Step-up transformer 208 to 480

[Foxman]

Hello everyone,

I'm having trouble trying to size a 3-phase transformer. I have a 480V 55A pressure washer that I need to connect to a 208/120V 3-phase system. I have an 800A 208/120V panel close to the heater. I just need to know I'm sizing this right before I start ordering the wrong items. The specs of the Pressure washer come from the manufacture's literature, not actual amperage reading.

I took 480V*55A*1.732 (for 3-phase) to get 45.7248 kVA as the demand that the secondary side needs.

Do I have to go up to a 75kVA transformer or can I use a 45kVA transformer?

 

[augie47]

Some of the guys here that are real familiar with transformers say that when motors are involved a multiplier of 2 or 2.5 is common.

 

[Foxman]

There will be a small motor on the pressure washer to run the water pump. After seeing the motor in the diesel model, I couldn't imagine it would be more than a 2HP motor. Majority of the load will be in the heating elements.

 

[coop3339]

I would go with the 75kw. If it were purely resistive load you could probably use a 45. With the 45, the motor could have trouble starting because there would be voltage drop caused by the small transformer.

 

[kwired]

I would go with the 75kw. If it were purely resistive load you could probably use a 45. With the 45, the motor could have trouble starting because there would be voltage drop caused by the small transformer.

Sounds like from what he said the load is mostly resistive heating and only 2HP motor. Chances are high the heat is not on when starting the motor either so there would be plenty of starting capacity for the motor. Most pressure washer pumps can not take high temp water - so heating occurs on the high pressure side of the pump, and heat source is cycled on and off as needed depending on water flow demand.

 

[Foxman]

I got my hands on the spec sheet for the pressure washer today. The pump motor is a 6.2HP motor. Just from my personal experience with pressure washers, when the water reserve runs out, the pump and the heating elements can be on at the same time. it think the safer bet would be to go with the 75 kVA transformer.

 

[kingpb]

I got my hands on the spec sheet for the pressure washer today. The pump motor is a 6.2HP motor. Just from my personal experience with pressure washers, when the water reserve runs out, the pump and the heating elements can be on at the same time. it think the safer bet would be to go with the 75 kVA transformer.

Can't imagine a actual motor rated 6.2Hp, most likely a 7.5Hp operating at 6.2Hp on the curve. Which means upon starting it will draw approx. 6 x rated FLA.

 

[kwired]

I got my hands on the spec sheet for the pressure washer today. The pump motor is a 6.2HP motor. Just from my personal experience with pressure washers, when the water reserve runs out, the pump and the heating elements can be on at the same time. it think the safer bet would be to go with the 75 kVA transformer.

I'd take the expected demand time into consideration - if it only runs for short periods of time the 45 kVA is likely fine, starting the motor mostly only gives you issues with holding overcurrent protection - it will not be too much of an issue for the transformer - unless you have a really high number of starts in a very short time period.

 

[Jraef]

Can't imagine a actual motor rated 6.2Hp, most likely a 7.5Hp operating at 6.2Hp on the curve. Which means upon starting it will draw approx. 6 x rated FLA.

IEC motor, most likely an 7.5kW 415V 50Hz motor. When used at 460V 60Hz the V/Hz ratio is close enough to use, but because the speed is higher, the rating comes out weird.
Here's how that works.

746 W/HP

7.5kW x .746 = 5.6HP

But because the kW rating is a function of torque and SPEED and the speed will be 6/5 (120%) faster by going with a 60Hz supply, it affects the "HP" rating of the motor. 5.6HP x 1.2 = 6.72HP

But also because the V/Hz ratio is not EXACTLY the same, it changes again. 415/50 = 8.3 V/Hz, 460/60 = 7.67V/Hz, which is lower, so torque is lower by 7.67/8.3 = .924

6.72HP x .924 = 6.2HP

Whenever you see an oddball "HP" value on a motor or machine, that's usually what's going on. The machine came from Europe or Asia and was originally designed to be used there. When they export it to here and know it will be used at a different frequency and voltage, they re-rate the motors for us, but it often comes out oddball like that.

 

[winnie]

If you have 746 W/hp, then 7.5kW = 7.5/.746 =10.1 hp.

-Jon

 

[GoldDigger]

If you have 746 W/hp, then 7.5kW = 7.5/.746 =10.1 hp.

-Jon

Yup. Unfortunately W²/hp is not a very useful number for characterizing a motor.

 

[kwired]

If you have 746 W/hp, then 7.5kW = 7.5/.746 =10.1 hp.

-Jon

But motors (NEMA ones anyway) are rated for output power, you need to factor in efficiency to get input power.

 

[Besoeker]

IEC motor, most likely an 7.5kW 415V 50Hz motor. When used at 460V 60Hz the V/Hz ratio is close enough to use, but because the speed is higher, the rating comes out weird.
Here's how that works.

746 W/HP

7.5kW x .746 = 5.6HP

Think you did the conversion the wrong way round.

 

[Besoeker]

Yup. Unfortunately W²/hp is not a very useful number for characterizing a motor.

Not sure how it would be useful for anything.

 

[kwired]

Anyone familiar enough with IEC or other non NEMA motors to know if a 7.5 kW rated motor has rating based on input or output? Something tells me it is likely input power @ full output rating, where as I mentioned before with NEMA motors or any other motor typically rated in HP, the HP rating is usually full load rated output.

 

[Besoeker]

Anyone familiar enough with IEC or other non NEMA motors to know if a 7.5 kW rated motor has rating based on input or output? Something tells me it is likely input power @ full output rating, where as I mentioned before with NEMA motors or any other motor typically rated in HP, the HP rating is usually full load rated output.

For IEC, it is the shaft output power and stated in kW. It is usual, perhaps mandatory, for the nameplate to give input current so you can work out the supply kVA required.

 

[under8ed]

Hello everyone,
I took 480V*55A*1.732 (for 3-phase) to get 45.7248 kVA as the demand that the secondary side needs.
Do I have to go up to a 75kVA transformer or can I use a 45kVA transformer?

Transformers will operate fine up to a percentage over the rated current, 125% being generally considered the limit. The drawback is excessive heat. Your own calculations, (correctly done), barely exceed rating. Sizing the primary and secondary feeders & fusing at the correctly anticipated levels I would not expect any problems unless operating conditions are at the worst extremes.. hot climate/conditions and/or continuous operations.