2300hp motor rtd's running hot

[philly]

We have a 2300hp 4.16kV motor whose RTD's seem to be going into alarm alot lately. The alarm is indicated on the multilin 269 protection relay with the alarm setpoint set to 135deg C and the trip setpoint set to 140deg C per the motor datasheet. Full load on the motor is 312A and the rtd's have been seen going into alarm anywhere between 285-300A. All 6 stator RTD'a are reading around the 135deg C mark.

My question is why these RTD's are going into alarm when the motor is still a little bit off from Full load current. We also have the same exact motor and application in another location here and at 280A the rtd's seem to only be around 60deg C or so.

What could be the cause of a motors RTD's running hot when it is not near full load especially when another similar motor and load has its rtd's operating at lower temps for a similar load.

This motor does have about a 5% current imbalance. Can this cause this additional heating.

Could the location of the motor play a factor. The motor is rated for a 85deg C rise in an ambient of 40deg C. If the motor is in a location that has a higher ambient temperature can the RTD's get hotter for a given load then it would see in a cooler ambient enviorment?

 

[Jraef]

We have a 2300hp 4.16kV motor whose RTD's seem to be going into alarm alot lately. The alarm is indicated on the multilin 269 protection relay with the alarm setpoint set to 135deg C and the trip setpoint set to 140deg C per the motor datasheet. Full load on the motor is 312A and the rtd's have been seen going into alarm anywhere between 285-300A. All 6 stator RTD'a are reading around the 135deg C mark.

My question is why these RTD's are going into alarm when the motor is still a little bit off from Full load current. We also have the same exact motor and application in another location here and at 280A the rtd's seem to only be around 60deg C or so.

What could be the cause of a motors RTD's running hot when it is not near full load especially when another similar motor and load has its rtd's operating at lower temps for a similar load.

The RTDs are looking at real measured temperature at the spot they are inserted into the motor. Current has an indirect effect on motor temperature, but it is not the only thing that affects it. Blocked cooling passages, high ambient from adjacent machines, radiant heat from sun exposure all contribute to motor heat.

This motor does have about a 5% current imbalance. Can this cause this additional heating.

Absolutely! Imbalance creates negative sequence current, which in turn creates negative torque that essentially fights the motor's normal output torque and creates heat in the motor that is disproportionately high with relation to normal heating from the amount of current flowing to it. Check your voltage, a very small voltage imbalance creates a very high current imbalance. If your voltage balance is within 1%, then you may have a bad connection somewhere. Do a thermal scan if you can.

Could the location of the motor play a factor. The motor is rated for a 85deg C rise in an ambient of 40deg C. If the motor is in a location that has a higher ambient temperature can the RTD's get hotter for a given load then it would see in a cooler ambient enviorment?

Yes, see above. heat is like a bucket with a small hole in it, anything that contributes added heat fills the bucket, only radiation and convection drain the bucket. Too much fill or not enough drain leads to overflow.

 

[kingpb]

A small voltage unbalance can cause a significant current unbalance in the windings. Have you checked the voltage unbalance? NEMA recommends 1% or less.

As the voltage unbalance increases, you must derate the motor or damage can occur.

 

[philly]

Absolutely! Imbalance creates negative sequence current, which in turn creates negative torque that essentially fights the motor's normal output torque and creates heat in the motor that is disproportionately high with relation to normal heating from the amount of current flowing to it. Check your voltage, a very small voltage imbalance creates a very high current imbalance. If your voltage balance is within 1%, then you may have a bad connection somewhere. Do a thermal scan if you can.
.

I do not have voltage readings on this bus, but for the other similar motor I do have voltage readings and the voltage unbalance is about 0.5% which results in a current unbalance of about 5% in this motor as well. Are these numbers to be concerned with?

Is there a way of correlating a %unbalanced current to an %unbalanced voltage?

Typically I have seen an unbalance alarm setting of 10% with a trip point of 15% for MV motors. Are these standard values? Should the 5% be anything to be concerned about?


Yes, see above. heat is like a bucket with a small hole in it, anything that contributes added heat fills the bucket, only radiation and convection drain the bucket. Too much fill or not enough drain leads to overflow.

So is the 85deg C rise about ambient to account for the heat given off by the motor itself during operation (windings, friction etc..?) In other words when you run the motor at full load the motor temperature will rise 85deg C from whatever its at, and if you run it above full load it will get hotter. I'm also assuming this 85deg rise is calculated with designed cooling capacity.

So anything that is added to the ambient temperature (room temp, sunlight etc...) will add to the ambient and add to the temp rise created by the motor itself to give the operating temperature.

An 85deg C rise at 40deg ambient gives a total of 125deg C. Why does the motor manufacturer then give alarm and trip points a 135deg C and 140degC respectively for this class F insulation if the rise of 125deg C is less than both of these?

If a motor is somehow placed in a cooler than ambient enviornment (lets say 20deg C) can you then run the motor above full load as long as the motor stays within the 125deg C total stated by the 85deg rise at an ambient of 40deg? Should RTD's take priority over current values when it comes to operating a motor within ratings?

 

[charlie k.]

How is the motor cooled? If it is water cooled I would make sure you have an adequate flow. Also check the supply and return pipes temperature. If it is air cooled make certain that the air flow is not blocked. Check to see if there are filters that are blocked or baffles that are closed.

Charlie

 

[jcole]

Are u sure the the rtd's are detecting winding temp? Maybe they are detecting bearing temp and your bearings are going bad. Just a thought.

 

[philly]

Motor is fan cooled and appears to be working.

So the 85deg C is the total rise that the motor is allowed and any ambient temperature adding to this will allow less heat that the motor could produce from current and friction.

 

[topgone]

IMO, an unbalance of more than 2% will definitely increase the motor temperature! At 5% imbalance, I roughly compute the percent increase of the motor temperature to about 50% of the normal motor temp under balanced voltage.
Recommend to scan the motor temp to verify temp or measure the RTD output resistance and compute for the temperature.

 

[markstg]

An 85deg C rise at 40deg ambient gives a total of 125deg C. Why does the motor manufacturer then give alarm and trip points a 135deg C and 140degC respectively for this class F insulation if the rise of 125deg C is less than both of these?

Based on information given you have a motor with Class F insulation with Class B temperature rise. Class F insulation is good to 155 deg C. Since the RTD's do not necessarily read the "hot spot" termperature in the motor winidngs, 10 deg C is used as the difference between what the RTD measure and what the hot spot temperature is. 155-10 = 145 deg max therefore trip at 140, somewhat conservative. Alarm is set abit lower and 135 is typical for this.