# Choosing a motor for VFD speed control



## John_Dennis (Feb 5, 2022)

I want to build a 2x72 belt grinder using a 3 phase motor and VFD.  I have a 1.5 HP inverter duty 1750 and a Gold Baldor 2 HP 1750.  Would I be better off to find a 3450 motor or will the ones I have work well?

Is id better to speed up a 1750 or slow down a 3450 or slow down a 1750?

My local salvage yard had a nice looking 3HP 3450 but I dont think I want to pay for a 3HP+ VFD

Thanks
JD


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## Watchwatch (Feb 5, 2022)

Big drive wheel, 3400 rpm, 2HP motor, and VFD is the way to go on a 2x72.

Buy once, cry once 


Sent from my iPhone using Tapatalk


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## Firebrick43 (Feb 5, 2022)

A 1750 4 pole motor will have the same hp at 3450 as it does at 1750.  With a vfd, once the motor is above rated speed HP is constant.  IE as the speed goes up the torque goes down proportionally.  So there is little difference in power comparatively if you double the speed or compared to a 2 pole 3450 motor.    Most inverter rated motors in that size go to at least 5500 rpm if not 6000.  This is more a function of centripetal force on the rotor. 

On the other had a 2 pole motor looses power if you run it at half speed with a vfd as torque will not go up as rpm comes down like a 4 pole motor (from 3450 to 1750).  Of course below 1750 a 4 pole motor torque remains constant but speed drops and therefore HP drops.


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## John_Dennis (Feb 6, 2022)

The Baldor 2HP I have is a 4 pole continuous duty motor. https://www.baldor.com/catalog/EM3558T Will it handle intermittent use at 3450 using a VFD?  What Hz would it need to run that fast?  I am not going to run this for hours at a time, I have a hobby shop.


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## mksj (Feb 6, 2022)

A 1750 RPM motor will have 2X the torque at 1750 RPM (60 Hz) vs. a 2 pole motor at 30 Hz at the same RPM. Four pole motors for the most part will turn 2X their base speed (to 120 Hz) with no issues, the bearing are the same, their is some slight performance drop off above ~100 Hz. Above the base speed the Hp will remain the same, torque does fall off but will be no worse than a 2 pole motor at 3500 RPM. You do want a totally enclosed motor so either a TENV or TEFC. If you get a motor with a higher rated top RPM you can also adjust the motor pulley size smaller which will give you more applied force to the belt due to the mechanical advantage.
This would be a very good motor to use BALDOR IDNM3584T 1.5HP MOTOR: https://www.ebay.com/itm/175086857097
Alternate: Baldor EM3558T 3 Phase AC Electric Motor 2HP 1755RPM 60HZ 208-230/460V https://www.ebay.com/itm/284397944262

Many people opt. for the KBAC sealed drive for the belt grinders, they work well for the most part although there have been a few cases where the  motor/VFD combination was not a good match. This may have to do with the tunability of the VFD to the motor.
Baldor Aegis 2 HP KBDA (KBAC+) VFD 2x72 Ultimate Grinder Package https://www.ebay.com/itm/175135808374


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## Firebrick43 (Feb 6, 2022)

The 1.5hp inverter duty yes, but the super E, I wont stick my neck out and say yes. 

You would drive the motor at 120hz to get 3450 rpm. 

Page 2 of the following link have a similar motor but has an auxilary constant speed fan instead of a shaft mounted fan and they say it can be ran at 120 hz. 
baldor super e
Most sub 10 hp motors share the same frame/bearings whether they are 1800 or 3450. 

Contact a baldor rep, they will get you the info


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## rabler (Feb 6, 2022)

For reference I’m building my 2x72 based on a 2HP 1750 rpm three phase TEFC  WEG inverter duty motor, 143TC frame, and a 6” diameter drive wheel.  Not the highest quality motor.  I’m using a Teco L510 series vfd, rated at 3HP because I had a spare and am familiar with their setup.  It’ll need to be enclosed., I’ll mount it so the frame acts as a heat sink.

You can pretty much accomplish the same grinder belt speed with either a 2 pole or 4 pole motor.  Just make the drive pulley 2x the dia on the 4 pole as on the 2 pole.  A vfd doesn’t change that equation unless the motors have have different variable torque speed ratio, a fancy way of saying how high you can turn up the vfd frequency for that motor.


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## Firebrick43 (Feb 6, 2022)

rabler said:


> For reference I’m building my 2x72 based on a 2HP 1750 rpm three phase TEFC  WEG inverter duty motor, 143TC frame, and a 6” diameter drive wheel.  Not the highest quality motor.  I’m using a Teco L510 series vfd, rated at 3HP because I had a spare and am familiar with their setup.  It’ll need to be enclosed., I’ll mount it so the frame acts as a heat sink.
> 
> You can pretty much accomplish the same grinder belt speed with either a 2 pole or 4 pole motor.  Just make the drive pulley 2x the dia on the 4 pole as on the 2 pole.  A vfd doesn’t change that equation unless the motors have have different variable torque speed ratio, a fancy way of saying how high you can turn up the vfd frequency for that motor.


Yea, but if you build as if you had a 2 pole motor,(motor) pulley size) but use a 4 pole inverter duty motor you can have your cake and eat it to.  It will perform the same at 120 hz as a 2 pole motor at 60 hz but it will have twice the torque and still have full rated hp at 60 hz but the 2 pole motor would have half the torque/hp at 30 hz


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## akjeff (Feb 6, 2022)

You've already got the 2HP Baldor in hand, that would make the choice pretty easy for me. Like rabler suggested, use a 6" pulley, and rock on. If you don't find it has enough cajones, you can always swap it out later if need be. I just finished building one of the Jer Schmidt 2x72's, and used a 3 phase 3450 rpm 2HP powered with a TECO 510 VFD, and love it! Metal eating machine.


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## rabler (Feb 6, 2022)

Firebrick43 said:


> Yea, but if you build as if you had a 2 pole motor,(motor) pulley size) but use a 4 pole inverter duty motor you can have your cake and eat it to.  It will perform the same at 120 hz as a 2 pole motor at 60 hz but it will have twice the torque and still have full rated hp at 60 hz but the 2 pole motor would have half the torque/hp at 30 hz


Assuming you run those motors to the same maximum VFD frequency, your example you loose half your belt SFPM.     For example if 120Hz is your max frequency:

1750 RPM motor (4 pole),  3" pulley, 120Hz  = 2,750 SFPM,   24 lbs belt pull (72 in-lbs torque @ 3 in)
3500 RPM motor (2 pole), 3" pulley, 120Hz   = 5,500 SFPM,   12 lbs belt pull (36 in-lbs torque @ 3in)

If you put a 2x pulley on the 4 pole motor, the torque vs RPM curve are basically identical for the same HP motor.  Changing the motor poles doesn't change this.  Effectively, this is what HP means.

You could argue that you can overspeed a 1750 RPM motor to a higher frequency (say 240Hz vs 120Hz for a 2 pole 3500 RPM motor).  From a rotor and bearing RPM standpoint that may be true, but from a motor electromagnetic design you are going to loose a lot of efficiency trying that.  The idea that a inverter motor maintains HP above 60Hz is only true for a limited range.


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## Firebrick43 (Feb 6, 2022)

rabler said:


> Assuming you run those motors to the same maximum VFD frequency, your example you loose half your belt SFPM.     For example if 120Hz is your max frequency:
> 
> 1750 RPM motor (4 pole),  3" pulley, 120Hz  = 2,750 SFPM,
> 3500 RPM motor (2 pole), 3" pulley, 120Hz   = 5,500 SFPM
> ...


You cant run a 3450 rpm motor at 120 hz. The rotor and bearings are not rated to take that rpm, depending on the motor, either 2 pole or 4 pole inverter rated max out about 5500 to 6000. 

You are correct about the efficiency, it starts dropping about 200 hz.  Which is 5800 rpm and is at or above the rpm limit anyhow for the 4 pole and only 100 hz on a 3450.  

But my whole point is with same size pulley, at max speed of the motor, things will be equal down to 120 hz on the 4 pole and 60 hz on the 2 pole or 3450 rpm.  Below that the 4 pole will have a torque/hp advantage.


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## akjeff (Feb 6, 2022)

I went with a 3500 RPM ( 2 pole ) simply because our local supplier had one a customer ordered and bailed on. It was collecting dust, and he gave me a screaming deal on it. Saved me from shipping one from the lower 48, which costs a fortune up here. I'm using a 4" drive pulley, which makes it around 3600 SFM at 60 hz.


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## ttabbal (Feb 6, 2022)

I use a nice 4 pole 2hp motor with a 5" pulley on the drive side. I have the VFD range from 30-120hz and it works great for me. I've never noticed a lack of power even when pushing HSS into the belt as hard as I like. 

I was a little concerned about speed when building it, but I got a great deal on the motor so I went for it. I have no complaints.


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## rabler (Feb 6, 2022)

Firebrick43 said:


> But my whole point is with same size pulley, at max speed of the motor, things will be equal down to 120 hz on the 4 pole and 60 hz on the 2 pole or 3450 rpm.  Below that the 4 pole will have a torque/hp advantage.


Your exact words were to the effect of have your cake and eat it too.  If you believe you really can run a 4 pole motor @ 240Hz with the same efficiency as a 2 pole motor at 120Hz, that is true.  But I'd challenge you to find electric motor efficiency curves for otherwise similar 2 pole vs 4 pole motors that supported that.   Most motor ratings that I have seen go to somewhere about 90 - 120Hz on the top of their curves.

Otherwise, you loose high end SFPM.  Which doesn't match your initial assertion.

My point is by changing pulley size, you pretty much make it work either way.  Without getting into wild speculation about what the motor will do well beyond what the manufacturer publishes specs for.


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## mksj (Feb 6, 2022)

If you look at motor recommendations (Marathon and Baldor) as to over speeding (non-vector motors) it is typically 2X the base speed for 4 pole motors and 1.25X for 2 pole motors with mechanical fan cooling. Only vector motor's in the 10 Hp and less are rated to 2-3X their base speed and will maintain full Hp up to the rated over speed. Mechanical speed may be higher but performance suffers. Standard inverter motor's typically will have the Hp start to fall off above 100Hz and the torque may also have a steeper decline. A standard inverter motor at 200 Hz, they are not designed for those speeds and cooling would be adversely effected on a TEFC with potential for fan failure. Every motor is slightly different, my lathe motor which is a 4 pole inverter type that specifically rates the maximum over speed as 72Hz, although I run it to 80.  Pulley size should be optimized to the motor type and operating range, and in this case a 4 pole motor running in the 60-120 Hz range is probably a more optimal approach then a 2 pole running 30-60Hz.


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## Firebrick43 (Feb 6, 2022)

Firebrick43 said:


> You cant run a 3450 rpm motor at 120 hz.  It will run out of torque and the the rotor and bearings are not rated to take that rpm





rabler said:


> Your exact words were to the effect of have your cake and eat it too.  If you believe you really can run a 4 pole motor @ 240Hz with the same efficiency as a 2 pole motor at 120Hz, that is true.  But I'd challenge you to find electric motor efficiency curves for otherwise similar 2 pole vs 4 pole motors that supported that.   Most motor ratings that I have seen go to somewhere about 90 - 120Hz on the top of their curves.
> 
> Otherwise, you loose high end SFPM.  Which doesn't match your initial assertion.
> 
> My point is by changing pulley size, you pretty much make it work either way.  Without getting into wild speculation about what the motor will do well beyond what the manufacturer publishes specs for.


I don't know where you keep getting this 240 hz thing.  I very specifically said that 200 hz is the top for a 4 pole and 100 hz for a 2 pole because of rpm limits.

IDNM3587T 2.0 hp 1740 name plate

max speed, 5500 rpm or 190hz



IDNM3584T 1.5 hp 1740 name plate
max speed, 6000 rpm or 206hz


I cant find a 2hp 2 pole industrial motor that is rated any higher in speed that 5500 rpm or a 1.5 higher than 6000 rpm either.  Or about 100 hz max.

For you to keep saying 240/120 hz is irresponsible as someone might try it, and blow up the motor. 

Baldor doesn't even make a 2 pole IDNM motor in this size, they tell you to run the 4 pole with vfd  to what ever speed you want as long as its under the rpm limit. 

To be very specific I am talking about 1.5-2hp 145t framed inverter motors.  The bigger the motor, the lower the top rpm due to the effects of mass/diameter of the rotor and the structural integrity/safety of said motor


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## mksj (Feb 6, 2022)

Blador comparison of 4 pole Super-E run to 120Hz (3500 RPM) vs. Vector type (IDNM/ZDM) run to 200 Hz (6000 RPM)


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## rabler (Feb 6, 2022)

mksj said:


> Blador comparison of 4 pole Super-E run to 120Hz (3500 RPM) vs. Vector type (IDNM/ZDM) run to 200 Hz (6000 RPM)


Thanks, good to see actual numbers for real motors vs broad generalities.  Motors can be designed to all sorts of parameters and price points.  Do you have a link to those online?  Image quality on here doesn't make reading the graphs easy.


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## rabler (Feb 6, 2022)

Firebrick43 said:


> For you to keep saying 240/120 hz is irresponsible as someone might try it, and blow up the motor.


You can buy a bearing rated for much more the 6000rpm.  Same with a rotor design.  Just depends on what the designers used for specs. Broadly generalizing that to any one specific rpm is just guesswork.

20% is (100Hz vs 120Hz, or 200 vs 240) noise level in my book.  But if it makes you feel better, use 200/100hz.  My argument doesn't change.  Adjust the pulley size and you're going to get in the same ballpark.  There might be some incremental improvements, Different motor manufacturers and price points, etc.  But 2x for free, as your cake comment implied, is bogus.  Otherwise 6 or more pole motors would be the solution for every problem.


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## Firebrick43 (Feb 7, 2022)

rabler said:


> You can buy a bearing rated for much more the 6000rpm.  Same with a rotor design.  Just depends on what the designers used for specs. Broadly generalizing that to any one specific rpm is just guesswork.
> 
> 20% is (100Hz vs 120Hz, or 200 vs 240) noise level in my book.  But if it makes you feel better, use 200/100hz.  My argument doesn't change.  Adjust the pulley size and you're going to get in the same ballpark.  There might be some incremental improvements, Different motor manufacturers and price points, etc.  But 2x for free, as your cake comment implied, is bogus.  Otherwise 6 or more pole motors would be the solution for every problem.


  The laminated core rotor is the limit as the bearings in that size are usually good for 10K. 

You still don't get what I was talking about having your cake and eat it to. 

If you use the same size pulleys on a 2 pole motor and a 4 pole motor, at 3600 rpm they will have the same torque/hp.  At 1800 rpm the 2 pole will have half the torque and half the HP that the 4 pole will at 1800 rpm 

Further more you stated that 200 hz was not possible for a 4 pole inverter rated motor yet mksj posted an article from baldor stating that operation to 6000 rpm is possible with full rated torque or more. 

6 pole or 8 pole motors are bigger yet for the same hp and therefore are even more limited in rpm due to inertia on the rotor.  Also the flux time on the windings become limiting. 

4 pole inverter duty motors are the sweet spot.


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## Dabbler (May 4, 2022)

For a novice VFD user, without any engineering experience I recommend that thye set the limits on the VFD to 10-120 Hz, not matter what motor they have.  This gives them forgiveness for things limke not knowing what kind of motor they have, condition of bearings, and other nicities.  Once tehy have hours under their belt, then they can branch out to widening the range.

For a typical mill with a 4 pole motor, this limits the top end to around 3000 rpm.  On my big mill, the spindle bearings heat up fast at these speeds.  On my tow smaller mills they begin to complain around 3300 RPM.  I get squeamish thinking of running a Bridgeport style head at any speeds higher than this.  I do it from time to time, but I have a lot of hours under my belt, and know what to look for.

There is lots of stuff you *can* do, but at least some stuff you *shouldn't*...


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