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Ramp start vs current limit on AC motors

Ramp start vs current limit on AC motors

Ramp start vs current limit on AC motors

I would like opinions on the relative virtues (vis a vis motor lifetime/incremental damage)of ramped voltage start and I limit start for lightly loaded 350/400 HP induction motors. The load is a 2 stage, gaseous helium screw compressor,compression ratio of 20:1 and throughput of 80 SCFM at start, rising to 600 SCFM fully loaded. The motor load is not hi inertia, the supply V is relatively stiff and the time up to speed is not important to the process. The motors (continuous duty, ODP type) reflect the conditions of their installations, which range from somewhat dusty to downright grimy. They are subject, on average to ~40 starts/year, though individual machines may be occasionally be started 4-5 times/hour.
 Can the start current be correlated to motor (insulation)lifetime?
 Does the high inrush of I limit(currently set to 425% of LRA) mechanically over-stress the stator windings? Rotor? Bearings?
 Does one mode provide greater thermal shock/ higher spot heating of insulation or is it only the I^2t of the start process that is relevant?
 Is one of these modes preferrable for a hot motor re-start?    

RE: Ramp start vs current limit on AC motors

1. The start current can be correlated to the insulation aging and lifetime over temperature (R x I**2).
2. The inrush current of 425% of LRA is not supposed to over-stress the stator, rotor and bearings, if this value is within the motor design limits. The design limits are suppose to have sufficient design margin for the motor to last long time, e.g. twenty years or more.
3. The ramp voltage start or its closest approximation shall be verified with the manufacturer since the time interval of starting will increase. This may have impact on other motor parameters, e.g. temperature, vibrations, etc.
4. The one which is motor desined for by the manufacturer.

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