retainer".
(he lets me park my RV in his parking lot, and use the computer
after hours.)

Thanks,
Rich

If your DC motor is 250V then you may be able to buy a unit that
converts from 3 phase to DC. This is more expensive to start with than
just buying a XMFR but it would save money on maintaince. If your DC
motor is 600V then you have to buy a step up XFMR any way I suggest
an autotransformer 75 Kva

 

[Rich Grise]

33kv xformer is what you need.

I assume you mean "33KVA", right?

Thanks,
Rich

 

[Bjarne Bäckström]

I assume you mean "33KVA", right?

I assume you mean "33kVA", right?
--

 

[Rich Grise]

On Thu, 18 Oct 2007 00:58:39 +0000, Rich Grise wrote:

Thanks for all the replies. I told the boss to get a 100KVA
transformer and the panel will need to be at least 150A per
phase.

Thanks,
Rich

 

[Paul Hovnanian P.E.]

I've googled the formula for 3 phase power calcs, and sadly,
I'm stumped.

The PHB just bought a used machine with a 60 HP motor; he
says the motor is DC, but the machine comes with a 440V
3 phase motor-generator, to make the DC.

His shop has 220V three phase, and he wants to know if he
can run this thing off a 100A panel; he needs to buy a
220->440 3ph. transformer, and needs to know how many amps
of tranformer he needs. But transformers are rated in Volt-Amps,
which is another confusion factor.

I never learned the 3-phase fomulas in the first place, and
it's been so long since I've done any trig and stuff that
I've pretty much forgotten it all. )-;

In other words, he's got
220V 3 phase -> transformer -> 440V 3 phase -> MG -> 60 HP motor.

Can someone either point me to a simplified formula that I can
grasp, or want to do my homework for me?

I'm not getting paid for this, it's included in my "retainer".
(he lets me park my RV in his parking lot, and use the computer
after hours.)

Thanks,
Rich

If you want to do this per code, look for a nameplate value on the
equipment. The M-G set will introduce some inefficiency into the system,
making the direct conversion from mechanical HP to KVA somewhat less
then accurate.

The quick answer is: You can't do this off a 100A panel. NEC Table
430.250 gives a full load current for a 60 HP 230V motor of 154A. As
your situation is quite a bit more complex than this, I wouldn't even
try to venture a guess as to what parts to buy to power this gizmo up.
Heck, I don't even know where you are and what code is applicable.

 

[Paul Hovnanian P.E.]

Eek. I've just gone and probed a panel, and it's 240 phase-to-phase,
and 208 to "neutral" on one phase, and about 120 to "neutral" on the
other two. Sounds like delta with one winding center-tapped, which
is why I say EEK!

A common arrangement. For the purpose of calculating motor loads, you
have a 240V 3 phase system.

You may also want to look around for nameplates and/or connection
diagrams. Its possible that the M-G set might be dual voltage and be
configurable for a 240V supply.

 

[YD]

Late at night, by candle light, John Larkin

That's the way our building is wired. The 208 line is called the
"stinger." Ignoring the neutral, it's a delta with 240 line-to-line.

Is the motor 440 l-l delta?

Hey, I spent most of the day pulling wires inside crawl spaces and
trying to to figure out what's connected to what. Why is it that
buildings don't come with schematics?

John

They do, sort of, for the benefit of the installers during
construction. They invariably vanish after a very short while.

- YD.

 

[Charlie Edmondson]

Late at night, by candle light, John Larkin
<[email protected]> penned this immortal
opus:




They do, sort of, for the benefit of the installers during
construction. They invariably vanish after a very short while.

- YD.

Actually, they did, before any construction. Of course, as built is
nothing like the schematic... ;-)

I have seen as-builts being produced. Never seen the final product though!

Charlie

 

[Rich Grise]

A common arrangement. For the purpose of calculating motor loads, you
have a 240V 3 phase system.

Yeah, I've seen the setup before - apparently the delta is totally
floating, and the CT winding gives 240 CT "lighting" power. The
thing that kinda weirded me out is that the panel I looked at
(actually, a sub-sub panel, right next to another machine) had
Black, Red, and Blue going to 3 big fuses, and white going directly
to the box frame, with green on the other side going to the machine.

You may also want to look around for nameplates and/or connection
diagrams. Its possible that the M-G set might be dual voltage and be
configurable for a 240V supply.

I saw a couple of very plausible answers today, and the PHB popped
his head into the office and asked, "What did you pay these guys
for these answers?" I said, "nothing, it's a public discussion group."

He grumbled a little, and said, "I guess I'm gonna have to get somebody
in here." and I said, "Yeah - your best bet would be a real electrician -
you're going to need somebody licensed to wire it up anyway." That
at least got him off my back.

Thanks,
Rich

 

[Rich Grise]

If you want to do this per code, look for a nameplate value on the
equipment. The M-G set will introduce some inefficiency into the system,
making the direct conversion from mechanical HP to KVA somewhat less
then accurate.

The quick answer is: You can't do this off a 100A panel. NEC Table
430.250 gives a full load current for a 60 HP 230V motor of 154A. As
your situation is quite a bit more complex than this, I wouldn't even
try to venture a guess as to what parts to buy to power this gizmo up.
Heck, I don't even know where you are and what code is applicable.

Southern California, so the code is probably pretty strict - the
California politicians LOVE to pass rules and regulations.

I finally told the PHB that he needs a real electrician for this one.

Thanks,
Rich

 

[Paul Hovnanian P.E.]

Southern California, so the code is probably pretty strict - the
California politicians LOVE to pass rules and regulations.

It'll be the NEC down there. But this should be a moot point since
California doesn't have enough surplus power left to run a 60 HP motor
anyway. At least that's what I've been told. ;-)

I finally told the PHB that he needs a real electrician for this one.

If this is work in a commercial building, his insurance will insist upon
it even if the local inspector lets it slide.

 

[Paul Hovnanian P.E.]

[snip]

I saw a couple of very plausible answers today, and the PHB popped
his head into the office and asked, "What did you pay these guys
for these answers?" I said, "nothing, it's a public discussion group."

Hey, at least you'll have some good puns ready for him. ;-)

 

[Jamie]

I assume you mean "33kVA", right?

Yes, sorry, I left off the A.
that's a bare bone minimum . that size will get
hot and should be well vented.
A better choice would be 50kva+

 

[Jamie]

On Thu, 18 Oct 2007 00:58:39 +0000, Rich Grise wrote:

Thanks for all the replies. I told the boss to get a 100KVA
transformer and the panel will need to be at least 150A per
phase.

Thanks,
Rich

Hope you have space for that