Suggestions on a 1ph -> 3ph motor swap 2

[itsmoked]

I'm building a control panel for a large machine. It's a 480V panel. At the last hour (rather than minute) I've been informed to not forget this motor (I've never seen or been told about).

My options as I see them are to:

1) change to the control transformer UP to something that will support this motor

or

2) change the motor over to a 480 3 phase equivalent.

What would you do?

If you're inclined to stick with this motor how big does the transformer need to be to support starting this cap-start motor?

BTW: The 480 at the site is WYE sourced.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Another possible option is a 480-240 V transformer (or autotransformer) in the panel. Motor full load is somewhere around 1.1 kVA. 2 kVA of transformer capacity would probably be enough. Maybe 1.5 kVA. The motor does have a 1.25 SF.

My first inclination would be to replace the motor with a 460 V 3-phase. You'd need a small (NEMA 0 or 00) starter including OL relay. That's probably $500. Maybe you can find a refurbished motor, but 1/2 hp 3-P motors are a little unusual. l

 

[GregLocock]

I'd guess the transformer is cheaper than a 3ph motor. I got one roughly that size for about $70, 240 to 120V in my case. I used it to run an immersion heater in the hot water system at 1/4 power.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[waross]

I would not feed it from the control transformer. The starting surge may cause unacceptable voltage sag.
I like the idea of a 480 Volt to 240 Volt transformer.
My transformer sizing:
4.4 Amps x 1.25 SF x 240 Volts x 1.25 by code = 1650 VA. I would use a 2 KVA transformer.
If you consider that the 4.4 Amps includes the SF, you are down to 1320 VA and a 1.5 KVA transformer.
The I²T of the starting surge is short enough that the transformer will withstand the short starting overload.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

I like the 240V gambit and after a quick epay check for a 3P replacement I'll probably go with the 240.
Thanks Bill for the KVA work-up.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Some comments on your choice. You know this but the information may be helpful to others following this thread.
Line to line versus line to neutral:
A neutral connection is not always cheaply available on 480 Volt circuits. Avoiding the use of a neutral connection makes the circuit more universally useful.
480/240 Volt transformers are more easily sourced than 277/240 Volt transformers, not just today, but in the future should a replacement transformer be required.
Transformer versus autotransformer:
A line to line autotransformer will put a common mode voltage of 277 Volts to ground on the motor leads. This is not a problem for the motor insulation but it is a little more hazardous for a repairman.
120:240/240:480 Volt transformers are common. The center point of the secondary may be grounded with the maximum voltage at the motor being 120 Volts to ground.
Line to neutral autotransformer: a 240/36 Volt transformer should work well if you go this route. 240V + 36V = 276 Volts. That is an excellent match for a 277 Volt circuit.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Who says 1/2 hp 3-phase motors are unusual? They're so inexpensive I would seriously considering switching to 3-phase 480v and skip the transformer altogether. $388 from McMaster means you could probably find one for half that price elsewhere.

Electric Motors, 208-230/460V AC, NEMA 56, 1/2 hp, 1725 rpm, Open Dripproof

https://www.mcmaster.com/6136K111

 

[edison123]

With 1 ph motor, you have two more points of failure, the cap(s) and the trafo.

Muthu

http://www.edison.co.in

 

[itsmoked]

They liked the new motor idea. They believe they may even have 3 new 240/480V motors bought to run fans that turned out to need more than 1/2hp. I'll keep ya all informed.

Keith Cress
kcress -

http://www.flaminsystems.com