HV motors - speed switch

[RRaghunath]

The speed switch and prolongd start protection based on sped switch operation is recommended in cases where the starting time of the drive is greater than the thermal withstand time of the motor.
In one of clients' specifications, it is said that the hot withstand time at 110% rated voltage shall be greater by atleast 3seconds than the drive starting time at the lowest voltage of 80%.
To me, this appears overspecification and perusal of motor ratings reveals that if we apply the criteria as above all the motors would require to be provided with speed switch.
Solicit your opinion / experiences on the subject.
Just to clarify, I am only talking about pump drives.

Thanks in anticipation. Raghunath

 

[electricpete]

I think I can understand the issue you are identifying. The client seeks to ensure that he won't have to install any dedicated locked rotor protection... can just protect with normal overload. Since the cost of locked rotor protective relaying would likely be small in comparison with the cost of overdesigning the motor (especially large motors), it doesn't make economic sense.

My understanding is that large motors are more likely to be rotor limited during starting, possibly due to skin effect of large rotor bars.

I looked at our plant (~20 years old) . We have five different applications (30 motors total, all associated with pumps) of 13.2kv motors above 2500hp. (presumably the best candidates to be rotor limited).

4 out of the 5 have no dedicated locked rotor protection relaying. I looked at one set of the curves and 80%voltage start time is 5 seconds, while locked rotor time limit is 14 seconds at 100%voltage (we don't have 110% curve). So it meets the criteria, and can be protected for locked rotor using the overload protecton.

I looked at the specification used to order the motors. There is no special requirements on starting or rotor performance other than that it has to safely accelerate the load at 80% voltage.

I wonder how it is that for large motors which are expected to be rotor limited, we ended up with so much margin when we didn't specify it. Maybe there is another spec in MG1 covering this apsect?

By the way the 5th application also has a flywheel with tremendous inertia and requires dedicated locked rotor protection. We used a distance relay to supervise and overcurrent relay for locked rotor protection.

 

[electricpete]

I guess that our 110% locked rotor withstand time could be estimate assuming that heating depends on I^2 and therefore V^2.

That would mean our heating rate at 110% would be approx 110%^2=121%. The withstand time would be adjusted down 20% to approx 11 sec. Still has 6 second margin beyond starting time at 80% voltage (5 seconds).

 

[busbar]

Small aside: Stan Zocholl [selinc.com] has a paper or two on the protection aspects of motor thermal modeling.

 

[xabproject]

Dear RRN,
Are these motors meant for frequent starting?
Speed switch is recommended also in frequent starting case, in addition to the case that you have mentioned.
The switch is then used to supress tripping if rotor condition is good. Sacrifice in terms of life of insulation and any other condition relegated in view of process continuity!
Further, considering that the stating current exists for nearly 80-90% of the starting time (rapidly decreases thereafter) with 'less' effective cooling during the period of adiabatic heating of insulation due to starting current, the specified margins might be just adequate instead of being overspecified.
See if this falls in line with logic or reasoning!
Best regards