A 1000 volts on a 575 motor is low. I would suspect a loose fuse holder before a bad wire until you said the breaker tripped. Not sure how much sleep I would loose over two trips.

 

15yo fuses get brittle and fail. Motor single phases, breaker trips.

Replace B phase and get another 15 years.

 

15yo fuses get brittle and fail. Motor single phases, breaker trips.

Replace B phase and get another 15 years.

I have seen that elsewhere here but the fuses looked recent when I replaced them(all 3) the first time.

Is it blowing the fuses during startup or while it is at steady speed? You mentioned it is a ventilation motor, so if it happens during startup only, it could be rotating in reverse when the request to start occurs. Negative pressure in the building can cause exhaust fans to spin in reverse when not running. The excess current drawn to stop the motor rotating, then get it rolling in the right direction can blow fuses and trip OLs. I run into this occasionally with VFDs running ventilation fans. There is a parameter on some VFDs to allow a rolling start to prevent nuisance trips.

The ventilation guy says that the pressure is generally well balanced so I don't think that it should be a factor. They don't keep a log for very long so I don't have any way to tell whether it's on startup or not. The motor is on a daily schedule from 6:00-22:00 so if it was on startup I think it would be more apparent.

 

Is it blowing the fuses during startup or while it is at steady speed? You mentioned it is a ventilation motor, so if it happens during startup only, it could be rotating in reverse when the request to start occurs. Negative pressure in the building can cause exhaust fans to spin in reverse when not running. The excess current drawn to stop the motor rotating, then get it rolling in the right direction can blow fuses and trip OLs. I run into this occasionally with VFDs running ventilation fans. There is a parameter on some VFDs to allow a rolling start to prevent nuisance trips.

 

When replacing fuses do you replace the one blown or all three? I replace all three fuses figuring that the other two fuses were stressed during the event. Yep throw the other two out.

 

That time period is perfect for cap banks on the POCO side to be turned on and off.
Not saying that is the issue. Do you have anyway to measure PF in your plant hour by hour?

 

Have you inspected the contacts of the starter? Check voltages on load side and across contacts for voltage drop. Visually inspect contacts and springs.

 

I haven't looked at the starter because it's upstream of the disconnect and fuses. The voltage at the disconnect was good so I didn't think the starter would be a factor.

That time period is perfect for cap banks on the POCO side to be turned on and off.
Not saying that is the issue. Do you have anyway to measure PF in your plant hour by hour?

I will have to check about the power factor but I don't see why it's an issue. How would the utility turning on/off the capacitor banks affect the motor?

With an old motor it could be going. As they get close to total failure often you get intermittent trips.

Sounds not too large. Do a surge test or inductive imbalance and resistive imbalance with a 4 wire (Kelvin bridge) ohm meter (milliohm or microohm).

On a 600 V motor the correct test is 500 V for 1 minute (60 asconds).On a room temperature motor so you can do temperature correction. Do the test correctly or your numbers are meaningless.

On a motor in service or even out of the shop over 4 GOhms sounds very suspect. 1500-2500 Megaohms right out of the shop sounds more like it. There might be no ground or paint under tge screw. You probably don’t have a good ground to the frame. Run a test wire to it or check at the peckerhead or something.

I did wonder about the bonding continuity so I checked and there is continuity(0-1 ohm) between the bonding screw of the disconnect and all the exterior parts of the chassis. I don't have the equipment to do anything more in-depth than a megger or a hi-pot.

One fuse indicates a short to ground generally not in a motor. Normally with a motor i would expect to clear a minimum of two fuses. (2 fuses doesn't always indicate a motor due to blowing one causing a single phase event until you clear another)

As you can replace the fuse and the fault has cleared i would suspect phase to ground wire (flex to motor?) or snail trails in a contactor or disconnect that may only show in the closed position. I suspect the event is enough to remove a small amount of material which seems to clear the fault.

I megged the wires between the disconnect and the motor and they seem good as well. They look good visually as well but I haven't pulled them out of the flex to see the full length. What do you mean by a "snail trail"?

 

With an old motor it could be going. As they get close to total failure often you get intermittent trips.

Sounds not too large. Do a surge test or inductive imbalance and resistive imbalance with a 4 wire (Kelvin bridge) ohm meter (milliohm or microohm).

On a 600 V motor the correct test is 500 V for 1 minute (60 asconds).On a room temperature motor so you can do temperature correction. Do the test correctly or your numbers are meaningless.

On a motor in service or even out of the shop over 4 GOhms sounds very suspect. 1500-2500 Megaohms right out of the shop sounds more like it. There might be no ground or paint under tge screw. You probably don’t have a good ground to the frame. Run a test wire to it or check at the peckerhead or something.

 

One fuse indicates a short to ground generally not in a motor. Normally with a motor i would expect to clear a minimum of two fuses. (2 fuses doesn't always indicate a motor due to blowing one causing a single phase event until you clear another)

As you can replace the fuse and the fault has cleared i would suspect phase to ground wire (flex to motor?) or snail trails in a contactor or disconnect that may only show in the closed position. I suspect the event is enough to remove a small amount of material which seems to clear the fault.

 

What type of fuse are your using? Since there is a breaker in the circuit, the fuses need to be class 30 slow-blow fuses. Maybe you are replacing with the wrong type of fuse?

 

I couldn’t find any reference to that motor designation, but “E-Line” makes me think this is an energy efficient (EE) motor design. It’s well known that many EE motors attain their high efficiency by messing with the magnetics, often resulting in very high magnetic inrush currents which can blow fuses and trip breakers. It will appear random because there are other factors involved in allowing it to happen, but when it is a repeating event like this, that is my suspicion.

This is the reason why an exception was added to 430.52 to allow mag trip settings to go as high as 1700% if it is demonstrated that lower settings result in nuisance tripping. As for fuses, you absolutely must use time-delay type, but often you must go to the maximum allowable fuse size to avoid nuisance clearing. Given the thorough investigation you have already done to eliminate other obvious issues, these measures are warranted IMHO.

 

I couldn’t find any reference to that motor designation, but “E-Line” makes me think this is an energy efficient (EE) motor design. It’s well known that many EE motors attain their high efficiency by messing with the magnetics, often resulting in very high magnetic inrush currents which can blow fuses and trip breakers. It will appear random because there are other factors involved in allowing it to happen, but when it is a repeating event like this, that is my suspicion.

This is the reason why an exception was added to 430.52 to allow mag trip settings to go as high as 1700% if it is demonstrated that lower settings result in nuisance tripping. As for fuses, you absolutely must use time-delay type, but often you must go to the maximum allowable fuse size to avoid nuisance clearing. Given the thorough investigation you have already done to eliminate other obvious issues, these measures are warranted IMHO.

They are definitely time delay fuses, i'm pretty sure it's this 600V model Ferraz Shawmut / Mersen Sales | TR & TRS Series / Class RK5 / Time Delay
On monday I will have a look at the spec sheet and see what it says about energy efficiency.

 

What kind of fuses are used on it? RK5?

 

I looked at the spec sheet and the motor is indeed a "Premium Efficiency" motor so that seems to be the cause.

I found an ABB white paper on the subject so I have attached it if anybody is interested in learning more. I didn't know about this until @JRaef brought it up. Apparently the new energy efficient transformers also have higher inrush currents than previously seen.

thanks to everybody for the help.

 

I looked at the spec sheet and the motor is indeed a "Premium Efficiency" motor so that seems to be the cause.

I found an ABB white paper on the subject so I have attached it if anybody is interested in learning more. I didn't know about this until @JRaef brought it up. Apparently the new energy efficient transformers also have higher inrush currents than previously seen.

thanks to everybody for the help.

You are not alone in discovering this the hard way...

Back in the early 90s I got pulled into a lot of projects where people were getting utility rebates for changing out old motors for new EE motors. But right away, we started seeing a LOT of tripping circuit breakers and blown fuses, as in 75% of installations! The utility rebates were being pushed by the US Dept. of Energy under a program called "Motor Matters" and they got involved right away to have someone study it. That study was done by the Electric Power Research Institute (EPRI) who was already doing a lot of studies on motor efficiency and failure rates at the time. EPRI concluded that the change in magnetics (and air gap reduction) to attain the higher energy efficiency also had the side effect of lowering the winding resistance and initial impedance (before the motor starts to rotate), resulting in much higher magnetic inrush than had been previously seen. Most of the papers now released by motor manufacturers and others in North America are all based on the study done by EPRI in the 1990s. I know this because I was so deeply involved in the rebate programs in the Pacific Northwest at that time, I was given an associate membership in EPRI and was able to access their papers. But I no longer am and have to pay-to-play like everyone else, so I don't have the name of that study to link to it any more. But papers like the one previously linked are probably using that data (although ABB being a foreign company, appear to claim it all came from Europe...). But here is a much later (2012) paper from the successor of the Motor Matters program in the DOE, dealing with just the Mag-Only Breaker issue, the problem extends to fuses too.

https://www.energy.gov/sites/prod/files/2014/04/f15/avoid_nuisance_motorsys_ts6.pdf

 

Lol, yeah I had a few uncomfortable conversations about the slight change in speed too. On most fans it took a simple sheave change to correct for it, but direct drive pumps were harder to deal with. We had to have them add throttling valves if they didn’t already have them, which represented an energy loss, so the net change in energy use was often zero or close to it. Trimming the impellers was the real solution in some, but they had to wait to take them out of service for a PM to get that done. As I said, “uncomfortable”…

 

I had a boiler randomly blowing a fuse then tripping the overloads. The overloads made sense as a blown fuse equals single phasing.

So over the period of a few months we lost 4-5 fuses and we presumed the motor had been rewound as energy efficient so that was most likely the problem especially as everything tested good. Fuses were max size for the disconnect so changing them was not a option and a power quality meter said we were with in spec for starting the motor on that fuse size.

I was working on another boiler (room is kinda dark) and i notice a small flash somewhere behind me and heard the boiler spooling down. One fuse has blown and again all tests show good at least according to a meg and a ATpro AT33IND motor tester.
Operations was unhappy so we decided to pull the motor and take a closer look (Honestly we did this because removing the motor is a sucky job and most of the time operations who have to open the boiler etc will suddenly be ok with the random problem plus the flash seemed a little odd even if i had no idea where it came from).

As soon as we knocked the ends off it was obvious something was beating the hell out of the rotor. We found a small washer that had been bouncing around inside (its air cooled winding). Lucky for us the washer had a small amount of red varnish which means it was there when they dipped it. Rewind shop admitted it was a balance weight and swallowed the cost of a complete rebuild.

Moral of the story is 99% of the time you can presume that something happened the other 1% is just the electrical gods giving you a hard time.