Troubleshooting a 2 Pole motor running at low speeds on a VFD

[Organic nutrition]

I run a small food manufacturing operation. We process nuts and seeds. We have several grinders/food processors that run at variable speeds. The speed of the motor is varied using an Emmerson VFD. The turning speed of the grinder is very low ( 50-100 RPM) arrived at using a sprocket and chain gear. Typically the load varies with product type, quantity in the grinder and amount of time the product has been grinding. Some are very viscous and take a long time to break down. Others contain more oils.

The loading process is tedious as we have to add the product slowly to each batch as it is ground. If too much product is added at a time, the VFD overloads and the speed has to be lowered until the overload condition stops.

We have been using 2hp 4 pole motors 3 phase 240V.

I decided to change to 3HP motors to increase the torque and hoped to be able to load the grinders more quickly. This did not happen. The new 3 HP motors run for a while but then cut out. Resetting gets them going again but it happens continually. It's not the thermal overload. The motors do not appear to be heating up.

On inspection, I found that I had received 2 pole (3 HP) motors rated at 3600 RPM. The 2 HP motors are 4 pole 1800 RPM rated. Presumably, this means that the frequency has to be much lower to achieve the same RPM of the grinder? I checked the settings on the VFD. All seems to be correct.Does anyone have any idea why the motors/ VFD would just cut out? As the grinder speed is the same as before, I wonder if the lower frequency needed to run the motors more slowly is causing voltage drop/lower torque /more current. The motors are not running hot.

I tried changing the sprocket on the chain drive to speed up the motors ( from 22 teeth to 14-which is the smallest I can get-at the shaft) It doesn't seem to have done the trick.

Sorry for the layman's explanation.
I'd be very appreciative of any ideas that can get me going properly.
I don't really want to just go buy more 4 pole motors unless I have to :>(
Thank you.

 

[anorlunda]

I checked rough numbers. A 3HP VFD drive will cost roughly $250. A new 3HP motor will cost about $375. Is is worth all the troubleshooting and the risk of it not working?

I think your first choice might be to return or to sell the motor you bought that is wrong for the application and to use that money to buy the correct motor.

 

[Asymptotic]

turning speed of the grinder is very low ( 50-100 RPM)

Is this the speed range required to grind the various types of nuts?

I tried changing the sprocket on the chain drive to speed up the motors ( from 22 teeth to 14-which is the smallest I can get-at the shaft) It doesn't seem to have done the trick.

I'm taking it this is the motor sprocket. How many teeth are on the driven sprocket?

What is the Emerson drive model number? Is is capable of driving the 3 HP motor at full load amps?

Not knowing the details, let's consider a possible scenario.
60 Hz is motor nameplate frequency for full speed operation. The drive is a simple scalar (volts/hertz) controller rather than a flux vector of one sort or another.
2 HP motor at 240V/3ph is about 6.8 full load amps; 3 HP is about 9.6 FLA.

If you had been operating the 4 pole motor at 80% of full drive speed (48 Hz) to get 100 RPM grinder speed, motor shaft speed would have been about 1400 RPM with about 192 volts fed to the windings. To obtain 1400 RPM with the 2 pole motor, frequency and voltage would need to be halved (40% speed, 24 Hz, 96 volts).

Ignoring power factor for the time being, maximum electrical power is 1.732*volts*amps. Counterintuitive as it may be, the 2 HP motor rated 6.8 amps is capable of delivering more shaft power than the 3 HP motor rated 9.8 amps (2.3 kW versus 1.6 kW) under the above conditions.

Using a 2 pole motor in place of the 4 pole is definitely part of the new problem.

If VFD trips can be averted, does it matter how long it takes to grind the nuts after they have been overfed? If no, then several approaches suggest themselves.

 

[Organic nutrition]

I checked rough numbers. A 3HP VFD drive will cost roughly $250. A new 3HP motor will cost about $375. Is is worth all the troubleshooting and the risk of it not working?

I think your first choice might be to return or to sell the motor you bought that is wrong for the application and to use that money to buy the correct motor.

The horse is out of the barn on that one :>) We own the motors and the drives. We already have the 'risk' the question is why doesn't it work? IMO, At question is does running the 3600 rpm motors with low Hz/volts create a condition that would produce an overload in the motors under higher load? we have been through drive settings with Emmerson tech help and they say its 'good to go'. When I got involved, I realized that the motors were rated for 3600 not 1800. So presumably this because they are 2 pole motors? Is it a good assumption that running the 2 pole motor at very low Hz/volts could result in higher than rated Current conditions?

 

[anorlunda]

Too bad the horse is out.

A motor is like a rotating transformer. Volts/hertz is an important parameter for transformers. Yes, too many volts/hertz can cause overheating. Aircraft use 400 Hz transformers instead of 60 Hz or 50 Hz because they can be built much smaller and lighter without overheating -- that illustrates the volts/hertz effect.

 

[Asymptotic]

The horse is out of the barn on that one :>) We own the motors and the drives. We already have the 'risk' the question is why doesn't it work? IMO, At question is does running the 3600 rpm motors with low Hz/volts create a condition that would produce an overload in the motors under higher load? we have been through drive settings with Emmerson tech help and they say its 'good to go'. When I got involved, I realized that the motors were rated for 3600 not 1800. So presumably this because they are 2 pole motors? Is it a good assumption that running the 2 pole motor at very low Hz/volts could result in higher than rated Current conditions?

At 60 Hz., the magnetic field in a 2 pole motor rotates at 3600 RPM (the rotor lags behind by a bit, 3500 RPM is ballpark). For a 4 pole motor they are 1800 RPM, and 1750 RPM, respectively, a 6 pole motor would be 1200 RPM, and so on.

The V/Hz ratio for a 230V, 60 Hz motor is 230/60, or 3.83 V/Hz. At 60 Hz (full speed) line voltage will be 60 * 3.83 (230V), and drop to progressively smaller values at lower speeds (115V at 30 Hz, about 81 volts at 21 Hz, etc.). Most scalar drive parameter sets have one or more frills such as "voltage boost" to help starting loads from zero speed, but other than that the V/Hz relation is maintained throughout the speed range.

It takes x amount of power to grind these nuts.

The following table doesn't bear close scrutiny (I'm assuming both motor efficiency and power factor are 75% for both 2 and 3 HP motors), but it is close enough to illustrate the point. See how much power can be produced at motor full load current rating for the (blue) 2 HP motor versus (yellow) 3 HP motor at a 1400 RPM shaft speed. Apparently you were already up against current limit when using the 4 pole, 2 HP motor. Changing to a 4 pole, 3 horse motor would probably have worked out, however, it is necessary to operate the 2 pole motor at half the voltage to obtain the same speed, and now it ends up limited at less power than before (or, looking at it the other way, a need to go even higher above motor FLA rating than before) hence the worsening of overload trips.

My suspicion is if Emerson tech support is giving a "good to go" it's because the nameplate values for the new 2 pole, 3 HP motor have been properly entered into their drive, and they aren't looking at the broader loading question.

 

[Organic nutrition]

Is this the speed range required to grind the various types of nuts?I'm taking it this is the motor sprocket. How many teeth are on the driven sprocket?

What is the Emerson drive model number? Is is capable of driving the 3 HP motor at full load amps?

Not knowing the details, let's consider a possible scenario.
60 Hz is motor nameplate frequency for full speed operation. The drive is a simple scalar (volts/hertz) controller rather than a flux vector of one sort or another.
2 HP motor at 240V/3ph is about 6.8 full load amps; 3 HP is about 9.6 FLA.

If you had been operating the 4 pole motor at 80% of full drive speed (48 Hz) to get 100 RPM grinder speed, motor shaft speed would have been about 1400 RPM with about 192 volts fed to the windings. To obtain 1400 RPM with the 2 pole motor, frequency and voltage would need to be halved (40% speed, 24 Hz, 96 volts).

Ignoring power factor for the time being, maximum electrical power is 1.732*volts*amps. Counterintuitive as it may be, the 2 HP motor rated 6.8 amps is capable of delivering more shaft power than the 3 HP motor rated 9.8 amps (2.3 kW versus 1.6 kW) under the above conditions.

Using a 2 pole motor in place of the 4 pole is definitely part of the new problem.

If VFD trips can be averted, does it matter how long it takes to grind the nuts after they have been overfed? If no, then several approaches suggest themselves.

First question: There is a speed above which the product gets too hot. We aim to keep the product below 110 F. Other than that, if the grinder turns too fast then it throws the product out! (depending on the product). So, between those two parameters, we aim to grind as fast as we can. My guess is 50-100 rpm at the drum. In order to achieve optimum speeds of grinding, we try to run the grinder as fast as it will turn without tripping the VFD. Once the grinder is full then it is left to grind from (say) 3 hours to 24 hours depending on the product and the micron size required.
I don't recall the number of teeth on the drum sprocket 80 maybe(?) I changed the small sprocket because I thought it would be better for the motors to run neaer rated rpm. It is not easy to change the larger sprocket as it impacts the frame.
I am sorry I don't know the Emmerson drive number. We're snowed in here for a while!
Given the little I know, I would say that it is more likely that the 4 pole motor is turning at around 5-600 RPM (?)and the 2 pole presumably half that. So the frequency oculd be waaay down from rated 60 HZ. I can imagine that such a low Hz/Voltage could create a power problem if the torque increased.

Too bad the horse is out.

A motor is like a rotating transformer. Volts/hertz is an important parameter for transformers. Yes, too many volts/hertz can cause overheating. Aircraft use 400 Hz transformers instead of 60 Hz or 50 Hz because they can be built much smaller and lighter without overheating -- that illustrates the volts/hertz effect.

I think that we have the opposite going on. Too low HZ/Volts as the motor is slowed down. Thanks for the input!

 

[Organic nutrition]

At 60 Hz., the magnetic field in a 2 pole motor rotates at 3600 RPM (the rotor lags behind by a bit, 3500 RPM is ballpark). For a 4 pole motor they are 1800 RPM, and 1750 RPM, respectively, a 6 pole motor would be 1200 RPM, and so on.

The V/Hz ratio for a 230V, 60 Hz motor is 230/60, or 3.83 V/Hz. At 60 Hz (full speed) line voltage will be 60 * 3.83 (230V), and drop to progressively smaller values at lower speeds (115V at 30 Hz, about 81 volts at 21 Hz, etc.). Most scalar drive parameter sets have one or more frills such as "voltage boost" to help starting loads from zero speed, but other than that the V/Hz relation is maintained throughout the speed range.

It takes x amount of power to grind these nuts.

The following table doesn't bear close scrutiny (I'm assuming both motor efficiency and power factor are 75% for both 2 and 3 HP motors), but it is close enough to illustrate the point. See how much power can be produced at motor full load current rating for the (blue) 2 HP motor versus (yellow) 3 HP motor at a 1400 RPM shaft speed. Apparently you were already up against current limit when using the 4 pole, 2 HP motor. Changing to a 4 pole, 3 horse motor would probably have worked out, however, it is necessary to operate the 2 pole motor at half the voltage to obtain the same speed, and now it ends up limited at less power than before (or, looking at it the other way, a need to go even higher above motor FLA rating than before) hence the worsening of overload trips.

My suspicion is if Emerson tech support is giving a "good to go" it's because the nameplate values for the new 2 pole, 3 HP motor have been properly entered into their drive, and they aren't looking at the broader loading question.

View attachment 222722

Thanks for the input. If I may summarize: The motor is turning too slowly to generate enough power without overloading the FLC rating? So I can change the gearing or change to a 4 pole? Yes,?

 

[Asymptotic]

Thanks for the input. If I may summarize: The motor is turning too slowly to generate enough power without overloading the FLC rating? So I can change the gearing or change to a 4 pole? Yes,?

If the driven sprocket is 80 tooth, and you've already put the smallest sprocket possible on the motor then there isn't much more that can be done on that front.
A 3 HP, 4 pole motor may do the job (so long as the drive is also rated for 3HP).

What I'd suggest is getting back to us once you have the drive info and sprocket ratios. If you already aren't operating at high speed settings it may make more sense to install a 6 pole, 3 HP motor to better match the motor/drive speed range to grinder.

From your description the practice is to set drive speed to just below the value that results in overload trips. An alternative approach that may yield better results (so long as the drive can be set up this way) is to set drive torque limit to whatever value is just under the tripping point. That way, if speed setpoint is pushed too high for the load, all that will happen is speed will fall off once the torque limit is met rather than shutting off the drive on an overload fault.

 

[jim hardy]

Counterintuitive as it may be, the 2 HP motor rated 6.8 amps is capable of delivering more shaft power than the 3 HP motor rated 9.8 amps (2.3 kW versus 1.6 kW) under the above conditions.

Using a 2 pole motor in place of the 4 pole is definitely part of the new problem.

Agreed.

Given two motors of equal horsepower , one 1800 RPM the other 3600 RPM,
the 1800 makes twice the torque of the 3600 RPM.
That's because $torque = \frac { horsepower} {RPM~ X (a ~constant~ which~ is) 1.904 X 10^-4 }$
so your first motor's rated torque is $\frac{2}{1800 ~X~1.904 X 10^-4}$ = 5.84 foot-pounds
and your second motor's rated torque is $\frac{3}{3600~X~1.904 X 10^-4}$ = 4.38 foot-pounds
both motors must be run very near their rated speed to deliver that torque without excessive current draw

so you need to either change the motor or your gearing.
I'd put the old ones back
and try them with your new 14 tooth sprocket

At question is does running the 3600 rpm motors with low Hz/volts create a condition that would produce an overload in the motors under higher load?

Yes, definitely
because at any value of torque the 3600 is more heavily loaded(or overloaded?) than was the 1800 .
by the ratio 5.84 / 4.38 = 1.33
.

 

[russ_watters]

This thread has been dormant for a year, so I'm locking it. Sorry Jim...