VFD Heat Loss on Single-Phase Input

[MitchusMaximus]

Hey everyone, in my region we help a lot of farmers set up 3-phase motors for their grain augers or dryers from their single phase 240V service by using oversized VFDs. Occasionally I will have a customer who needs a weatherproof enclosure in order to install the drive outside near the equipment, so I will size an enclosure based off of the Watt loss (or heat loss) rating for the particular drive I'm selling and add in some fan cooling if necessary. Because the Watt loss rating for a drive tends to be given assuming the motor is being run at full load, I was wondering if this value would change at all because you are driving a smaller motor, and therefore the output transistors are handling a lower current.
Eg. I have a 40HP 230V drive running off 3-phase power, and it has a rated Watt loss of 900W. If I use the same 40HP 230V drive to run a 20HP motor off of single-phase power (ensuring the input current is rated to handle the full current of the motor between only two input lines), will the Watt loss of the drive be equal to, less than, or greater than 900W? If this does reduce the Watt loss, I can look at down-sizing the enclosure which will help save the customer some money, and may reduce my lead time on providing the enclosure. If it actually produces more heat, I definitely would want to be aware of that to prevent an enclosure from turning into an oven.

Thanks for your input.

 

[jraef]

The watt losses are mostly in the transistor switching losses, so will be based on the actual connected load.

I would determine the load in watts and use 3% of that value as the peak losses. So 20HP motor, 746 W/HP, so 14,920W, x 0.03 = 448W losses. It might be a LITTLE higher because the DC bus caps have to work a little harder due to the deeper ripples from rectifying only 2/3 of the input, so I would call it 500W to be conservative.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I hope that you are sizing your motors based on about a 20% overload.
Many augers have a slider that regulates the amount of grain and the power demand.
If the slider gets knocked further open when the auger is being moved to another bin, many farmers will run it anyway until something smokes.
Oversized motor and conservative current limit.

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

 

[MitchusMaximus]

Thank you for the input, it sounds like I can use this model and could be able to look at smaller outdoor enclosures when I'm derating drives in this way. I'd love to do some experiments to measure the temperature when a drive is fully loaded vs using a smaller motor off a single phase input, but this is helpful to think about.