1a) When a portable fan is first turned on it draws 4.25 A (rms current). After
ID: 1374287 • Letter: 1
Question
1a) When a portable fan is first turned on it draws 4.25 A (rms current). After it has come up to speed it draws 0.60 A (rms current). Assuming the fan runs off a 120 V (rms) outlet, calculate the back emf while the motor is running.
1b) A very long solenoid with a circular cross section and radius r1= 1.20 centimeters with ns= 300. turns/cm lies inside a short coil of radius r2= 4.50 centimeters and Nc= 27 turns.
If the current in the solenoid is ramped at a constant rate from zero to Is= 2.80 Amperes over a time interval of 95.0 milliseconds, what is the magnitude of the emf in the outer coil while the current in the solenoid is changing?
1a) When a portable fan is first turned on it draws 4.25 A (rms current). After it has come up to speed it draws 0.60 A (rms current). Assuming the fan runs off a 120 V (rms) outlet, calculate the back emf while the motor is running. 1b) A very long solenoid with a circular cross section and radius r1= 1.20 centimeters with ns= 300. turns/cm lies inside a short coil of radius r2= 4.50 centimeters and Nc= 27 turns. If the current in the solenoid is ramped at a constant rate from zero to Is= 2.80 Amperes over a time interval of 95.0 milliseconds, what is the magnitude of the emf in the outer coil while the current in the solenoid is changing?Explanation / Answer
1)
a)
120 = 4.25 * R
R = 28.23
120 = E + 0.6*R
E = 120 - 0.6 * 28.23 = 103.062 V
b)
e = A*N2*N1*uo*di/dt
= 3.14*0.045^2*27*30000*4*pi*10^-7*2.80/95*10*10^-3
= 0.1908 V
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