Erratic behavior from VFD

[Ischgl99]

I think you need to go back and read the very article you quoted.
HP = T * rpm/5252.

If you get the same torque for the same current, regardless of rpm (poles), why would anyone run anything other than 2 pole motors? You could gear a 2 pole motor down by a 2:1 pulley and get double the torque of the motor using the same amount of electricity to run a 4 pole motor.

The article is about overspeeding a motor on a VFD and has nothing to do with two speed motors.

I screwed up, that is a constant HP motor he has. What I posted earlier only applies to two speed motors that are commonly used in air handling applications.

 

[Ischgl99]

Thanks for clarifying. Some time I’ll try 3400 rpm in 2-pole vs. 4-pole config to see what difference I can detect.

Ignore what I said, you have a constant HP motor so you shouldn’t see a decrease in performance, I confused myself with a different type of two speed motor that are commonly used in air handling applications where the power does decrease at the lower speed setting.

 

[jwmelvin]

Ignore what I said, you have a constant HP motor so you shouldn’t see a decrease in performance, I confused myself with a different type of two speed motor that are commonly used in air handling applications where the power does decrease at the lower speed setting.

So although my motor has similar current ratings in 2- and 4-pole configs, it will make about double torque as a 4-pole? (Each running at 60Hz)

Then, running the 4-pole at 120 Hz will reduce the torque to about the same level as the 2-pole config (meaning either way it will be 2hp at 3400 rpm)? Overdriving the frequency should be smoother torque but less efficient and potentially harmful to the motor?

 

[Ischgl99]

So although my motor has similar current ratings in 2- and 4-pole configs, it will make about double torque as a 4-pole? (Each running at 60Hz)

Then, running the 4-pole at 120 Hz will reduce the torque to about the same level as the 2-pole config (meaning either way it will be 2hp at 3400 rpm)? Overdriving the frequency should be smoother torque but less efficient and potentially harmful to the motor?

Yes, the torque should be about double in the 4 pole configuration, so overspeeding with the VFD should get you close to the 2 pole speed torque at the same hp.

Overspeeding the motor puts more stress on it, so you could reduce its life the more you run it at the higher speeds. I chose to not overspeed the 900 rpm motor on my Millrite due to it not being an off the shelf motor and would likely be pricey to replace, but the benefits of a wider variable speed range might be worth that risk to others.

 

[rabler]

So although my motor has similar current ratings in 2- and 4-pole configs, it will make about double torque as a 4-pole? (Each running at 60Hz)

Then, running the 4-pole at 120 Hz will reduce the torque to about the same level as the 2-pole config (meaning either way it will be 2hp at 3400 rpm)? Overdriving the frequency should be smoother torque but less efficient and potentially harmful to the motor?

More or less. I expect over driving it all the way to 120Hz will reduce torque by a bit more than 1/2 due to inefficiency. Without having detailed data from a manufacturer on the motors performance at 120Hz as you might get in an expensive inverter-rated motor, your best way to test that is by measuring heat buildup. You're trading off wear on the motor vs risk to the VFD from what may be an erratic drum switch.

 

[mksj]

You are technically not over speeding the motor running it on 4 pole and 120 Hz, the motor is running at its rated speed for 2 pole. I also have not seen any information to indicate the effect of 60Hz vs. 120Hz on either insulation or heating of the coils, the biggest factor is the carrier frequency and very long cables. Older motors sometimes people will use a dV/dT filter between the VFD and the motor. Dual voltage motors run on their lower voltage windings, typically do not have an issue with insulation breakdown.

Torques does fall off in a non-linear fashion above the base speed and also Hp may dip a bit in the 100-120Hz range, the difference between that and the 2 pole setting would most likely be small. Also an older motor would not have a wider CT ratio so the torques would probably start dropping off below 20-30 Hz, and also motor cooling becomes an issue below 15 Hz. Newish 2-speed motors that are constant Hp, I have always run on the higher pole setting and run them to 2X their base speed with no issues. The 2 speed motors that are constant torque, I have done several VFD installs and they did not perform well on either pole setting with all the parameters adjusted for either configuration. I ended up replacing them with single speed.

 

[strantor]

the biggest factor is the carrier frequency and very long cables.

Carrier frequency and cable length are both critical factors, so agreed on that.

You are technically not over speeding the motor running it on 4 pole and 120 Hz, the motor is running at its rated speed for 2 pole.

Also agreed on that, if we add the stipulation of "you aren't mechanically overspeeding the motor..."

I also have not seen any information to indicate the effect of 60Hz vs. 120Hz on either insulation or heating of the coils

For a better understanding of this, you may find documentation of 400Hz motors interesting. 400Hz was/is used on aircraft because you can achieve the same quantity of horsepower from a motor 15% (60Hz/400Hz) the size. 400Hz motors are also used for some industrial applications like concrete saws which are run from a special 400Hz generator, to allow the operator to get into tighter spaces with smaller and more powerful tools that would usually be much larger. If you look into the construction of 400Hz motors you'll find that their iron laminations are of a special alloy and much thinner (think < 1 thou shim stock) than 60Hz motor laminations. The reason is reluctance, the magnetic equivalent of electrical resistance. Alternating magnetic fields cannot alternate fast enough through typical 60Hz motor stator to run on 400Hz. If they could, then any old induction motor could be rewound for higher voltage and we could run them at the 400-500Hz and get 7x the nameplate horsepower from them. But it doesn't work, sadly.

60Hz motor laminations are manufactured to meet the demands of 60Hz service. You can usually get away with running them a little higher, especially inverter duty motors which typically have a separate rating for "base frequency" and "maximum safe speed" and can usually be ran up to 120Hz, maybe a bit beyond, wil no issue. But for a motor made before the idea of VFDs were ever conceived, you can put money on them not performing to expectations at 120Hz. I don't know exactly what that means as I've never tried it, maybe overheating, but I would be surprised to see it work reliably.

One other thing I've never tried, but should be possible per my understanding of things, is doubling the HP of a 240v/480v inverter duty motor by wiring for 240V but powering it with 480V @ 120Hz from a VFD. The HP drop-off as shown in the graphs in this thread so far is contingent upon an [unplotted but present] identical graph of V/Hz, where the V:Hz relationship is linear up to nameplate ratings but thereafter voltage remains constant as frequency continues to increase. But if instead, you continued to increase voltage as you increase Hz, the HP would continue its linear upward trend. The inverter duty motor, due to its "skookum" insulation designed with multi-kV spikes in mind, (I think) should easily withstand a doubling of its applied voltage without issue. I keep meaning to try this but never have... I figure if I keep dropping bread crumbs on the internet, maybe some day someone else will get intrigued and try it for me...

 

[rabler]

For a better understanding of this, you may find documentation of 400Hz motors interesting. 400Hz was/is used on aircraft because you can achieve the same quantity of horsepower from a motor 15% (60Hz/400Hz) the size. 400Hz motors are also used for some industrial applications like concrete saws which are run from a special 400Hz generator, to allow the operator to get into tighter spaces with smaller and more powerful tools that would usually be much larger. If you look into the construction of 400Hz motors you'll find that their iron laminations are of a special alloy and much thinner (think < 1 thou shim stock) than 60Hz motor laminations. The reason is reluctance, the magnetic equivalent of electrical resistance.

Being picky, the reason is eddy currents, not reluctance. Otherwise you are right. Higher frequencies cause loss unless the motor is designed for that frequency. Same with transformers.

 

[Ischgl99]

One other thing I've never tried, but should be possible per my understanding of things, is doubling the HP of a 240v/480v inverter duty motor by wiring for 240V but powering it with 480V @ 120Hz from a VFD.

I believe I read somewhere that Haas runs their machines that way.

The centrifuge company I worked for ran one of the motors on the machine that way, but used a delta-wye motor wired for 230v and 50 hz (it was a European company) on a 400 volt design feed into the VFD. Instead of typing it out, this article describes how it works.

87Hz motor control for Delta Wye VFD operated motors - KEB

This article gives an overview of the 87hz motor operation for delta/wye motors. The trick allows a motor to be oversped while keeping the torque constant. Effectively increasing the available motor power.

www.kebamerica.com

 

[mksj]

I agree with you on all points, the caveat being that one rarely runs a motor on a manual mill at maximum speed. There have been a number of real world evaluations of running a "standard" motor (if I recall from Toshiba) above the base speed and typically the full performance is maintain to around 100 Hz on a 4 pole motor, and in some cases the torque was higher than predicted. Above 100 Hz and the motor characteristics play a more significant factor. The motor is not so old that I would have significant concerns about the insulation.

With regard to your discussion of wiring a dual voltage motor for low voltage (240) and running the motor in to 120Hz/480V, this are a number of postings over in the Practical Machinist where they have done that in industrial motors. It would require 480V 3 phase service, or the means to convert single phase to 3 phase, step it up to 480V and then run it into a VFD. Phase Perfect also offers voltage doubler models, but we are talking some serious $$$.

Learn how to push VFDs above 60 Hz to widen speed range, improve stability, and reduce cost:

How to Maximize Gearmotor Speed Range

Learn how to push VFDs above 60 Hz to widen speed range, improve stability, and reduce cost.

www.machinedesign.com