Motor Running 1.2 times of FLA

[tankfarms]

Hello All,
I have a motor (3-ph, 2300V, 300HP, FLA-67.5amp, SF-1.0) constantly running at about 82 amps. Which is about 1.2 times of its FLA. It's used to drive a water pump. The motor has class B insulation. I have checked the amp meter to make sure it's not the meter reading higher. I don't have any flow information thus not able to check pump curve to see any horsepower information. Knowing the SF is 1.0, I thus wonder how bad would it be to run the motor like this. I know that this will make the temperature go up, but I'm not sure how much that will be; and of course we all know that "rule of thumb" is every 10 deg.C increase will halve the motor life. Thus I'm trying to get some advice here.

Also, my thermal protection on this motor is an old GE electromechanical type relay, which is fed by a 100/5 CT, and has thermal setpoint at 3.52 amp on the CT secondary; so that makes it 70.4amps to trip? how come my motor is still running then if that's the case. did I intepret it right?

Thanks.

 

[Jraef]

Hello All,
I have a motor (3-ph, 2300V, 300HP, FLA-67.5amp, SF-1.0) constantly running at about 82 amps. Which is about 1.2 times of its FLA. It's used to drive a water pump. The motor has class B insulation. I have checked the amp meter to make sure it's not the meter reading higher. I don't have any flow information thus not able to check pump curve to see any horsepower information. Knowing the SF is 1.0, I thus wonder how bad would it be to run the motor like this. I know that this will make the temperature go up, but I'm not sure how much that will be; and of course we all know that "rule of thumb" is every 10 deg.C increase will halve the motor life. Thus I'm trying to get some advice here.

Also, my thermal protection on this motor is an old GE electromechanical type relay, which is fed by a 100/5 CT, and has thermal setpoint at 3.52 amp on the CT secondary; so that makes it 70.4amps to trip? how come my motor is still running then if that's the case. did I interpret it right?

Thanks.

To avoid repeating myself, read this thread first. It's about a low voltage motor but the issues are basically identical.

So how that relates to your questions on the OLR setting is the "Pickup Point" I mentioned before. OL relays are not necessarily set to trip at 100% FLA, there is an allowance for tolerances. It looks to me as though yours might have been based on the 125% rule, although that would have only applied if the motor had a 1.15SF, it should have been lower for a 1.0SF. But without knowing the details of the OLR you have and how the settings relate to the pickup point, we really can't tell for sure.

It's also possible that the OL setting is correct, and your OLR is not functioning. I would have it tested. Actually, I would replace it with a decent motor protection relay anyway. MV motors, and the downtime costs typically associated with them, are too expensive to take a risk on being cheap about the protection scheme in my opinion.

As to the 1.0SF motor running at 120% FLA, that rule of thumb is likely in full play. You have the equivalent of that motor smoking 6 packs of cigarettes a day... But the real thing to worry about is winding temperature. I suppose there are no RTDs embedded, right? Without that, you can only assume the damage is being done.

 

[nollij]

The process is the most probable cause of overloading the motor.

edit: nevermond... thought it was 480V, small motor.

 

[weressl]

Hello All,
I have a motor (3-ph, 2300V, 300HP, FLA-67.5amp, SF-1.0) constantly running at about 82 amps. Which is about 1.2 times of its FLA. It's used to drive a water pump. The motor has class B insulation. I have checked the amp meter to make sure it's not the meter reading higher. I don't have any flow information thus not able to check pump curve to see any horsepower information. Knowing the SF is 1.0, I thus wonder how bad would it be to run the motor like this. I know that this will make the temperature go up, but I'm not sure how much that will be; and of course we all know that "rule of thumb" is every 10 deg.C increase will halve the motor life. Thus I'm trying to get some advice here.

Also, my thermal protection on this motor is an old GE electromechanical type relay, which is fed by a 100/5 CT, and has thermal setpoint at 3.52 amp on the CT secondary; so that makes it 70.4amps to trip? how come my motor is still running then if that's the case. did I intepret it right?

Thanks.

Check if there is a difference between the nameplate voltage and the actual supply voltage. You may be able to change your supply transformer tap to match the nameplate.

Without built-in RTD's it is difficult to determine what is the actual winding temperature, which would be the real indicator how muc - if any - trouble you are into. In lieu of this, if you have the motor available at your command, you can:

1. shut it down, wait 8-24 hours to come down to 'cold' temperature,
2. measure the winding resistance with a low range, high accuracy Ohm-meter,
3. star it up, run it for 8-24 hours at load,
4. shut it down and measure the winding resistance to calculate how much your winding heats up.

Older motors had a lot of 'margin' in them and from the voltage it could be an older motor as 2300V systems are being obsoleted and you mentioned that your OL realy is an old thermal GE. So you could be OK as far as insulation life goes.

Invest in an electronic oveload, preferebly one that has thermal module to mimic the motor profile, based on startup characteristic. This will offer you a better protection and longer life for your motor.

Investigate if any of the operating conditions have changed and if those can be reversed to bring the motor back to its service factor rating?

Have a game plan for either repair or replacement with a larger motor IF the load had actually and irreversibly increased.

 

[BJ Conner]

"I don't have any flow information thus not able to check pump curve to see any horsepower information. "
IT's easy to overload a pump motor by tweeking a few valves. Get some one from operations to adjust some valves on the discharge side and watch the current.

 

[hurk27]

Seal packing tighten down too much or not being lubricated, bearings bad, wrong RPM motor for pump installed, 50hz motor installed on a 60 hz supply, pump being fed with a positive pressure with a pump design for negative inflow, cutting down the supply (liquid) to the pump (centrifugal) will lower the draw on the motor.

the rest of problems which have been mentioned is winding problems, supply voltage to low causing slippage of sync, etc...

 

[hillbilly1]

I dont know how your motor is lubricated, but I had a treatment pond aerator (much smaller motor that you have though) that was drawing about 125 % above FLA, figured the motor was going bad, paddled out to get the numbers off (thats another story) the motor, when I noticed the grease fittings were very clean, like it had never been greased. When I got back to shore I asked the maintance guy when was the last time they were greased? He said that it had been running 24 hours a day for the last 4 years and they had never greased it. Paddled back out with a grease gun, lubed them up, amperage dropped back down where it was supposed to be.

 

[dbuckley]

Given this is not a mass produced motor out of a sweatchop, maybe the nameplate is wrong?

 

[tankfarms]

Check if there is a difference between the nameplate voltage and the actual supply voltage. You may be able to change your supply transformer tap to match the nameplate.

Without built-in RTD's it is difficult to determine what is the actual winding temperature, which would be the real indicator how muc - if any - trouble you are into. In lieu of this, if you have the motor available at your command, you can:

1. shut it down, wait 8-24 hours to come down to 'cold' temperature,
2. measure the winding resistance with a low range, high accuracy Ohm-meter,
3. star it up, run it for 8-24 hours at load,
4. shut it down and measure the winding resistance to calculate how much your winding heats up.

Older motors had a lot of 'margin' in them and from the voltage it could be an older motor as 2300V systems are being obsoleted and you mentioned that your OL realy is an old thermal GE. So you could be OK as far as insulation life goes.
Invest in an electronic oveload, preferebly one that has thermal module to mimic the motor profile, based on startup characteristic. This will offer you a better protection and longer life for your motor.

Investigate if any of the operating conditions have changed and if those can be reversed to bring the motor back to its service factor rating?

Have a game plan for either repair or replacement with a larger motor IF the load had actually and irreversibly increased.

Thanks. You know lots of what you mentioned here are exactly what I'm thinking except we're in a refinery, and most of this are just "I wish" type of things. But could you please elaborate a bit more on the insulation life part like you mentioned here? Also, how exactly measuring winding resistance would calculate heat/temp. rise? comparing with some sort of charts?

 

[rcwilson]

Measuring motor winding temperature

Measuring motor winding temperature

Also, how exactly measuring winding resistance would calculate heat/temp. rise? comparing with some sort of charts?

For copper, R1= R2 x [(234.5 +T1)/(234.5+T2)], where R1 = resistance measured at T1 in degrees C and R2 is at temperature T2. Measuring the temperature and resistance when the motor is at ambient gives the base line R2 & T2. Measuring resistance immediately after the motor has been under load for long enough to stabilize temperature (at least 4 hours) gives R1 and you can calculate T1.

It will take a few minutes to safely connect to the motor terminals or cables and make the resistance measurements. The motor will have cooled down some. Make several measurements and record the exact time since motor shutdown for each measurement. Graph the calculated temperatures against time after shutdown then extrapolate that curve back to tiem zero for a good estimate of winding operating temperature.

For the most accurate results, do this at the motor terminals to avoid the small errors caused by the starter to motor power cables and terminations. The cables and terminations will not change temperature as much as the motor winding so the error is not that great if you take the readings at the starter. Use a high accuracy ohmeter that can read milliohms.

Thsi is the method recommended by IEEE for motor and transformer winding measurements.

I would also verify the accuracy of your CT's. Your meter may be calibrated but the CT's could be off.

 

[weressl]

Thanks. You know lots of what you mentioned here are exactly what I'm thinking except we're in a refinery, and most of this are just "I wish" type of things. But could you please elaborate a bit more on the insulation life part like you mentioned here? Also, how exactly measuring winding resistance would calculate heat/temp. rise? comparing with some sort of charts?

Here is a fairly short explanation and a tool to calculate winding temperature from resistane mesurement of cold to hot.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html

Concerning the insulation life I was refering to the fact that manufacturers design their motors differently from each other. As the result each motor run at different temperatures when reaches equivalent loads. Ex. a 10HP motor from seven manufacturers reached the following temperature rises above ambient in C*: 47, 61, 86, 59, 52, 56 and 68. All manufacturers had the same insulation class. So it can be said that from the thermal failure standpoint the motor with the 86*C rise will fail first and the one with 47*C rise will last the longest should they be subjected to the same overload. Some motors run for years overloaded and some fail within weeks or months. Magnitude of overload, load profile and base of thermal design are the factors.

When you watch a compressor loading, you will see that as the cycle nears the high-pressure cut-off points, the motor actually runs above it's nameplate amperes. It is done by design, since the motor will shut off and will have a predictable length of cool-down time after which it will start again by the low pressure cut-in switch, but now the current is significantly lower that the nameplate current.

 

[weressl]

For copper, R1= R2 x [(234.5 +T1)/(234.5+T2)], where R1 = resistance measured at T1 in degrees C and R2 is at temperature T2. Measuring the temperature and resistance when the motor is at ambient gives the base line R2 & T2. Measuring resistance immediately after the motor has been under load for long enough to stabilize temperature (at least 4 hours) gives R1 and you can calculate T1.

It will take a few minutes to safely connect to the motor terminals or cables and make the resistance measurements. The motor will have cooled down some. Make several measurements and record the exact time since motor shutdown for each measurement. Graph the calculated temperatures against time after shutdown then extrapolate that curve back to tiem zero for a good estimate of winding operating temperature.

For the most accurate results, do this at the motor terminals to avoid the small errors caused by the starter to motor power cables and terminations. The cables and terminations will not change temperature as much as the motor winding so the error is not that great if you take the readings at the starter. Use a high accuracy ohmeter that can read milliohms.

Thsi is the method recommended by IEEE for motor and transformer winding measurements.

I would also verify the accuracy of your CT's. Your meter may be calibrated but the CT's could be off.

Good comments, just two points to emphasize. Steady load will give data that is sueful, varying loads are much harder to interpolate and be meaningful. The length of continuous run to reach temperature equilibrium depends on the motor mass, construction and size. A TENV motor will reach it faster than a TEFC. A 10HP will be faster than a 1000HP and so on.

After reading, short the two measuring leads and deduct that resistance from your read value.

Target your reading that it will occur some time - 30-45 min. - after the hottest part of the day. Be aware that significant humidity change can influence the cooling of the temperature rejection into the atmosphere.

Oh, and don't forget to ground-bleed your MV motor before trying to connect your leads to read the Ohms....