Heat Gain from Electric Motors and their Loads Formula and Calculator

Related Resources: calculators

Heat Gain from Electric Motors and their Loads Formula and Calculator

Heat Transfer Engineering and Design
Electric Motors Design and Engineering

Heat Gain from Motors and their Loads Formula and Calculator

Preview Heat Gain from Electrical Motors Calculators

Instantaneous rate of heat gain from equipment operated by electric motors within a space.

q_em = ( P / E_M ) · F_UM · F_LM

Where:

q_em = heat equivalent of equipment operation, kW
P = motor power rating, kW
E_M = motor efficiency, decimal fraction < 1.0
F_UM = motor use factor 1.0 or <1.0 (proportion operating)
F_LM = motor load factor 1.0 or <1.0

When motor is outside the conditioned space, but load is inside,

q_em = P · F_UM · F_LM

When motor is inside the conditioned space, but load is outside,

q_em = P [ ( 1.0 - E_M ) / E_M ] · F_UM · F_LM

Heat output of a motor is generally proportional to motor load, within rated overload limits. Because of typically high no-load motor current, fixed losses, and other reasons, F_LM is generally assumed to be unity, and no adjustment should be made for under loading or overloading unless the situation is fixed and can be accurately established, and reduced-load efficiency data can be obtained from the motor manufacturer.

Unless the manufacturer’s technical literature indicates otherwise, motor heat gain normally should be equally divided between radiant and convective components for the subsequent cooling
load calculations.

Derived from resources provided by:

ASHRAE Pocket Guide for HVAC, 2013