Standard Motor Catalog

Section TR Technical Reference Guide

To properly size a motor for the varying duty cycle described in the preceding horsepower versus time graph, a table should be made as shown quantifying each part of the cycle. The first three columns of the table list data from the graph, and fourth column is derived by squaring the horsepower and multiplying by the time for each part of the cycle. Using this data, sum the periods of time and (Hp) 2 t, and evolve the RMS horsepower as follows: Table 15 Part of Cycle Time Sec. (t) Required Hp (Hp)²t 1 15 32 15,360 2 40 74 219,040 3 30 27 21,870 4 5 32 5,120 5 148 66 644,688 6 200 27 145,800 7 12 32 12,288 8 70 27 51,030 520 1,115,196

The motor must also be capable of carrying the peak horsepower (torque) value from the duty cycle at 90% voltage. Since motor breakdown torque is reduced by the voltage squared, the required breakdown torque (BDT) is determined by the equation:

NP Hp = Nameplate Rated Horsepower %BDT = Percent Breakdown Torque NOTE: Margin of 20% added to prevent inefficiencies of operation too close to actual breakdown torque.

For Example, with a 50 Hp motor at 1.15 SF:

%BDT + [74/50 x 123+20] = 202%

For 60 Hp at 1.0 SF:

%BDT = [74/60 x 123+20] = 172%

RMS Hp = Root - Mean - Square Horsepower

Conclusion: Since the NEMA breakdown torque for a 50 Hp or 60 Hp motor is 200%, the best choice for the application is the 60 Hp rating which only requires 172% at the peak load horsepower.

The load RMS horsepower is used to determine the required motor thermal capability at constant speed. To allow for ±10% voltage variation and the resulting additional motor heating, particularly at peak loads and 90% voltage, a 10% allowance is added to the RMS horsepower calculation:

Required Hp =

RMS Hp x 1.10 46.3 x 1.10 = 50.9 Hp

The motor’s usable horsepower is determined by the nameplate Hp x service factor and must be equal to or greater than the required horsepower. In our example, there is a choice of either a 50 Hp motor with a 1.15 service factor or a 60 Hp motor at a 1.0 service factor. From a thermal viewpoint, either of these ratings is suitable for the required load Hp of 50.9, but the peak load still needs to be considered.


Data subject to change without notice. 02/23 •

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