Drives Question

Guy's

I have a lot of oversized drives in the plant on purpose. In a lot of places we have a 20 HP drive connected to a 10 or 15 HP Motor to reduce spares.

This works well for us but i found a issue today that makes me ask the inverse of this setup. We have some equipment taht is just a little over a year old and has AB Powerflex 4 drives for conveyors and augers. There is 15 drives total.

All the motors are 460 volt 3 hp 1705 rpm 4.5 FLA and the drives are frame size A @ 2HP. Techs say they have replaced a lot of drives down there enough to bring it to my attention.

Can having a drive that is smaller than the driven motor cause problems? If so what? If the material jams the overcurrent protection in the drive kicks in but i am not sure if the larger motor can cause other problems.

I would think so because the motor is still trying to magnetize to capacity and the volts per hz has to be off a little also. This drive is running in scalar mode / open loop.

Can having a drive that is smaller than the driven motor cause problems? If so what? If the material jams the overcurrent protection in the drive kicks in but i am not sure if the larger motor can cause other problems.

I would think so because the motor is still trying to magnetize to capacity and the volts per hz has to be off a little also. This drive is running in scalar mode / open loop.

Kid
I think the big reason to not install undersized equipment is cooling. In your case there may be enough cooling for the undersized drives in that location to get away with it. BUT you are replaceing them are you not?

I prefer to install 20% oversize relays wire etc and find I get long life service. Maybe it is more a factor of ability to absorb heat generated (higher mass) and dissipate absorbed heat.

Dan Bentler

If this is a design flaw and they really should not be like that then i can get capitol money and scheduled downtime to change them all at one time and be done with it.

Make our lives easier and it won't hit my budget.

I know it is not the ideal method but is it very wrong is that why drives are dying so rapidly.

Dan i too like to oversize when i design something. You are right you do get more life out of the equipment when you do.

This is a oem machine that a smaller company contracted the controls for the machine oem.

The workmanship displayed in that panel tells me they are not very experienced in this game.

heat dissipation is sometimes the reason why bigger amperage rating is used but in the case of a VFD drive their is more parameter in the game

Usually, A bigger motor have a lower impedance and need an higner inrush current VS the small motor running the same load...The current draw by a locked rotor use the same math laws...(few % difference should works for a while but if you talk about a 50% more amperage even if the motor isn't loaded...you'll experience problems for sure..)

Electronics components are more sensible to inrush current and compared to an overloaded wire, it doesn't just become hot until it burn...it can burn instantanously just by the inrush being higher than is limit like a fast acting fuse...

DC bus filtration is also selected by VFD amperage and overcharging it may result to a bad filtration and bad motor voltage at the output and probably more...

Every parts of a VFD system should be selected according to motor power.

Actually using a 2 Hp drive on a 3 hp motor will be an issue in itself. The issue comes from the sizing of your componets inside the drive. In this case you have a drive sized for light duty (80 percent motor rated compacity) of a 2 hp motor.

Since the actually motor is 3 hp you are basically cooking your drive. It is just not built to run a 3 hp motor. Drives are sized based on DC capicity or also refed to as the DC Bus.

I would explain in polite terms that this should have been a minimum of 3 hp drive.

This doesn't even take into account derating the drive due to the motor needing 80 percent+ compacity or cable lengths.

The really bad news is a 3 hp drive is a larger frame size than the 2 hp. Good news is you will never cook the motor with that drive. The drive will let out it's magic smoke long before the motor is hurt.

in the other side, if you have oversize VFD it is nice but you shouldn't have oversized input and output filter (Choke) as the syntonisation to get the best efficienty is tunned with the motor reactive impedance....

You should also be aware of the application. VFD are rated somewhere like overload class 10-20-30 being
-variable torque (Light load with 110% oL capacity)
-Constant torque with a 150% ol capacity (Conveyor...)

a 10 hp VT can also be rated 7.5 CT ....

So when you replace a burned one why not buying the right size instead of just buying the same part # ?

Jeff23spl said:

So when you replace a burned one why not buying the right size instead of just buying the same part # ?

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It's not that simple. They zero stacked the drives and they did not leave enough room in that cabinet to add a pack of cigarettes.

I told our idiot of a ME not to do buisness with these people anymore because the last equipment we got from them was **** and never worked right and we ended up fixing all of it in house.

We will have to build a new cabinet to fix this. some of the motor leads are 150 feet and they did not use any vfd cable or load reactors just THHN straight out of the drive.

The people at that company are just plain stupid from the president on down. I have dealt with most of them before. Most of there shop and field people are house electricians at best.

I just gets me pi%^&ed having to tear our and build a new control system on new equipment.

Drives are designed to protect themselves and the motor so an undersize drive is not at risk as several have described above. Instead, you are at risk for all kinds of nuisance tripping or faulting as the drive defends itself against the larger motor.

If the drive is not faulting, I would conclude that it is happy doing whatever it is doing and not worry about it. Remember, the actual load might be 2hp or less and the motor was oversized for cooling purposes.

The real point here is that a drive know nothing about hp. It makes volts, hertz, and amps and, as long as the load requires those things within its design limits, it's good to go.

Having said that, the PF4 is hardly a jewel of reliability so, if the failure rate gets intolerable, you might want to replace it with something that has a better record. Not necessarily larger but more reliable.

I consider a drive the same as a regular motor starter. According to the NEC there are certain size requirements for a particular HP motor. A good rule of thumb taught by my master electrician is that it never hurts to go a bit larger, but never smaller. A motor is going to pull x amount of amperage and an undersized starter or drive just is not capable of handling the larger load.

The Plc Kid said:

They zero stacked the drives and they did not leave enough room in that cabinet to add a pack of cigarettes

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Based on this statement, I wonder if you have adequate heat dissipation in the cabinet.

I'm led to believe that this is new equipment purchased as an overall package. If so, why aren't you demanding the parts and design to be fixed or replaced under warranty?

Steve

Steve Etter said:

Based on this statement, I wonder if you have adequate heat dissipation in the cabinet.

Heat is no problem the cabinet has a temp monitor and we maintain 70 degree's F in the cabinets.

I'm led to believe that this is new equipment purchased as an overall package. If so, why aren't you demanding the parts and design to be fixed or replaced under warranty?

Steve

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It was not my project and the ME whose project it was will not do it. He keeps doing buisness with them even though we get stuff like this from them.

Yes a VFD is designed to protect himself...you can short some of them and push start while it just fault without burning or blowing...But it is not made to operate this way all day long....

It's also easier to detect a solid fault than a heavy operating demand that is very close to correct operating but a little bit more...

It isn't made to operate beyong is rated capacity for extended period...If you use it for class demo or a low duty equipement, yes it may works...But if you design that for industrial use....be ready to look for new customers...

Maybe an old or DC drive doesn't worry about motor torque or HP but 2000's drive use motor reaction and current/phase feedback to determine magnetic slip. This can be used to do a closed loop speed control without adding a tach feedback reading etc. The same data is also used to display an accurate shaft power (HP) and efficiency in manny models

In some way, they probably had to tweak parameter a little bit to made it works with bigger motor and this can affect everything like some protection capacity....


About panel space vs VFD cooling...if there isn't free space, i expect that VFD installation requirement as not been adressed either and probably that operating temperature inside the panel would be higher than recommanded from Rockwell resulting in a shorter VFD life cycle even if the VFD as built in temperature protection...

It's like a car engine cut-off, it prevent it from breaking but if you drive it all year long at that limit, you should expect to rebuild the engine before 3-400 000km

The Plc Kid said:

It was not my project and the ME whose project it was will not do it. He keeps doing buisness with them even though we get stuff like this from them.

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This is when you get a stick and take him behind the barn...

One thing you may try since it is no longer corret to give someone the beating they deserve is to bring the subject up in from of the vendor, the engineer over the project and that engineer's boss. Let them know that a slight (not really slight but you have to be polite) issue was overlooked when the machine was installed. Aparently the motor was sized as a 3 hp but the drive was sized as a 2 hp.

Reality is this is one of those times in your career that you are just screwed. But if you get those guys together and go over this issue the BS that get shoveled about should be pretty entertaining.

Even better the plant manager came and asked me what was going on and why all the issues with this new equipment and i showed him.

His is a EE so he sort of understood. He has a power generation background not much on controls and such. But he got that.

He said after tommorrow that i would not have this problem again.

I will have to wait til tommorrow and see what that means.

He was not happy at all.