DC Motor - battery life and more.

Archiver

I have a need to control a 24 volt DC motor, but it will be running off of
deep cycle batteries, much like a trolling motor in a boat.

Battery life will be a big issue, so I would like to know how the pro's do
it. Not step for step, just a push in the right direction.

Below is a excerpt from a trolling motor manufacturer, maybe it can better
explain what I am after:

"The Dura Amp is simply a switch that turns the connection to the battery
on and off 30,000 times per second.
If 'on' time is longer compared to 'off', the motor receives more power. If
'off' time is longer, the motor gets less power. Since there is minimal
power lost to accomplish this, the overall motor is very efficient."

The motor will draw up to 20 amps, if that helps any.

Thanks,

-John

Archiver

In a message dated 5/17/2003 7:35:31 PM Pacific Daylight Time,
john@l... writes:

> I have a need to control a 24 volt DC motor, but it will be running off of
> deep cycle batteries, much like a trolling motor in a boat.
>
> Battery life will be a big issue, so I would like to know how the pro's do
> it. Not step for step, just a push in the right direction.
>
> Below is a excerpt from a trolling motor manufacturer, maybe it can better
> explain what I am after:
>
> "The Dura Amp is simply a switch that turns the connection to the battery
> on and off 30,000 times per second.
> If 'on' time is longer compared to 'off', the motor receives more power. If
>
> 'off' time is longer, the motor gets less power. Since there is minimal
> power lost to accomplish this, the overall motor is very efficient."
>
> The motor will draw up to 20 amps, if that helps any.
>
> Thanks,
>

Do you intend to control the speed of the motor?

What is described by the excerpt is a method to give the motor variable speed
(assuming this trolling motor is electric).
From what I know (which isn't much), the "deep cycle" battery does not really
have much to do with the Dura Amp switching 30,000 times/sec.

In summary, the 30k/sec is a method we call PWM (pulse width modulation) and
is a common technique for speed control.

For your 24 dc motor, you can control the speed with PWM and there are a
multitude of ways to do it. If you are fairly new to electronics, probably
and easy way to control your motor is to have a 555 timer running in the
Astable mode (continuous pulses) feed the trigger circuit of another 555
timer set up in the Monostable mode. The monostable mode 555 then has a
variable resistor to control the pulse output, giving you PWM

Again, this is only one a many ways to do it, and I am sure many other ideas
will be posted.

If you need further circuit assistance, write back. I have a schematic I can
fax that shows the 555 hookup and external component values.



[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

In a message dated 5/17/2003 7:35:31 PM Pacific Daylight Time,
john@l... writes:

> I have a need to control a 24 volt DC motor, but it will be running off of
> deep cycle batteries, much like a trolling motor in a boat.
>
> Battery life will be a big issue, so I would like to know how the pro's do
> it. Not step for step, just a push in the right direction.
>
> Below is a excerpt from a trolling motor manufacturer, maybe it can better
> explain what I am after:
>
> "The Dura Amp is simply a switch that turns the connection to the battery
> on and off 30,000 times per second.
> If 'on' time is longer compared to 'off', the motor receives more power. If
>
> 'off' time is longer, the motor gets less power. Since there is minimal
> power lost to accomplish this, the overall motor is very efficient."
>
> The motor will draw up to 20 amps, if that helps any.
>
> Thanks,
>
> -John
>

Also, the PWM I explained in my other posting wastes very little power
compared to other methods of speed control, and that may be why the Dura Amp
is good for battery life compared to other methods.


[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

>Do you intend to control the speed of the motor?

Sorry, I left out one of the main questions, yes I do intend to control the
speed.

I thought the Dura Amp part had something to do with efficiency, not just
speed control. The site also talked about speed coils, if that means anything.

If I'm controlling the motor with PWM (have played with PWM a little) I
understand the stamp would only be paying attention to PWM, so are (2)
555's the way to go, or some kind of motor controller?

How do I actually switch that kind of power? I assume some kind of Mosfet.

Thanks,

-John

Archiver

Well, it looks like I asked before I was done looking. I found a really
good article at:

http://www.solutions-cubed.com/Stamp/sep99.pdf

But please still give me any info you see relevant.

Thanks,

-John



>How do I actually switch that kind of power? I assume some kind of Mosfet.
>
>Thanks,
>
>-John

Archiver

In a message dated 5/17/2003 9:51:24 PM Pacific Daylight Time,
john@l... writes:

> Do you intend to control the speed of the motor?
>
> Sorry, I left out one of the main questions, yes I do intend to control the
>
> speed.
>
> I thought the Dura Amp part had something to do with efficiency, not just
> speed control. The site also talked about speed coils, if that means
> anything.
Yes the Dura Amp does have a lot to do with efficiency. Very little wasted
power.
> If I'm controlling the motor with PWM (have played with PWM a little) I
> understand the stamp would only be paying attention to PWM, so are (2)
> 555's the way to go, or some kind of motor controller?
>
Two 555's are only to generate the PWM


> How do I actually switch that kind of power? I assume some kind of Mosfet.
>
You are right on with this point. A mosfet with very low on resistance should
be selected and ultimately driven by the PWM pulse

If all you want is simple speed control, simple PWM the Basic Stamp is
OVERKILL.



[noparse][[/noparse]Non-text portions of this message have been removed]