Re: Throttle Control

[Dave Everett <deverett@idx.com.au>]

At 21:32 23-06-98 PDT, you wrote:
>My one weakness in building my sub is definitly the electronics,  I have 
>never been able to understand it.  So please bear with my "stupid" 
>questions! For instance, I know that a few of you use relays to meter 
>the power flow from the batts to the motor.  Is it possible to use 
>something like a regular household light dimmer switch for a throttle 
>control?  Something that I can run all 36 volts into and have control of 
>what comes out to my motor.

A light dimmer only works with AC signals. It needs the zero-crossings to
switch the triac off after each enable pulse. 

Now, I know that last sentence will mean nothing to you <g>.

The most efficient way to control motor speed is with an H-bridge driven by
a pulse-width modulated signal. Many electric cars use such a system, the
most popular controller made by Curtis Electronics.

An H-bridge is like 4 switches wired so that the Pos or neg of the battery
can be switched to either pole of the motor.

+         +
|         |
S1        S2
|         |
-----M-----
|         |
S3        S4
|         |
-         -

In the crude drawing above you can see that if I enable S1 and S4, the
motor will turn one way, if I enable S2 and S3 the motor will turn the
other way, of course if I enable S1 and S3 or S2 and S4 you can say goodbye
to the battery.

Now if I want to control the speed of the motor I simply turn the required
switches on and off very quickly. If I leave S1 and S4 on all the time I'll
have full speed, but if I turn S1 and S4 on and off for equal amounts of
time, I'll get half speed as the average voltage to the motor is halved. By
changing the width of the on pulse I can vary the motor speed from zero to
full.

Please note that the switches in the drawing are semiconductors like
Mosfets or IGBTs, not relays.

I'd suggest you contact a local electric vehicle club. They'll be able to
show you the devices in question and recommend where you buy them.

Dave Everett.