Erratic behavior from VFD

[jwmelvin]

Wasn’t Mark’s point that running 4-pole at 120 Hz is the same as running 2-pole at 60 Hz? The torque from 4-pole decreases over 60 Hz but the torque from 2-pole is less to begin with, as there’s no other way the two configurations would have the same power (because power is rpm*torque).

 

[rabler]

A drum switch that's rated for high voltage and current may be making erratic contact to the VFD also, which would cause the VFD to try to use the wrong parameters.

5.8 amps at 230 volts 3phase is about 2.3KW. 5.4 amps translates to about 2.15KW. Both are believable figures at 746 watts per HP, if a little inefficient, for roughly 2HP output. So I'd *guess* you're getting double the torque by running four pole.

Motor temperature is a pretty good guide to how well it's handling different operating parameters. I'd be tempted to try it 4 pole at 120Hz and see if it starts running hot.

 

[Ischgl99]

I thought there was reason not to push an older motor like this past 90 Hz or so?

Mostly I did it because it was easier than removing the H/L switch and I thought it would be an enjoyable experiment to see the difference myself.

With 4-poles vs. 2-poles, isn’t there a torque difference for the same current?

Usually you don’t want to overspeed a 2 pole motor too much due to vibration problems, but by running it on 4 pole, doubling the speed brings it back up to what it can mechanically handle on 2 pole operation. However, given it’s age, and it has E class winding insulation and not F, the windings might not be able to handle the stress of VFD operation as well as a newer motor with better insulation.

 

[rabler]

Not exactly, he'll have constant torque and variable HP up to 60 hz, then constant HP above 60 hz, but the torque will decrease as the speed increases to maintain the constant HP as well as overcoming the inefficiencies of the worsening V/f ratio. Running above nameplate speed could be useful for small cutters that need high speeds and low cutting forces, but not very good if you want to hog out material.

This is a good article on it. http://pumped101.com/hp torque vfd.pdf

The problem with that article is that it totally ignores efficiency in there. Motors are optimized for 60Hz, so the farther away from 60Hz you get, the less applicable the graph is. Inverter motors are somewhat less optimized for 60Hz, and designed to maintain efficiency over a wider frequency range. This gets into complex issues like how the laminations in electromagnets are designed, etc.

Ultimately what determines the life of a typical motor is heat. Insulation breaks down faster, and bearings wear faster, when things run hotter. Inefficiency means running hotter. You can make up for inefficiency to some extent by improving the cooling.

 

[Ischgl99]

Wasn’t Mark’s point that running 4-pole at 120 Hz is the same as running 2-pole at 60 Hz? The torque from 4-pole decreases over 60 Hz but the torque from 2-pole is less to begin with, as there’s no other way the two configurations would have the same power (because power is rpm*torque).

No, torque on a 2 pole motor is only less than a 4 pole motor if they are the same HP. Since the current on the nameplate of the motor above is similar for both 2 and 4 pole settings, the torque will be similar for both settings. The current is actually a bit less on 4 pole, so the HP will actually be a bit less than half the 2 pole rating.

Two speed motors are also two HP motors unless the windings on the 4 pole setting can handle higher current than on 2 pole. I don’t think I have ever seen a motor that can do that.

 

[Ischgl99]

The problem with that article is that it totally ignores efficiency in there. Motors are optimized for 60Hz, so the farther away from 60Hz you get, the less applicable the graph is. Inverter motors are somewhat less optimized for 60Hz, and designed to maintain efficiency over a wider frequency range. This gets into complex issues like how the laminations in electromagnets are designed, etc.

Ultimately what determines the life of a typical motor is heat. Insulation breaks down faster, and bearings wear faster, when things run hotter. Inefficiency means running hotter. You can make up for inefficiency to some extent by improving the cooling.

You are completely correct, there are a lot of things that will reduce the actual performance. The article is a generalization to explain what happens, it’s not meant to say that is what your actual torque will be when running at higher than nameplate speeds.

 

[rabler]

No, torque on a 2 pole motor is only less than a 4 pole motor if they are the same HP. Since the current on the nameplate of the motor above is similar for both 2 and 4 pole settings, the torque will be similar for both settings. The current is actually a bit less on 4 pole, so the HP will actually be a bit less than half the 2 pole rating.

To be blunt, that is wrong. Torque is NOT directly related to current. By that logic, a typical motor running on 460V would generate half the torque as the same motor running on 230V, since the nameplate current is about 1/2.

Current * voltage / 746 = horsepower. Or current * voltage * sqrt(3)/746 if its is 3 phase. Actual horsepower is related to that by efficiency, and if you want to really get complicated, you throw in power factor (phase angle).

Use the HP = RPM * torque/5252 to compute torque.

If you're comparing the same number of poles, same voltage, then yes, torque follows current, ignoring efficiency and power factor.

 

[Ischgl99]

To be blunt, that is wrong. Torque is NOT directly related to current. By that logic, a typical motor running on 460V would generate half the torque as the same motor running on 230V, since the nameplate current is about 1/2.

Current * voltage / 746 = horsepower. Or current * voltage * sqrt(3)/746 if its is 3 phase. Actual horsepower is related to that by efficiency, and if you want to really get complicated, you throw in power factor (phase angle).

Use the HP = RPM * torque/5252 to compute torque.

If you're comparing the same number of poles, same voltage, then yes, torque follows current.

We are not comparing different voltages, we are comparing different numbers of poles in the motor.

 

[rabler]

We are not comparing different voltages, we are comparing different numbers of poles in the motor.

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.

 

[jwmelvin]

No, torque on a 2 pole motor is only less than a 4 pole motor if they are the same HP. Since the current on the nameplate of the motor above is similar for both 2 and 4 pole settings, the torque will be similar for both settings. The current is actually a bit less on 4 pole, so the HP will actually be a bit less than half the 2 pole rating.

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