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60hz compressor conversion
2

60hz compressor conversion

60hz compressor conversion

(OP)
i bought a direct drive screw compressor with a 1977 Schorch 3ph 30kw 50hz, 1440rpm, 380v Y/220v delta 12 wire motor that was supposedly modified for 230v 60hz and I assumed this meant this modified motor was rewound to slip down to around 1500 rpm and so I install a 28 kva 230v delta/delta transformer.

It turns out that the importer changed nothing in the motor but only connected it YY with instruction to supply 230 volts but it is drawing 35 amps over the 220 volt nameplate FLA of 105A while pulling the supply voltage down around 210v even without full load.

i realize now that i have a 1750 rpm motor overspeeding a compressor by 20% while trying to produce 36kw but being under supplying the required 60hz voltage of 115% or 260v. I re-tapped the transformer to 240v and beefed up my cabling but this is not working out.

I can manage my overload cycling to control transformer and motor temperature but it seems i have to reduce the load and/or increase the voltage enough to get some hours out of this setup.

A bigger distribution transformer is out of the budget but i had bounced around the idea of having the tap modified by dropping primary turns or adding secondary turns to get the voltage up but i have to acknowledge that i am up against the power limit already.

From what i learned since yesterday, i dont believe an ungrounded delta boost transformer is an option.

the VFC option is out of the budget and i also wonder the quality of that solution anyway given the low supply voltage.

While i have yet find a reference, i am considering ways to add a belt drive to slow the pump down and have a sketch here of offseting the motor latterally with an adapter plate and adding a cutout of the bell housing. Any comment would be appreciated.

i found that for NEMA T frame motors a 15% undervoltage reduces max torque by around 15% and increased full load amps by around 15%.

Can i assume that my pully ratio will target the nameplate FLA of 105 amp at 220 volts? I have a rule of thumb for air compressors generally using 2.5 Amps of 230V 3ph per HP an the factory paired a 40HP motor.

Do i get any credit for supplying an additional 10 or 20 volts to the 220V nameplate voltage?

if i have a 1.2 service factor does that give me license to exceed the FLA 20%?

What are the relative affects of each an overload and an undervoltage?

I guess I am looking for numbers that will burn up this thing after a few months rather than rite away while i am shopping for another 30kw 230v motor.

RE: 60hz compressor conversion

Changing voltage will do nothing (or, rather, not much) to the speed. You need to apply a VFD to get the right operating conditions. Rewinding the motor is not an option either because you can only have 3600, 1800, 1200 and so on RPMs. Sorry. But there it is.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 60hz compressor conversion

Yep VFD or pulleys and belts and the inherent losses they come with.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 60hz compressor conversion

Electric motors specifically designed for direct drive usually have bearings (and maybe shafts) that are _not_ designed for the radial forces ("chain pull") associated with a belt drive.

It's something you need to check before you get all lathered up over solving your problem with a belt.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: 60hz compressor conversion

Yes. Of course you also need to keep the V/Hz ratio. But you also shall keep the speed right. And, as Mike says, there are mechanical considerations, too. You may need to use external bearings and a shaft with a pulley.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 60hz compressor conversion

The motor should be supplied with (220 x 6/5)V minimum regardless of other changes. The torque should stay the same but because of the increased speed, the HP will increase X 6/5.
The new motor rated voltage will be 264V but it is common for the supply voltage t be higher than the motor rated voltage.(motor = 460, supply = 480)
This will take your supply voltage up to 275V.
I haven't been able to find a reference to your primary voltage, so I cannot make any suggestions as to a possible transformer choice.
I have found an open delta auto transformer circuit to be an economical way to adjust motor voltages.
What is the speed torque characteristic of your compressor?
Does your compressor have a capacity control device?
I suspect that Gunnar's suggestion of a VFD may be the best solution, but if the torque requirement of the compressor is fairly constant then 275V or even 280V may do the trick. You have the 1.15 service factor in your favor.
Also, if the compressor has a capacity control you may be able to run it a little below full capacity.
Your motor kW will increase from 30 kW to 36 kW with the proper V/Hz ratio at 60 Hz but the increase will be i speed alone, torque will stay the same.
A real kludge hack, if the correct V/Hz ratio gets the current fairly close may be to reduce the load by restricting the inlet.
Depends on the budget and the overall operating plan.
If you do go to belts, you will be able to go above the original 1500RPM. If you can get a speed/hp curve for the compressor, you should be able to go up to he 36 kW point on the curve.
What is your plant voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 60hz compressor conversion

VFD's for this size motor are fairly inexpensive these days. If you haven't done any price shopping, it might be worth a look.

Alan
----
"It's always fun to do the impossible." - Walt Disney

RE: 60hz compressor conversion

What is your available voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 60hz compressor conversion

"The motor should be supplied with (220 x 6/5)V minimum regardless of other changes. The torque should stay the same but because of the increased speed, the HP will increase X 6/5."


If you run this unit at 60Hz the power will increase 60/50^3 x 30 = 50Kw  

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