Motor Hot withstand time 1

[SSKLC]

Motor Specification from one of the reputed consultant in India states following :

For motors with starting time upto 20 secs at minimum permissible voltage during starting, the locked rotor withstand time under hot condition at highest voltage limit shall be at least 2.5 sec more than starting time.
For motors with starting time more than 20 secs and upto 45 secs at minimum permissible voltage during starting, the locked rotor withstand time under hot condition at highest voltage limit shall be at least 5 secs. more than starting time.
For motors with starting time more than 45 secs. at minimum permissible voltage during starting, the locked rotor withstand time under hot condition at highest voltage limit shall be more than starting time by at least 10% of the starting time.
Speed switches mounted on the motor shaft shall be provided in cases where above requirements are not met.

Can someone throw some light on this? Is this coming from IEC/IEEE standard ? because I am not able to find this in any of the standard.

 

[RRaghunath]

IEEE C37.96 Motor protection guide has the following:
"...the time-delay setting should be selected to provide 2-s to 5-s margin above the starting time at rated voltage. Due consideration should be given the integrating effect of the motor-starting current on the relay. When the start time is in the 5-s to 10-s range, a 2-s margin is satisfactory. For a start time of 40 s to 50 s, a margin of 5 s is more appropriate.
When the time delay cannot be set to obtain the desired margin above starting current and still protect the motor (i.e., trip before the motor thermal limit is exceeded), it may be desirable to supervise the TOC with another device (see 5.2.8)."
Hope it helps.

 

[Gr8blu]

Part of the answer is in how accurate the initial site information (load inertia, available line voltage, grid stability/voltage drop, etc.) is to the machine designer. It really doesn't take all that much to cause a 5% swing in acceleration time.

Another part is that the possible acceleration times varies quite widely - so someone arbitrarily created some "buckets" (i.e. under 20 seconds, 20-45 seconds, 45+ seconds) to simplify matters. And then used a rule that would fit the worst-case4 condition (which would be the longest acceleration in the "bucket").

The last part is the recommendation from IEEE C37.96 (use time delay settings that are longer than acceleration times by some amount) given by a previous poster in this thread.

 

[SSKLC]

Thanks for above replies..
I have checked IEEE C37.96 reference. The IEEE clause 7.2.10.9.1 talks about time delay settings (Margin) to be provided above motor starting time for motor protection. However, the clause I am referring to is the Margin (difference) between Motor Safe Stall time (Hot Withstand time) and motor starting time.

The specification from other reputed consultant says following:
" Thermal withstand time (hot) in locked rotor condition at rated voltage shall be more than the acceleration time of the motor with full load connected, at minimum starting voltage, by 5 seconds for drives having acceleration time more than 20 seconds and by 2.5 seconds for the motors having acceleration time less than 20 seconds. However, in all the cases it shall be ensured that minimum cold thermal withstand time under locked rotor conditions shall be 15 seconds and same under Hot conditions shall be minimum 5 seconds."

The above specification dictates the margin between Hot Locked rotor withstand time @100% voltage and Starting time @80% rated voltage for design of the motor.
To above specification, reputed motor vendor has clarified that there is no such requirement in IEC/IEEE and also, it is not correct to compare Hot Locked Rotor withstand time @100% voltage with Starting time@ 80% rated voltage and margin should be checked with respect to Starting time @100% rated voltage.

The 'Industrial Power Engineering Application Handbook' by K.C. Agarwal mentions the similar requirement at section 7.19. It says that CEA publishes the manual based on the actual feedback obtained from various thermal sites, electricity companies, leading consultants and the NTPC (National Thermal Power Corporation), in addition to motor manufacturers.

I have tried to search above in CEA regulations also, however no success.

 

[waross]

Apples and oranges.
The heat generated during locked rotor time is much more than the heat generated during a motor start that takes the same time.
The safety margins may be tighter than they first appear in the event of a stalled load resulting in full voltage into a locked rotor condition.
We had an issue with planer motor protection.
From time to time a board would explode in the planer and stall a motor already at full load temperature.
We lost a couple of 40 HP motors when the protection didn't act quick enough.
We added zero speed switches to immediately detect and disconnect stalled motors.
If I was faced with a possible lcoked rotor condition on an expensive motor I would consider a zero speed switch with a time delay of 1/4 to 1/2 of motor starting time. (The exact time setting may depend on the characteristics of the zero speed switch.)

 

[RRaghunath]

Thanks for above replies..
I have checked IEEE C37.96 reference. The IEEE clause 7.2.10.9.1 talks about time delay settings (Margin) to be provided above motor starting time for motor protection. However, the clause I am referring to is the Margin (difference) between Motor Safe Stall time (Hot Withstand time) and motor starting time.

The specification from other reputed consultant says following:
" Thermal withstand time (hot) in locked rotor condition at rated voltage shall be more than the acceleration time of the motor with full load connected, at minimum starting voltage, by 5 seconds for drives having acceleration time more than 20 seconds and by 2.5 seconds for the motors having acceleration time less than 20 seconds. However, in all the cases it shall be ensured that minimum cold thermal withstand time under locked rotor conditions shall be 15 seconds and same under Hot conditions shall be minimum 5 seconds."

The above specification dictates the margin between Hot Locked rotor withstand time @100% voltage and Starting time @80% rated voltage for design of the motor.
To above specification, reputed motor vendor has clarified that there is no such requirement in IEC/IEEE and also, it is not correct to compare Hot Locked Rotor withstand time @100% voltage with Starting time@ 80% rated voltage and margin should be checked with respect to Starting time @100% rated voltage.

The 'Industrial Power Engineering Application Handbook' by K.C. Agarwal mentions the similar requirement at section 7.19. It says that CEA publishes the manual based on the actual feedback obtained from various thermal sites, electricity companies, leading consultants and the NTPC (National Thermal Power Corporation), in addition to motor manufacturers.

I have tried to search above in CEA regulations also, however no success.

I too faced this issue and agree it becomes impractical / hugely expensive if we insist on the margins as required in the quoted specification.
The numerical motor protection relays available in the market take the data that corresponds to 100% voltage at motor terminals and can calculate internally the trip time corresponding to the actual voltage at the time of motor starting. This should allay any fears as to the safety of motors.

 

[LionelHutz]

That doesn't make much sense. The reduced voltage start will also reduce the current which reduces the I^2t heating of the motor and rotor compared to a full-voltage locked rotor condition.

 

[Gr8blu]

I sat down and carefully re-read the OP's quotations from the reputed consultants.
It sounds to me like the means of determining whether the machine has successfully started is a single condition - i.e. the machine is not up to the identified speed in the time allotted for acceleration. There is (apparently) no means to detect whether the rotor is not actually moving (which would be a locked rotor condition) - which SHOULD have a separate pass/fail criteria.

Most modern protection schemes incorporate both - a speed (or position) feedback signal to determine actual shaft motion, and a time delay coupled with a speed feedback to determine whether operating speed has been reached in the allotted interval. Without having BOTH parts of the scheme active, the anecdotal result of waross' 40hp planer motors (stalling after some period of partially-loaded operation) is going to cause issues.

 

[LionelHutz]

I see very few induction motors with any means to measure the shaft speed or determine shaft motion. This is not part of most modern protection schemes.

You won't get a motor failure if you properly model the motor thermal characteristics in the overload. This requires a fairly complex overload though and not a simple one current dial unit.

The planer motors were failing because the overload wasn't doing it's job. Still, tripping a motor quicker than when the thermal overload would trip certainly isn't a bad thing in that kind of application. A good modern overload would have a "shear pin" or overcurrent trip ability that can be setup to trip the motor when the current rises above a set level for a few seconds. No need for any shaft speed feedback to get that function today.

To above specification, reputed motor vendor has clarified that there is no such requirement in IEC/IEEE and also, it is not correct to compare Hot Locked Rotor withstand time @100% voltage with Starting time@ 80% rated voltage and margin should be checked with respect to Starting time @100% rated voltage.

This from the motor makes much more sense. If you calculate that the motor will take 15s to start @ 100% voltage then saying you want the hot locked rotor time to be at least 17.5s is a simple way to ensure the motor is capable of always starting the load.

The request in the specification is something the consultant made up to be very conservative.