VFD Breaker Tripping

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tmacdonald

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Saginaw, MI
Hello,
I have a project that contains 3 air handlers. Each unit has a VFD that controls the motor.
Unit 1: 5 hp motor with 15A breaker ahead of the VFD
Unit 2: 5 hp motor with 15A breaker ahead of the VFD
Unit 3: 7.5 hp motor with 20A breaker ahead of the VFD
(No input line reactors or filtering has been installed on either side of the VFD).

I am getting reports that the breakers on unit 1 and 3 trip out at the same time. This problem doesn?t occur every day but they do trip at the same time of day (aoubt 6:00am).
Unit 2 does not trip.
My first thought is the utility.
Does anyone have some insight to this problem?
 
Hello,
I have a project that contains 3 air handlers. Each unit has a VFD that controls the motor.
Unit 1: 5 hp motor with 15A breaker ahead of the VFD
Unit 2: 5 hp motor with 15A breaker ahead of the VFD
Unit 3: 7.5 hp motor with 20A breaker ahead of the VFD
(No input line reactors or filtering has been installed on either side of the VFD).

I am getting reports that the breakers on unit 1 and 3 trip out at the same time. This problem doesn?t occur every day but they do trip at the same time of day (aoubt 6:00am).
Unit 2 does not trip.
My first thought is the utility.
Does anyone have some insight to this problem?
The breakers are tripping either because of a high instantaneous current or a sustained overcurrent. neither is real likely on a VFD. have you checked to see what size CB is recommended by the VFD manufacturer for that VFD? Even for a 480V unit they seem a little undersized to me.
 
The breakers act as the main disconnect for the enclosure that houses the VFD.
Referencing an old Allen-Bradley Type 2 motor starter overcurrent protection guide and a similar Square-D guide, they list the overcurrent protection sizes as 15A for the 5hp and 20A for the 7.5hp.

The manufacturer of the air handler provide the enclosures. Of, course I'm assuming because they are from the same supplier, they are all built the same.

The clients maintenance guy has been doing some research on is own and believes the problem is cause because the VFD doesn't have input line reactors.

I don't understand why the absence of line reactors would cause the breaker ahead of the drive to trip.
 
The breakers act as the main disconnect for the enclosure that houses the VFD.
Referencing an old Allen-Bradley Type 2 motor starter overcurrent protection guide and a similar Square-D guide, they list the overcurrent protection sizes as 15A for the 5hp and 20A for the 7.5hp.
You cannot use the motor starter charts for VFDs. Get the manual for the VFD and see what it says.

I have an AB 70Flex manual on my hard drive. It says max CB size:

5HP
208V - 70 or 100 A (depends on whether it is normal or heavy duty)
240V - 60 or 125 A
480V - 40 or 60 A

7.5HP
208V - 100 or 125 A
240V - 80 or 110 A
480V - 30 or 40 A

YMMV with your VFD.
 
Breaker size

Breaker size

I think breakers are undersized. You may put a 32 amps breaker for 5 hp units and a 50 amps breaker for 7.5 hp.

What is the voltage at 6:00 am Is it lower or higher than day time voltage?

The input reactor (some call it choke) is to reduce the current harmonics and will not alleviate the breaker tripping problem.

Cheers.
 
If these breakers are on the supply side of a VFD then the size of the motors have nothing to do with sizing the breakers.

430.122(A) requires the conductors supplying a speed drive to be rated at least 125% of speed conversion equipment rated input current.

430.128 requires the disconnecting means ahead of the speed control equipment to be rated at 115% of the equipments rated input current.
 
Hello,
I have a project that contains 3 air handlers. Each unit has a VFD that controls the motor.
Unit 1: 5 hp motor with 15A breaker ahead of the VFD
Unit 2: 5 hp motor with 15A breaker ahead of the VFD
Unit 3: 7.5 hp motor with 20A breaker ahead of the VFD
(No input line reactors or filtering has been installed on either side of the VFD).

I am getting reports that the breakers on unit 1 and 3 trip out at the same time. This problem doesn?t occur every day but they do trip at the same time of day (aoubt 6:00am).
Unit 2 does not trip.
My first thought is the utility.
Does anyone have some insight to this problem?
You don't give the VSD input voltage so it isn't possible to estimate VFD input current. And, does the same problem occur on all three AHUs?

Given that two drives of a different rating trip at the same time as each other and at around the same time of day, does suggest that the problem may be related to the supply rather than the VFDs.
Do the breakers have an undervoltage trip? If so, something heavy starting up might be causing a dip sufficient to drop the breakes out.

The absence of input line reactors will make the input RMS current higher but that wouldn't easily account for the time of day at which the breakers trip.
 
Here is one sceanrio I can think of.

If for some reason, the power is failing at 6 AM (or earlier and coming back on at 6 AM), there is an initial inrush of current that occurrs when the capacitors are charging. This is a very fast rise because it is charging the capacitors almost instrantaneously. Many VFDs have various methods of mitigating that, i.e. charging resistors, DC bus chokes or using SCRs on the front-end instead of diodes, but some do not. If yours do not, the level of inrush current may exceed the instantaneous trip setting of your CBs. Some CBs ave adjustable instantaneous trip settings that allow them to be set at 10x the CB rating, some do not. Of those that do not, some are pre-set for very low levels, i.e. 250% - 400% of rating.

So for this to be the case, you would need to have a power failure or initial power up at 6 AM, VFDs that have no mitigation of charging current inrush, and CBs with fixed instantaneous trips set too low for this application. That's a lot of ifs, but I have seen it happen, especially with panelboard mounted breakers.

Line reactors or DC bus chokes would slow down the rise time of that charging current, and they provide a level of protecton for both the VFDs and the other connected equipment FROM the VFD harmonics that almost always makes it worth the investment.
 
TM,

Check the time setting on ramping up the VFD for starting. It could be starting to quick for the size breakers you have on unit 1 & 3. Check the Manufacturer's recomendations for breaker sizes for those pump sizes. Also the motors may be getting worn or belts are too tight and the FLA is higher than nameplate, depending on age of units. Check with your ampmeter to see the draw on startup.

- JWC
 
Just a point to consider;
If it were any problem at the load (motor) side, the DRIVE should be tripping, not the breaker feeding the drive. So check the settings for the motor protection as set up in the VFD as well.
 
Thank you all for your input.

The system is rated 480V, 3phase.
When the units were installed, initial startup tests were performed.
The voltage ratings measured were +or- 490V.

I am on my way to visit the site to get a better idea of what was actually installed in the panel.
 
My findings:
The circuit breaker that is tripping is serving as a disconnect to the enclosure that houses the VFD.
Each unit has a 15A breaker (even the 7.5hp motor unit).
Between the breaker and the VFD is a set of fuses (Fusetron type JJS).
Unit 1 with the 5hp motor has 20A fuses.
Unit 3 with the 7.5hp motor has 25A fuses.
No fuses have blown.

It appears there is a problem with the over current protection coordination.
However, the owner is convinced the problem is there are no input reactors.

I will recommend the circuit breakers get changed out with a more appropriate size.
However, I would like to discuss line reactors with the owner a little more intelligently.
Does anyone have a good link that addresses VFD line reactors?
 
TMAC,

So, what is the input rating of the VFD, motor HP ratings are meaningless. You are really wiring a VFD, not a motor. IMO.
That's right.


So t macdonald,
Do NOT assume that because the fuses are larger that this is the appropriate OCPD size for the circuit. Fuses for VFDs are often selected only to satisfy UL listing, NOT to really protect the drive and the circuit conductors. For instance in order to UL list the drives, they needed fast acting fuses like the JJS, but the VFD mfr does not want customers complaining that the fuses blow, so the purposely over size them. As an example I just looked at a Danfoss manual that says they use a 50A fuse on a 7.5HP 230V drive. But the VFD maximum input current is only 22.2A, so the NEC says that the circuit conductors must be capable of at least 125% of 22.2A or 27.75A. That could be done with #10 wire, but clearly, 50A is way too big of a fuse for #10 wire.

Sick to the NEC here. Check the wire size against the VFD max amps, then check the OCPD for the wire size. Forget what the VFD mfr did, that is not necessarily per code because meeting code is the installer's responsibility, not the mfrs. You don't want to fix a circuit protection problem by over sizing devices unless you first prove that a mistake has been made by someone before you.
 
I just got a call from the owner.
He tells me unit 3 went down this morning at 6am.
And all 3 units just went down at 2:35pm.

I have requested VFD specification information from the HVAC manufacturer.
Hopefully this will shed some light as to their overcurrent protection recommendations.

Thank you all for your input.
 
Here is one sceanrio I can think of.

If for some reason, the power is failing at 6 AM (or earlier and coming back on at 6 AM), there is an initial inrush of current that occurrs when the capacitors are charging. This is a very fast rise because it is charging the capacitors almost instrantaneously. Many VFDs have various methods of mitigating that, i.e. charging resistors, DC bus chokes or using SCRs on the front-end instead of diodes, but some do not. If yours do not, the level of inrush current may exceed the instantaneous trip setting of your CBs. Some CBs have adjustable instantaneous trip settings that allow them to be set at 10x the CB rating, some do not. Of those that do not, some are pre-set for very low levels, i.e. 250% - 400% of rating.

So for this to be the case, you would need to have a power failure or initial power up at 6 AM, VFDs that have no mitigation of charging current inrush, and CBs with fixed instantaneous trips set too low for this application. That's a lot of ifs, but I have seen it happen, especially with panelboard mounted breakers.

Line reactors or DC bus chokes would slow down the rise time of that charging current, and they provide a level of protecton for both the VFDs and the other connected equipment FROM the VFD harmonics that almost always makes it worth the investment.

We had this happen on a couple of drives a while back. We have a 138 kV utility substation at our facility with a capacitor bank. The switching transient would pass through the 138kV/34.5kV transformers, the 34.5kV/4160V transformers, and a 4160/480 transformer and cause this exact problem with the drives. This occurred before my time but I believe the manufacturer's fix was to add a choke or reactor of some sort like jraef is saying. Knowing the time of day is a very powerful troubleshooting tool!
 
This is where having the proper tools come in, like say a power quality analyzer. You could connect it to the line side of the vfd or the 480 distribution pnl and let it record for a couple of days and the problem most likely would be solved. I would just about bet something was turning off at about 6 every day and there is a spike in the voltage that's tripping the breakers.
 
We had this happen on a couple of drives a while back. We have a 138 kV utility substation at our facility with a capacitor bank. The switching transient would pass through the 138kV/34.5kV transformers, the 34.5kV/4160V transformers, and a 4160/480 transformer and cause this exact problem with the drives. This occurred before my time but I believe the manufacturer's fix was to add a choke or reactor of some sort like jraef is saying. Knowing the time of day is a very powerful troubleshooting tool!

We had a similar type problem in our plant with motor protection relays and surges taking them out. We installed a SOLA isolation transformer and these issues went away.
 
We had this happen on a couple of drives a while back. We have a 138 kV utility substation at our facility with a capacitor bank. The switching transient would pass through the 138kV/34.5kV transformers, the 34.5kV/4160V transformers, and a 4160/480 transformer and cause this exact problem with the drives. This occurred before my time but I believe the manufacturer's fix was to add a choke or reactor of some sort like jraef is saying. Knowing the time of day is a very powerful troubleshooting tool!

This is one of the reasons for adding line chokes.

One would think that three transformers would have been effective at mitigating the issue in the first place though.
 
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