Doubling motor frequency and voltage with VFD
Doubling motor frequency and voltage with VFD
(OP)
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
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





RE: Doubling motor frequency and voltage with VFD
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Doubling motor frequency and voltage with VFD
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-
RE: Doubling motor frequency and voltage with VFD
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Doubling motor frequency and voltage with VFD
RE: Doubling motor frequency and voltage with VFD
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.
RE: Doubling motor frequency and voltage with VFD
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
RE: Doubling motor frequency and voltage with VFD
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Doubling motor frequency and voltage with VFD
http://www.usmotors.com/Products/ac_hsc.pdf
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Doubling motor frequency and voltage with VFD
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.
RE: Doubling motor frequency and voltage with VFD
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RE: Doubling motor frequency and voltage with VFD
A LPS for your last post. Any more freebies behind my ear ?
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