Duty cycle, electric motor

For induction motors, the total current can be approximated by the vector sum of the torque producing component and the magnetising or flux producing component of motor current. These two current vectors are in quadrature, and as the hoist torque approaches zero, the motor current approaches magnetising current. Electrical RMS calculations for hoist motors must be based on motor current, and so the calculation based on torque current only is not accurate for induction motors, and under-estimates the induction motor current. For greatest accuracy the duty cycle must be calculated with the correct motor characteristics. 

Motors are classified by size, apphcation, electrical type, NEMA design letter, and environmental protection and cooling methods. They are rated for special standard environmental and operating service conditions by performance and mechanical configuration voltage and frequency, locked-rotor kVA, service factor, horsepower, speed, torque, locked-rotor current, performance, temperature rise, duty cycle, and frame size. 

Choose an appropriate service factor for the power soiuce, type of driven load, daily duty cycle, and operating conditions. Table 7-1 provides a list of base service factors for a few different types of eqnipment. For example, a centrifugal blower application powered by an electric motor would have a service factor of 1.3. Extensive lists of base service factors are available in the ASME B29.2 standard and from most chain manufacmrers. 

The power fed to the fan shaft (or the total electrical power fed to the motor if within the duct) appears as sensible heat in the system. During a heating cycle this is useful. As a mle of thumb, fan shaft power is 17 x flow m x mm swg watts. This commonly lies between 2 per cent of the cooling duty for small systems to 10 per cent for large systems. 

Vehicles with high cycling duty (>0.1Ah km ). These include taxis with a comprehensive array of electrical consumers. A similar requirement is exhibited by emergency vehicles such as ambulances and by motor homes, where cycling duty is of major importance due to extended engjne-off periods with electric equipment in operation. Operating temperatures should, however, be moderate. 

While it is generally accepted that the motor electrical thermal cycle be approximated with a steady state calculation, this assumption ignores the shorter time constant of the drive, as well as some parts of the motor. Close attention must therefore be paid to the short term RMS duty of the drive inverter. 

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