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-   -   Kv - voltage vs freq. (https://www.wattflyer.com/forums/showthread.php?t=79671)

Bigron 12-05-2018 12:48 PM

Kv - voltage vs freq.
If the Kv rating of a brushless motor is based on voltage and the motor speed is determined by frequency, how is Kv determined? Is it at a set frequency? At a given frequency does and increase of voltage (lets say 3s vs 4s battery) increase motor speed? Does an ESC have a max frequency? What frequency is half throttle? Just trying to increase my understanding of motor speeds.

Wildflyer 12-06-2018 05:54 AM

ron_van_sommeren is the master of motor tech. He can answer far more than I can.

KV rating of a brushless motor is a complex thing.

Things like the strength of the magnets affect the KV of the motor, and many other things.

An ESC applies a burst of power to 2 leads of the motor then sort of looks at the signal it gets from the motor on the 3rd wire, this lets the ESC know what relation the magnets and poles of the motor are in. from here on out it gets real complicated.

Looking at the current you may see on a watt meter is a bit deceiving, that is the average current. Actually the motor is receiving short burst's of full battery voltage across the windings, so it is a series of full current pulses and times with no current at all. These average out to what you see on a meter.

KV works kind of like this, If you have a motor rated at 750 KV, it would turn (in theory) 750 RPMs with 1 volt applied, if you applied 10 volts it would turn 7500 RPM, 20v = 15000 rpm, these are no load voltages. Running with a prop you get around 75% of those speeds.

Trying to come up with a frequency number may be kind of hard.

There are limits as to how fast the chips in a certain ESC can do all the necessary switching, so some ESC's may allow a certain motor to go faster. BUT many of the ESC's I have say they can handle an inrunner 2 pole motor at up to 220,000 RPM, so I don't think I will worry about the high end limits.

The ESC responds to the pulse from the transmitter, 1/1000 of a second is off on the ESCs I have and 2/1000 is top end. Just what would give you 1/2 the RPM of the top may not happen at 1500 ms, that is why higher end Tx's have a throttle curve adjustments. I don't think there is a freq. that is a standard 1/2 throttle

Make a search for Ron's posts on how these crazy things work. He has many in-depth things to read. Brushless motors and ESCs are a wonderful thing for the hobby. Once you start to understand the workings of an ESC, you will wonder how come they are not at least $100.00

ron_van_sommeren 12-07-2018 07:04 PM

/* Oops. Wrong planet. */

ron_van_sommeren 12-07-2018 07:07 PM

The controller is not a VFD, it's the motor that 'tells' the controller when to switch/commutate, via the BEMF signal.

A brushless motor is a brushed motor with electronic switching (the ESC) instead of mechanical switching (commutator+brushes).

A brushless motor+ESC behaves the same as a brushed motor, same formulas for rpm, current and power drawn apply.

Kv says nothing about a motors rpm, max.torque, max.current or max.power.

It's all about what the motor wants to do versus what the motor can do.
Kv matches rpm and voltage, nothing more.
  1. Kv and voltage determine how fast motor wants to run. Kv matches rpm and voltage, nothing more.
    rpm_noload = voltage Kv
  2. Rpm and prop determine torque, torque in turn determines current.
    current = torque Kv (in SI units!)
  3. Max.current and max.power determine whether that voltage/motor/rpm/prop combo can run without going up in smoke.

ron_van_sommeren 12-07-2018 07:07 PM

An excellent quote from
brushless motors Kv?.

Originally Posted by scirocco (Post 27290157)
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.

  1. Decide your peak power requirement based on the weight of the model and how you want to fly it.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.

The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

ron_van_sommeren 12-07-2018 07:12 PM

Several simple methods for determining Kv
www.bavaria-direct.co.za (RCG user Skylar)
→ Motor constants
→ Kv constant

ron_van_sommeren 12-07-2018 07:17 PM

Kv has a huuuuge effect on current and power drawn. Bigly.

Motorcurrent is proportional to pitch, voltage, Kv and diameter⁴.
Power-drawn is proportional to pitch, voltage, Kv and diameter⁴.

Without the exponentation
extra current with one or two cells added, simple table

So, changes in setup (and lousy Kv specifications!) can have surprisingly considerable/huge effects.
E.g. doubling voltage will four(2)fold current, doubling Kv will eight(2)fold current, and doubling prop diameter will sixteen(2⁴)fold current.
Even a small 10% change/difference in Kv will already lead to a 30% difference in current.

Bigron 12-07-2018 08:52 PM

I appreciate the responses They have given me some info to digest. I guess I am interested in what voltage waveform the motor sees from the esc. I am curious how the voltage changes I have a basic understanding of PWM Maybe I am barking up the wrong tree in trying to understand the relationship between voltage and rpm (no load). I have to find someone who has scoped the output of an esc to help me.

I am so thankful for the wealth of knowledge on this forum. I have learned a great deal.

Thank You.

ron_van_sommeren 12-07-2018 11:01 PM

You sure you are big Ron? Because this Ron is 6'7".

Do you have a background in electronics?

Do you want to design a brushless controller?

Controllers specify their maximum electrical rpm, i.e. for two-pole motors.
Max. shaft-rpm = 2 e-rpm/#magnetpoles (try it for a 2-magnetpole motor).

As stated above by my learned friend, most controllers can handle somewhere between 150 - 250k e-rpm.

However APD controllers can handle 750.000 - 2.000.000 e-rpm, 500W tot 20kW.
Voorzien van 32bit processor(en).
www.powerdrives.net / shop.powerdrives.net

Max. shaft rpm for give nmake/type controller and number of magnetpoles
www.bavaria-direct.co.za (RCG user Skylar)
→ More Info
→ ESC Max. RPM Limits (below colourful winding diagram table)

Bigron 12-08-2018 12:30 AM

You got me beat, I am only 62. But I am the biggest Ron in my flying club (Ithaca Radio Control Society).

I am a retired Industrial Electrician. I worked on both control circuits and 480v and higher 3ph motor and power distribution systems. And did a lot of PLC programming. Worked with many vfds.

I am not interested in designing anything. Just trying to understand what happens between the battery and the motor.

I appreciate you taking time for me.

Btw, I am located in Ithaca, NY

ron_van_sommeren 12-08-2018 07:49 PM

/* Oops. Wrong planet. */

ron_van_sommeren 12-08-2018 11:54 PM

/* Oops. Wrong planet. */

ron_van_sommeren 12-08-2018 11:59 PM

/* Oops. Wrong planet. */

ron_van_sommeren 12-09-2018 12:21 AM

A PMBLDC motor & controller is an isosynchronous system, however, permanent magnet brushless DC motors can operate synchronously.
Three Phase Alternator - Three Phase Motor? - WFF

ron_van_sommeren 12-09-2018 12:59 AM

Powersignal scope traces


Also, wire thickness and number of parallel wound wires have no effect on Kv.
Kv is inversely proportional to number of winds.
For a given motor:
Kv #winds = constant

Wildflyer 12-09-2018 02:00 AM

Powersignal scope traces


Thanks Ron, my scope is only a dual trace and I wanted to see the scope traces also but hadn't got around to trying to hook it up. I just kept thinking of a way to see the third trace.

I did however prove to myself that a cap plugged into the Rx could effectively keep a Spektrum Rx from losing voltage and showing a signal loss. I had a weak output from a special ESC I wanted to use.

ron_van_sommeren 12-09-2018 06:35 PM

Links to other, less clean, scope traces in this thread's OP
Too long battery wires will kill ESC over time: precautions, solutions & workarounds - RCG

See paragraphs
  • ... ...
  • Explanation, water hammer/knock video&analogy, theory
  • Measurements & scope traces

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