oversize motor on undersized drive

[Gil47]

The issues we are seeing is motors kicking out via their overtemp switch, while no overcurrent faults have taken place. Russ

Is the motor itself becoming hot enough that you can not hold your hand on it. ?
Are these over temp cut outs inside the motor ?
or mounted seperatly.
Are the over temp cut outs a bimetal switch, ?
These when run on a VFD can suffer from internal heating, caused by high PWM Frequency that is passing thru them, or being induced into them, that is additional to the current flow heating effect.

 

[russrmartin]

Again, more good suggestions. You are correct, this is exactly a bandaid. Bandaids are the result of either lack of cooperation from management, or a lack of viable options for the conditions, or a lack of effort. This instance is the middle circumstance. We need 2 of these units to run ASAP, and anything outside of this week is just too long. Permanent solutions can be considered later. These are EX motors, and are run off VFDs, so while the 215 frame size is correct, once one throws in the inverter duty rating, the frame size becomes 254. If we continue to see issues, my best guess is that gear reduction will be the next step, however, getting a couple available motors in here to turn the equipment while that work is done is critical. Thank again to all.

BTW, service factor of 1.15 means that the overload device can be selected at 125% of the FLA rating on the motor nameplate. The factor of 1.0 means that the overload device can be selected at 115%. At 115% of the rating on the motor we have specified, we are under the current hardware limitations. There seems to be some interest in this, so I'll try to post the results after some time has been passed to analyze them. I'm very interested in whether we prolong the kickouts, or if the added heat sink capacity eliminates the problem.

 

[allscott]

and anything outside of this week is just too long. QUOTE]

Disconnect the temp switch, put a box fan in front of it, and cross your fingers. Seriously.

You should be able to get the black max I suggested overnight as it is a very popular motor.

 

[DickDV]

russrmartin, I know you are in a hurry but we simply need more info to help you. I see you have a 5hp probably 460V rated drive. That would be somewhere close to 9 amps continuous output. I need the motor nameplate FLA.

You say the motor is kicking out via the overtemp switch!! What is that? Normally, the drive software provides the overload protection. This can't be a motor internal switch like on single phase motors, or is it?

I can't really tell whether the motor is overheating, the drive is overheating, or the system simply trips open without anything getting too hot. Please advise what's tripping and why.

In your second post you say you've found a 10hp motor with a FLA less than the current rating of a 5hp drive. Now, I don't think so!!!!!!!!!! Something is ****eyed here if that is so.

Please fill us in on these questions.

 

[Baja Joe]

You had mentioned that time is of the essence. A very small amount of forced air will transmitt a serious amount of heat.

Take (ie borrow for awhile) someones personal office fan and get a 110 volt extension cord and duct tape the fan at the motor. If you dont have one run to the hardware store.

The bigger the fan the better.

You can even used compressed air source for cooling air when you are in a bad spot.

Keep in mind that you are only trying to move, at the most, 5 hp of heat. So a small 110 v fan should really help.

 

[russrmartin]

russrmartin, I know you are in a hurry but we simply need more info to help you. I see you have a 5hp probably 460V rated drive. That would be somewhere close to 9 amps continuous output. I need the motor nameplate FLA.

You say the motor is kicking out via the overtemp switch!! What is that? Normally, the drive software provides the overload protection. This can't be a motor internal switch like on single phase motors, or is it?

I can't really tell whether the motor is overheating, the drive is overheating, or the system simply trips open without anything getting too hot. Please advise what's tripping and why.

In your second post you say you've found a 10hp motor with a FLA less than the current rating of a 5hp drive. Now, I don't think so!!!!!!!!!! Something is ****eyed here if that is so.

Please fill us in on these questions.

DickDV,

I'm only replying now because I value your input more than most on this subject. The drive is actually rated for 5 HP heavy duty, 7.5HP normal duty. It is a Powerflex700. I used to user manual charts in Appendix A to get the drive ratings. You are correct, input current rating is 9.5 Amps. Output rating is slightly less than that of the motor at 11 amps, so my previous post was incorrect. The new 10HP motors FLA rating is 13 Amps, so that is correct that we will not realize all of the HP potential of the motor, and we knew this already. The motor cabling is 14 gauge, so even pushing this motor with a 10HP drive should not exceed it's limits.

The motor is kicking out via external motor temp switches which monitor the motor temp(hazardous location precaution from way back when), and shutdown the motor circuit if the temp limit is exceeded. The actual motor size on the mixers today is 5HP heavy duty rating, but the process engineer is completely dissappointed in their performance. (This is an issue they've been putting up with for years.)

Originally a project was launched to fit these mixers with 7.5HP motors to match the drive capability, but size the gearbox and motor frames such that if the move to 10 HP was required, the move would involve electrical hardware, and the same gear reducers could be used. So, gearboxes and motors were specified. The gearboxes have a 1 week lead time, unfortunately, the 7.5HP motors as I mentioned before are a 6-8 week lead time.

Also not a possibility is drive replacement coupled with new motor cable as it would require new piping in a classified area and would take time that we simply don't have.(This is also why a new cooling unit is not a possibility) So, the only solution at the moment we're pursuing is getting the 10HP motors which have a very short lead time onsite, get the new system together, and attempt to run them. The hope is that the 7.5HP drive will flux up and run the 10HP motors without an issue.

I had never seen this done before, hence my post here. While I think everyone here agrees that long term this should not be the solution, but it appears to be the only one which yields a result of possibly having runable systems in place in 5 days or less.

I believe the long term plan will be to see if the 7.5 HP drive becomes the weak link, or if it is rated large enough to serve the process. If not, the drive system will be replaced with 10HP hardware, which surely should cover it. However, this would be done during a planned shutdown of the process. If it will run it, I would surely hope that we would order the 7.5HP specified motors and install them at a known specified time.

Given that information, my hope is that you might be confident in the drive's ability to at least flux up and run the motor temporarily. i.e. 6-8 weeks.

 

[DickDV]

russbmartin, am I understanding that this is an XP rated atmosphere that the motor is operating in? If so, I assume you know that the motor must be nameplated not just for the CL, Div, & GRP of the hazard but MUST HAVE INVERTER DUTY OPERATING PARAMETERS on the nameplate as well. This would include "PWM Power", the rated frequency range, and the Current Limit setting that the drive must have in its settings. If the motor doesn't have this extra stuff, IT'S NOT RATED FOR XP VARIABLE SPEED.

Assuming that that has all been taken care of properly, these overheat devices should not be in the motor leads and should be shunted or removed. The PowerFlex drive has perfectly good overload calculation software that outsmarts these overload blocks anytime.

Now that we have a 10hp 13amp motor directly connected to a 9 amp drive, let's see just how much that motor will produce in shaft torque. Magnetizing amps on that motor will be very close to 3.5amps so, solving for the full load torque-producing amps gives us the square root of the quantity (13x13 minus 3.5x3.5) which equals 12.5amps. Since the drive is limited to 9 amps, the limited torque-producing current equals the square root of the quantity (9x9 minus 3.5x3.5) which equals 8.3amps. So, the available motor shaft torque will be the nominal times 8.3/12.5 which equals .66 or 66%. The 10hp motor's output torque will be reduced by one-third on the undersized drive. It will have the cooling capacity of the 10hp rating but will be able to output only 6.6hp so it should be cool enough. You will even gain a little over the 5hp motor running right at its rated output.

You must input the correct motor data into the drive software where it asks for it. Then set the overload protection exactly as specified by the XP label on the motor. The weakest link is now the drive and the only fault you should see is a Drive Overcurrent or Drive OverTemp fault. You could even eliminate those by setting the drive max current output down just below its rated output. Just be sure that is less than or equal to the current limit on the motor's xp label.

Now, with the drive programmed that way, if you load the motor to the point where the drive is reaching overload, the drive will slow the motor down until the current falls to acceptable levels. If the operators see this happen and complain about it, you have trapped them into admitting that the mixer is way overloaded and no more is available without resizing most of the components upward. No more tricks that I know of, especially since it is xp.

Hope this clears up some of the questions and makes your limits clearer.

 

[kamenges]

If you are going to use the drive to model motor temp make sure you set up the speed dependent derating curve. Many XP motors are only rated to a 10:1 speed range. Below that you need to derate the continuous current value. Since you said you are running at low RPM alot this is most likely why the motor thermals are tripping before the drive is. If you are used to using TENV motors this will be something a little different.

Outside of how you are going to get your hands on a 10HP XP motor in under a week, DickDV's evaluation about available torque is correct. The good news is if you were running a 5HP motor and you are moving to a 10HP motor you should have the required torque. And as stated earlier the motor will run cooler with the larger frame.

Keith

 

[derekw]

If the motor itself is getting hot and opening the Klixon (cheap bi-metal) switch , I would zip tie a small pancake fan to the back of the motor or use the office fan or compressed air as suggested. It will probably solve the problem if the motor speed is low as you say.

 

[Gil47]

We had a case where we had multiple fan motors running on 1 VFD, and had an individual bimetalic overload fitted to each fan motor, now these overload units would nuiscance trip, until we took them to 140% of each motor rating, even though the combined load was nowhere near full load of the overloads, or the VFD, the suppliers eventually informed us that the composition of some bimetalic elements, can suffer from induced heating when run on VFD.

My obsevations of what you wrote, These are mixers that are not new, and initially had no VFD to run them, a later addition is adding VFD, now the origional bimetal overloads are reacting to the Pulse Width Modulated Frequency from the VFD, and are creating nuiscance tripping,

As DickDV says bridge the bimetalics out, the VFD is capable of doing your overload protection, and so possibly the 5HP motor with the bimetals bridged out is probably capable of doing its required service, without over heating or overloading.

The drive is actually rated for 5 HP heavy duty, 7.5HP normal duty. It is a Powerflex700.

The motor is kicking out via external motor temp switches which monitor the motor temp(hazardous location precaution from way back when), and shutdown the motor circuit if the temp limit is exceeded. The actual motor size on the mixers today is 5HP heavy duty rating, but the process engineer is completely dissappointed in their performance. (This is an issue they've been putting up with for years.)

 

[russrmartin]

Double Check

I did check the parameters that you mentioned. The motor is listed for Class 1, Group D, which is the enviroment it will be placed in. It's is composed of Class F insulation, and is designed for inverter use, but the cut sheets I have on it do not give the PWM power ratings you mentioned. I'll have to take a look at the nameplate when they arrive.

Thanks much for the explanation on the available torque. I'm going to have to sit down when this all blows over and look at the mathematics of what you gave.

I had never seen this done before, and thus, was worried about the motor torque issue. Now seeing that we should have at least the available torque that they have with the 5 HP motors in, I believe they should be able to limp by until the long term solution can be implemented. Thanks again!

 

[russrmartin]

Final decision

Thanks to all for the assistance on this. I've learned a lot in the last week about placing a motor in a classified area. What we did find is that while the motor we had specified(10HP) was rated for C1 Group D, it did not meet the ratings for the auto-ignition temp rating. We have gone back to the drawing board and were able to find a 7.5HP unit with a T3B temp code rating that also had a smaller frame size. We opted to go with this and force the mechanical guys to redo their end due to extended lead time and overall uneasiness of using the 10HP motor with the 7.5HP drive. I think we all feel that it was just the best thing to do in light of all the information given over the last week. Thanks again, you all make me smarter.

Russ

 

[Bob A.]

My first thought after the obvious slow speed issue was in regard to harmonic heating in the motor. I'm not an expert here, but you may find that an inductor between the motor and the drive may reduce motor heating.
I would also investigate volts / hertz. In early drives, it was very important to reduce this value until the drive began to slip at a given frequency. You may be able to reduce this in your drive config.
This will reduce the power dissipated in the motor windings. I would also seek out a motor with lower base speed, if it is possible. While moving air will improve cooling, this is a band-aid.

The very best way to get rid of a problem, is don't get it in the first place! (If it was properly designed, the problem would not exist)

Best Regards,

Bob A.