SRVB #5-HT BWCP rotor to stator magnetic gap restack criteria

There has been much discussion over when to restack a BWCP rotor or stator for electrical performance reasons (i.e. not mechanical damage, but actual or anticipated performance degradation).  There are four primary concerns with an increased magnetic gap due to corrosion in wet stator BWCP motors:

1. The motor power factor starts dropping; this results in an increase in running amps without an associated increase in kilowatt draw (this increase in current causes additional heating inside the stator).  Some customers have reported this current increase being so high that the control equipment does not allow the motor to run cold (the hot operation is not an issue due to the reduced load on the motor). 
2. Debris from the lamination material (steel particles) would be circulating in the motor and over time may build up enough to cause bearing wear and cause rotor/stator contact on cold start.
3. When the lamination erodes to expose the rotor bar, the mechanical support is reduced and the rotor bar could be thrown outwards and lead to rotor/stator contact during running.
4. Limited movement of the rotor bar could lead to damage at the brazed connection to end rings and “broken rotor bar” condition which could increase the Unbalanced Magnetic Pull (UMP) and cause rotor to stator contact on cold start.
   

Points two to four above can only be assessed by judgment; however, HT engineering have issued the following summary to assist in evaluating point one:

• The corrosion limit (increase in magnetic gap) is dependent on cold duty amps (site operation) and design max rated amps (full load amps including service factor load if applicable).
• Cold duty amps corrected for corrosion must be less than the maximum rated amps.  When cold duty amps is within 2% of max rated amps the rotor pack, stator pack or both should be re-stacked to reduce cold duty amps to an acceptable value.  Every contract should be reviewed based on rated max amps and reported site cold duty readings.
• Unless reliable site information on cold duty amps is available it would be safer to use the following guidelines (and example).  Note: there will be exceptions to this guideline; one is where design cold duty amps are close to max amps, this could recommend restacks with small or no gap increase.  The first step below is specifically to address this issue:

1. No restacks for gap increases under 50% (0.150” on 0.100” gap) unless site reports problems with high cold duty amps.
2. From the contract manual determine:
   The Max Amps (normally in protection setting section, or use FLA x service factor as an estimate – example 130A).
   The Cold Duty Amps (Operating data section – example 120A) – typically max amps will be 5 to 10% above cold duty amps.
3. Determine magnetic gap increase – new gap from electrical spec (C spec) and measured gap (example: 0.100” design, 0.160” measured would be a 60% increase).
4. Use ‘rule of thumb’ approximation to determine current increase:

Current increase ~ 10% of gap increase percentage (example – 10% of 60% = 6%).

• Estimate cold duty amps with current increase, cold duty amps from manual multiplied by current increase (example – 120A x 106% =  127.2A)
• Compare estimated cold duty (example – 127.2A) with 98% of max rated amps (example – 130A * .98 = 127.4A).
• 98% of max Amps is 127.4A, therefore estimated cold duty amps of 127.2A is borderline but acceptable.