# Question on matching VFD KVA and Motor HP

• posted

I have an Allen Bradley 1333-YAA Series D VFD which I plan to run on single phase 240V power, to drive a TBD motor for an 11" Logan lathe. The 1333 manual gives the following ratings:

For input of 3 Phase 230V, output is 2 HP, 8.0 Amps, 3.2KVA

For input of 1 Phase 230V, output is 1HP, 4.9 Amps, 2.0KVA

Although not rated for it, I would like it to drive a 1.5HP motor (don't think 1HP will be quite enough to drive a 11" metal lathe at var. freq.). I've read the RCM archives that it is not good to mate a bigger motor than the VFD, but...

If I calculate the HP from this formula, I get this:

HP = KVA * 1000 * (Eff * PF) / 746 (I read in a Siemens pub that (Eff * PF) for small 3 phase motors is ~=.70)

HP = 2000 * .7 / 746 = 1.88 HP

I know motors give considerably more than their nameplate amps & HP during startup and peak loading (150%?). But, the 1333 says it is rated for 150% starting torque amps and 140% overload amps for 1 minute. And, 1.5HP 230V motors are typically 5.0 nameplate amps which is only 0.1 amp off of the output rating of the 1333 (maybe amps are more relevant than HP and KVA anyway?) So, it seems to me a 1.5HP motor would work especially given the intermittent duty of a lathe. But I'd appreciate the thoughts of the more experienced folks out there!

Thanks, David

• posted

OK, the reason to not run motors larger than the VFD rating is that the VFD is "tuned" for the inductance of the specified motor. Larger motors have lower inductance, and therefore can draw more current than the VFD's transistors can handle.

The reason to derate for single phase is to limit the current on the rectifiers, and also the AC ripple current in the filter capacitor bank.

Within reason, you can bend some of these rules. If you don't demand full power out of the 1.5 Hp motor except for short periods, I'm sure you'll have no problem. Note these VFDs are rated to run in equipment cabinets and mechanical rooms at 40+ C temperatures, at 100% rated load, for 5 years continuously! Your service in a typical home shop, or even a light commercial shop, is going to be a LOT more gentle than that.

So, it seems to me a 1.5HP

Yes, I think you'll have no trouble. I will give one data point: I have a Magnetek GPD-333 1 HP rated VFD on a 1 Hp Bridgeport mill. The VFD is rated for 3-phase input ONLY, but I am running it on

240 single-phase, and have had no trouble whatsoever with it in the last 5 years. So, I am getting away with full power and no derating, even! I also run a high-speed spindle motor that runs 4.5 A at 80 V and 400 Hz, and it DOES get the VFD to run warm. I put a fan on it, and it seems to be OK with that, too.

Jon

• posted

Go for it. The VFD will self protect with it's internal current limit circuit on steady-state loading by reducing the output freq. or by refusing the command to got to a higher freq. if it is running at current limit. If reducing the freq does not get the output current back under the limit, it will ultimately trip off. It will also trip if you abuse it on start-up. It does not know KVA or HP, all it monitors is current and time and it is very good at protecting itself. Given hobby shop duty, you should have no problems except maybe a lack of power on very heavy cuts.

Randy

• posted

Thanks to both of you for the helpful, informative replies. Interesting points about rated conditions (industry v. home shop) and internal protections. I will give it a go! David

• posted

Since your VFD is rated for 2 hp with three phase in, you shouldn't have to worry about the motor inductance. With single phase power in and a 1.5 hp motor, you may be drawing more current thru the input diodes than their rating. And the capacitors on the input side will be getting single phase rectified power instead of three phase power. Which means they will be having more ripple current.

You could add capacitors in parallel with the ones already there to help with the ripple current. You could put in bigger diodes so they are used within their ratings. Or you could use a rotary converter to supply three phase input to your VFD.

I would probably add some capacitors and call it good. And as Jon says that may not be needed.

Dan

• posted

How about the flip-side? I have a 10hp VFD and don't have any motors that come anywhere near that size that I would like to speed control. Probably run various 1/2 - 2 hp motors. I was thinking of putting it on a cart and using it as a mobile power supply. I can't think of any reasons why it would be a bad thing, but the "tuning" reference made me wonder.

JW

• posted

JW:

If you do use it as a portable 3-phase vfd, make sure you do something to make sure you don't disconnect the load when the VFD is operating. They tend to self-destruct if you remove the load while they are operating.... If it were me, I'd use a connector scheme that makes certain that you can't unplug the load inadvertently. (In my shop, I use twist-lock connectors for all 220 volt receptacles, but that's because I like twist locks...)

Thanks

--Rick

Jeridiah wrote:

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