VFD HP change - 60HZ to 50HZ

[m3rheino]

I have a 460vac/60HZ/3HP drive that I am going to use in a 380VAC/50HZ application. How do I determine the drop in HP at 50HZ?

 

[LionelHutz]

HP is proportional to speed/frequency if you keep the V/Hz ratio constant.

 

[m3rheino]

I know the drive's new HP value is going to be less than 3HP, I just don't know how to determine the exact value.

 

[Masbibe]

If you are speeking only about drive (VSD) derating will be 3HP * 380 / 460.

Milovan Milosevic

 

[jraef]

The "HP" rating of a VFD is a contrivance that gives us a short cut to selecting one, but is otherwise meaningless. The VFD is actually rated in amps, which will not change. So if your VFD is rated for 4.8A at 480V, it's rated for 4.8A at 380V.

The motor you are able to connect to that VFD which will have a FLC rating of 4.8A or less may end up having a lower HP (or kW) rating at 380V than at 480V, but that is 100% dependent upon the motor itself.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

You can't figure it out using HP is proportional to speed or hp is proportional to frequency?

Hint - You'll be running at 5/6th of the 60hz frequency.

 

[ScottyUK]

Lionel,

Re-read jraef's post. You're describing the behaviour of a motor, not a VFD.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

The VFD itself, pretty much the ratio of voltages or just make sure you keep the VFD within it's rated current.

 

[rhatcher]

I am seeing a different question than the one that has been answered.

I have a 460vac/60HZ/3HP drive that I am going to use in a 380VAC/50HZ application

Is the OP is asking what will be the result of connecting a VFD that is designed for a 480V/60hz power supply to a different power supply of 380V/50hz? The answer to this question is that it will not work unless the VFD is rated for both supply voltages. He should consult the manufacturer if he is not sure.

The other way to view this questions is what happens when a 380V/50hz motor is connected to a 460V/60hz VFD? In this case, I agree with the previous answers.

A 460V/60hz VFD can operate a motor rated for 380V/50hz power supply since the voltage per hertz ratio is the same. Specifically, a 460V/60hz VFD will be putting out 380V at 50hz for any motor that is connected (480/60 = 380/50 = 7.6-7.7).

For a constant torque 3hp load at 60hz synchronous speed, any motor that is connected will have to produce about 2.5hp at 50hz speed. The amp draw will be the same since this is constant torque.

 

[jraef]

A VFD in and of itself couldn't care less what the frequency is, all it does is rectify it. So the only other issue is voltage; it cannot create voltage that isn't there, so using a VFD at a lower voltage limits the maximum output voltage to the input voltage (for the most part). So you cannot put 380V into a VFD and get 480V out of it. But again, that has nothing to do with the current rating of the VFD.

Now that said, sure, the maximum HP/kW capacity of the MOTOR you can connect to that VFD will change because of that, but that is the answer to a question not asked.

And just for the record, 99% of all low voltage VFDs come in 3 voltage classes: 240V class, 400V class and 600V class:
240V class VFD cover 200-240V inputs,
400V class VFDs cover 380-480V inputs,
600V class VFDs cover 550-690V inputs (although 690V is more rare)

So rarely is there a difference between a 380V and a 480V VFD, and even if there were, a 480V VFD would almost certainly accept a lower input voltage. There are always exceptions, these are generalities.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[rhatcher]

jraef,

A VFD that is designed for a power supply of 460V/60hz may not notice the frequency difference on the input voltage but it would certainly notice the voltage difference and would certainly register a fault in the 'power module' of the VFD because of 'Low DC Bus Voltage'.

The front end, 'control module' of the VFD may or may not operate enough to even turn on the VFD because the 120VAC control power from the internal CPT will only be supplying 99V to the 'control module' and the VFD display.

In summary, a VFD that is designed to operate from a 480V/60hz power supply should fault if operated on a 380V/50hz supply.

As I said before, some VFD's are rated for both voltages. If so, usually there are software changes and voltage tap changes on the CPT that must be made to make the voltage change.

 

[jraef]

Sorry, but I haven't seen a VFD that uses a CPT in years. Most either get their control power off the DC bus or have a separate SMPS in the front end with a wide input voltage range, 2:1 is very common.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[DickDV]

jraef is correct. Most drives rated 480V are actually rated 380-500V when you read the fine print. And, no, you don't have to make any hardware changes and generally no software changes either.

But note his comments about the output voltage being limited to the input voltage. That pretty much restricts what kind of motor you can use on the output when the input is down around 400V.