Motors & Drives Information

Typical Question

I have a 200 hp motor with a 250 hp adjustable frequency drive (AFD) on it. Because the load is sometimes erratic, I have used a dynamic resistor to absorb regen. Through no fault of my own, we ended up using a NEMA A motor.

As the motor reaches synchronous speed, the drive faults out on locked rotor current. The current tries to go through the roof. It only happens when I run towards 60 Hz. If I run at a reduced speed—say 90% of sync speed—it runs fine.

Can anybody help me understand why? Is this symptomatic of NEMA A? Should I be able to tweak the AFD to adjust characteristics to overcome this or am I screwed until I get a NEMA B on there?

Discussion Group Answers

• I have a hard time understanding how your symptoms have anything to do with type A vs B. Yes, a type A has higher locked rotor current. However, you shouldn't be drawing anything near locked rotor current if you ramp up the speed slowly.
  
• As a first approximation, the motor current at breakdown torque is close to locked rotor amps. Thus, 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.
  
• 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 short-term 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.
  
  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. 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 acceleration ramp just enough to avoid the fault if the variable load conditions ran the overload up too high.

Full Story

OK, here is the full story. I have a 1336 II, 250 hp, 575-volt drive powering a 200 hp Siemens, model D-91-056 at 575 volts +/- 10%, with a 180-amp, 1,792 rpm motor. It is used to mix 3,000 lbs of dough, which I have done successfully many times.

As the load is thrown up, the weight can cause regen, hence the extra size on the AFD. Again, nothing new. The acceleration is set to approximately 10 seconds, and I have had it up to 30 seconds. As 60 Hz approaches, it bombs out. When running at 50Hz, it will run forever and a day. Allen Bradley has been in and says that we have this problem because we are using a NEMA A motor.

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 and de-sensitize the drive, it should not hit this "meltdown point."

Another question: The main's voltage is 575 volts (probably closer to 600), which raises the bus voltage. Is it possible that I am closer to certain limits than at 460 volts and more susceptible to problems?

Full Answer

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 horsepower requirements at 1,800 rpm are (1800/1500)^3 = 1.728 times. Some “design B” motors have much higher slip than a “design A” motor, and if you try to overload the motor, the speed drops and the power consumption is not increased that much.

# posted by Medical Information : 3:27 AM