NEMA A vs NEMA B 1

[smeg]

I have recently stumbled into a problem. I admit that I am no expert on NEMA and or motors, I can control and write the code for one so long as it runs. Here is the prob. I have a 200HP Motor with a 250HP afd on it. The load an be erratic and in the past I have used a dynamic resistor to absorb regen. Through no fault of my own we have wound up with a NEMA A motor on it (I read some of your comments about getting a cost reduction - exactly that!!) Anyway, as the motor reaches synchronous speed, the drive faults out on locked rotor current. The current tries to go through the roof. If I run at a reduced speed say 90% of sync speed, it runs fine. Only when I run towards 60HZ this happens. Can anybody help me understand why? Is this symptomatic of NEMA A? Should I be able to tweek the AFD into not being so clever and adjust characterisics to overcome this or am I screwed until I get a NEMA B on there?
Any help will be gladly received
Thanx

 

[electricpete]

I have a hard time understanding how your symtpoms have anything to do with type A vs B. Yes, a type A has higher locked rotor current. But you shouldn't be drawing anything near locked rotor current if you ramp up the speed slowly.

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[electricpete]

One thing comes to mind - you mentioned the erratic load. Can you tell us some more about it. And this dynamic resistor how does it work and is it still there?

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[electricpete]

As a first approximation, the motor current at breakdown torque is close to locked rotor amps. So if your erratic load is momentarily pushing you toward breakdown torque, it will momentarily push you toward locked rotor current, which is higher for the NEMA A.

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[DickDV]

Sounds like you should lengthen your acceleration ramp time a little. The NEMA A motor may have a slightly steeper current characteristic as it goes into the shortterm overload range and that may be causing the overcurrent fault on the drive.

Remember that, when operating a motor on an inverter, you never really operate anywhere on the curve except on the "front face" which is the nearly linear section below and just above the nameplate rating point. So, we must be seeing slightly different characteristics between the two motors in that zone. What happens to these motors in the breakdown and locked rotor part of the curve is of no consequence because the system never operates there.

Actually, in my view, a NEMA A motor is about equally suitable as NEMA B on an inverter.

One other thought, maybe you can set up your inverter to hit current limit before the drive faults. That would simply extend the accel ramp just enough to avoid the fault if the variable load conditions ran the overload up too high.

 

[smeg]

OK, here is the full story. I have a 1336 II 250HP drive powering a Siemens NEMA A 200HP motor. It is a mixer, mixing dough, 3,000lbs which has been done many times before. As the load is thrown up, the weight can cause regen, hence the extra size on the AFD. Again, nothing new. The acel is set to approx 10 secs and I have had it up to 30secs. As 60 Hz approaches, it bombs out. If running at 50Hz, it will run forever and a day. AB have been in and say that it is because we are using a NEMA A that we have this problem. They say if we change to an impact drive all out problems will go away. I smell a rat somewhere, something does not ring true. Surely if I disable (not able to get to site easily to test) vector control, de-sensitize the drive etc. etc. it should not hit this "meltdown point" Another question, the mains voltage is 575V (prob closer to 600) which raises the bus voltage, is it possible that I am closer to certain limits than at 460V and more susceptible to problems?
Still scratching head on this one. Any more ideas, all are more than welcome.
Thanx

 

[mc5w]

Holy Cow! If you are running a 480 volt drive on 575 volts you should be toasting the drive! Baldor carries some drives for 600 volt power input but that is a nonstandard application. For G*d's sake buy a drive isolation transformer and knock the voltage down to 480 volts.

I have had similar problems with drives if the IR compensation is set too high. If the drive is close to the motor any amount of IR compensation **WILL** cause the drive to trip.

You might also try lieing to the drive and telling it a nameplate voltage that is less than the actual nameplate voltage. This will have an effect equivalent to a negative IR compensation which will reduce the tendency to push too many amps into the drive.

 

[DickDV]

The problem is NOT the NEMA Design A motor.

Are you operating a 460V rated drive on 600V?

Exactly what drive fault is occurring?

Can you display the motor current and observe what it is doing as it nears the trip point? (assuming Overcurrent Fault)

 

[aolalde]

Hello smeg

“As 60 Hz approaches, it bombs out. If running at 50Hz, it will run forever”

Your problem is not the motor but the load.

Mixing dough at high speed will increase the power requirements with the speed ratio cubed.
That means the HP requirements at 1800 rpm are (1800/1500)^3 = 1.728 times.

Some “design B” motors have much higher slip than a “design A” motor, and then if you try to overload the motor, the speed drops and the power consumption is not increased that much. I wonder if in reality this very system has worked properly in the past with a 200 HP design B motor.

Provide the current demand and motor rpm speed at 50 HZ and 60 HZ so we could have references to give you our opinions.

 

[smeg]

To dispell any doubt. This is in Canada where normal mains voltage is 575V. The drive is a 1336 plus II AB 575V, 250HP I think it will piss them off to put a transformer in !!

aolalde, I think you are heading the right way. In days of old with no clever stuff on the front i.e. a vector drive, the motors would have taken probably more abuse than anyone knew, but they lasted. In the early days of AFD's I believe the same. In AB's own words, the drive has to be much more closely matched with the load today. Todays inverters seem to have much more control than before, perhaps too much for their own good in some applications. Perhaps the load causing slip on a NEMA B is within tolerance but the load wanting to make the NEMA A slip more than it wants to is consuming the extra trying to hold speed.
I will supply all motor details this evening when I get back in hope some light can be shone on this.
Also, the burning question is, AB is pushing to get us to change the motor from NEMA A to B or change the drive to an impact drive (torque) each option carries a hefty $k's price tag so I want to explore all options including foolng the drive/motor setup. There must be a tweak master pro for the 1336 out there smewhere who knows where the hidden settings are like "Motor type A or B" ... I wish.
Load, slip, NEMA, Torque drive? If I can not tweak the 1336, I'll have to spend the bucks, that seems to be the bottom line.

 

[aolalde]

Smeg:

Is it possible to work your system at a frequency below 60 HZ? say 57 HZ?
The frequency could be adjusted to the current that the motor and the driver can handle without damage.

Instead of spending the money.

 

[smeg]

It would if I had an engineer who was being belligerent. It makes little difference other than it will not run at 60Hz!!
Have more data to post and I have an engineer heading back to site so I can get more info. Thanx for your continued help. Friday night says I am about to enjoy a scotch though. Post soon.

 

[DickDV]

smeg, check the drive software for a current limit setting as I suggested above. By setting this level below the fault level (I'm assuming OverCurrent Fault since you haven't said otherwise) you can work the motor up to the current limit without tripping. When the load is lighter the frequency may well reach 60hz. When the load is heavier, the frequency will limit a bit lower to avoid the fault.

Replacing the motor and/or the drive will not help unless you come to understand why you have the present problem.

 

[electricpete]

smeg - did you record how high the current got before the trip? If I am reading the original post right, the current was recorded at approximatley locked rotor current.

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[smeg]

Motor Name Plate details
Siemens, model D-91-056. 575V+/- 10%. 180A, 1792rpm. Anything over 50Hz causes the problem. When it happens, usually 8 mins into mix of 15 min mix (as the dough becomes stiffer) the power will instantly double and trips out on overcurrent. I too am getting some new details.

 

[DickDV]

Notice that this is an extremely low slip motor--only 8 rpm (.5%) from no load to full load. I suspect that the drive is sized at less than 150% short-term overload so it doesn't take much to cause the drive to trip.

Rather than stiffen such a tight motor with vector control etc etc, you need to soften the speed regulation. First and most important is to set your current limit at a level just under the drive overcurrent, second,, set any slip compensation or torque boost parameters to zero (no boost or slip comp), and third, as a last resort, set your drive output voltage max limit down a little--even a couple % less than motor nameplate.

Of course, if the actual running conditions of the mixer are such that the drive is being pushed into overload frequently and for extended periods, then you simply have an undersized drive situation. You need the next size larger drive to deal with the overload conditions that the motor is seeing.

Changing the motor is not the answer.

 

[smeg]

Thanx DickDV. Have already told engineer, heading to site to get rid of slip and sens vector. There is a function called drop (inverse slip) more load, more speed drops, told hime to try that. As far as output volts, will advise him now. Will that just decrease the ability of the motor to hold speed at 60HZ? I am convinced that the motor is not overloaded and does not sustain constant overloads. Just transient peaks.
Thanx, will let you know.

 

[DickDV]

That would be "speed droop". It is a deliberate softening of the speed response to load changes and may be a good thing, in your case.

That is the tightest motor I've ever heard of.

 

[rconnett]

Just my 2 cents worth -
If I understand all the above dialog, you applied a vector drive to the application. With the application, a vector drive works very hard to maintain constant speed on the shaft. When a transient torque is applied (when the dough is 'thrown up' as you put it) the vector drive output works very hard to try to maintain the constant speed & the VFD will go into overload more easily.

The 'old fashioned approach' accommodated transient torques by allowing the motor to slip. I would strogly suggest disabling the vector feature on the drive.

As far as the 'A' vs 'B' issue, NEMA MG-1 limits locked rotor amps for Design 'B' motors much more than 'A' motors. (Therefor, when an 'A' motor goes into the breakdown region of the curve, it's current would, indeed, be higher than a NEMA 'B' motor. Under vector control the phenomena is aggravated.

 

[DickDV]

rconnett, due to the current limit setpoint in the drive, the motor will never be brought to the breakdown point, much less the locked rotor point. All motor operation will be on the "front face" of the torque-speed curve.

smeg, I had overlooked the fact that you are operating the drive in vector control mode. In that case, instead of dealing with slip gains, speed droop, etc. simply drop the P and I gains in the speed loop setpoint controller so the speed regulation goes real soft. Also set the current limit down as I mentioned earlier.

With these P and I gains (there should be no D gain) set down, the motor speed will simply sag down under heavy overload conditions hopefully avoiding the overcurrent trip.