Doubling motor frequency and voltage with VFD 3

[bill318]

I have an application with wide motor RPM and torque requirements and could use some help / verification. Here is what the motor shaft will see for loading.

Continuous Duty torque curve:
302 RPM @ 50Nm (37 ft/lb) ~ 1.5KW (2 HP)
1800 RPM @ 65Nm (48 ft/lb) ~ 12KW (16 HP)
2200 RPM @ 75Nm (55 ft/lb) ~ 17KW (23 HP)
3100 RPM @ 100Nm (74 ft/lb) ~ 33KW (44 HP)
Note: highest duty cycle is in the 1800-3000 RPM range

Load acceleration (Worse Case Overload)
218 Nm (161 ft/lbs), 11 second duration, repeats @ 5-minute intervals worse case.

Since motor weight is a large concern, the smallest commercial motor I have come up with is a 25HP, 4-pole motor wired for 230V/60Hz but supplied by a VFD at 460V/10-120Hz. I would like to use a totally enclosed aluminum frame, external blower cooled motor.

Questions:

1) Am I off base when pressing the 25hp motor through this load range?

2) With a 460V VFD running a 230V motor operating at low RPMs, will rotor heating be more of an issue than it would if the VFD was running a 230V input? As near as I can tell, the RMS current would be the same… Narrower pulses @ 460V vs wider pulses @ 230V.

3) What is the best way to calculate motor overload torque capability and duty cycle? I know what my load is doing, but what can the motor handle?

4) I’m looking at ABB and Siemens motors and ABB ACS 800 series VFDs … are there other motor or VFD makes I should consider in this application?

Thank-you, Bill

 

[Skogsgurra]

Besides the usual concern about winding insulation and mechanical speed limits (balancing, bearings), there doesn't seem to be any problems in steady state operation.

But, during acceleration, you will have load torque plus torque needed for acceleration. They will sum up to 318 Nm at highest speed. It is not something a standard 25 HP @ 1800 RPM motor can produce. It has something like 100 Nm nominal torque and more than three times nominal is difficult. Even with an inverter drive. There is a definite risk that the motor cannot follow the acceleration ramp.

The cooling doesn't seem to be a problem at all. If you can accept a longer acceleration time, I think this will work.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[bill318]

Thank-you Skogsgurra,

If possible, I would like the VFD to limit the overload torque to about 225Nm total. I'm guessing this can be handled by setting a motor current limit in the VFD.

-Bill-

 

[Skogsgurra]

Yes, but you will not get the acceleration you need. Or, are the 218 Nm total torque (acc + load)? In that case, no probs.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

I would guess you can probably set an actual torque limit in the ACS800 rather than simply current limit. You should be able to set this in Nm if the drive has been commissioning correctly.

 

[DickDV]

ozmosis is correct. The ACS800 operating in DTC mode will provide a calculated torque limit which will be much more accurate and repeatable than a current limit. Look in Group 20 Limits for Torque Limit and Torque Selection. If the application is reversing, be sure to set both + and - torque limits.

A 25hp four pole motor will peak its overload torque at about 165ft-lbs or 223Nm. So, as Skogsgurra points out, you may be short of acceleration torque if the stated torque is in addition to the load torque.

It looks to me that an ordinary TEFC motor would be ok based on the low speed load data given. An auxiliary blower won't hurt anything but is extra cost and will probably include an intake air filter which is another unneeded maintenance item.

 

[bill318]

Thank-you for the replies, it looks like I should be able to work out the rest of the details now. And yes the overload torque listed above is indeed the total load + acceleration torque.

I'm still a bit unsure on the overload capability for a given motor. I see some motors list up to 3.5 X max continuous torque, which puts it up above the starting torque. Is there some "rule of thumb" based on starting torque or max continuous torque values for a safe overload period of 30-60 seconds with X amount of cooling inbetween?

Bill

 

[Skogsgurra]

The generic asynchronous motor has a speed/torque curve that starts with a rather low torque at zero speed, the starting torque. It then usually falls slighgtly down to the so called saddle torque at around half nominal speed. Torque then increases up to the peak torque, which can be two, three or four times nominal torque. Torque then falls down to zero at synchronous speed. Nominal torque is just below synch speed, often a few pecent below.

A vector drive can often utilize the peak torque, but it is not considered good practice (if you do not have an encoder fitted) to do so. With an encoder and the right programming, you can use the peak torque for a few seconds during acceleration and retardation. That is what makes the lowly asynch induction motor quite useful as a servo motor.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Found the paper I was looking for. Pages 22 and 23 show torque vs speed curves for different motors.

http://www.usmotors.com/Products/ac_hsc.pdf

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

To add to Skogsgurra's post, remember that the current to produce the peak or breakdown torque is also much higher than the rated torque. It can be 3x to 4x the rated current. A typical VFD is capable of providing 1.5x the continuous rated output current for a short period of time. So, to run the motor at the breakdown torque may require a VFD that is sized for 2x or 3x the motor rated current.

The motor manufacturer should be able to provide a speed vs torque and speed vs current curve for the motor. Now, you are only going to be operating the motor between the breakdown peak and 100% speed on these curves but they are still useful to know the overload torque the motor can produce and the current required to produce this torque.

I would deal with the motor manufacturers to confirm that the motor will not overhead during your acceleration and deceleration periods. You'll want to check that the motor you chose is suitable for the higher speed operation anyways.

 

[gepman]

NEMA has a useful publication for the application of VFDs which includes the maximum speed (varies with size and number of poles of motor) that a standard NEMA motor can be run. It is available at

http://www.nema.org/stds/acadjustable.cfm.

You can buy a hardcopy for $101 or download it for free!

 

[edison123]

gepman

A LPS for your last post. Any more freebies behind my ear ?

* I would go green if only I were not yellow *