ECN Forum Forums Technical Discussion Electrical Theory and Applications Hooking up VFD

I will be hooking up two VFD's on an upcoming project.

There are two existing 3phase pumps, currently started using Nema 1 starters.
We are putting VFD's in place of starters.
Each of the two "buckets" in the MCCenter, have a 3phase 25a braker with a starter.
I beleive the motors are 7 1/2 HP @208V
Will the 25A breakers be enough with the VFD?
I dont know if the Drive has any type of amp draw in itself

Adjustable Speed drives are covered in 430.120. The ampacity of the conductor must be at least 125% of the drive rating.
A 7 1/2 HP has a FLA of 24 amps. I don't think the 25 amps breaker will do.

Also, if the breakers were part of a UL listed MCC bucket assembly, they might be (probably are) "Mag-only" breakers (aka MCPs), UL "recognized" (UR) only to be used specifically in a listed motor starter assembly. If you don't have the starter, those breakers CANNOT be used as feeders to another device, i.e. your VFDs. They will not have thermal trip elements and are NOT UL listed by themselves.

I got a hold of the notes on the print, and they say to replace breaker with 60a.
Where does it state in the code I cannot use breaker in MCC bucket w/out thermal protection/starter?

PS I saw your site. (jraef) Nice to have guys like you around.

[This message has been edited by ayrton (edited 10-03-2006).]

[This message has been edited by ayrton (edited 10-03-2006).]

One really must read the directions. As well as follow the instructions.

VFD's often greatly reduce the power required by a motor. They do this by incorporating a "soft start" control, as well as letting you run at a much lower level.

To use one example, a blower that was pulling 28 amps (single phase), was pulling only 8 amps (three phase), while the motors were both of the same horsepower rating. Why? because the blower was only moving the air needed- and not running full speed, fighting a damper.

As for lacking "heaters," well, the few VFD's I've installed all had equivalent circuitry in them. They also had protection for lost phases, etc- comparable to any 'electronic' starter.

I am NOT aware of the UL issue alluded to, and would appreciate some references. As I understand it, breakers (OCPD's) and overload deviced are intended to address very different issues. As the Brits would say, they are as different as cheese and chalk. I suppose "assembly" is the key word there.

"Where does it state in the code I cannot use breaker in MCC bucket w/out thermal protection/starter?"

240.4 states that you must have over-current protection for all conductors, then goes on to say that motor circuits are covered under article 430.

430.52.C.3 then states that Instantaneous trip circuit breakers can only be used as part of a listed combination motor controller assembly that includes an over-current device (overload relay). If you use the MCP breaker all by itself, it is no longer part of that listed assembly.

Don't get me wrong, this has only to do with MAG-ONLY breakers, commonly referred to as MCPs or Motor Circuit Protectors. Standard Thermal-Magnetic circuit breakers are fine as stand alone feeders.

Thanks, jraef. I've just become accustomed to 'inverse time breakers'. What you describe - an assembly- is another thing alltogether.

OOPS