What's so bad about low impedence?

[Punx0r]

It all goes back to supply voltage. The power you can get out of a load is given by its resistance (at DC) or its impedance (at AC.)

well there's the flaw in your thinking.
generally power goes into a load.
hard to believe you actually went to MIT.

This is irrelevant semantics. A motor is a load from an electrical perspective and a power source from a mechanical one.


Regarding impedance matching of amplifiers and speakers, just do a word search for "impedance" on this page:

https://sound-au.com/articles.htm

There's a lot of solid, practical electrical theory in those pages described in a way that's easy to understand. Recommended reading for those of us not well educated in the subject.

 

[madin88]

Regarding impedance matching of amplifiers and speakers, just do a word search for "impedance" on this page:

https://sound-au.com/articles.htm

There's a lot of solid, practical electrical theory in those pages described in a way that's easy to understand. Recommended reading for those of us not well educated in the subject.

Yes

The thing with impedance, as far as i understand, a motor at 0 RPM has no impedance since the frequency is also zero, right?
So important is A) it's inductance, and B) it's resistance. Both will effect di/dt (current rise per time).
Low resistance motors with low inductance are a nightmare for most controllers to control at start.

If we talk about speakers, same here: at steady state (no movement) there is quasi also no impedance.
The lower the inductance, the faster the current and power will rise in the speaker coil and the better it will respond (subwoofer for instance this is important). Once it does swing at given Hz, we talk about impedance, right?

 

[JackFlorey]

Just my $0.02 ... the whole impedance story is wrong. The motor generates a voltage (the backemf) in which you are putting a current -> the amoujt of mechanical power you get is voltage times current. I dont see why this would not work with a superconductor.

Agreed. Back-EMF becomes an issue once the motor is spinning at some fraction of its base speed. Impedance is still an issue, but you are seeing a higher effective impedance, since you see less current flow for a given voltage across a motor winding due to the lower delta V. Eventually, of course, the current becomes zero when you are at base speed, and the effective impedance becomes infinite.

You are confusing V/I and dV/dI ... impedance is the latter.

Back-EMF changes the dV term. Once the motor is running, the back-EMF serves to reduce the voltage delta the motor winding sees. Instead of (say) 36V across the inductance of the motor, you might see 24V effective across the inductance (battery voltage of 36V minus 12V of back-EMF.)

(It also has DC effects of course.)

 

[nowholeself]

@ op, did they tell you a number for inductance?

At which voltage and RPM you would like to use it?

One idea, which probably would make it running, would be to split the delta winding and connect it in star. By doing so you would get about 3 times higher inductance.
The loss of RPM (1,7x less) you could get back with field weakening to some point.

The engine is a Neumotors 8025. It's 150kv with a torque constant of 63.789 mNm/a and .0177ohms. I planned to run it on a 52 volt pack max 90amp output to start with, but can add a bigger pack if need be. I make my own packs out of 21700 cells. No BMS. I balance charge (if that matters).

 

[peters]

You can try to add serial inductors on the phase wires if the motor inductance is too low for the controller. Should be air-cored, because ferrite cores would be too heavy or go into saturation, probably at least 10uH, thick phase wires winded on plastic tubes, and avoid coupling between them.
Here are some calculators:
https://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml
http://www.66pacific.com/calculators/coil-inductance-calculator.aspx

I didn't try, but I think it should work. However, too much inductance may limit the frequency (rpm).
The problem with low inductance is that the phase ripple current within a PWM cycle can be too high, because the BEMF and the battery voltage is different and the inductance is between them, and may exceed an overcurrent trip threshold (or something else..).

 

[marcos]

The problem with low inductance is that the phase ripple current within a PWM cycle can be too high, because the BEMF and the battery voltage is different and the inductance is between them, and may exceed an overcurrent trip threshold (or something else..).

^exactly this