Overloads in front of a VFD

[itsmoked]

Recently on several kludgy machines I've seen motor-starters, a contactor with an electronic overload, in front of VFDs. I suspect the VFDs were added to DOL setups and the motor-starters are 'just there'.

Is this kosher? I always wonder what electronic overloads think of feeding rectifier charged capacitors.
What issues could I expect from a setup like this?
Does it provide any useful protection?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[iceworm]

Does it provide any useful protection?

Assuming you mean a combination starter with a CB;
And, the VFD provides it's own overload protection.
From an NEC POV:

The electronic overload can be set out of the way to prevent nuisance trips. Yes it is pretty useless. Although I have customers that use an AB E3+ to monitor the currents - circuit breaker feeder, no contactor, all the protective functions are shut off.

The starter CB provides the short circuit protection? If so, I'd check to see if it is an instantaneous CB. If So, likely does not meet the VFD MFG requirements. If a TM CB, still check if it meets the MFG requirements.

One other thing is the NEC requirement for the disconnect to be in sight of the controller. Usually the VFD is considered the controller, not the contactor. So if the VFD is around the corner and not next to the MCC with the starter, the VFD may need a disconnect next to it. Although I have seen more that one AJH that did not care and were easily persuaded the contactor was the controller.

ice



Harmless flakes working together can unleash an avalanche of destruction

 

[jraef]

If the OL relay is there to protect the motor, it could be problematic. The VFD will correct the motor's power factor as seen by the line side, so the input current TO the VFD will be lower than the output current as seen by the motor. If you sized/set the heater elements in the OL relay ahead of the VFD to the motor current, it will not trip when it is supposed to.

But as you know, MOST modern VFDs now include OL and Short Circuit protection for the motor connected to it. UL has required that since around 2005, but not all VFDs are UL listed (or are UL listed as "Power Converters" which is a trick to get around that). So if listed as a "motor controller" as most of the major brands are, all the VFD needs is a circuit breaker ahead of it.

Technically, that now must be a Thermal-Mag breaker, you can no longer use Mag-Only breakers for that. You used to be able to (if you were the VFD mfr), but UL changed their minds on that a few years ago and although preexisting designs were not forced to change, UL has been aggressively enforcing this issue with the mfrs in that if they make even the SLIGHTEST change to an older VFD design that requires UL to re-evaluate it, they lose the ability to allow the use of a Mag-Only breaker. Now, in that vein, if you leave in the Thermal OL relay of a starter that uses a Mag-Only breaker, does that constitute an acceptable circuit as a Branch Feeder for protecting the VFD? That's something that the AHJs are kind of variable on it. I've seen it accepted, I've seen it bounced. It's a craps shoot...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[iceworm]

If the OL relay is there to protect the motor, it could be problematic.

I would think so - but for a different reason. Mild disclaimer - I may have the physics screwed up here. I am definitely not a VFD internals design whiz.

Consider the VFD can not limit the current to the motor - Only reason overloads would be needed.
Now consider the motor is being operated a low frequency, but overloaded: Frequency is way down, voltage is down, motor current is way up. VFD power out is low compared to a motor at normal speed and full load. So the VFD input power is low. Other than internal VFD losses, Input power equals output power

VFD input voltage is normal and as you said, input pf is close to unity. That should leave the input current low.

If I'm even close on the physics, input current is not necessarily a function of the output current.

Just thinking

Different issue:
As I noted in another thread, interestingly, or perhaps disturbingly, the NEC allows an instantaneous CB to feed a VFD. I'm not saying it is a good idea - just that it does. (edit to add) and I'm not saying one could even get there considering the listing issues.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[davidbeach]

The VFD has absolutely no difficulty limiting the current to the motor and will do so to protect itself.

 

[iceworm]

David -
With no animosity intended, and intentionally jerking your chain with mild sarcasm:

Really? It will?

I think we all knew that. Truly I do.

So thinketh the worm

Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

I defence of David, I also thought that that had to be said.

Consider the VFD can not limit the current to the motor

I was mislead by this statement.
On rereading it twice I now understand what you meant, but it was very easy to misunderstand.
Let's just forget it, move on and all be friends.
In respect to the original post:
I suspect that these installations were maintenance additions without the benefit of codes or inspections.
Just add the VFD and make it work. If it trips due to non-sinusoidal wave forms and/or distortion power factor, set it higher until it stays online.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

One thing worth noting, if the starter is use to open line power every time there is a Stop command, it will hasten the death of the VFD in a lot of cases. Most smaller VFDs (<50HP) use a current limiting resistor and shunting relay as a pre-charge circuit for the DC bus. Cycling line power over and over eventually stresses that resistor and once it fails, the drive dies shortly thereafter.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

ice;

Assuming you mean a combination starter with a CB

No, just a contactor with an electronic overload. Most the time short circuit protection is provided by a circuit breaker in the power panel down the hallway..

The in-sight disconnect is the twistlock plug next to the machine.

Jeff; I doubt these motor starters are "there for overload protection" they're there cuz they were there when the machine's motor was DOL run and now remain only because someone thinks they had some "back-up utility".


Ice; That 'different thread' was what reminded me of this issue which I've seen several times in the last 2 weeks. One machine even had overloads in series with lesser overloads running smaller loads.

Since these machines tend to be fed from typical power panels they're probably thermal breakers.

Bill;

I suspect that these installations were maintenance additions without the benefit of codes or inspections.

No question! That is exactly what's going on.

If it trips due to non-sinusoidal wave forms and/or distortion power factor, set it higher until it stays online.

That was my thinking.

Jeff; You must be clairvoyant.

if the starter is use to open line power every time there is a Stop command

That is exactly what I was faced with on a machine this week. They added a VFD and were having difficulties because the RUN switch closed the motor starter feeding the VFD. They were using the poor VFD like it was a contactor.

Thanks everyone for sharing your knowledge.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

I am not sure if anyone mentioned this about the line disconnect at the motor. Its common practice in my industry to put an aux contact on the disconnect if you have a vfd as the controller. This way the plc would pull the enable for the vfd, or you wire this aux disconnect into the vfd enable. I never put an aux contact for the disconnect on just a contactor/starter combo unless customer requests it.

Saying that this is a retrofit? I would check for an aux disconnect in the motor disconnect.

 

[itsmoked]

Hi Controls; On machines that are plug-disconnected, if the motor is bigger than about 2HP and VFD fed I put a contactor in front of the motor that only closes when the front panel power switch is turned ON. That way no poor sucker is plugging in a large capacitor bank in their hand. The only thing they're plugging in is the little control transformer that's used to run the contactor.

As for aux contacts the IEC contactors I use are always dripping with multiple aux contacts which often come in handy. You have a good point on using the supply contactor aux to disable the VFD since a VFD can continue to run on the stored energy. I also see parameter settings that disallow hold-up running so maybe a wire/aux isn't needed. Course most stuff I work on has E-STOPs which a panicked user would likely use first and uses the VFD to rapidly stop the motion.

Keith Cress
kcress -

http://www.flaminsystems.com