breaker size to feed a vfd?

[willmitch]

So I have a cheap China vfd and I'm going to use it to feed my 3 phase mill.
The tag on the vfd says input is 200vac + or - 15%, 14.2 amp. so do I feed this with a 15 amp breaker or do I go to 20 amp to go by the 125% rule?

Not sure it would matter but the vfd will feed a 1hp 3 phase motor. 3.3 amps at 208 volt.

 

[manwithtools]

You may need even higher than 20 - depends on the manufacturer recommendation. Not sure if you have thoroughly read the manual, but frequently they will specify the breaker / fuse size and it may be more than 125% of the rated input current to accommodate for motor startup.

 

[alfredeneuman]

The motor itself is where you apply the 125%, not the maximum rating of the VFD.
The VFD doesn't automatically draw 14.2 amp on it's own.

 

[Norcal]

There is very little help availible for cheap China VFD's, 'cause you get what you pay for.

 

[wyliesdiesels]

What does the manual say?

 

[budmur]

VFDs eliminate the startup surge. You can go with 15 amp.

 

[manwithtools]

VFDs eliminate the startup surge. You can go with 15 amp.

Negative, they CAN eliminate the startup surge. They don't always do so. You need to read the manual. For example, if you set the VFD to run 120hz at start with zero ramp time, they will create a demand on the supply that is much larger than the rated input amperage.

Here is an example: Allen Bradley PowerFlex 525 drive (perhaps the most commonly used drive in industry today) 240 volt single phase input- max input amperage 18 amps - basically a 2HP VFD.

AB specifies 40 amp fuses or a 25 amp motor protection circuit breaker - this a rated device - far different than a 25 amp breaker in your panel board - it's a breaker and a thermal overload combined.

Read the manual....

For you reading enjoyment :http://literature.rockwellautomation.com/idc/groups/literature/documents/pc/520-pc001_-en-p.pdf

 

[Bert_]

3 phase motor amps X 1.732 = single phase amp draw, This number + 125% is the minimum breaker size. The 250% rule still applies here so the breaker could be sized to a maximum of 250% of the SINGLE PHASE amp draw

The large startup draw normally associated with motors can mostly be eliminated by programming a slower ramp up time. Even a second or two will make a tremendous difference with many lower inertia loads.

*I did notice you say the VFD is rated 200V +/- 15%, that is a max input voltage of 230V. That could pose a problem assuming you have a 120/240V single phase service, it would not be uncommon to see 245 or even more on such as system.

 

[matt_i]

I doubt you can draw the 15A on the mill, ever. Even in a locked-rotor overload condition the VFD should limit current due to its own inability to supply it.

 

[Aceman]

What make and model drive?

I'm not sure how the other posters are able to come up with wire and breaker sizes without knowing if this VFD is designed as a single phase input/3 phase output drive or if it's a 3 phase input/output drive being fed single phase.

It does make a difference on breaker, wire and VFD sizing.

 

[willmitch]

The vfd is specified to have single phase input and 3 phase output.

Yeah, it came with a manual, I read it (ok, maybe not thoroughly), and saw no listing of what size breaker. I thought it would be in the "wiring" section?

This is the one.

https://www.amazon.com/dp/B01DKJWM62/?tag=atomicindus08-20

 

[Falcon67]

My 1 HP 3 phase mill using a Teco VFD is on a 20A 240V circuit. The circuit is shared with the 2HP 240V lathe motor.

 

[budmur]

Negative, they CAN eliminate the startup surge. They don't always do so. You need to read the manual. For example, if you set the VFD to run 120hz at start with zero ramp time, they will create a demand on the supply that is much larger than the rated input amperage.

If a plausible use case is going from 0-200% speed with zero ramp, then I'd _definitely_ recommend a 15 amp breaker. It'll help protect the equipment from morons at the control.

 

[sberry]

The 125% is the wire size requirement for motors, not always the breaker. Its not really extra for start up but to protect the wire should a larger breaker need to be used. The over size is the thermal protection for the wire.

 

[manwithtools]

With no manufacture OCPD information available, supply over current protection based on the drives input rating. Size your supply at 125% of the input current. Be sure to set the drive parameters to those of the connected motors voltage, hp and current if it has those variables.

14.2 * 1.25 = 17.75, round up to 20 @240 for your supply circuit. For your application it's likely 15 amp would work just fine as well because you are using the drive at less than it's max output potential. I probably over complicated my earlier reply for what you are doing. Just be sure wire sizes are correct for your breaker.

Good luck with your project!