# 12v to 24v

#### Tobias

Joined May 19, 2008
158
I have a 12v alternator in the system right now and need to run a 24v system for four brush-type motors.The motors are rated for 24v, I need to double my speed. I have found this device in the link below that will do the trick and let me keep the 12v alternator. That is I understand what this deal does correctly. The only problem is I need it in a couple of days and they are in Australia. I had 10amp fuses inline with the motors and never blew the fuse. So I only need 5amps at 24v. Any ideas how I can step up the voltage to 24 using two 12v batteries and keep the 12v alternator?

http://redarc.com.au/products-and-se...ger-12v-to-24v

#### SgtWookie

Joined Jul 17, 2007
22,230
Are the four 24v brush-type motors the only thing the alternator is powering?

Any chance of adding a 2nd alternator?

#### Tobias

Joined May 19, 2008
158
The four motors are the only thing the alternator is powering. A second alternator is the second route I am looking into at the moment.

Are the four 24v brush-type motors the only thing the alternator is powering?

Any chance of adding a 2nd alternator?

#### SgtWookie

Joined Jul 17, 2007
22,230
OK then, why not just modify the regulator so that it outputs ~27.6v instead of 13.8v?

You might do that by using something in the feedback path from the output of the alternator to the input of the regulator. Tell the regulator a "little white lie". You might accomplish that with an opamp driving a power transistor.

I'm not sure how much current the field takes (actually, the rotor in an alternator) in order to generate the output; but it'll certainly vary over the RPM range; much more @ idle then at speed.

The regulator will probably need at least 11v or so to supply current to the field. Then you might scale the rest of the output something like 15v in = 11.5v to the regulator, etc. and finally 27.6v is 13.8v at the regulator. It's just a rough idea.

#### Tobias

Joined May 19, 2008
158
I screwed up in answering your question. The existing alternator is powering alot of other things on the tractor. This is going on a farming application. The motors are turning metering plates for fertilizer. After testing and realizing a few things in the math for the metering plates we realized we needed to have almost double the rpm on the electric motors.

OK then, why not just modify the regulator so that it outputs ~27.6v instead of 13.8v?

You might do that by using something in the feedback path from the output of the alternator to the input of the regulator. Tell the regulator a "little white lie". You might accomplish that with an opamp driving a power transistor.

I'm not sure how much current the field takes (actually, the rotor in an alternator) in order to generate the output; but it'll certainly vary over the RPM range; much more @ idle then at speed.

The regulator will probably need at least 11v or so to supply current to the field. Then you might scale the rest of the output something like 15v in = 11.5v to the regulator, etc. and finally 27.6v is 13.8v at the regulator. It's just a rough idea.

#### strantor

Joined Oct 3, 2010
6,685
if you're considering adding a 2nd alternator, why not just get a 24V alternator?

#### SgtWookie

Joined Jul 17, 2007
22,230
Well, there goes that alternator mod idea.

That Aussie site's products are for battery chargers, not for constant output like you're wanting.

have a look here:
http://www.powerstream.com/dc12-24-doubler.htm

One of these would be much better:
http://www.powerstream.com/dc1-extreme-12-24-15A.htm

On something that gets heavy-duty use under sometimes really lousy conditions, you need something that's weatherproof and a good bit over the required capacity. If you bought the minimum supply, you may wind up overloading it just trying to turn the motors on. Even with a 360W 15A supply, I'd suggest staging the motor turn-on so they don't try to start all at once.

#### SgtWookie

Joined Jul 17, 2007
22,230
You may be able to get a military surplus alternator. A typical alternator from a reasonably recent military vehicle puts out ~65A @ ~27.6v.

[eta]
Also, many buses have 24v electrical systems. If there's a truck/bus junkyard near you, you might have a look.

#### Tobias

Joined May 19, 2008
158
I am hoping to not to fabricate up a PTO drive for a second alternator but I might be heading that route. Thank you for your input.

#### SgtWookie

Joined Jul 17, 2007
22,230

#### strantor

Joined Oct 3, 2010
6,685
Oh yeah, I missed the deadline part; With that time crunch I would hit up the junkyard for a 24V alternator, preferably the military high output type SGT suggested, or if not a big truck alternator might do it.

#### JMW

Joined Nov 21, 2011
137
Many marine applications (not USMC) use 24VDC. There are alternators available as well as 12 to 24 volt inverters.
Interesting site.
Just found it as a link from somewhere else.

Regards,

Jim W