Motor pulling excessive current.

[sparkie]

To answer everyone:

I didn't see a post from n0rM anywhere and I don't have anyone on my ignore list.

The application is a steam-jacketed caramel cooker. The motor is a scraper. It's a pretty viscous liquid and would load it down. Maintenance went through the mechanical, and the team on it are pretty solid in that regard and very familiar with these. This is brand new equipment.

The company that builds these has been doing it for some time, which is why I'm hesitant to pursue the undersized motor thing. They have ordered a new motor and drive from the company and will deal with necessary warranty issues there. We also had another guy much more capable than me go through the system and he says everything looks normal.

I'm not surprised by the equipment pulling 22A unloaded. It has a rather large belt drive with a two pullies, so I'm not all that surprised that 1/3 of the power is needed just to turn the empty equipment.

I'll have to see if I can find a nice white paper that succinctly breaks down the sizing various mechanical and electrical drive systems. Now that I'm done with school I have a bit more time to dig into things like that above the superficial level.

 

[plvlce]

I didn't see a post from n0rM anywhere and I don't have anyone on my ignore list.

He means the user who has the handle ndzied1, post #9 is the post in question.

Look at the signature if you're still confused as to why he was referred to as nOrM.



As a general comment I'll note that it's better to stick to usernames in a situation like this as not everyone may have the option to view signatures enabled.

 

[sparkie]

He means the user who has the handle ndzied1, post #9 is the post in question.

Look at the signature if you're still confused as to why he was referred to as nOrM.



As a general comment I'll note that it's better to stick to usernames in a situation like this as not everyone may have the option to view signatures enabled.

I just found out they ordered a larger drive in case the current drive is undersized. My cohort and I are both going to step away from this one if they can't be bothered to contact the OEM before shotgunning parts at a brand new system.


I will update this thread once they figure it out.

 

[ndzied1]

We made some mistakes ourselves when starting to apply more variable frequency drives. In industrial hydraulics there are a lot of rule of thumb calculations that are all based on motors spinning at base speed.

Trying to apply those at different speeds get you burned.

Understanding the VFD modes of operation below and above base speed is critical to successful applications. All basic VFDs I have seen operate with "constant torque" below base speed and "constant horsepower" above base speed. I put the terms in quotes because at very low or very high speeds there are other considerations that need to be taken into account.

Bottom line is you have to do your homework and then you'll be ok.

 

[sparkie]

We made some mistakes ourselves when starting to apply more variable frequency drives. In industrial hydraulics there are a lot of rule of thumb calculations that are all based on motors spinning at base speed.

Trying to apply those at different speeds get you burned.

Understanding the VFD modes of operation below and above base speed is critical to successful applications. All basic VFDs I have seen operate with "constant torque" below base speed and "constant horsepower" above base speed. I put the terms in quotes because at very low or very high speeds there are other considerations that need to be taken into account.

Bottom line is you have to do your homework and then you'll be ok.

Maybe know a good paper or book that gives a decent crash-course on applications at more an intermediate level?

 

[JesperMP]

Motor nominal current @ 60 Hz 460V delta is 52 A.
VFD nominal continous current is 52 A.

They can run it at 80% [speed ?] and it runs at about 50A,

as at 60 Hz it pulls it's FLA when being ran

So both VFD and motor are maxed out at nominal speed. Simple as that.

They are trying to run it at 80 Hz, and getting an overcurrent condition.

A motor and VFD can run for a short time above nominal current. But for a longer time, the VFD will try to avoid overheating the motor and itself.

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.

More likely the drive is trying to limit the current by reducing the speed.

The motor is a scraper.

For a scraper, probably the load increases linearly with the speed.

I'm not surprised by the equipment pulling 22A unloaded.

That is typical. It does not indicate a mechanical issue.

I just found out they ordered a larger drive [the VFD ?] in case the current drive is undersized.

I am guessing the scraper shall run continously and for a longer time at the increased speed. So both motor and VFD must be dimensioned accordingly. It is not enough to change the VFD only.

 

[Tom Jenkins]

A scraper would probably be a constant torque load, like a conveyor. That means that the power demand would increase linearly with speed. Running above nominal speed, assuming the drive and motor are sized to the load at nominal speed, would undoubtedly result in overload.

The drive supplier should be made aware of the constant torque nature of the load. VFDs on constant torque applications are usually oversized a bit compared to variable torque applications because of heat dissipation concerns.

 

[kamenges]

Originally posted by Tom Jenkins:

A scraper would probably be a constant torque load, like a conveyor.

I'm not so sure of this. From the OP:

Originally posted by sparkie:

The application is a steam-jacketed caramel cooker. The motor is a scraper. It's a pretty viscous liquid and would load it down.

I think of this almost more like a mixer. It is there to get the hottest material off of the vessel side walls and into the center of the mass during the heating process. So the faster it goes through a highly viscous material the more torque will be required.

Keith

 

[Tom Jenkins]

I'm not so sure of this. From the OP:


I think of this almost more like a mixer. It is there to get the hottest material off of the vessel side walls and into the center of the mass during the heating process. So the faster it goes through a highly viscous material the more torque will be required.

Keith

You may well be right - which is even a worse case than constant torque! Seems to me some basic engineering was neglected in his application.

 

[sparkie]

Yea, it isn't a scraper. I haven't actually seen inside of it, so I'm not sure exactly how it works, but I did ask if it scraped the sides and it doesn't. I was given incorrect information there. At the moment the drive size was checked and upgraded for a heavy-duty application. I'm not sure if it was installed yesterday or not. I just work weekends, so I came in this weekend and when I was told that the relief engineer came in and told them that at minimum the drive was undersized, I just told them that I'm sick of them calling me over to look at something, asking for an opinion, then ignoring it because they don't like what they hear. They will go to three or four different people and have each of us waste hours on something because they don't like what they hear.

I was also given incorrect information. The 80% value they gave the mechanics is 80% of speed, not frequency. So it ends up being something like 52 Hz. I spoke with the relief project engineer, and no one had informed him that those windings were way too hot, and the vendor is trying to cover their butts, of course. So we are going to get the new drive in, run the system. Either way we will put a new motor on and send these back to a vendor. The new motor will be a 50 HP motor.

Then, to top it off, the entire piece of equipment is 33% too small for the line rates they need, so it's coming out anyway. I'm glad I'm not the one that has to explain all of this to the bean counters.