I bought a fairly antique 20" Monarch bandsaw. It came with a gigantic (~16" diameter) 5HP electric motor. I'm not sure a bandsaw of this size needs this much motor, but since I have it, I'll try to use it.

The motor appears to be 5HP 3Phase, and probably 1730RPM. It is old enough that I can't find information about it on the 'net. In the US, power is supplied to residences in single phase, so I would like to size and buy an appropriate VFD/Inverter/starter to run it from 240VAC 1-phase at 220VAC 3-phase. It would be great to choose the run speed, have soft start so there's not a huge inrush current, and possibly use dynamic braking.

I've seen less expensive Chinese manufactured VFD's on ebay, but there's typically no consultation with the vendors, and caveat emptor. I have seen recommendations for sizing at multiples of up to 6 or 8 times Full Load Amps. Also I've seen it said with 1 to 2 HP motors that 2 x FLA is sufficient. The label is faded enough that I can't be sure about the listed FLA (12? 12.68? 12/6.8 at 220/440?) nor the motor efficiency.

One formula I found online shows to use ((HP x 746watts/HP) ÷ Efficiency) x √3(for 3phase) to get the KW required to size the VFD properly. It sounds right, but I can't read the necessary info from the motor badge so I wondered if anyone is familiar with this gear enough to make a recommendation?


Antique GE Induction Motor, 5HP - 3PH - 220VAC

  • I would suggest using a smaller, single-phase motor. While you might be able to run it on a rotary converter, (or possibly find a VFD in that size and single-phase in, 3 phase out) if you don't have one that's likely to cost as much or more as a suitable single-phase motor of more reasonable size. – Ecnerwal Mar 3 at 21:36
  • I agree with Ecnerwal I have done static caps , rotary and vfd. VFD is the best worked much better than either of the others, next was rotary and I ended up with a start / run setup for good performance. Static not very good at all had to hand start but then it ran (that’s what I built first) Today I stop at 3hp with VFD and go to rotary with larger motors. – Ed Beal Mar 3 at 23:32
  • what is "TS Photo" – Jasen Mar 4 at 8:52
  • I license images. A © statement is not necessary in the US as creative works generally are covered from the time of first publication/or their creation, but it helps control image use. If the label weren't so worn off or I'd found info on it online, I probably would have swung for a low price VFD...but as in my question, I woulda needed to know specifics. I'll very likely look for a 3HP motor, and fabricate a mount for it, and a rolling stand for both saw and motor; likely sell this working antique...it's a pretty nice piece of disappearing industry. I really wonder when it was made? – Old Uncle Ho Mar 4 at 18:02

It can be done with a vfd? Possibly. I would want to Meg the motor prior spending the $ for a vfd. There are also ways to do it with capacitors. And capacitors plus another 3 phase motor.

If the voltage is the same 220/240v it would be 1.732 (square root of 3) plus the vfd losses (we don’t make 3 phase without losses)

by code you would need to use the book value 3 phase 15.2 amps that x 1.732 = 26.32 and the book value for a 5 Hp 230v single phase is 28 amps so those work out close by the time you add some for the vfd.

So yes it can be done. Yes I have done it. Yaskawa drives can be purchased from automation direct by the general public although inexpensive have been good drives.

You might get some sticker shock when you find out you should use a heavy duty 5 or a general purpose 7.5 rated drive.

As for a soft start, don’t plan on doing two much with this old motor, the windings are not insulated for doing two much and you may find the bearings start disintegrating after a short time, this was a big problem in the 70’s & 80’s with VFD’s running anything other than 60 hz.

I mentioned having the motor megged at the start. Before spending anything I would want to check all 3 phases to ground and verify a minimum of 10k at 1000v Higher is better, then a simple ohm test a-b a-c b-c all 3 resistances should be similar. If the motor megs low I might heat it to 140f or so for 24 hours and see if the values increase but don’t try below 10 meg @1000 or the overloads if set properly will probably kick out in 10 to 30 seconds depending on the class of overload used in the vfd (10 is normal for most equipment like this).

So yes it can be done, however you may find a 5 Hp 240 single phase motor is less expensive.

  • What does "megging" or "meg" a motor mean? – George Anderson Mar 3 at 23:40
  • 2
    @George See 2nd to last paragraph it’s a high voltage ohm meter. Most industrial electricians have them or regularly use them and HVAC techs to see if the oil has gone acidic In the compressor and eaten the varnish the windings will short to ground, a low resistance (bad) a really good number is +500meg ohm at 1000v . I believe book value is 100 meg 1000v but since using only 240v rms I have found as low as 10meg ohm will spin up. On 480 I won’t spin up under 30 meg unless a big motor 150+ Hp. So the number varies with experience ( I have had motors with good meg & phase to phase not work) – Ed Beal Mar 4 at 0:00
  • Thanks Ed Beal! You obviously have had a wide variety of experience. I learn something every day I participate here. I'm just a humble homeowner but with a decent amount of residential electrical experience. No electrical license, but it seems to come naturally to me. Nearly all the electrical work I've done has passed inspection the first time or with only minor corrections. Never charged anybody for it bc that would be illegal...just did it as charity work for friends and relatives. – George Anderson Mar 4 at 1:28
  • A decent 3HP motor seems to run under $300, while a reasonable looking VFD of the capabilities you state would cost that or more, so a new, continuous duty, TEFC motor now planned. I don't think I could measure Mega (million) Ohms with my cheap meter, much less at greater than DC battery voltage (~9V.) If the windings compared well, and the running load were ~1.5HP, would a VFD with a lower rating than 28A suffice to run it? Doesn't look that way.... – Old Uncle Ho Mar 4 at 18:15
  • A megger is a high voltage meter home meter won’t do it it may measure high resistance values but won’t have the high voltage. As far as the 3hp that might be the way I would go unless it is a metal cutting model. I can’t read the type I was looking for a frame type today’s motors are like T184, 215, 254 ,,, these 3 are the ones I use most in 5 Hp this is important because a 3 Hp t184 will have the same foot bolt pattern , foot to shaft offset, and shaft size are all the same so if you can find a 3 Hp that matches you can use the same everything mechanical. VFD would need 3 phase HD at min. – Ed Beal Mar 5 at 0:14

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