In later years, motors improved and could be run directly from a 60 Hz power supply. As a result, 25 Hz power systems shrank and disappeared. However, there were many perfectly-good working 25 Hz motors in factories around the country that owners were not ready to discard. To keep them running,

Question

In later years, motors improved and could be run directly from a 60 Hz power supply. As a result, 25 Hz power systems shrank and disappeared. However, there were many perfectly-good working 25 Hz motors in factories around the country that owners were not ready to discard. To keep them running, some users created their own 25 Hz power in the plant using motor-generator sets. A motor generator set consists of two machines connected on a common shaft, one acting as a motor and the other acting as a generator. If the two machines have different numbers of poles but exactly the same shaft speed, then the electrical frequency of the two machines will be different due to Equation (3-34). What combination of poles on the two machines could convert 60 Hz power to 25 Hz power?

$f_{s e}=\frac{n_{s m} \ P}{120}$ (3-34)

[latex]f_{s e}=\frac{n_{s m} \ P}{120}[/latex] (3-34)

Accepted Answer

From Equation (3-34), the speed of rotation of the 60 Hz machines would be

[latex]n_{s m 1}=\frac{120 f_{s e 1}}{P_1}=\frac{(120)(60 \ Hz )}{P_1}=\frac{7200}{P_1}[/latex]

and the speed of rotation of the 25 Hz machines would be

[latex]n_{s m 2}=\frac{120 f_{s e 2}}{P_2}=\frac{(120)(25 \ Hz )}{P_2}=\frac{3000}{P_2}[/latex]

If the machines are tied together in a motor-generator set, the shaft speed must be the same for both machines, so

[latex]\begin{aligned}& \frac{7200}{P_1}=\frac{3000}{P_2} \& \frac{P_1}{P_2}=\frac{7200}{3000}=\frac{72}{30}\end{aligned}[/latex]

This result says that a 72-pole machine at 60 Hz would rotate at the same speed as a 30-pole machine at 25 Hz. Each machine would rotate at 100 r/min

[latex]n_{s m 1}=\frac{120 f_{s e l}}{P_1}=\frac{(120)(60 \ Hz )}{72}=100 \ r / min\n_{s m 2}=\frac{120 f_{s e 2}}{P_2}=\frac{(120)(25 \ Hz )}{30}=100 \ r / min[/latex]

Question 3.11: In later years, motors improved and could be run directly fr...

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