Underpowered VFD....will it work? 1

[pamcinto]

I have a 100HP/460V synchronous motor sitting in the back of my shop along with a 40HP/460V VFD. I am hoping to hook the two parts together. The motor will be very lightly loaded with a gear box. The only thing I am concerned about is actually starting from 0RPM (kind of like a locked rotor situation). Am I going to be able to start this motor or will I need a pull-start (just kidding of course)?

 

[Lewish]

In simple terms, you ain't gonna start a 100HP motor with a 40HP VFD. So, get your starter rope ready, and hope you don't blow the rectifiers out of the VFD.

 

[electricpete]

I'm not all that familiar with vfd's, but I was under the impression that starting duty was mild since the frequency is slowly increased. Couldn't any starting problems be overcome but programming a slower start?

 

[dpc]

I would think an unloaded motor could be started and run without overloading the VFD. The VFD will current limit and shut down if it's overloaded, so I don't think you'll destroy anything.

 

[pamcinto]

All decent VFDs have current limiting to ensure that the inverter doesn't blow itself up, therefore I am not worried about blowing any rectifiers up either(as per dpc's comment).

Most VFDs are rated for a temporary starting overload of 200% of rated. This is for start-up since it resembles a locked-rotor condition. I am not worried about anything but the start-up.

I will most definitely work with your suggestion "electricpete", but I am still unsure as to whether I will be able to get that rotor moving.

Any other comments?

 

[UKpete]

I would say try it. Incidentally there is a good free NEMA application guide available on VFDs (previously posted by electricpete - thanks):

http://208.156.24.64/index_nema.cfm/1427/0DDF742A-8790-4A21-88D139CFC069648B/

- read in particular the comment on locked rotor current in section 4.2.2 (actually referring to design A motors but applicable here I think). With VFD operation the motor is always operating on the final slope of the conventional 60HZ DOL torque-speed curve, regardless of actual motor speed. So the DOL starting current doesn't apply.

 

[HomeMadeSin]

I would say that it is borderline. We had a 40HP motor (2 pole) that we wanted to run at 4pole speed (pump load would've been 5HP), but our 10HP drive wouldn't do it. We had to get a 20HP drive to run the test. Where the line is between the two, I don't know (as in 50% of motor size???).

 

[DanDel]

pamcinto, programming a 'soft-start' may take care of the inrush starting current problem, but I would be more concerned that a VFD rated for 40HP may not be able to handle the continuous load of a 100HP motor, even if it is lightly loaded. I know that you stated that "The only thing I am concerned about is actually starting from 0RPM (kind of like a locked rotor situation)", but I would recommend checking what the maximum continuous current rating of the VFD is, and comparing that to the maximum continuous current that you expect to draw from it for the motor. You may find out that a lightly loaded 100HP motor may draw more current than the FLA of a 40HP motor.

 

[jbartos]

Suggestion: Contact the VFD manufacturer tech support for any recommendations.

 

[GusD]

Hi Pamcinto

No reason why you can't run your 100 hp Sync motor on a 40 hp VFD.the VFD will give you the soft start you need.
As you mention, it will be lightly loaded, and the VFD current limits will let you know when you have exceeded it.
You say that the Sync will be driving a gearbox,but you don't mention what is the gearbosx doing.
Don't forget that you can start the Sync,but the Sync will be using nearly 40 hp just iddling.
It will have to be a very light load indeed.

Good luck

GusD

 

[Modula2]

Perhaps not directly related, but where is the VFD overload protection located and is there duplicates. I mean, on the output to the motor and on the dc bus side also?

 

[jbartos]

Suggestion: The VFD overload protection is on the VFD input side and motor overload protection is on the VFD output side.

 

[WallyGroot]

Are you in a position to connect the motor in a 6 wire (inside Delta) connection? This enables a softstarter of approx. 1/2 the value of the motor current to safely start the motor. This causes the starter to only handle the phase current which is less than line current. Note that the soft starter must be capable of operation in the 6 wire connection format. 6 wire connection of non 6 wire connection softstarter will result in major damage to the softstart. Consult the manufacturer of the softstart to see if this is possible.