KV rating
 

How is the rating deturmined,and wgat is the motors KV out put when boosted/ thanks!

A motor's Kv rating is the so-called "back EMF constant". It can be measured by spinning the motor at a known speed and measuring the peak voltage across any two of the output terminals; since the units of Kv are RPM/V, the math is pretty simple. It can also be calculated by using the physical geometry of the motor (stator "shoe" surface area, magnet thickness, stator/rotor airgap width) as well as the magnet material properties. This will get you in the ballpark (within 15% or so); much closer approximations can be made via the use of generic magnetic FEA modeling software or motor-specific software (SPEED is quite a nice tool for this).

The motor Kv does not change when "boosting" (using phase advance), as the Kv parameter is an property of the motor itself. However, the effects can be approximated by dividing the motor Kv by the cosine of the electrical phase advance angle, although there are several flaws with this approach.

Just as an example, a motor with a Kv of, say 5000 RPM/V might be run with 25 degrees of advance, and thus one might assume that it would have roughly the same performance as a motor with a Kv of 5500 RPM/V. But this neglects the fact that the motor with the higher Kv should also have less winding resistance (approximately .82x that of the 5000 Kv motor), and thus would have a substantial advantage in efficiency.

Hope this helps!

Thanks,if kv is rpm/volts,my 10-5 is about 3300 kv? when it is boosted my motor doesn't turn more rpm's?same gearing with both yet double the acceleration and top end when boosted.You gave a great explanation but my though cognisant ability has diminished over the years

Kv ratings are done at a set and known timing point, this can and will vary by brand though. When you add timing/boost I would think the Kv would change. I don't know that there is way, on paper, to say "If I add 15deg of timing my kv will go from 3500 to 3xxx."

from a racer stand point, your ending Kv/RPM isn't the main concern when adding timing.

Nope - Kv has nothing to do with the controller. It is a property of the motor itself.

Once again, the Kv does not change - it is call the "speed/voltage constant" for a reason!

The effects of adding phase advance ("boost" or "timing") is indeed predictable, but to model its effects properly requires some knowledge of motor and controller behavior that is not easily accessible to a hobbyist.

This is an accurate statement. How the car performs is the only concern; don't spend a lot of timing trying to calculate things that have very little meaning to an end user. If you're trying to design a motor/controller system, then that is another matter altogether.

Not sure what brand motor that you are using. Different models of LRP 540-size motors range from 3600 RPM/V (Vector and X12 StockSpec) to 4100 RPM/V (Eraser). The difference here is the rotor; the Vector and X12 use a sintered rotor, which typically provides more flux and thus lowers the Kv.

Make no mistake - while phase advance does not alter the Kv of a motor, it can provide huge performance improvements (usually at the cost of efficiency, but not always). It's a great tuning aid, but it's also not an exact substitution for a higher-Kv motor.

I was referring to motor timing when companies measure Kv ratings, nothing to do with esc timing.

You seem to know quite a bit about this, so do you have a layman's version of what's going on when you add timing. Not asking how a boosted esc works, I get that but adding timing increases RPM and normally looses torque. The general public seems to think increased RPM means higher kv.

That's because the general public typically looks at the motor and the battery and thinks there's a direct feed between the two. I think most people may be thinking of "kv" in the wrong way. As the guy said, it's an (indirect) physical property of the motor. In a hopefully non-blowhardical way, you cannot change kv without physically altering the motor itself.

Kv should be derived from doing a measurement of back EMF, which is essentially a measurement of how the motor performs as a generator. When performing this test on projects in the past, I use the method stated in my first post in this thread. Sensor timing has absolutely nothing to do with this.

If Kv is calculated by the maximum unloaded speed when driven by an ESC and a supply of fixed voltage, then timing is but one source of measurement error - the various losses in the motor (hysteresis and eddy-current losses in the stator steel, windage and friction losses from the rotor, ohmic losses in the stator winding, etc.) will cause significant variation in this measurement. I would not be satisfied with this type of measurement.

Basically, the "ideal" operation of a brushless motor assumes that the current in the winding rises and falls in phase with the back EMF. This best utilizes the flux linkage between the rotor and stator, and would be the definition of "neutral" timing. To do this, the ESC applies voltage of the right magnitude and phasing to the winding (as a side note, the voltage and current are not in phase due to winding inductance, and thus the ESC must advance the voltage as speed and load increases just to maintain "neutral" timing, and any shift of the rotor magnetic field with increased load may also require additional ESC timing).

Now, as the motor's back EMF increases with higher shaft speed and approaches the battery voltage, less current can be induced in the winding and thus torque decreases. If the phase of the applied voltage is shifted ahead in time ("advanced"), we can get some more current to flow in the winding before the back EMF rises during each cycle. Since this current is not properly timed with the flux linkage, it is not quite as efficient at producing shaft torque - but the net effect is still an increase in overall mechanical output power. This is what makes the motor act like it has a higher Kv constant. It still has the additional winding resistance of a motor with lower Kv, though, and if you combine this with the additional current that we are putting through the winding, the losses can be quite considerable. This is why adding timing is not the same as switching to a motor with higher Kv.

As a side note, this idea of using phase advance has been used in much larger permanent-magnet motors for a least a few decades, and the rotor designs of such motors are actually designed such that the motor speed can be increased more than 2x with large amounts of phase advance.

So the rating kv/wind size of the brushless motors is not applicable,and the rpm's do not change when the motor is boosted as the older brushed motors dropped in winds.Thanks for the info,albeit over my head.

No, that is not what I am saying - the speed/torque curve of the motor does indeed change as the timing is changed, similar to how a brushed motor responded (the major difference being that brushed motors had only one timing setpoint, where as brushless motors can have a continuously-varying advance curve).