[metastable]

With a copper stator winding there appears to be a direct relationship between electron drift velocity at rotor stall and rotor velocity at no load.

For example doubling the number of turns halves the electron drift velocity at stall and halves the rotor speed with no load. Doubling the voltage doubles the electron drift velocity at stall and doubles the rotor speed with no load. Doubling or halving the winding cross section doesn’t change the electron drift velocity at stall and doesn’t change the rotor speed with no load.

Changing the winding material from copper to silver (without changing the geometry) changes the electron drift velocity at rotor stall, but does it change the rotor velocity with no load?

 

[Tandy0]

Current consumed by a motor is (pd applied- back emf) divided by internal resistance. Therefore presumably silver windings would have less resistance, so current would be more. Therefore presumably would run faster under no load?

Not sure if that would increase the back emf, so that current reduced, back to what it was at the start.

 

[metastable]

If I’m not mistaken the free electron density in silver is lower but the drift velocity for a given length and voltage is higher...

With a copper winding if we double the conductor cross section but same number of turns, the resistance decreases but the rotor no load speed for a given voltage stays the same as does the drift velocity.

 

[donald]

...and rotor velocity at no load...

... the rotor no load speed for a given voltage stays the same ...

What is your definition here?

Surely a rotor can run at all sorts of speed with no load?

If there was a speed at which a rotor could not be at zero load then once you go past it you might never stop!!! How would you pass back through 0kW (no load)?

 

[Tandy0]

If I’m not mistaken the free electron density in silver is lower but the drift velocity for a given length and voltage is higher...

With a copper winding if we double the conductor cross section but same number of turns, the resistance decreases but the rotor no load speed for a given voltage stays the same as does the drift velocity.

Ah, but if you double the conductor csa, you double its weight also. Therefore each little extra jolt of force is just spread over a larger bit of copper. I suspect silver is a better conductor for a given weight, so will turn faster.

 

[metastable]

What is your definition here?

[bldc motor constant KV] * battery voltage = no load rpm

 

[Tandy0]

BTW, are we doing your Physics homework for you?

 

[metastable]

I have these hub motors for an electric skateboard and I'm wondering if I rewind them with the same gauge wire and same # of turns with silver wire instead of copper if I'll get exactly the same KV (rpm/volt) motor constant...

 

[metastable]

The formula is:

Original Turns Per Tooth # * Original KV = New Turns Per Tooth * New KV

which can be rearranged:

(Original Turns Per Tooth # * Original KV) / New Turns Per Tooth = New KV

which can be rearranged:

A = Change Factor of KV (rpm/v)
B = Change Factor of Conductor Resistivity (ohm-meters)
C = Change Factor of Conductor Volume (meters^3)
D = Change Factor of Conductor Resistance (ohm)

A=sqrt(B/(C*D))

...but this implies changing from copper to silver won't change the KV... I wonder if it's true or if there is a simple answer?

 

[donald]

.... bldc motor constant KV ....

What does this mean?

If you cannot communicate 'engineering' there's no point pretending you're doing any.

 

[cah197]

What does this mean?

If you cannot communicate 'engineering' there's no point pretending you're doing any.

Stop being a poo-poo-er. He’s going to wind his motors with silver wire and nobody can stop him.

 

[Spiny]

Silver is around $0.57 a gram, more as wire and then it needs insulation. Its also a bit denser (heaver) so the motor will be heaver and vastly more expensive for no gain. Do not forget silver will tarnish unless protected at connections.
This could be the worlds most expensive skateboard!

 

[metastable]

What does this mean?

If you cannot communicate 'engineering' there's no point pretending you're doing any.

Brushless Motor Kv Constant Explained • LearningRC

Brushless motor Kv tells how motor RPM relates to voltage. This article gives an in-depth explanation of this important motor constant.

learningrc.com

"The more turns a motor has, the slower it will spin."

MOTOR TERMINOLOGY

www.astroflight.com

 

[metastable]

When the conductor is copper, changes to the drift velocity when the rotor is stalled are directly proportional to changes to the rotor velocity at no load... but in switching to silver, the drift velocity changes, but does the no load rotor velocity? Why or why not?

 

[ColA]

It’s like those nonsense poems; just when you think you understand it you realise you don’t get it. I worked for a motor manufacturer for quite a few years and the term “electron drift velocity” was never mentioned. Nor was Fermi velocity, nor was quantum theory (except when we had a few too many drinks). I have never heard so many half truths added together to sound clever but be total rubbish. Motor theory is not that difficult so if this gentleman wants to design a different motor he should go to school first.

 

[metastable]

I worked for a motor manufacturer for quite a few years and the term “electron drift velocity” was never mentioned. Nor was Fermi velocity, nor was quantum theory (except when we had a few too many drinks). I have never heard so many half truths added together to sound clever but be total rubbish.

^doubling voltage doubles the drift velocity and rotor rpm @ no load — doubling the length halves drift velocity and rotor rpm @ no load — changing the conductor thickness doesn’t change drift velocity or rotor rpm @ no load — changing from copper to silver changes the drift velocity — but does it change the rotor rpm @ no load?

 

[donald]

Brushless Motor Kv Constant Explained • LearningRC

Brushless motor Kv tells how motor RPM relates to voltage. This article gives an in-depth explanation of this important motor constant.

learningrc.com

"The more turns a motor has, the slower it will spin."

MOTOR TERMINOLOGY

www.astroflight.com

If you are talking about constant V/Hz (that I mention occasionally - the point where V/Hz is no longer constant is the peak torque on the power curve, after which you get constant power), then to say that this applies to PM machines, you appear to be running a SR machine there.

If you can find enamelled silver wire, I'd be interested to see a supplier of that. You do realise it will have to be 'enamelled' wire (PA, these days) to make a winding, don't you?

 

[Spiny]

@donald enamelled silver wire is and has been about in the radio world for ages, used in RF coils, though these days usually only plated for the skin effect at high frequencies, the jewelry trade has some clear enameled, keeps it shiny but I doubt it has much in the way of electrical insulation properties.
Expensive!

 

[donald]

@donald enamelled silver wire is and has been about in the radio world for ages, used in RF coils, though these days usually only plated for the skin effect at high frequencies, the jewelry trade has some clear enameled, keeps it shiny but I doubt it has much in the way of electrical insulation properties.
Expensive!

What's the point of solid silver wire for RF?

(FWIW, Skin depth of copper is actually deeper than silver, so net resistance is about the same to RF)

 

[Spiny]

@donald no point for solid wire now, but plated is used for coils at radio frequencies.
Somehow I don't think this skateboard runs at a frequency needing the skin effect. Thick enameled silver wire will be special order though