Choosing a motor for VFD speed control

[Firebrick43]

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

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.

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.

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.

 

[akjeff]

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.

 

[ttabbal]

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.

 

[rabler]

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.

 

[mksj]

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.

 

[Firebrick43]

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

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

 

[mksj]

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

 

[rabler]

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.

 

[rabler]

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.

 

[Firebrick43]

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.