kv's timing and real life....

[jimmyhorns]

Just thought I would post a few observations from my recent work with motors. Some of it will probably be common knowledge, but just thought I would share.

1) what is printed on the motor wont be the 'real' kv.

2) different manufacturers must calculate/evaluate kv differently - a 500kv scorpion runs faster than a 510kv align for example.

3) kv isnt consistent on a per volt basis

4) timing can make a massive difference (up to about 10%), especially auto-timing methods

As I said, none of it really is particularly suprising, but just thought I would share... interestingly none of the stock motors I have tested so far come out with a kv above what is printed, they are all below by varying amounts depending on timing frequency used.

This is becoming a bit of a headache for me when it comes to the 'new venture'. If someone needs a motor slowing down, say 560kv to 500kv, perhaps im going to have to change it proportionally if when on testing the 560 turns out to be a 530!

James

[dieselcarb]

in that scienario your right but manufacturers lie to a degree to sell this is common knowledge and not always iliegal. timing from 1 motor to the next will always have a different affect and don't for get heat kills the magnatizum in the magnets. if someone want a 500 kv motor I assume they have done the maths rather than oh thist motor spins too fast but if somebody want a 4000kv motor and they get 4000kv compared to a shop moter they gonna be more than happy

just remember a stronger magnet yields more torque and the gap between magnets does something special too (just cant for the love of money remember) :P

[dieselcarb]

oh yeah also a hand wound motor will always massively out perform a MACHINE wound back in the day we would buy machine wound corally motors as these were half the price of hand wound. being carpark bashers this was perfect but on the track it was the edge to get hand wound

[jimmyhorns]

Yes it wasnt really a moan about the manufacturers lying, more that until now I had taken for granted that my 600mx was a 510kv motor, and that if I wanted, I could slot in any other 510kv motor and wouldnt have to think twice about my gearing/gov headroom.

[trillian]

Yes it wasnt really a moan about the manufacturers lying, more that until now I had taken for granted that my 600mx was a 510kv motor, and that if I wanted, I could slot in any other 510kv motor and wouldnt have to think twice about my gearing/gov headroom.

Yeah, that's annoying because we choose a motor with the claimed KV usually being an important part of the decision.

In my experience in practice I often find the headspeed I'm getting is higher than I expected based on the motor KV and using the various formulas or headspeed calculators.

[jimmyhorns]

Yeah, that's annoying because we choose a motor with the claimed KV usually being an important part of the decision.

In my experience in practice I often find the headspeed I'm getting is higher than I expected based on the motor KV and using the various formulas or headspeed calculators.

I guess part of the problem is that kv is determined off load, so freely spinning. Its therefore difficult to work out what that would then equate to in real terms once a load is applied, and thats going to vary from one motor to the next, yet again.

[cjcj1949]

Perhaps we could start by getting the units right. Shouldn't it be k per v? k/v . My first reality check was with rubbish gyros, called competition class. Things get better all the time though.

[trillian]

Perhaps we could start by getting the units right. Shouldn't it be k per v? k/v .

Sorry but no, the use of 'KV' is very well established in the labelling / terminology of brushless motors and it's entirely proper. We think of it as meaning rpms per volt but the term actually comes from being an abbreviation for a velocity constant.

[jimmyhorns]

Perhaps we could start by getting the units right. Shouldn't it be k per v? k/v . My first reality check was with rubbish gyros, called competition class. Things get better all the time though.

No, sorry.

Its an established system for defining the 'constant' that is rpm per input volt. As I have elluded to, its not a constant. Some companies use KV, some use kv, and I havent seen rpm/V or k/V written on any motors so far.....

you could call it c, s, alphabettyspagetti, whatever you like, its just a constant

[pgkevet]

I guess part of the problem is that kv is determined off load, so freely spinning. Its therefore difficult to work out what that would then equate to in real terms once a load is applied, and thats going to vary from one motor to the next, yet again.

I don;t see there's any oher way of doing it.. you couldn't have a standard load since clearly a 450 size motor hasn't the oomph of an 800 class. To create a load related to power output brings in a host of the other variables. about the only thing you could use load for is determining power output and when you burn your motor out and efficiency...
And you know there's never no load since the motor bits and it's bearings provide some.

[cjcj1949]

Is it the output volts at 1000 rpm?

[Snaaakeey]

Diffferent ESC brands effect apparent kv as well. I remember seeing a scorpion document listing all their popular motors and the kv they make with different brand ESCs. YGE made the highest on comparable settings.

[trillian]

Is it the output volts at 1000 rpm?

The letters K and V as applied here do not relate to volts or 1000 as in 1K of something. The K is being used as a symbol for a constant and the V is for velocity.

[cjcj1949]

Motor constants - Wikipedia, the free encyclopedia

So the motor constant K subscript v is the motor velocity constant which has units of rpm per volt. So we use a corrupted constant terminology with no units stated. The generated voltage should be very close to the noload rpm when driven at that voltage from simple theory. There will be losses though that will make the two interpretations different.

[Gixxer]

Kv is a calculation of how many RPM the motor needs to rotate at to produce 1V of back emf. When this voltage is equal to the input voltage, the motor can't be spun any faster, and that determines the Kv of the motor.

If the motor was 100% efficient, then the motor RPM would = the input V multiplied by the Kv rating but motors aren't 100% efficient, so an RPM equation would need to include the motor efficiency.

Or something like that.