Significance of Kv

Electric Motors and Controllers
Post Reply
pengyou   1 kW

1 kW
Posts: 340
Joined: Oct 27 2007 10:38am
Location: Beijing

Re: PMDC Motor theory - formulae etc.

Post by pengyou » Aug 07 2010 7:31pm

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

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?

User avatar
AussieJester   100 GW

100 GW
Posts: 8736
Joined: Mar 11 2008 3:33am
Location: Perth Western Australia

Re: PMDC Motor theory - formulae etc.

Post by AussieJester » Aug 07 2010 8:57pm

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

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

KiM

User avatar
Miles   100 GW

100 GW
Posts: 11030
Joined: Mar 16 2007 4:15pm
Location: London UK

Significance of Kv

Post by Miles » Aug 08 2010 2:52am

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

User avatar
liveforphysics   100 GW

100 GW
Posts: 14137
Joined: Oct 29 2008 1:48am
Location: Santa Cruz, CA, USA

Re: PMDC Motor theory - formulae etc.

Post by liveforphysics » Aug 08 2010 5:23am

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.
Each carcinogen vapor exposure includes a dice roll for cancer.

Each mutagen vapor exposure includes a dice roll for reproductive genetic defects in your children.

Each engine start sprays them into a shared atmosphere which includes beings not offered an opportunity to consent accepting these cancer experiences and defective genetics life experiences.

Every post is a free gift to the collective of minds composing the living bleeding edge of LEV development on our spaceship.

pengyou   1 kW

1 kW
Posts: 340
Joined: Oct 27 2007 10:38am
Location: Beijing

Re: PMDC Motor theory - formulae etc.

Post by pengyou » Aug 08 2010 6:24am

Wow! ....and I thought that Chinese was difficult! 我不明白

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.

Post Reply