- Location
- New Jersey
- Occupation
- Journeyman Electrician
Has anyone ever heard of using a drive with a rated current less than the FLA of the connected motor and using the drive limit the current by never allowing the motor to run at full speed?
Variable Frequency Drives
- Thread starter infinity
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You can but bear in mind that the motor magnetising current can be about 30% of full load current without the motor doing any work.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
It is not an unusual practice to upsize motors that are used on VFDs. It is much less common than it once was.
I think that it is a dubious code thing to put a 10HP drive on a 15 HP motor though.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Someone raised the question of a 400 amp circuit and a 400 amp VFD with a chiller rated at 426 MCA.
kwired
Electron manager
- Location
- NE Nebraska
Is the 426 MCA for a single motor or a multimotor piece of equipment? Likely the VFD itself is only going to drive a single motor.
426 MCA also is likely already taken 125% factor for continuous load, for at least the largest motor if a multimotor unit.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Details are sketchy but from what I know it's a chiller with a nameplate of 426 MCA. I would assume a single motor since the control panel is set up for one single feed.
weressl
Esteemed Member
Motors are seldom loaded to their full capacity. Rather than limit to speed, the limit should be set on the current.
weressl
Esteemed Member
Feeding an entire chiller unit WITH a control panel from an ASD? Usually you have control power transformers and other elements such as oil pump, oil heaters, etc also feed from the single feed to the control panels besides of the contactor for the compressor. You may also have multiple compressors. So you would 'insert' the ASD into the circuit where the compressor combination starter may have been. The control of the ASD is a whole other issue, especially with complex control sytems such as a chiller. I really would not attempt to do this without talking to the manufacturer of the chiller.
kwired
Electron manager
- Location
- NE Nebraska
Exactly why I asked if the 426 MCA was for a single motor or multimotor piece of equipment. VFD that does supply more than one motor will need to be a situation where all motors run at same frequency at same time, and will all start, stop, accelerate, decelerate at same rate and at same time. All different loads you may run into in a chiller probably will not work out so well to supply from a single drive.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I was thinking about that also. Right now we're working off of a one line diagram that shows a conduit to the VFD then another conduit to the chiller which seems odd for the reasons you've menitoned.
weressl
Esteemed Member
...and of course all motors must be the same size, speed and even model. Even then it is an iffy proposition, in other words not just cut-and-dry, need to look at the application, minimum desired speed, accuracy and certainly not expected to perform high precision synch drive, like turing two wheels of a gantry crane.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
That just seems really odd.....:? Wouldn't you want a fullsized drive anyway just so it isn't working so hard?
weressl
Esteemed Member
- Motors are seldom loaded to their nameplates.
- Chillers/Compressors are especially cyclical loads, going from virtually unloaded to over nameplate loading and then unloading.
- Since the ASDs are thermally sized to continuous load and have typical overload capability of 10%(normal duty) or 50%(heavy duty) for one minute within a ten minute cycle. (that is allowing the motor to load to 100% for the remaining 9 minutes.)
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