VFD questions

Yes... you can use a 3 phase rotary to power in a 3 HP VFD... just be sure your rotary is fairly well balanced as VFDs can be alittle touchy. As yoou corrrectly pointed out, you will not have to de-rate your 3 HP VFD and can run a real 3 hp motor.

And yes you can run any motor that is equal or less than the current out of the your VFD... so running a 1 or 2 hp motor with a 3 hp vfd is no problem.

Probably, but why bother, if you can use single-phase input?

If you only have single phase power, get a VFD that works with it.

All I have ever seen can be configured to smaller motors.

opinion..worth what you paid for it....

A 3ph motor is designed to deliver its rated rpm at 60 hz. When the AC freq is doubled, it doubles the rated rpm..ie a 1750 rpm motor turns at

3500 rpm at 120 hz and when its halved..it runs at half speed..ie 875 at 30 hz. and so forth.

As you lower the frequency, most motors tend to loose torque. The slower you go, the less "power" it has. Its not until you get down in the 150 or so RPM ranges does the torque really start bottoming out.

So Id not run 1:1 pulleys on it.. as at 120 hz..that blade is Smoking However..if you simply use a pulley ratio on the drive motor and band saw that gives you your maximum lowest SFM without lowering the motor rpm below that magical 150 hz (Id suggest calculating that the lowest motor rpm be no less than 300 rpm simply for decent cooling of the motor), you should have plenty of power and SFM for most cutting. The larger driven pulley sure does help out on the torque side of things.

As far as Im concerned..only 3 ph motors will work with a VFD. And a

3ph motor has more torque per HP than a single phase motor of the same rating.

Many VFDs are underated and will provide nearly as much "power" when running on single phase as on true 3 ph input power. Nearly. Almost. Its gets most noticable when you turn the motor at low rpms. (hence the above comment about 300 rpm minimum plus better cooling).

if it wasnt so late, Id calc the pulleys and give you a number..but its been a long day and my brain hurts. This is a good exercise for you

But yes..in a nut shell..given the minimum motor RPM of 300, and the maximum of 3000 (which is what I recommend for most motors) you should be able to figure the pulley ratios which gives you your minimum 60 sfm and your maximum 2700 sfm (or a bit higher, though its really over kill)

Id still get a 3hp vfd to run a 2hp motor. I do this for a living..and there is nothing more embaressing than having your pride and joy bog down the first time its used...by the paying customer.

Gunner

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Errata...150 rpm

Gunner

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Gunner, Quick question concerning these.... If you have a motor, designed by the manufacturer to run at 1725RPM and you use a VFD to crank that badboy up... what's the chances of throwing the windings??? If it's designed for 1725, it would stand to reason that the armature is wound to withstand the forces at 1725, and revving it up would increase those forces. Ken.

The VFD should slow your motor down fine, but you won't like the power you have at the slow speed. You'll probably have about the same amount of torque from the motor as at a normal speed, but that may not be enough to cut steel.

I've got a 1 HP motor on my wood/metal bandsaw. Wood cutting, I've stalled it on occasion cutting large green logs. Metal cutting, I've never stalled it, although the band has slipped on the tire. But, because of the gearbox, I'm still getting the full 1 HP when cutting metal.

If your 1 HP saw normally runs at 3,000 feet per minute for wood, and you use the VFD to slow it to 60 fpm for steel, you'll be cutting with about one fiftieth of a horse.

The jackshafts and gearboxes are there for two reasons - speed and torque.

John Martin

The thing about slowing down a motor with a VFD is that although the torque is constant (proportional to current, which VFD holds to a maximum), the _power_ drops proportionally to the speed. So at 1/2 speed (RPM), you are getting 1/2 power. Unlike pulley speed reduction which increases the torque proportionally to the speed drop, so max power is constant.

I have a problem with my VFD-driven drill press with this. Drilling a big hole at low speed pops the VFD breaker a lot. (The d-p motor is 3/4 hp.)

One way around this is to oversize the VFD & motor such that when the speed is lowered you still have the power you need. E.g., if you need 1 hp at 1/2 speed, use a 2 hp motor/VFD. Also, you can use the VFD's ability to overspeed to get a wider range. E.g., using a VFD range of

30-60 Hz is 2:1 range, but 30-120Hz is 4:1. BTW, I limit my VFD to a 90Hz max - I just don't like the idea of running my 3450 rpm motor at 6900 rpm.

But you'd like a 10:1 speed range. Using a VFD only, that could be

12-120Hz (20-200%). Using a 2hp motor, that would only be .4hp (20% of 2hp) at the slowest speed. To get 1 hp at 12Hz, you'd need a 5hp motor!

Good luck, Bob

While this does not really pertain to my question, do they also make such a thing as a speed controller for a 115VAC motor or is it normally just 3 phase motors and DC motors?

I have a like new 23.1:1 gear motor, continuous duty, 2 hp 115/220 volts single phase. Came off a huge industrial (Ollie North type) paper / document shredder. This would probably be fine for power transmission for a bandsaw along with a series of different pulleys.

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Ken, I honestly dont know. Ive never had one fail for that reason, but..shrug. Ive had to replace a drive motor or two on older machines I retrofitted , with very old motors that puked when the bearings failed at 3000 rpm, however. Since a goodly number of motors run at 3600 from the start, shrug

Gunner

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No

Yes

Just remember that there are no 'windings' per se inside the rotor of a three phase motor.

Jim

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Some of the Marathon inverter duty motors I have used lately do list a maximum speed on the nameplate. As I recall, one of the 1750 motors had a max speed of 4000 rpm. The electrical engineer I worked with had me insert code to shut down the VFD's at ~3600 rpm, or 120 Hz. This fellow is about ready to retire, and is really sharp. He said the motors have and will come apart if run too fast.

Pete Keillor

Yes -- and that might be the way to go if you need to use the full power rating of the VFD. But be sure that the rotary is well balanced before you depend on it allowing you to use the VFD at full power rating.

The major advantage is that you don't need to make a rotary converter to feed them -- so they run quieter and have less waste energy (the efficiency of a VFD is higher than that of a rotary converter). Of course, if you have power-company three-phase, this does not apply.

The disadvantage is that it will draw a bit more current from the single-phase than it would from three phase.

You can use anything from its full rating on down to something which you're likely to lose under a micrometer. :-) It just doesn't care.

Note that running a motor from a VFD under about 25% or so of full speed risks overheating -- especially if the motor is depending on its internal fan for cooling.

It looks as though you will need about 20 RPM on the wheels at your lowest speed, and working from that 25%, it suggests that your motor's natural speed should be around 76 RPM -- which suggests a gearmotor not a direct shaft output, which you might be able to find in about 650 RPM for an 8-pole motor -- which would even be rather hard to find, I think. (Of course, I am neglecting the speed reduction of the pulley and belt connection between the motor and the drive wheel -- but I don't think that will get you you where you need to be.

Also -- you will get something approximating constant torque from a VFD running below the line frequency -- *not* constant horsepower, so you are losing the benefit of the torque boost by the gearing. I would consider the VFD nice for getting variable speed to optimize the cutting behavior of the blade (in the absence of a variable-speed pulley driving a gearbox for the low speed., but it would be poor practice to depend on it in the place of gearing/belting so the motor runs most of the time near its rated speed.

They are neat devices, and I use them regularly -- but they are not a cure-all -- and I think not the ideal solution to your problems.

Good Luck, DoN.