## Significance of Kv

Electric Motors and Controllers
pengyou   1 kW

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Joined: Oct 27 2007 10:38am
Location: Beijing

### Re: PMDC Motor theory - formulae etc.

Thanks! I have just been researching this and tadaaa! you put it up! Clairvoyance, for sure

I have one question about motor specs. I have read that Kv is rpm per each volt at "no load". My understanding of "no load" is that the motor is not hooked to anything, i.e. propeller, bike gear, etc. Why is this statistic useful?

AussieJester   100 GW

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Location: Perth Western Australia

### Re: PMDC Motor theory - formulae etc.

pengyou wrote:Thanks! I have just been researching this and tadaaa! you put it up! Clairvoyance, for sure
AutoBoT!!! Miles

Thanks Miles..i was lost not long after the word 'etc.' though

KiM

Miles   100 GW

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Joined: Mar 16 2007 4:15pm
Location: London UK

### Significance of Kv

Kv is the velocity constant.

With the 3 basics: velocity constant, no-load current and phase resistance, you can calculate how a motor will perform, how efficient it is etc.

If you know Kv you also know Kt (torque constant). Using Nm for torque and radians/sec for velocity: Kt = 1/Kv

liveforphysics   100 GW

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Location: Santa Cruz, CA, USA

### Re: PMDC Motor theory - formulae etc.

pengyou wrote:I have one question about motor specs. I have read that Kv is rpm per each volt at "no load". My understanding of "no load" is that the motor is not hooked to anything, i.e. propeller, bike gear, etc. Why is this statistic useful?

It's the RPM that the motor is not capable of any torque production. So, you learn a whole lot of things from this right off the bat. Due to the relationship between voltage and current, and RPM and torque, it lets you roughly know the torque constant of the motor (roughly because many of the efficiency losses increase at non-linear rates, so the measured KV is not a perfect relationship to calculate torque, but it's pretty darn close.)

Next, assuming you're not at a stator saturation point, you learn the RPM that the motor is able to make it's power peak, simply 1/2 of the KV*V.

If you know these points, and you know the power curve is an inverted parabola with the known power peak point as it's maxima, and you know power equal zero at peak RPM (KV*V), then you can plot the power vs RPM curve, which you can plot the torque curve from, because if you know power and RPM, you know torque, and if you know the no-load current, and the motor resistance, you can fill in the amps scale for that torque line, which then knowing amps, voltage and power, you can draw the efficiency curve, and have a pretty good idea of the behavior of the motor, just from knowing the KV, no-load current, and motor resistance.

*** In real life though, motors have saturation points, so the low RPM area of the curve actually has a flattening out point in it's efficiency and torque numbers, and in the case of very high RPMs, the curve can get pretty skewed and distorted by the squared and cubed growth functions of some of the losses.
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pengyou   1 kW

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Joined: Oct 27 2007 10:38am
Location: Beijing

### Re: PMDC Motor theory - formulae etc.

Wow! ....and I thought that Chinese was difficult! Ã¦Ë†â€˜Ã¤Â¸ÂÃ¦ËœÅ½Ã§â„¢Â½

liveforphysics wrote:
pengyou wrote:I have one question about motor specs. I have read that Kv is rpm per each volt at "no load". My understanding of "no load" is that the motor is not hooked to anything, i.e. propeller, bike gear, etc. Why is this statistic useful?

It's the RPM that the motor is not capable of any torque production. So, you learn a whole lot of things from this right off the bat. Due to the relationship between voltage and current, and RPM and torque, it lets you roughly know the torque constant of the motor (roughly because many of the efficiency losses increase at non-linear rates, so the measured KV is not a perfect relationship to calculate torque, but it's pretty darn close.)

Next, assuming you're not at a stator saturation point, you learn the RPM that the motor is able to make it's power peak, simply 1/2 of the KV*V.

If you know these points, and you know the power curve is an inverted parabola with the known power peak point as it's maxima, and you know power equal zero at peak RPM (KV*V), then you can plot the power vs RPM curve, which you can plot the torque curve from, because if you know power and RPM, you know torque, and if you know the no-load current, and the motor resistance, you can fill in the amps scale for that torque line, which then knowing amps, voltage and power, you can draw the efficiency curve, and have a pretty good idea of the behavior of the motor, just from knowing the KV, no-load current, and motor resistance.

*** In real life though, motors have saturation points, so the low RPM area of the curve actually has a flattening out point in it's efficiency and torque numbers, and in the case of very high RPMs, the curve can get pretty skewed and distorted by the squared and cubed growth functions of some of the losses.