Choosing a motor for VFD speed control

[John_Dennis]

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

 

[Watchwatch]

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

 

[Firebrick43]

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.

 

[John_Dennis]

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.

 

[mksj]

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

 

[Firebrick43]

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

 

[rabler]

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.

 

[Firebrick43]

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

 

[akjeff]

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.

 

[rabler]

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.