300 HP Motor, DC versus AC efficiency

[DickDV]

Generally, it wasn't much of a problem until IGBT's came along. These output switches switch so fast that it is easy to couple the high frequency components from the motor stator to the rotor across the air gap. To make matters worse, as the efficiencies of induction motors has been pressed upward, the air gap has been reduced. Where a standard efficient motor used to have a 1/16" air gap, a premium efficient version of that motor will have only a 1/64" air gap. The trend toward higher drive carrier frequencies has made it worse too.

The tricky thing about shaft currents is that it is very hard to predict when it will occur. It doesn't seem to follow any particular frame size pattern, motor name brand, or motor lead length. And it doesn't pop up often enough to, as a blanket rule, add shaft grounding all the time as a precaution. If there is any good news in this it is that there is a peculiar pattern etched in the races of failed bearings so a good motor shop can tell you specifically what your problem is.

Using insulated bearings or insulating sleeves in the bearing cups isn't an "always" fix either. Some of the pulses in the shaft are of such high frequency that they capacitively couple right thru the insulation to ground.

Conductive grease has been proposed and usually works very good but, if you live in the same work world I do, as soon as you turn your head, someone is going to put standard grease in that bearing and you know what happens next!

No, I like the little grounding rings by ElectroStatic Tech and Parker best. Check them out if you've got this problem.

 

[milldrone]

Thank you DickDV, for another "golden nugget"!

 

[russrmartin]

Power Factor Non-issue

Hey guys.

Thanks for all the interest, looks like I hit a good topic. We currently have power monitoring throughout the facility, and I have partial numbers for this system. That is, my values are taken from a power distribution panel feeding the extruder, not directly from the isolation transformer for the drive.

That said, we have a 4.5" extruder which was changed a few years back to AC. The power factor reading on that drive alone is .84. The PF on the PD panel feeding the 6" extruder is .79. Basing the following statement on that data and the fact that the 6" extruder is about 30% of the PD panel load, I am not expecting a large PF correction effect. Beyond this, our plant PF is about .88 (way better than I expected, no doubt due to all the VFD's already in service), and even if it were not that good, we are not billed on it in any way.

What the nuts and bolts of this post was really was that I am trying to come up with some set of criteria to judge applications and say "the best fit for this application is X" with regard to energy consumption to achieve the same work. Or, in management terms, which one costs me more to run.

 

[Tom Jenkins]

The grounding and arcing through bearings issue just came up. I've done dozens of applications and never encountered it, but now a competitor is running around screaming "beware". Of course neither the competitor or my customers are well informed, and I'm hardly an expert, so I'd like some help if possible.

1) Is there a difference between IEC and NEMA motors in this regard? The screamer is using IEC motors.

2) The article cited is by a supplier of grounding rings, so it is hardly an unbiased opinion. Does anyone have any statistical info on actual failure rates from this cause?

3) Is there any difference in the rate of occurrence if the motor frame is grounded or not grounded? Are there code problems if the motor frame is not grounded?

Tom

 

[Psymon]

...and so before submitting the AFE, I have been asked to recreate the justification...

For a second there I thought you meant active front end. Anyways, I'm assuming there are some other drives on the system. There would be a benefit if they all had a common bus system and lets say for instance that there were some AC driven unwinds on there that constantly regenerate, then whatever drives were on that bus would use the regen bus voltage. Personally, I think that for an application such as an extruder, the 30% number on savings sounds high unless you factor in upkeep on the DC motor vs. AC.

 

[DickDV]

Hi, Tom! Regarding your questions, (1) there is no discernable difference that I can see between IEC and NEMA motors, (2)in my experience the problem surfaces in maybe 1 in 20 motors. I see no particular pattern with size or base speed but, the higher the carrier frequency, the worst the problem seems to be. (3)The motor frame has to be grounded and usually is anyway. It is fairly hard not to ground a motor. If you use insulated bearings or try to unground the motor, the shaft currents simply look for somewhere else to go to ground. That could be thru the coupling into a pump or fan bearing, gearbox bearing, etc. None of that is pretty.

In my opinion, the shaft grounding rings are the way to go. Simple and cheap! Parker uses a conductive lip seal style and ElectroStatic Tech uses a bunch of carbon fiber whiskers to contact and ground the shaft. Either is ok by me.

Psymon, I believe the number is 6% not 30% energy savings.

 

[plchacker]

I would check with the drive manufacturer to see if they have any selling points for the swap.


DickDV, do line conditioners help? The last drives I installed had both isoation xformers on the line side, and conditioners on the T-Lead side. I have not noticed any bearing failure, but that will make anyone who has installed lots of drives take note.

 

[DickDV]

Line conditioners like reactors and input isolation transformers will have no effect on bearing current problems. That is because they are dealing with input power and the bearing current problems are entirely and output power issue.

Motor lead reactors will tend to reduce the chances of having bearing current problems but they also reduce efficiency and increase motor speed error. And, they are also more expensive that the grounding rings and are harder to retro-fit after you think the installation is complete.

 

[rdrast]

Just an adder, We have added some new lines, based entirely on AC drives, that are functionally equivilant of older DC based lines. The new lines have 450HP of extrusion capacity each, as do the older ones, in addition to 15 or so other speed controlled drives.

Both old and new run the same products, at the same output levels, but the new lines use less than half of the power that the old ones do. After lots of searching on this, I'm attributing the power savings back to the fact that the motors are almost never running at base speed.. Normally 50% or less actually. That seriously hurts on the DC Lines, but the AC lines are much much better at below-base speed operation.

On another line, where the extruders run continuously at base speed, there is little power savings between comperably machines.

 

[plchacker]

I'm attributing the power savings back to the fact that the motors are almost never running at base speed.. Normally 50% or less actually. That seriously hurts on the DC Lines, but the AC lines are much much better at below-base speed operation.

You may have hit on something there. I watched a 300 hp 240VDC motor peak at around 800A, but maintain that peak for about a quarter of its cycle time. This was due to acceleration and deceleration during most of the run cycle.

AC drives on the same system, although smaller did not maintain peak curent nearly as long. Even with the same accel and decel rates they were not quite as power hungry.

Low speed troque on the DC motor was amazing.

 

[Tom Jenkins]

Thanks for the info, Dick!

 

[rdrast]

Low speed troque on the DC motor was amazing.

Low speed torque on modern vector or some sensorless vector AC drives have the same if not better low and zero speed torque control.

 

[russrmartin]

Thanks guys

As alwways, I've gotten a lot of good responses. My conclusion based on what I've learned here and from other outside sources, it appears to me that our main energy benefits are going to come in the form of improved PF. This doesn't really give us financial justification, but will probably still get the AC implementation based on plant standardization. Thanks to all.

Russ