Page 350 - Improving Machinery Reliability

316 Improving Machinery Reliability

For NEMA performance ratings, ambient temperature is assumed to be 40°C. The
ambient temperature of actual applications may be higher or lower, affecting the
amount of temperature rise that the insulation system may withstand without exceed-
ing its class limits.
Most of the temperature rise results from watt losses, the power required to mag-
netize the stator and rotor and convert electrical energy to mechanical energy. Nev-
ertheless, frictional heat from poor bearing lubrication may be a contributing factor.

Watt Losses = electrical energylN - mechanical energyouT

The rate of heat generation (dH/dt), as the current passes through the windings and
rotor, increases in proportion to the square of the current (I2) and the resistance of
the wire and rotor bars (R).
dH/dt = 12R

While the resistances of the windings and the rotor remain relatively constant, the
amount of current increases along with the load. Hence, the motor runs progressively
hotter as the load increases.
Motor ventilation systems are designed to achieve “watt loss equilibrium” and dis-
sipate enough heat to keep the insulation within its temperature limits-as long as
the motor remains within its rated load capacity (rated horsepower) and service fac-
tor.
The capacity of the ventilation system is determined by the thermal conductivity
of the materials (aluminum versus cast iron and steel), degree of mechanical contact
with radiation surfaces, available radiation surface area (stagger-stacked laminations,
fins, etc.), and amount of air flow over the radiating surfaces. Air flow is affected by
fans, shrouds, baffles, etc.
If the motor is loaded beyond its rated capacity and service factor, the added heat
from greater watt loss can exceed the capacity of the ventilation system, raise the
temperature of the insulation beyond its rated limits, and rapidly destroy it.
The greatest amount of current flow, and thus heat generation, in a motor occurs
during locked rotor conditions or at the moment of start-up before the motor begins
to generate “back electromotive force.” Rotor current at start-up in a NEMA Design
B motor is approximately six times as high as it is at its rated load. With some of the
newer high-efficiency designs, it may reach eight to ten times full-load current (see
Figure 6-2).
At six times the current, heat is generated 36 times as fast (I2 = 62 = 36) as under
full-load conditions. This is why repeated starts in a short period of time can be so
damaging to motor life; the heat builds up faster than the ventilation system can dis-
sipate it. Also, difficult-to-start loads that require a long time for the motor to reach
synchronous speed create excessive heat buildup.