DIY Electric Car Forums banner

1 - 10 of 10 Posts

·
Registered
Joined
·
70 Posts
Discussion Starter #1
Hi All,
>From what I gather, Current in a motor, relates to mechanical
torque... more current, more torque.

If, for a given wattage available from a battery pack, you could vary
the voltage from it (somehow), to give more current,less volts, for
say accelerating from a standing start.
Then change it a higher voltage less current for a cruising speed.

Would this make more efficient use of the available power ?? and is
it possible ? or pointless ?
Grant

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #7
A motor is just a transformer. If you can get the current varying in one
winding it will induce/interact with the other winding. When you look in
the 300 page college paper you will discover they model motors as
transformers.

Go unplug the wall wart transformer on your cell phone charger and read
the text on the bottom.
There will be one wattage listed only. but two sets of voltage and current.
mine doesn't actually mention the watts. (but that can always be
calculated from the Volts and Amps)

100-240Vac .2A in
5V 1A out

obviously these are max ratings. ie when pulling 100V then .2A max
watts is watts
100*.2 = 20
5*1 = 5

OMG! bad example only 25% efficient! (I Guess I wouldn't advertise my
watts either. Can you say parasitic load?)

Chinese PC power supply "400W" ( removed from my PC cause it was loud
and pumped to much heat in the room)

115V 6A = 690W in

5V 30A 150
12V 11A 132
-5V .5A 2.5
-12V .5A 6
3.3v 28A 92.4
5V 2A 10
============
393W 57% efficient.

The real point is not how grossly inefficient these examples are it is
that the high voltage low amps gets turned into lower voltage and higher
amps.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #8
Why do some controllers use 1.5KHz for low speeds and 15KHz for high speeds?
Why specifically is a lower frequency better for low speeds?


Thank you,

Dave Delman
1981 Electric DeLorean Project



************************************** Get a sneak peek of the all-new AOL at
http://discover.aol.com/memed/aolcom30tour
_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #9
[email protected] wrote:

> Why do some controllers use 1.5KHz for low speeds and 15KHz for
> high speeds?
> Why specifically is a lower frequency better for low speeds?

So everyone can hear you coming <ducking>

Paul

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 

·
Registered
Joined
·
70 Posts
Discussion Starter #10
[email protected] wrote:
> Why do some controllers use 1.5KHz for low speeds and 15KHz for high
> speed? Why specifically is a lower frequency better for low speeds?

The switching frequency isn't associated with vehicle speed. In general,
lower switching frequencies allow slower transistors and diodes to be
used, which are cheaper for a given power level. Lower frequencies also
reduce switching losses in the transistors and diodes, which makes them
run cooler and raises efficiency.

But... frequencies in the audible range of human hearing can be heard.
The motor acts as a loudspeaker, and people complain. So, most modern
controllers accept the higher price and lower efficiency to get the
switching frequency above the audible range.

Curtis originally designed their controllers to switch at 15 KHz, which
is high enough that most people can't hear it. However, they also
skimped on the parts, so they are barely strong enough. When people put
these controllers in a heavy road-going EV with a huge motor, the
controllers were over-stressed and failed. Rather than redesign it with
bigger parts, they chose to make it drop down to 1.5 KHz under heavy
stress conditions. So, you'll hear them "whistle" when hot or trying to
start a large motor.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
 
1 - 10 of 10 Posts
Top