Motor Relay Settings - Medium Voltage

[Knoll149]

I have SEL-710 application and we're using stator/rotor/ambient RTD settings. We have enabled TRIP points for these RTD's per motor manufacturer data sheet.

Is it still necessary to include time between starts & starts per hour lockout protection since the RTD's protect the motor against over temperature damage? We were basing these settings on NEMA MG-1 recommendations, 20 min and 3 starts per hour. I'm thinking these settings are not necessary with RTD protection?

We would prefer to remove the previous lockout settings because the 710 outputs to control relay that have seal-in contact and it causes operator have to manually reset the MV starter. The starter scheme has other contacts and is not easy to rewire.

Thoughts?

 

[dpc]

Not sure the RTDs will respond fast enough to protect against overheating due to multiple restarts. But RTDs in the rotor are not common so I don't really know. But using either multiple restarts or starts/hour is totally optional.

 

[waross]

Contact the manufacturer and enquire about warranty coverage with only RTD protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I started to read two main standards about starting induction motor IEC 60034-12 and NEMA MG-10.
Both state motors shall be capable of withstanding two starts in succession (coasting to rest between starts) from cold conditions and one start from hot after running at rated conditions.
IEC 60034-12 does not indicate the coasting duration but NEMA MG-10 does.
Then I saw what did dpc said: the rotor temperature will be more elevated than stator at starting.
However, in an overload situation the stator winding temperature will be more and the RTD are mounted on the stator in order to control the overload.
Standard thermal overload relays will not protect against an excessive number of starts
or less coasting to rest between starts.
See: NEMA MG10/2001 Table 7 ALLOWABLE NUMBER OF STARTS AND MINIMUM TIME
BETWEEN STARTS FOR DESIGN A AND DESIGN B MOTORS.
I agree with waross, it will be better to consult the motor manufacturer.

 

[Knoll149]

I will double check the design code, also Need to confirm RTD locations. I agree they are typically not in rotor.

I do not see any comment from dpc about stator vs rotor temp? Why does rotor temp get more elevated than stator during starting?

 

[7anoter4]

In my opinion, the winding heating depends on two factors: the conductor resistance and the current flowing through. Since the stator winding resistance depends merely on temperature and less than skin effect[as current frequency is unchanged] this resistance stays almost unchanged.
The rotor resistance will rise 300% from rated- at least- at start when the frequency is 60 or 50 Hz [at rated rpm is 3-4 Hz]
So, at start, the rotor heat losses will be 2-3 more elevated than the stator heat losses.
However, in an overload case -even the increased slip determines a minor frequency rise in rotor-resistance will stay almost unchanged. The same thing with the cooling of rotor. Then the rotor is less sensible to overload, at minor change in velocity, than stator winding.
Also, the thermal constant of the stator winding is less than in rotor and then in stator temperature will increase faster. So in this case ,RTD is an effective protection.

 

[VTer]

If the starter includes autotransformer for starting, you need to confirm the number of starts on the autoransformer as well. Not sure if it applies in this case but wanted to point this out as well before you remove the protection.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[waross]

Is it still necessary to include time between starts & starts per hour lockout protection since the RTD's protect the motor against over temperature damage? We were basing these settings on NEMA MG-1 recommendations, 20 min and 3 starts per hour. I'm thinking these settings are not necessary with RTD protection?

It is not often a large investment in time or money to include these settings.
After a few decades in the field you will see a lot of failures that shouldn't have happened.
The RTDs are a backup for the lockout protection.
The lockout protection is a backup for the RTDs.
But the RTDs were good when they were installed, why would they ever fail?
The relay was in good order when it was installed, why would it ever fail?
Wait! The motor was in good shape when installed. The operators have been told to never overload the machine. Why would the motor ever fail?
I have seen a lot of various types of equipment damage when a single safety interlock failed. Almost all of those incidents would have been avoided by redundant interlocks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hoxton]

Good post Waross

OP - please remember, as Waross states that rotor RTDs are not common, because of the expense / difficulty of getting the measurement from rotating to stationary parts.

So essentially, the rotor is protected by limiting the number of starts and the time interval between starts. Very simple, but very effective.

Also RTD and other temperature detectors (eg thermocouples) can be relatively small and can be located where they measure the typical temperature, not necessarily the hottest spot (Hot Spot). Insulation failure due to excess temperature will probably (sorry, but this type of discussion will be full of "will probably", "relatively", "can", "typically", since so many factors could affect the outcome.)

Where was I, oh yes, will probably occur at the hot spot, but this may not be measured by the RTD. For example, a stator RTD is typically fitted in the stator slot separator. That is the divider between upper and lower parts of the stator Winding. Normally between two fiber glass like strips that are there to give a bit of cushion between these two winding layers.

Now if your core length is 1000 mm, do you make the RTD 900 mm long, to capture the average temperature, or do you make it 100 mm long n order to try and capture the hot spot? This is assuming that the motor designer knows where the hot spot is going to be, especially after a few years operation when some of the ventilating ducts are blocked with dust?

Also remember that on manufacturers test, the temperature may have been take by measuring the stator winding resistance, this will be an average temperature, of course.

Sorry to go on, but I am trying to show how complex these inter related issues can be. At some point, some engineer, taking the duty of Designer has made some decisions. Having RTDs, over current, overload (thermal) protection etc are multi layer approaches to protect your ac motor and the value of the product it moves.

Before changing this, consider whether you are aware of what the Design objectives were.

You say that his is an "MV" motor, so above 1000 V and a relatively large motor (2000 kW?). I suggest you investigate and cure whatever is causing this motor protection to trip AT ALL before changing the protection settings.