motor sizing problem

[njlee]

Hi Gurus,

I am designing an agitator which can agitate an 3000lb object linearly at about 180 rpm. I will use a 10 to 1 gear ratio gear box to achive 180 rpm requirement. And, I will use an VFD drive to alter the speed of the agitation (from 60 rpm to 180 rpm).

I have a few questions and hope you can help me.

1. Is it true that VFD can only work with inverter duty motor (not the regular AC motor), what will happen if I use a regular AC motor with VFD drive?

2. What is the function of VFD? VFD will make the torque independent of the speed? meaning the VFD will try to maintain the speed (no matter how small the speed is) and enable the motor to provide enough torque (up to the breakdown torque/full-load torque??) to run the system at the same time. In other word, the full-load torque of the motor will be kept the same. what about the breakdown torque??


3. If I choose a motor which has a base speed of 1750 rpm, how much faster can I increase the motor speed using VFD in general?

4. When sizing a motor, what parameters do I need look at? make sure that

a: the motor starting torque is big enough to overcome the inertia (motor breakdown torque > system starting torque)

b: the motor full-load torque is bigger the maximum system running torque at 180 rpm? anything else?

5. What about speed? If I make sure that the torque requirements at the top running speed is ok, then I can assume that the motor can always run the system at lower speed using VFD, right?

Thank you very much!

Lee

 

[gcaudill]

Here goes.

1.) Standard AC motor will work with VFD, but is not recommended. The corona effect and large dv/dt transients from the switching action of the VFD will combine to cause premature failure of the standard AC motor magnet wire insulation. The dv/dt phenomenon is increased as cable length increases. I recommend using Marathon motors as they use Essex magnet wire which manufacturers a superior inverter duty wire.

2.) This is a tricky question and deserves its own post. There are different flavors of VFDs.

Volts/Hz - operates open loop and maintains a fairly fixed motor fux through the speed range by varying the voltage and frequency at a fixed ratio. Operates in a speed control method only.

Sensorless Vector - operates open loop similar to the volts/hz VFD and but uses advanced algorithims to maintain field control to improve low speed torque regulation. Operates in a speed or torque control mode (depending on manufacturer).

Vector Control - operates closed loop and often provides smooth, ripple-free torque from zero to full speed.

Starting and running torque for all VFD types is really a function of the VFD's ability to provide sufficient current. If sized correctly full motor torque is possible from zero to full speed (for the most part - varies by manufacturer). Vector control will provide the best starting torque and low speed control.


3.) General rule of thumb is 150%, or 90Hz for a 60Hz motor. Above rated base speed you go into the constant HP range and the torque decreases as RPM increases.

4a-4b.) First properly characterize the mechanical load. Next select the motor AND VFD accordingly. If the load is not constant torque consider the HP (namely torque) required through the entire speed range.

5.) Speed regulation is poor with the volts/hz, better with the sensorless vector and nearly perfecty with the closed loop vector type. If your target speed range is 25%-75% motor rated RPM and you do not care if you are +/- a few RPM go with the volts/hz. Ugrade accordingly based on your regulation requirements.

A motor operating at less than base speed needs to be built and packaged correctly. Inverter duty motors come in 10:1, 100:1, 1000:1 speed ranges. The speed range is the base RPM/minimum operating RPM. Select the proper speed range to ensure that overheating is not a problem. Also, TENV vs. TEFC vs. TEBC are to be considered based on environmental condtions.

 

[njlee]

Thanks gcaudill! your post is very helpful!

I have a few more questions if you don't mind.

I think I'm going to go with Volts/Hz VFD because I am not too concerned about the speed accuracy.

1. ""If sized correctly full motor torque is possible from zero to full speed (for the most part - varies by manufacturer). Vector control will provide the best starting torque and low speed control.""

what do you mean by sizing correctly? you mean choosing the right VFD (among Volts/Hz, sensorless vector / vector control) ?

2. What is the operating speed range for Volts/Hz VFD for continuous-run application (more than 10 hours)? 25% to rated motor speed? or 25% to 150% of rated motor speed?

3. ""First properly characterize the mechanical load. Next select the motor AND VFD accordingly. If the load is not constant torque consider the HP (namely torque) required through the entire speed range.""

What do you mean by "consider the HP (namely torque) required through the entire speed range"? do you mean the "maximum" torque load throughout the entire speed range should always be lower than the motor's full-load torque?

OR the "average" torque load should be lower than the motor's full-load torque? and counting on the motor's breakdown torque to overcome the maximum torque load?

4. any recommendation of good websites/books for this kind of info?

Thanks again!

 

[gcaudill]

1.) Any of the VFD/motor combinations should provide 100% rated motor full load torque from zero to full speed. The motor should be selected so that the full load torque is adequate for the load characteristics considering both accelerating and running torque. The problem with volts/hz is that when operating at low speeds you have no rotor speed feedback and no torque regulation. You can end up with motor cogging or irregular operation. In my experience this usually happes at 8-10Hz or less with a 60Hz motor although some manufacturers claim to be lower.

Once you know the motor that is required based on the load you will know the motor current requirements (full load amps). Knowing the motor current requirments you can select the appropriated VFD. You should oversize the VFD as necessary to provide for overload conditions. For example if you require a drive rated at 100A continuous to provide for 100 full load motor amps, size the drive to 125A to allow for a 25% overload.

2.) With a correctly selected motor I would feel comfortable operating the volts/hz VDF/motor combination within a 6:1 range continusously (approximately 10Hz).

3.) The motor full load torque should be greater than the maximum load torque. If the maximum load torque is intermittently high, say starting only for example, size the motor and VFD for the largest operating torque. Then make sure the drive is capable of delivering the required current for the overload condition for short periods of time and that the motor is also correctly rated for the short overload condition. Some volts/hz drives offer a "voltage boost" option for starting which increases starting torque above the rated motor full load torque.

4.) Here is site that seems suited to your application.

http://www.geindustrial.com.br/publicacoes/artigos/asdi.pdf

 

[busbar]

Small aside: Link posted here by others—currently for no-charge download.

http://www.nema.org/engineering/icsstandards/acadjustable.pdf

 

[gcaudill]

njlee:

In busbar's link, see section 5.2.1.4. This discusses breakaway torque and voltage boost for volts/hz applications. This is very important in starting your load.

 

[jraef]

Something struck me in the original post that I feel should be looked at.

//an agitator which can agitate an 3000lb object linearly...\\

If by "agitator" you mean a shaker/vibrator system there may be an additional problem. Vibration is often created by spinning an eccentric load on a shaft, i.e. an offset weight. When doing this with a VFD you may find that the cyclical nature of the load will cause a regeneration effect in the VFD which charges the DC bus faster than it can discharge. The VFD may give nuisance Overvoltage trips as a result. Some VFDs have features to help avoid this, Toshiba is a good example. The other way to do this is to oversize the VFD by at least 50% in order to get the increased capacitance on the DC bus. Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion to njlee (Mechanical) Mar 18, 2003 marked ///\\
1. Is it true that VFD can only work with inverter duty motor (not the regular AC motor),
///No, it is not true. The VFD can work with any standard motor provided that the VFD output is sinusoidal or nearly sinusoidal. However, this is not true for every VFD.\\what will happen if I use a regular AC motor with VFD drive?
///If the VFD has better output than the Utility Power supply, it will work properly. If not, then the standard or ordinary motor will experience harms or damages caused by the VFD deviations from the pure sinusoidal voltage waveform.\\
2. What is the function of VFD? VFD will make the torque independent of the speed? meaning the VFD will try to maintain the speed (no matter how small the speed is) and enable the motor to provide enough torque (up to the breakdown torque/full-load torque??) to run the system at the same time. In other word, the full-load torque of the motor will be kept the same. what about the breakdown torque??
///There are various principles of VFDs and various VFD outputs accomplished by VFD programming.\\
3. If I choose a motor which has a base speed of 1750 rpm, how much faster can I increase the motor speed using VFD in general?
///The motor speed can be increased, provided that various conditions are satisfied, e.g. the manufacturer's recommended max rpm, nature of motor load, nature of motor operation, e.g. starts/stops.\\
4. When sizing a motor, what parameters do I need look at? make sure that

a: the motor starting torque is big enough to overcome the inertia (motor breakdown torque > system starting torque)
///The motor terminal voltage must not decrease more than permitted by the manufacturer or motor design, e.g. 80% of rated motor terminal voltage.\\b: the motor full-load torque is bigger the maximum system running torque at 180 rpm? anything else?
///Make sure that:
1. There are not load oscillations
2. The motor shaft is not turning in opposite direction. This would require plugging type of motor duty.\\
5. What about speed? If I make sure that the torque requirements at the top running speed is ok, then I can assume that the motor can always run the system at lower speed using VFD, right?
///Not quite, there might be a motor cooling problem at the lower speed. Also, the motor-load vibrations may increase at the lower speed. There may be mechanical subharmonics developed.\\