VFD confusion again...5 HP 3 phase DC

[Thomas Marr]

I continue to be confused by VFD derating advice. I understand the requirement to derate a VFD designed for 3 phase input but connected to single phase power. I understand derating for certain environmental conditions (high altitude) or use circumstances. But it seems like there’s a rule of thumb to ALWAYS derate a VFD, even if it’s designed for single phase input.


I'm looking at a VFD designed for single phase input, three phase output. It’s a pretty nice unit with a good manual and useful features, such as parameter controlled “catch on the fly” restarts. The VFD is "rated" as 5 HP, 3.7 KW and 19.2 amps. The amp rating of my motor is 11.6 amps at 230 volt input (Baldor VEM3613T). The salesperson thinks this VFD may be too small and I should go bigger (but can’t explain why exactly). But, to me, it seems this VFD should have plenty of overhead to run a 3-phase motor in a blower application with reasonable accel/decel times (e.g., 5 seconds or more). Any thoughts appreciated as the “next” step up VFD is more than twice as expensive.

[Dan Friedrichs]

VFDs do not need to be derated provided the input stage is capable of handling the higher single-phase current. Ideally the datasheet would list the input current limit, but if there is a statement that it supports 5HP with a single-phase input, then you should be fine.

[Aaron Inami]

This has to do with the input source of VFDs.

A 5HP VFD that has a single phase input will be designed to specifically handle the power requirements for driving a 5HP 3-phase motor just fine.

The problem comes in to play when you try to connect only single phase power to a VFD that has 3-phase input. A 5HP VFD with 3-phase input is usually rated to handle about 9-11 amps of current on the 3 input legs of power (this is what is required to drive a 5HP motor at full-load amps). While you can certainly do this, the VFD will attempt to draw more amps from the single phase because it needs that amount of current to drive the 5HP motor fully. The estimated single-phase current requirements for 5HP motor is about 19-21 amps. The root of the problem is the diodes on the input of the VFD are typically not rated high enough for that 19-21 amps of current and will blow. The diodes are the devices which convert the A/C to DC power (which is used to artificially create the output voltage and frequency). So, in the 3-phase input VFD world, you essentially have to double the size of the VFD itself (basically getting 10HP 3-phase VFD when driving a 5HP motor off single phase input). This problem does not usually reveal itself until you reach the 3+ HP area.

If you look at the specifications for a 5HP single-phase input VFD, you will see that it is rated for somewhere in the 19.5A to 21A current area (enough to drive a 5HP motor).

[Chris Parks]

Why would you use a 3 phase VFD on a single phase input? I have been selling them for a long time and never heard of it being done.

[Jared Sankovich]

Why would you use a 3 phase VFD on a single phase input? I have been selling them for a long time and never heard of it being done.

Ive not seen much over 3hp units that were single phase only. I prefer the fuji frenic drives and they are single up to 12amps and 3ph input up to 60 amps. Those 3ph models are also labeled for single phase input with the factory de-rated output listed.

Op. Get a Fuji Electric model FRN0012C2S-7U. It's 12fla output and will work perfectly with your motor.

[Chris Parks]

Ive not seen much over 3hp units that were single phase only. I prefer the fuji frenic drives and they are single up to 12amps and 3ph input up to 60 amps. Those 3ph models are also labeled for single phase input with the factory de-rated output listed.

Op. Get a Fuji Electric model FRN0012C2S-7U. It's 12fla output and will work perfectly with your motor.

I guess single phase 110V has to be a factor but 240V single phase units go above 3hp, we sell single phase 3kw units and I know they go above that by a fair bit. I can't check where they top out on this computer but they are available and the brand we have used for 10 years are rock solid. One has been in use for about 8 years 5 days a week and is yet to commit suicide.

[Aaron Inami]

Several single phase input VFDs that are 5HP and higher. ATO makes one at 20HP.

https://www.grainger.com/category/mo...&filters=attrs

https://www.wolfautomation.com/produ...-7_5_hp-10_hp/

https://www.ato.com/single-phase-to-three-phase-vfd

[Jason Roehl]

Why would you use a 3 phase VFD on a single phase input? I have been selling them for a long time and never heard of it being done.

3 phase service is not available to many (most?) residential areas. A VFD that inputs single phase and outputs 3 phase thus allows many folks to use industrial 3-phase machines (often much cheaper than their 1-phase counterparts).

[Thomas Marr]

Op. Get a Fuji Electric model FRN0012C2S-7U. It's 12fla output and will work perfectly with your motor.

Jared, are you sure that's output amps...and not input?

[scott lipscomb]

Thanks for this succinct explanation Aaron. I always wondered why VFDs designed for 3 phase input needed to be downrated for 1 phase, but did not know why-this makes sense.

This has to do with the input source of VFDs.

A 5HP VFD that has a single phase input will be designed to specifically handle the power requirements for driving a 5HP 3-phase motor just fine.

The problem comes in to play when you try to connect only single phase power to a VFD that has 3-phase input. A 5HP VFD with 3-phase input is usually rated to handle about 9-11 amps of current on the 3 input legs of power (this is what is required to drive a 5HP motor at full-load amps). While you can certainly do this, the VFD will attempt to draw more amps from the single phase because it needs that amount of current to drive the 5HP motor fully. The estimated single-phase current requirements for 5HP motor is about 19-21 amps. The root of the problem is the diodes on the input of the VFD are typcially not rated high enough for that 19-21 amps of current and will blow. The diodes are the devices which convert the A/C to DC power (which is used to artificially create the output voltage and frequency). So, in the 3-phase input VFD world, you essentially have to double the size of the VFD itself (basically getting 10HP 3-phase VFD when driving a 5HP motor off single phase input). This problem does not usually reveal itself until you reach the 3+ HP area.

If you look at the specifications for a 5HP single-phase input VFD, you will see that it is rated for somewhere in the 19.5A to 21A current area (enough to drive a 5HP motor).

[Thomas Marr]

I was looking at the ATO unit. I'm not familiar with this brand. However, their manuals are outstanding and they appear to have an advanced feature set similar to premium brands.

I calculated the KW of my motor, considering Power Factor and other issues, and I'm getting 4.4KW. The "5 HP" ATO unit is rated at 3.7 KW (most "5 HP" seem to be 4 KW rated). Therefore, their concern about selecting a larger VFD appears warranted in my case. Right now, I'm using a "10 HP" Chinese VFD I purchased for $170 for another project with a cheap motor. It seems to be working fine for now. I use 2 second DC injection at 15v (selected arbitrarily) prestart up to ensure the motor isn't spinning (or isn't spinning much). I have no idea if this VFD uses catch on the fly restarts.

To me, Chinese VFD rating is false advertising. Why rate a VFD at 5 HP when the company itself says it can't be used with a 5 HP motor? I realize it "can" be used. And probably in many applications it will be fine...assuming the motor isn't driven at FLA most of the time...like in a blower application. I guess the rule of thumb to derate VFDs, even when designed for single phase power, still applies. However, based on my research to date, doubling the size isn't always necessary.

[Thomas Marr]

Thanks for this succinct explanation Aaron. I always wondered why VFDs designed for 3 phase input could be used for 1 phase, but did not know why-this makes sense.

Yes, good information in this thread. Thanks all who responded.

[Aaron Inami]

I'm using a Baldor CEM3613T 5HP 3 phase motor on a dust collector (also for blower) with an ATO 5HP single-to-three-phase VFD and it runs the system just fine. The specification on this motor is also 11.6 full-load amps (the single phase equivalent motor is 21.8 FLA). I'm not sure that you are going to actually use the full load amps of the motor like you would for a planer/sander/saw. On these cyclone dust collectors with a 16" impeller, the 5HP motor was recommended because the 3HP would be on the maximum edge of capability. Your situation might be different. If you feel at all concerned, then go for a 7.5HP VFD.

This is the VFD I have:

https://www.ato.com/5hp-vfd

Like you said, they provide absolutely excellent documentation. I chose this because I did not want to go bottom of the barrel cheapest Chinese product, but I did not want to spend $600-700 on the high end level.

One thing on the ATO VFD is that it has a small 2" cooling fan that runs and is somewhat loud/annoying. It's not anything that would cause hearing damage, but I made a muffler box for it anyways. Other VFDs will likely have the same thing.

[Bill Dufour]

1.73 x 11.6amps = 20.06 amps. Plus some loss of efficiency. So your motor needs 20+ amps input on single phase.
If that VFD is 19.2 amps input on single phase it is not big enough to work.
Is the 19.2 amps input or output?
Bill D

three phase requires (square root of three) times (the motor amps) for single phase input amps

[Jared Sankovich]

Jared, are you sure that's output amps...and not input?

100% positive. I'm running two of them on 5hp motors. They will push 18FLA for 1 minute ever 10 minutes (10% duty cycle at 150%)

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