# About Power Electronics

I have a question about an exercise. An electric resistance space heated at 1500W for a voltage source of V(t)= 120(2)^(1/2)sin(2pi60t)V has a thermostatically controlled switch. The heater periodically switches on for 5 min and off for 7 min.

My question is...is the power 1500 W the average power? Or the power 1500w is the instantaneous power per second; that is, instantaneous power P(t)= 1500*t while the switch turn on. Or the 1500W has another meaning? Thanks.

Sorry..I lacked something...the Problem is

"An electric resistance space heater rated at 1500W for a voltage source of V(t)= 120(2)^(1/2)sin(2pi60t)V has a thermostatically controlled switch. The heater periodically switches on for 5 min and off for 7 min.

Thank you for answering my question.

What I thought is the same as yours. However, I have another question. It's so weird that if 1500W is the power during the heater's turning on,then I can compute the resistance by the voltage source.Then I found the resistance is 4 ohm.

After that, I can compute the instantaneous power P(t)= V(t)^2/R =7200*(sin(2pi60t))^2.Then the maximun instantaneous power should be 7200W, right? (Becasue (sin(2pi60t))^2 is

equal or smaller than 1)

Therefore, I got confused why the circuit was designed the way that theinstantaneous power is bigger than the average power?(average power should be 1500W because when time is 0 to 5min, the power is 1500W, and time is 5min to 12min the power is 0. )

That is, I need to waste more power during heater's turning on, and then just get fewer power finally. Am I wrong on this conclusion? Anyway, thank you so much. Have a nice day.

Sincerely yours , Mark.

### 2 Answers

- 1 decade agoFavorite Answer
I believe that the power 1500 watt is the power rating of the mentioned heater at 'power on'. The power consumption will be zero at 'power off'. The reason that the heater has to be turned off 7 min after 5 min 'turn on', is to cooling down the hearter. This is called the duty-cycle, otherwise the heater will be over-heated and burned.

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- Anonymous6 years ago
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