Page 355 - Improving Machinery Reliability

Extending Motor Life in the Process Plant Environment 321

Fan Flow vs. Energy Consumption

100% flow requires
100% of power

I 80% flow requires
55% of power

0 20 40 60 80 100 120 140 160
Yo Flow

Figure 6-4. Fan flow vs fan energy consumption. (Source: Lincoln Electric, St. Louis,
Missouri.)

motor is still running at full speed and consuming much of its maximum power
requirement. In these centrifugal loads, the relationship between power consumption
and flow is not linear. As can be seen from Figure 6-4, a fan will still deliver 80% of
its maximum flow at only 55% of its speed. The use of an AC adjustable speed drive
(ASD) gives precise output control, saves energy, and allows the motor to run cooler.
A word of caution: Many of today’s adjustable speed AC drives rely upon pulse
width modulation (PWM) and insulated gate bipolar transistors (IGBTs) to provide the
needed frequency and voltage control. The wave forms generated by these drives can
contain voltage spikes of 1,600 volts and higher. Motors used with ASDs should be
designed to avoid damage to the winding insulation from these high-voltage spikes.

Economics of Oversizing
Oversizing or derating was once a common way of assuring long motor life and
dependability, along with providing for unanticipated load fluctuations or future load
increases. Driving a constant 25 HP load with a 50 HP motor will result in a much
lower heat rise, thus extending insulation and motor life. However, today’s energy
consciousness has discouraged many motor purchasers from oversizing.