Duty Cycle Question

[Traxstar1825]

So I have kinda a dumb question... If a siren is running an attack cycle, would the duty cycle be doubled because the chopper motor is only on for half the time?

[Arteelx]

That's actually an excellent question. I would love to know the answer, too.

[Chem_Boffin_6589]

So I have kinda a dumb question... If a siren is running an attack cycle, would the duty cycle be doubled because the chopper motor is only on for half the time?

Most attack timings are biased to having the motor on than off. (6/4 and 8/4 are good ones) Come to the UK and siren timings here are 4/4 and were 32/4 for flood warnings.

[uncommonsense]

So I have kinda a dumb question... If a siren is running an attack cycle, would the duty cycle be doubled because the chopper motor is only on for half the time?

I don't think it's a dumb question at all. It got me to thinking that's for sure. And ultimately, I don't think that the attack cycle would extend the duty cycle. The reason the duty cycle exists it's because of the amount of heat generated in the motor. If the motor is cycling fairly quickly, as in the attack signal, that wouldn't give the motor ample time to cool down.

[1939_Fedelcode]

I agree with Traxstar.

[Arteelx]

An electric motor draws the most current (hence it produces the most heat) when the rotor is accelerating from rest (or from a slow speed). Once it reaches it's steady-state running speed, the current draw is MUCH less. When the manufacturer specifies a certain duty cycle, he's assuming that the motor is running at it's steady speed (and low current draw) for the vast majority of that time. It's interesting to note that on some very large industrial motors, the duty cycle will usually be 100%...but there will be strict limits on how many times the motor can be started per hour due to the extreme heat that is generated during that initial acceleration.

When a siren is in "Attack" mode, it is true that the motor is de-energized for a good portion of the cycle, which should be beneficial to the duty cycle. However, the downside is that whenever the motor is re-energized, you're now accelerating the siren from a slow speed, which means that the "on time" is much more intense than would be if holding a fixed speed.

I'm not sure how these two competing factors translate into the final winding temperatures, but I don't think it's as simple as just deducting the "off time" from the overall duty cycle. It might actually be harder to cool a motor that is repeatedly being accelerated from a slow speed, despite the fact that it's turn off half the time.

Think of a car cruising down the highway at a nice steady 60 mph. Then think of one that's being floored to reach 60 mph, put into neutral to coast down to 30 mph, and then floored again to repeat the process. It's easy to see which car is having an easy ride, and which one is being severely abused.

[Traxstar1825]

Thank you all for these great answers!