March 2012 Electrical Apparatus

March 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus March 2012 featured technical article,  by Richard L. Nailen, P.E.    

A basic decision when purchasing an a-c motor is the choice of power rating. For a given speed and voltage, the larger the output rating, the lower the temperature rise at any load, and the more mass to absorb acceleration heating.

However, three conditions can make the choice difficult. First, the power required by the driven machine may be uncertain. It may involve fluctuations hour by hour or day by day. The motor must be able to sustain peak loads while operating much of the time at a lower output. Second, the process may change in the future, and the load with it. Third, the application may require frequent starting and stopping, or alternate between loaded and unloaded running. The IEC 60034 standards define most such variations (S1 through S10).

In the United States, uncertainty about load has been dealt with in two ways. One is deliberate oversizing. Confronted with “rapid cycling,” one user of a 2.2 kW motor was advised to change to the next larger standard rating of 3.7 kW. Some studies indicate that most motors operate at no more than 2/3 of their nominal power ratings. An advantage is that efficiency tends to be somewhat higher at that reduced load than at full load. The disadvantage (besides cost) is that accelerating current is higher, which adversely influences voltage drop on the system.

Users may choose to avoid this issue by specifying a lower motor rating with a service factor to sustain a higher load. Although not uncommon in the United States, service factor is not meant to allow continuous overload. And it will not add thermal capacity for frequent starts.

Any motor subject to on/off cycling requires accurate definition of the nature of each acceleration and deceleration, including the load inertia and torque characteristics. Simply deciding that a 15 kW motor should be used in place of a 7.5 kW unit is unlikely to be a cost-effective choice. 

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