Training Manual on Energy Efficiency - APO Asian Productivity ...

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Energy Efficiency in Electric Motors
replace it, because it is a cheaper option; but the disadvantage is that there
is not much quality assurance. During a motor failure, or in the stripping of
the winding from the stator core prior to rewinding, heating involved causes
exposure to high temperatures. These temperatures can, in many cases, affect
the electrical characteristics of the stator core steel and result in increased iron
losses and lower motor efficiency.
The only area of possible loss reduction available during the rewinding
process is the copper (l2r) losses in the motor. Reduction is achieved by
rewinding the motor with a larger-diameter wire so that the effective conductor
area is increased, where slot area permits, resulting in a lower winding
resistance. The effect is to lower the stator copper loss.
Rewinding for lower copper losses is possible only if additional slot space is
available in the original stator slot, often released by use of new, higher class
of insulating material replacing older, lower grade insulation. It follows that not
all motors are candidates for efficiency improvement by rewinding.
Many industries operate with a population of 40%–50% rewound motors,
and it would be useful to evaluate all such motors and introduce a phasingout
program and a quality assurance program for motor efficiency based on
measurement of load losses, stator resistance values, etc.
Optimize transmission efficiency:
Motor power transmission equipment, including shafts, belts, chains, and
gears, should be properly installed and maintained. When possible, use flat
belts in place of V-belts. Helical gears are more efficient than worm gears; use
worm gears only with motors under 10hp. As far as possible, it is better to
have a direct drive, thus avoiding losses in the transmission system.
Operate motors loaded in star mode:
When a delta motor is connected in star mode, the voltage across the windings
is reduced with a corresponding reduction in flux produced, current drawn, and
torque produced; in essence, the motor is de-rated electrically, with least cost.
Star mode operation is applicable as an energy-saving opportunity
with motors loaded to less than 50% of rating, as a low-cost alternative to
replacement.
Use soft starters for induction motors:
AC induction motors are frequently used at fixed speeds, and the
overwhelming majority of these are powered by direct on-line (DOL) starters
and Star Delta starters which are characterized by jerky startup operations,
high transient currents, maximum demand surges, and possible damage to
motor windings and insulation. Soft starters use electronic controls, PLC, to
gradually increase the voltage applied to the motor, starting slowly and limit
start up currents. Since voltage is reduced, current also comes down, and
there is reduction in iron and copper losses. Power factor is improved as the
reactive current is reduced due to lower flux strength requirements, which in
turn results in a higher power factor. (The starting current with a soft starter