I have ben working on building my metal beltgrinder and been looking for a suitable electric motor for it.
Because of the low price, I bought a 1.5 hp 3 phase Baldor motor that features 1750 rpm TEFC construction. Now this motor is a HVAC variable torque motor and as I am reading up on the term "variable torque" I am starting to have a nagging feeling that I should not have bought it. I understand that as the speed of this motor slows down the torque is diminishing as well as opposed to constant torque motors. Does this mean that if I put this motor on the grinder and start working with it, once I lean into it it will slow the motor down which in turn will produce less torque and the motor will basically stall, or will not produce enough torque for the work.
Most belt grinders use DC motors which can be slowed without losing torque. You can, however, use a combination of variable speed drive and 4-step belt drive. That way you can change belts to get close and then tweak the motor speed a little slower or faster.
You might also consider a 2 hp motor, 1.5 is a little light for a belt grinder.
I am a total noob at this. To further ilustrate that point, I just realized that a 3 phase motor - such as the one I bought - will not run properly in my home shop where only 1 phase power is available.
That would depend on the exposed belt area/contact area. If this is a platen grinder, I'd agree with you, if a contact wheel grinder, not. My Wilton Square-Wheel is a one horse, and that is a lot of power for offhand use on a
1 1/2" wide contact wheel. My Bader is a two horse, driving a two inch contact wheel. In normal use I'm not signifcantly slowing either of them (and this is rough grinding knives in a fair hurry). Now if "belt grinder" means 6 or 8 inches wide over a foot or two of platen, all bets are off: more power is definitely better.
Single phase motors are not very expensive, you can find a 2 HP motor for about $50 or so if you look a little bit. Last night, I brought home a 5 HP single phase Baldor motor for $58 (mounted on a Bullard free air pump for free air respirators, like sandblasting). Not sure if I want to part it from the pump (which works, but is in visually not so great condition) or keep it on the pump. Harbor Freight has motors that will probably work OK for your application and will give you years of service.
they put this stuff on sale from time to time.
You can also make a phase converter to run 3 phase equipment, but I think that it would be an overkill for your simple application.
Oh, gosh! Well, the advantage of 3-phase motors is they can be speed-controlled, with a device called a VFD. They will also run directly off a single-phase supply, with some power loss if you connect some run capacitors to it.
Sure it will. You either build a rotary converter, or if you want variable speed, get a VFD (electronic inverter) to generate the three-phase needed.
Enough of the articles have expired so I can't see much of what else was said. I'm not sure what you mean by variable torque in the subject line. Most motors will simply supply increasing torque with increasing load until you reach the limit, at which point the speed will drop drastically as slip takes over.
The 1.5HP is plenty. If you want to have a variable speed, you'd need to go with either:
- DC motor + DC speed control (PWM or SCR type)
- 3 Phase AC motor + VFD (variable frequency drive)
I went the latter way. Price-wise, TEFC DC motor, in 2HP capacity, is very expensive.
3 Phase 2 HP, on other hand, can be had new for $150. $50 ebay kills are common.
Having learned my lesson the hard way, I'd look for a __local__ distributor that you can get both 2HP 3phase motor AND VFD. Motor HAS to be TEFC. Controllers, as well, have to be well protected. If it comes in NEMA
4, then problem's solved. If it don't, either:
- mount it away from your grinder. You'd need to bring the controls close to grinder then: on/off, speed, direction. Better VFDs offer this as an option. You can always fashion your own.
- OR, put it inside of an air/dust tight enclosure. Make sure it is big nuff to where the controller won't overheat.
1.5 HP is plenty for 2x72. 3Phase motors have true rating on the plate. DC motors are not regulated like that and most of the time the plate rating will be suspect. Especially motors used in excersize equipment.
After getting VFD, think about what else, in your shop, you can control with it. The thing is great. Bandsaw, mill, lathe, etc etc . 3 phase motors are cheap.
About torque: most VFDs will give you area of constant torque. As you lower RPMs, the torque will be lowered as well, but you'd need lower speeds for control grinds where you don't need lotsa torque.
You need all the torque when you hog metal away. So you'd crank up RPMs and off you go. Be careful with sparks - when my grinder is really going, it lights up the workshop as if I am welding.
I am still working on mine, trying to scrounge up the pulleys too. I got the 3 phase 1.5 hp baldor motor off ebay for $30, it was a HVAC ventillator motor and was said to have variable torque. I read up on it and it means when the rpm drops so does the torque as opposed to the contant torque machines. That does not sound good I think, since it means if I put a speed controller on it and lower the speed then it would not have the power to grind at lower speeds. At max speed it still should be good though. I will look around for a VFD, but I am on a budget. By the way, wouldn't the motor loose power un the VFD ?
Henrik, in the USA, the term "variable torque" is always applied to a multi-speed motor. Describing a single speed motor as such makes no sense.
If the motor is rated 1.5 hp, it will deliver that at it's rated speed. Driven by a VFD, the motor should produce essentially constant torque from
100% speed down to about 50% without worrying about cooling problems. This also means that the motor will only produce 3/4 hp at 50 % speed. The actual speed range available is much wider, but the load capacity drops off rapidly below 50% speed. However, speeds below 50% are available for detail grinding at low removal rates because of the reduced power needed.
Even if you restrict the rig to a 2:1 speed range, it makes an grinder much more usable and adaptable to various materials. All belt operated machines suffer from mechanical resonances at some speeds. VFD drive allows easy speed control to eliminate running at those speeds, plus a 3-phase motor runs much smoother than any single-phase unit. You will marvel at how smooth your rig will run( provided the pulleys are balanced).
Since you already have the motor, I strongly suggest you get a VFD (single-phase input) to drive it.
I have purchased several VFD's from
with satisfaction. Prices are good, tech support is very good, delivery is fast.
I have to wonder if you looked for your motor type on the Baldor website. I'm gussing that you may have alloed Goog results to take you on a lot of non-specific wild goose chases, in search of lots of confusing, non-pertinent info.
Because I couldn't imagine Baldor was intentionally producing any motors that are designed to be easy to stall (your basic assumption), I looked for where the term "variable torque" was used at the Baldor website.
That wasn't particularly difficult, at all. When you read the description for your type of motor, you'll discover why the term is used by the manufacturer of your motor, not from some unrelated source.
You don't say how you intend to drive the motor, but when Baldor labels a motor with a 1.5HP rating, that's what the motor will produce for the specified input power. If your 3-phase source isn't balanced, you can expect a loss of performance. If you utilize a motor controller to effectively reduce the input power, you can expect a reduction in output.
If you have a suitable 3-phase power source, the motor will most likely provide adequate power for your grinder.
If you determine that your application requires full torque at reduced speed, then you should probably choose a suitably rated, industrial-duty permanent magnet DC motor and a proper drive controller.