The question was posed: Are efficiency improvements of motors being eaten up by additional transformer losses?
One consultant stated, “It’s difficult to measure savings. You save on efficiency, but you pay on power factor penalty.
“Bad power factor substantially increases costs. You can pay a huge premium.”
A representative from a manufacturer noted that some correction is built into the motors — power factor leading.
A utility employee remarked, “We always talk to customers about upgrading to higher efficiency motors.” However, he related, one customer installed a new high-efficiency motor and found that it wouldn’t start. It pulled nine times the inrush current of the previous motor. The company then had to change to a larger transformer just to start the motor. So a 2% gain in efficiency required the added expense of a new, larger transformer.
A second consultant said, “When you look at the overall system, you need to consider, ‘What does this mean? How do we measure it?’”
The first consultant stated that assumptions regarding transformers are laughable. “Equipment is way oversized for the load.”
Time To Rethink?So do we need to rethink how we size transformers and electrical distribution systems?
The second consultant said that guidelines are based on past experience, not on current systems. “The load profile has changed dramatically, and our rating systems haven’t kept up with it.”
The utility representative commented, “There are different application situations that need to be considered. More information is needed on the application of high-efficiency motors.”
“When you run at part load, power factor drops off quite a bit,” the manufacturer’s engineer said. “Power factor rating is at full load.”
“High-E motors have a higher peak,” said the utility person. Different starting “adds to the expense of the motor.”
With high-efficiency motor design, the engineer explained, “you end up compromising in some area.” You may have to sacrifice “startability” and accept a higher inrush current.
Another attendee asserted, “Bigger transformers negate higher efficiency motors. It’s a wash.”
Are new guidelines needed? The consensus of opinion was yes.
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Publication date: 03/19/2001