Subelement I: Power Sources— Topic 62: Motors & Generators
Question 3-62I5
Element 3 (GROL)A 3 horsepower, 100 V DC motor is 85% efficient when developing its rated output. What is the current?
Explanation
To determine the current drawn by the motor, we first need to calculate its mechanical output power in watts, then account for its efficiency to find the electrical input power, and finally apply the DC power formula.
1. **Convert horsepower to watts:** One horsepower (HP) is equivalent to 746 watts.
Output Power (Po) = 3 HP * 746 W/HP = 2238 W
2. **Calculate electrical input power (Pi):** Efficiency is defined as Output Power / Input Power.
Since the motor is 85% efficient (0.85), we can find the input power:
Pi = Po / Efficiency = 2238 W / 0.85 = 2632.94 W
3. **Calculate the current (I):** For a DC circuit, Power (P) = Voltage (V) * Current (I).
Therefore, I = Pi / V = 2632.94 W / 100 V = 26.3294 A
Rounding to three decimal places, the current is approximately 26.300 amps. This calculation demonstrates fundamental electrical power concepts essential for understanding power supplies and loads in amateur radio.
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