Motor pulling excessive current.

[sparkie]

Hey guys.


So I have a 50 HP 460/600V motor on a PF525. I've verified the motor is wired correctly. The service factor is 1.15 and the pf is 0.8 The name plate states that for 460 it should be wired in delta, and it's FLA is about 52A. They are trying to run it at 80 Hz, and getting an overcurrent condition. The current is also not a stable current. It the magnitude oscillates.

They can run it at 80% and it runs at about 50A, which makes sense. I can definitely smell the windings cooking when I open the JB. I've megged the motor and wiring, and they seem to be okay. It's been tuned.

This is new equipment custom built for us. I've asked for the motor sizing calculations to review them, but it's not likely I'll get them. I do suspect it's undersized, as at 60 Hz it pulls it's FLA when being ran.

I checked the fault logs and didn't see any kind of over-current or overload faults. People are leaning towards it being a bad motor, but I'm also wondering if it can be a bad drive / potentially be misfiring. I'm not sure how to check that out.

The drive is fed with shielded VFD cable, and the shield wires are clamped to a grounding bar prior to landing on the drive. There is no line reactor or output choke.

I feel like there might be some stuff to try, but I don't know what it might be at this point.

 

[padees]

PF525 only goes to 30HP.

 

[sparkie]

PF525 only goes to 30HP.

Then it's probably a 30. I was 14 hours into my shift when I looked at it last night.

 

[Bering C Sparky]

Can you post a photo of the motor nameplate?

 

[padees]

52 amps sounds like a 40HP motor.

30HP PF525 is rated 43 amps.

Depends on type of motor and what the load is.

Similar situation on a small extruder. 22Kw A-B motor. The load is fairly high, so ended up with a 40HP PF755. Been working well for the past 6 years. Next is to upgrade controls and remove all the Dnet, SLC500 stuff. Actually use a real E-Stop system. LOL

 

[sparkie]

Can you post a photo of the motor nameplate?

Yea I'll get one. I did another retrofit to go from a soft start to a drive yesterday under a lot of pressure and I think I'm conflating the two.


It's actually a 700 series drive. I'll get the info from both in a bit.

 

[padees]

Yea I'll get one. I did another retrofit to go from a soft start to a drive yesterday under a lot of pressure and I think I'm conflating the two.


It's actually a 700 series drive. I'll get the info from both in a bit.

That will help.

 

[Dale87]

80HZ???? Is the motor rated for 80HZ????

 

[ndzied1]

A couple comments.

All motors we use, WEG and Baldor, state that the service factor is 1.0 when run from a VFD. Even if the across the line SF is 1.15.

At 80Hz you are well into the constant HP region of the drive operation. Your torque capability at 80Hz will be about 75% of what it is at 60Hz. What is the application?

 

[JesperMP]

I do suspect it's undersized, as at 60 Hz it pulls it's FLA when being ran.

Well, isn't it obvious then ?
The motor is sized exactly to the load at nominal speed. There is no room for a higher load, and certainly no room to run at a higher speed.

 

[lfe]

If the windings smell cooking or burning, it is very likely that there are some short circuits between turns.

 

[sparkie]

Here is the nameplate. Forgot to get it last night. Had someone on site get it today. Sorry it is kinda blurry. I also got the VFD label. Just looking at this, though and given that it's wired for 460 delta, it looks like we are maxing out the drive.


I can't quite read the kW from the pics so I asked him to confirm it. Sure looks like a 60 to me, but surely not. That would explain the oscillations with the current though.


EDIT: Got them to confirm it's a 40 HP, so that must be a three.

 

[Robb B]

Motors are amazing, they will work themselves to death if you ask them to. This is why we have to make sure the overload protection (Heater, bi-metallic strips, electronics, whatever) are sized absolutely correctly to the motor, not the load. If the motor cannot move the load, then either the load is too big or the motor is too small.

A motor working itself to death has a very distinctive smell, like burning money.

 

[boardmaker]

Here is the nameplate. Forgot to get it last night. Just looking at this, though and given that it's wired for 460 delta, it looks like we are maxing out the drive.

Looks like you are maxing out more than the drive.

 

[Tom Jenkins]

Max rpm is a function of frame size and number of poles. 80 Hz on a nominal 750 rpm motor should be OK, but prudence dictates you check with the manufacturer.

You haven't answered nOrM's question about the application, which is crucial. Above nominal speed most VFDs are capable of operation at constant motor power. That means the torque capability drops. You need to determine the power demand and torque demand of the load at speeds above nominal.

Frankly, this looks like insufficient application engineering, not a bad drive or a bad motor. Just sayin'.