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.
Thanks in advance.
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.
Thanks for your advice Erwin !
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.
Midland, ON, Canada
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
with a device called a VFD. They will also run directly off a
with some power loss if you connect some run capacitors to it.
According to Henrik :
Sure it will. You either build a rotary converter, or if you
want variable speed, get a VFD (electronic inverter) to generate the
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.
why don't you just add a start capacitor to the 3 ph motor you already
have - look up Static Phase Converter in your favorite search engine, there
are many articles on how to build them - the simplest approach is a
capacitor and a momentary switch
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__
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
4, then problem's solved. If it don't, either:
- mount it away from your grinder. You'd need to bring the controls
to grinder then: on/off, speed, direction. Better VFDs offer this as an
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
About torque: most VFDs will give you area of constant torque. As you
RPMs, the torque will be lowered as well, but you'd need lower speeds
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.
Join bladeforums .
That grinder looks SWEET :)
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 ?
Variable torque is used on centrifuge and ventillator motors
supposedly. It is said that as the speed drops on these motors so does
the torque as opposed to contant torque motors.
D> According to Henrik :
VFDs allow you to have a pretty wide range with constant torque.
They do it by increasing voltage @ lower RPMs.
Assuming you feed a VFD via 220V 20A circuit, you WILL be able
to get full 2HP out of your motor, assuming VFD is capable of taking
single phase 220V in and is rated @ 2 HP.
Anything above that requires 3 phase input.
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
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
satisfaction. Prices are good, tech support is very good, delivery is
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
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
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