Full Torque at Zero Speed 1

[rickt]

I am using a nord 20hp gearmotor with a Siemens MM440 invertor in a hoist application.(With encoder feedback and resistor for regen) The system can lift 9000 lbs at over 8 inches per second in normal operating mode. When is comes to holding the load at close to zero speed, it can only handle 5000-6000 lbs before the platform looses control.

(Specs - Gearmotor Torque 13000 in lbs at 97 rpm)

Question:

Shoudn't the system be able to generate full torque at zero speed. (I am using encoder feedback for closed loop vector)

Is the motor undersized? I do not want to put blame on the invertor supplier if it is a motor problem. The VFD specs claim that my config can produce full torque at zero speed. (200% for a small period of Time)

Thanks

 

[Baldor]

What is the encoder resolution? I once saw a problem where the encoder resolution (2048) was finer than the mechanical backlash of the gears. At low speeds, the drive saw a greater percentage of speed error (the gear slop being constant), and would become erratic trying to corect for it. Switching to a 1024 encoder helped somewhat.
Also, check to see if the drive has a parameter for "allowable speed variations per XXXX" or something like that. I think its intent is to allow for backlash.

 

[electricpete]

I don't know that much about this application but it seems it will be challenging to get a motor to maintain high torque at zero speed for any length of time.

Just thinking out loud of possible problems:
- To obtain zero speed the motor has to be able to exactly match the load torque. Sounds like that will require sophisticated control.
- If you reduce sync speed you also have to reduce voltage which reduces torque.
- At zero speed there will be high current and zero cooling. This results in motor heating very rapidly.
- Rapid heating will increase rotor resistance which has dramatic effect on the torque speed curve and can further complicate the control problem.
- Heating can also damage the motor.

Let me repeat I am not that familiar with this type of application.

 

[ojayl]

If you are trying to hold a position that requires nearly the full controller current, it may be possible that the motor's inductance is somewhat limiting the peak current into the motor and therefore it's torque. Is this a permanent magnet brush or brushless motor? As long as the encoder is on the motor and not the gearbox output and the load is in one direction, I don't think it will be a backlash issue. I have had "position hold" applications where the controller had to be specified for a higher voltage to overcome motor inductance.
-ojay-

Jay Lehrbach

http://www.eCycle.com

 

[jraef]

You may have fallen into a semantics trap that is all too common. When a Vector Drive mfgr says "full torque at zero speed", they leave out the word "starting". If for instance you have a design B motor, starting torque may be 160% of FLT. Breakdown torque on that same motor may be 220% of FLT, and the Vector drive can allow you to use that torque after the motor is running, but it does that by taking advantage of slip in the moving rotor. That would explain why you can lift a moving load of greater mass than what you can hold stationary. At zero speed, your slip is essentially 1 so all you can hope to get out of the motor is that locked rotor (starting) torque. Add to that the amplification of the torque (loss) through the gear box and you might be in trouble.

For electricpete, this is done all the time in crane and hoist applications now. Good quality Closed Loop Vector Drives have what is called a "Torque Proving" algorithm (maybe by another name) which serves to ensure that the motor does in fact match the load torque requirements prior to releasing the mechanical holding brake. The motors are designed for this use, having independently powered cooling fans so that they get full air flow regardless of rotor speed. I worked on crane systems at Boeing where we lifted and held 747s and 777s with 4 20HP vector drives. If you drop one of those babies, run to the nearest Tibetan monestary to hide for the rest of your life!

Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion: Certain loads, e.g. hoists, elevators, valves, etc. require the high or highest motor torque at the motor start. Motor NEMA D design or special motor design may be considered for such applications.

 

[kenlacey]

Another issue could be that an encoder is a speed feedback device and is not the best suited feedback device for holding. Velocity feedback is a pure integration function and position is not so the problem may be related to the type of feedback relative to the application...Something to think about