NEMA A vs NEMA B
NEMA A vs NEMA B
(OP)
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
Any help will be gladly received
Thanx





RE: NEMA A vs NEMA B
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Eng-tips forums: The best place on the web for engineering discussions.
RE: NEMA A vs NEMA B
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: NEMA A vs NEMA B
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
Still scratching head on this one. Any more ideas, all are more than welcome.
Thanx
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
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)
RE: NEMA A vs NEMA B
“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.
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
Replacing the motor and/or the drive will not help unless you come to understand why you have the present problem.
RE: NEMA A vs NEMA B
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RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
Thanx, will let you know.
RE: NEMA A vs NEMA B
That is the tightest motor I've ever heard of.
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
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.
RE: NEMA A vs NEMA B
RE: NEMA A vs NEMA B
Thanx to all who helped.