Motor questions for you EE's
03-19-2007, 02:36 PM
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Motor questions for you EE's
Why do they rate some electric motors in kV? I'm not familiar with that term. Watts is simple W=Vi. With A/C there is KVa which takes into effect reactive current, but there is no reaction for DC. Does kV account for slip or something? Also, how does the speed control control the motor's speed, by reducing voltage? Does anybody know how to calculate rpm for a DC motor given V, i, and e.
I am confused, b/c I work with A/C motors. I am a plant engineer and we have a lot of VFD's floating around. So I know with an A/C motor that the motor must turn at a given rpm at a given frequency and it will just draw as much current as it needs . How does a DC motor act?
I am confused, b/c I work with A/C motors. I am a plant engineer and we have a lot of VFD's floating around. So I know with an A/C motor that the motor must turn at a given rpm at a given frequency and it will just draw as much current as it needs . How does a DC motor act?
03-19-2007, 02:49 PM
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RE: Motor questions for you EE's
I am an electrician but it has been a while since school. I seem to remember number of poles being part of the speed equation?
KV rating, although I don't know why KV is used because in electronics KV would suggest Kilovolt, is RPM per volt. so lets say a motor rated at 3000kv will turn at 30,000 RPMs at 10 volts (in an ideal situation) this gives you a ballpark figure to work with so you will know, given your choice of batteries, what gears you need to get a specific headspeed on your heli or prop speed on an airplane.
KV rating, although I don't know why KV is used because in electronics KV would suggest Kilovolt, is RPM per volt. so lets say a motor rated at 3000kv will turn at 30,000 RPMs at 10 volts (in an ideal situation) this gives you a ballpark figure to work with so you will know, given your choice of batteries, what gears you need to get a specific headspeed on your heli or prop speed on an airplane.
03-19-2007, 03:06 PM
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RE: Motor questions for you EE's
That clears up the kV thing and helps me alot.
Yes, the motor speed has something to do with the number of poles. Synchronous speed = frequency x 120 / poles, but that's for A/C. Hence my question.
I'm still a little at a loss for understanding how a DC motor responds to loading, etc. I've been able to figure out that the rpm ~ Vi/T where T is the torque. Presumably you drop V through the speed control then the thing slows down.
Yes, the motor speed has something to do with the number of poles. Synchronous speed = frequency x 120 / poles, but that's for A/C. Hence my question.
I'm still a little at a loss for understanding how a DC motor responds to loading, etc. I've been able to figure out that the rpm ~ Vi/T where T is the torque. Presumably you drop V through the speed control then the thing slows down.
