Motor Start Time vrs Motor Age

[rah]

Motor start time and applicable torque curves for different voltage levels are or can be provided by the Manufacturer at the time of purchase of large 4 kv induction motors (greater than 800 hp). Understanding this, will the start time for the same given voltage value and load torque increase as motor ages 10+ or 15+ years? Is there a good reference standard/paper on this subject?

 

[gordonl]

This isn't exactly the answer to your question, but I've seen starting times increase over time on fans because as the dampers get older they don't seal as well, so the fan requires more tourque to start, thus a longer time.

I'm not exactly a motor expert, but I can't think of any electrical parameters that are going to change over time if there is no damage to the system. (ie. interturn faults, bad rotor bar connections)

 

[electricpete]

I agree with Gord 100%. The torque speed curve can be computed directly from electrical parameters which should not change over time for NORMAL motor aging process.

The one possible exception would be rotor resistance, which plays a central role in determining torque speed characteristics. Brazed joints connecting copper rotor bars to the end rings can degrade overtime. The factors that make this type of degradation likely are:
Large Motors
High Speed Motors
Frequent Starts
Multiple starts without cooling between.
High-inertia load
Long starting time

One of the symptoms of rotor bar problems is in fact higher slip than expected. Also can be detected thru vibration analysis and current signature analysis, subject to some interpretation, as well as single-phase test of motor (turn rotor while applying low voltage single phase ac and measure current pattern).