Overload and Branch circuit protection for VFD or Servo Drives

[Ashu]

Hi,
I have a question about the infeed protection of the drives. Manufacturer specifies the max amp rating for branch circuit protection. In our case, we usually use oversized drives to fulfil peak load requirements, however, motor is quite small in size. In this situation, we usually set the drive's infeed protection per motor FLA, multiplying it by 125%. For example, a 17.9A@460V motor being fed by 25 Amp drive (Manufacturer's recommendation for max. branch circuit protection is 80A). We have set drive infeed protection device (MSP) to 22.4A. Is it ok, or it should be set according to VFD infeed current and not the motor? Thank you!

 

[petersonra]

Hi,
I have a question about the infeed protection of the drives. Manufacturer specifies the max amp rating for branch circuit protection. In our case, we usually use oversized drives to fulfil peak load requirements, however, motor is quite small in size. In this situation, we usually set the drive's infeed protection per motor FLA, multiplying it by 125%. For example, a 17.9A@460V motor being fed by 25 Amp drive (Manufacturer's recommendation for max. branch circuit protection is 80A). We have set drive infeed protection device (MSP) to 22.4A. Is it ok, or it should be set according to VFD infeed current and not the motor? Thank you!

There are two different things going on here.

The ampacity of the input conductors has to be 125% of the rated input current. Pretty unequivocal. 430.122(A).

The OCPD rating is determined by 430.130(A)(1), that refers you to 430.52(C)(1), (C)(3), (C)(5), or (C)(6) using the full load current rating of the motor load as determined by 430.6. 430.52(C)(1) refers you to table 430.52, which gives you a maximum rating based on the table FLA rating of the motor. So I think you are OK. However, I would point out that there is often a note in the instructions with VFDs that says something about a minimum rating for the OCPD of 125% of the input current. I have not been able to find this in the NEC anywhere, even though some VFD instructions claim this requirement is from the NEC.

I am not sure there is any special benefit to making the OCPD rating lower, other than maybe it costs a little less.

 

[Ashu]

I appreciate the prompt reply to the question. Thank you!

 

[Jraef]

Most VFD mfrs just give you a MAXIMUM OCPD rating, however the 430.52 rules still apply, which is also a maximum value. Minimum is up to you and your risk tolerance for nuisance tripping / clearing. But if you think about it, a VFD will have a maximum capability of 150% of its rated current for 1 minute (less if it is a VT rated drive), so that tracks fairly well with the 125% rating you are using, if you plot that on the trip curve. But in your case, the VFD is over sized, so that kind of skews things a little more because 150% o the VFD rating is going to be more than 150% of the MOTOR rating. If it doesn't trip though, I see no problem with it.

 

[garbo]

Hi,
I have a question about the infeed protection of the drives. Manufacturer specifies the max amp rating for branch circuit protection. In our case, we usually use oversized drives to fulfil peak load requirements, however, motor is quite small in size. In this situation, we usually set the drive's infeed protection per motor FLA, multiplying it by 125%. For example, a 17.9A@460V motor being fed by 25 Amp drive (Manufacturer's recommendation for max. branch circuit protection is 80A). We have set drive infeed protection device (MSP) to 22.4A. Is it ok, or it should be set according to VFD infeed current and not the motor? Thank you!

 

[garbo]

I would adjust the ampere on drive for the motor that it feeds. On important equipment my company would purchase the next size drive then adjust the overload to match motor nameplate amps. I questioned 2 different dive companies about reducing the fuses mounted in drive but they did not have an answer. Were a!ways taught to use the lowest over current setting or fuse to better protect motors. On so.e over sized drives I did reduce the fuse size and never had any problems.

 

[Macbeth]

Fuses are not straight forward. You have to first decide the reason for using a fuse. Protect Device, Protect Branch Circuit, or Limit Fault Current. Then select the proper Class and size.

The reason the VFDs come with full size fuses, that match the max fuse size, is the VFD manufactures are using them to limit Fault Current. Fuse manufactures limit expenses by only sending the max current fuse to UL for Fault testing, therefore only those fuses can be used to limit fault current. The UL testing requirements use to be 20%, every 5th fuse got tested. So fuse manufacture test 30A and 60A and nothing else between.

It seems sensible that replacing a 60A fuse with a 40A fuse would be safer but the 40A fuse was never tested by UL as a Fault Limiting. Sure it is still Current Limiting for Branch Circuits, But not Fault Limiting.

 

[milmat1]

Conductors @ 125% of the rated input current to the drive : 430.122 (A)
Branch circuit protection should be determined in accordance with 430.52. Using the FLA of the motor. : 430.130(A) (1).
Be careful of 430.130(A)(2) Follow the drive manufacturer's instructions...
Remember, the drives input current is directly related to the motor load requirements (FLA)
Just my opinions / thoughts ..

 

[ptonsparky]

The VFD will protect the motor if it's been properly be set.
The fuses don't really protect anything. If they blow the damage is already done.

 

[ptonsparky]

Well they may limit it somewhat.

 

[ptonsparky]

Auto correct or whatever drives me nuts.

 

[kwired]

Well they may limit it somewhat.

You mean it lessens the amount of "spectacular" the fireworks show might end up being.

 

[kwired]

I still disagree to some extent to NEC requiring input conductors to be sized to input rating. I have an install where we had an existing but not used 250 HP drive (driven load from previous plant owner but not used by current owner) and a 100 HP motor we wanted to use a drive on. Why buy another drive when we have one that will work, but also why supply it with around 2.5 times the supply needed as long as the conductors used have normal overcurrent protection levels? Thing has been that way for maybe 15 years, I haven't lost any sleep over it.

 

[petersonra]

I still disagree to some extent to NEC requiring input conductors to be sized to input rating. I have an install where we had an existing but not used 250 HP drive (driven load from previous plant owner but not used by current owner) and a 100 HP motor we wanted to use a drive on. Why buy another drive when we have one that will work, but also why supply it with around 2.5 times the supply needed as long as the conductors used have normal overcurrent protection levels? Thing has been that way for maybe 15 years, I haven't lost any sleep over it.

I wouldn't either, but the code rules are the rules. Many of the rules are pretty arbitrary, most are very conservative, and many times something cheaper could be done without becoming a safety problem. But, a rule is a rule.

 

[kwired]

I wouldn't either, but the code rules are the rules. Many of the rules are pretty arbitrary, most are very conservative, and many times something cheaper could be done without becoming a safety problem. But, a rule is a rule.

Problem with NEC sometimes is it has a one size fits all kind of rule. Hard to write such a rule book and cover everything you might encounter. I'd guess not too often is a VFD oversized other than when intentionally de-rated for the various reasons one does do that for. In most those cases probably is best to supply it with a conductor sized to the drive rating anyway though.

 

[Jraef]

Problem with NEC sometimes is it has a one size fits all kind of rule.

I think it kind of has to though, because when writing rules for everyone, you can’t know what level of expertise or intended use the user will have. In your 250 / 100 situation, you obviously knew what you were doing and quite frankly, I would have done it too. But if either of us had applied for a permit for that and an AHJ saw it, even if they would be in a position to judge whether or not we were competent, they cannot know if it was going to be misused by someone in the future. The NFPA and associated AHJs cannot infer intent.

I have actually seen the exact scenario that rule is intended to guard against on more than one occasion. In the past when that rule didn’t exist yet, some VFD mfrs who built their drives to KW rated motor sizes at lower IEC voltages, ended up with the drives being over sized for our HP ratings (larger Mitsubishi drives were notorious for that). People would size everything for the HP being used, then a few years later some manager would want to increase production and squeeze a bigger motor onto the drive based on the amp rating. But that would leave the input conductors undersized and they burn up.

 

[kwired]

I think it kind of has to though, because when writing rules for everyone, you can’t know what level of expertise or intended use the user will have. In your 250 / 100 situation, you obviously knew what you were doing and quite frankly, I would have done it too. But if either of us had applied for a permit for that and an AHJ saw it, even if they would be in a position to judge whether or not we were competent, they cannot know if it was going to be misused by someone in the future. The NFPA and associated AHJs cannot infer intent.

I have actually seen the exact scenario that rule is intended to guard against on more than one occasion. In the past when that rule didn’t exist yet, some VFD mfrs who built their drives to KW rated motor sizes at lower IEC voltages, ended up with the drives being over sized for our HP ratings (larger Mitsubishi drives were notorious for that). People would size everything for the HP being used, then a few years later some manager would want to increase production and squeeze a bigger motor onto the drive based on the amp rating. But that would leave the input conductors undersized and they burn up.

Owners/managers are good at doing that sort of thing. I've seen 50 HP motors that been replaced with 60 HP but no changing conductors contactors, etc. overload was probably cranked all the way up to begin with and that allowed the 50 HP to burn out. Or 7.5 replaced with a 10.

I sort of done wrong myself one time, 1/2 hp auger for a bag filling machine went bad. All that was on hand at the time was a 2 hp same 56C frame so we put it on there. Might have been fine temporarily, but this auger reverses, full across line reverse immediately on command, and the extra starting torque of the 2 HP motor stressed that outuput shaft and it literally splintered like a piece of wood when it did fail some fair amount of time later. I did put delay timers on so it can stop before being hit with torque in opposite direction even though we did put a 1/2 hp motor back on it after the shaft on the 2 hp failed.

 

[ptonsparky]

Owners/managers are good at doing that sort of thing. I've seen 50 HP motors that been replaced with 60 HP but no changing conductors contactors, etc. overload was probably cranked all the way up to begin with and that allowed the 50 HP to burn out. Or 7.5 replaced with a 10.

I sort of done wrong myself one time, 1/2 hp auger for a bag filling machine went bad. All that was on hand at the time was a 2 hp same 56C frame so we put it on there. Might have been fine temporarily, but this auger reverses, full across line reverse immediately on command, and the extra starting torque of the 2 HP motor stressed that outuput shaft and it literally splintered like a piece of wood when it did fail some fair amount of time later. I did put delay timers on so it can stop before being hit with torque in opposite direction even though we did put a 1/2 hp motor back on it after the shaft on the 2 hp failed.

Ouch