If my VFD outputs more current than my motor, but is the right HP, do I size my MCP for the drive, or for the motor?
There's a similar question a few threads back in reference to understanding this very issue;
Single motor protection with VFD per Article 430.130
I see people getting caught up in the nomenclature trying to interpret the Code by throwing crow-bars at it. The Manufacturer's Manual is spot on - or they're off the Market. Think of a typical Transformer with the values for the Primary & Secondary Fusing: 1kVA) 480V Pri. = 2A [Typical Fuse = 3A] and the Sec. = 8.33A (with typical Fusing maybe 12A]. You are not Fusing the Primary side based on a Load from the Secondary (i.e. if you put a 500W DC Power Supply going from 120V AC In / 24V DC Out; you are not Fusing the Primary of the Transformer based on the 20A DC Output).
A VFD (otherwise known by the NEC as Power Conversion Equipment) is in reverse of this. It's a 'Black Box' that has the same Power consumption as it is capable of Supplying (less a few % for losses - just like a Transformer). So we need to look at an example. A 1kVA VFD for a (1/2) HP 230VAC 3-Phase Motor from a 120V AC Supply. Compare the Ampacity difference of a (1/2) HP Motor on Table 430.248 (115VAC = 9.8A) to Table 430.250 (230VAC [3-Ph.] = 2.2A). It's the same HP - how can Fuse the incoming Power of the 1kVA VFD based on 2.2A? -You can't.
Everybody has an Ugly's Reference Guide right?
This is an introduction, more to follow ...
VFD's are Rated for worst case scenario. In fact, if there are Start-up Faults and other listed conditions mentioned in the Manual (and every Manufacturer is different), it says to go the next size up VFD for the same Motor.
Take a closer gander at the Motor - what is S.F.: 1.15 ?? -This is the "Service Factor". This tells you that you can go 115% of Rated Nameplate Information in reference to Ampacity - and still be 'good to go' and operate 100% Duty Cycle. Of course there are a number of underlying facts to consider such as the actual Voltage vs. Nameplate Voltage - Higher the Voltage - lower the Amperage. Is it starting under Full Load or not? Not too many Start/Stops, and so on.
The key to VFD applications are the Parameter Settings. The default settings are the VFD Rating Max. In order to have the correct OL Protection, Accel capability, Over-Amp Trip points and so on, you need to enter the Motors Nameplate info in the correct Parameter locations. We need to see an example:
So Parameter P0.00 needs to be changed from the default value of 240 to the Motor Nameplate value of 208.
The value for Amps is the same as the default.
The Input to the VFD is 9A, CB or Fusing is per 9A (not 2.5A). Note: Fuses are required for UL.
Here's the answer to your question:
You have to go over the Parameter options to dial in you application.
Parameter P6.00 is "Electronic Thermal OL Protection" which is the value of 100% over the speed range and up to 150% for 60 Seconds (described in the Manual). Depending on the application, this can be trimmed down to as much as 30% of VFD Rating (or 30% of 1kVA).
There's another important one called "Stall Prevention" which is Amps vs. Load during Start-up (200%), or the value entered.
There's Torque OL Type (which is Load vs Amps), under "Variable Torque" selection setting allows less Current vs Load or "Constant Torque" where the Current is maxed the entire Accel range.
And of course, the best reason for a VFD is the final operating Speed (60Hz typical) vs the Accel Time (or also known as Soft Start; don't make it too long though, that might require tweaking various other Parameters). And Decel is most ofter set to "Coast to Stop".
Your drive does not output current, it has an output and input rating only. Article 430.130 is only for manufactured and listed Power Conversion equipment, drive and motor combined. If it's not a listed and rated system Article 430.130 does not apply.
Drives should be selected by HP the same as any other motor controller ie soft start, NEMA 1 type, etc.
As a side note: Power Conversion Equipment as one example, would be an artesian well pump system, where they are using a VFD for speed control, phase conversion, etc.
If your designing or installing a drive for a motor that hasn't been listed and rated specifically with the motor your installing, your in Article 430.6. Forget it's a VFD and do your install as any other motor controller other than after your finished your install, now you have to do some programming to make it work correctly.
As mentioned by Robert, the drive has to be programmed to the motor and application parameters, keep in mind just because the drive may have higher parameters than your motor nameplate ratings, means nothing. the drive is programmable for different parameters based on application.
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