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Archives for September 2021

How a DC to AC Retrofit Saved this Customer Money and Time

Converting from DC to AC brings many benefits. This customer understood that the total cost of a new AC motor and new VFD was less than the cost for a typical DC motor rebuild. That alone was a driving force in their decision. However, that’s just ONE of the benefits in this case study.

  1. LESS EQUIPMENT
    1. The AC motor does not require a tachometer for speed control in this application so there is a reduction in cost and one less major component leading to greater reliability.
  2. LESS MAINTENANCE
    1. The maintenance on the new AC motor and Drive is fraction of the DC Motor and DC Drive, a reduction of 85% to 90% less maintenance.
  3. GREATER RELIABILITY
    1. The AC motor will have far greater meantime between failure (MTBF) due to reduced components, circuits/windings, and wear/consumable components.
  4. BETTER SAFETY
    1. We were able to design the motor as vertical, so it has proper lifting provisions to stabilize the machine during install. This greatly aids safety for the technicians.
  5. INCREASED MOBILITY
    1. The AC motor weighs less than 60% of the DC motor so it is much easier to maneuver.

Click on photos to enlarge.

RCFA: A Key Element to Improved Reliability

A Decatur Industrial customer had back-to-back motor failures on a 300HP boiler fan. This was a critical application and the failures greatly impacted plant production.

Issues often occurred during startup from maintenance outages while the fan was being energized or a short time later when the unit had reached speed.

We needed to do two things: On-site evaluation + motor disassembly and RCFA in shop.

The motor was found to have failed due to moisture condensing on the windings. When the units were offline for maintenance, they failed to have the proper accessories to prevent moisture.  D.I.E. installed silicone space heaters on the winding.

On-site inspection found a steam leak in an adjacent area that vented into the room where the motor was sitting idle during the outages. The customer needed to the change the venting to send the energy outside the building for the steam to be released.

 

Lastly, we reviewed the motor for opportunities to improve performance. The OEM winding design was not optimal as it produced high voltage stresses within the stator. D.I.E. redesigned the winding for better performance and increased reliability. The new winding has:

  • 50% less voltage, at 77 volts per coil
  • 300% more phase-to-phase and coil-to-coil insulation
  • Air gap harmonics mitigation 
  • Optimum Chord factor where each turn produces higher torque
  • 10% increase in the circular-mil-per-ampere area of wire allowing the motor to operate cooler under the same load due to reduced copper losses
In this case study, RCFA was necessary in-shop as well as at the customer site to identify and isolate issues. The savings in motor repair or replacement cost alone is significant.
  • Rewound/Repaired Motor  $15,000
  • Replacement Motor $25,500