# Alternator voltage problem

1. Feb 12, 2010

Hi All,

I am new to this forum and would appreciate some help from you experts!!

I am trying to turn a turbine (@40RPM) using an alternator via a gearbox.
I have a 12v/110A alternator which i am running at 1120RPM (On its lower limits) and a 25A constant load. The voltage at this speed is 12.6v which is fine but the cable run from the battery to the alternator will be over 60m so i will have a huge volt drop. If i increase the regulator voltage setting this does not make any difference as the alternator is tunning so slow?

How can i increase the output voltage without changing the alternator?

Thanks

2. Feb 12, 2010

### sophiecentaur

One way is to take the voltage sense signal to the regulator from the remote load rather than directly (internally) from the regulator output. It is possible to do this, I believe, in most alternators. Alternatively, you can make up for the remote voltage drop by including some forward biased diodes in the voltage sensing circuit. You'll need to be careful, though, if your cable resistance is not VERY low. (What is the resistance?) Can you be sure that you will have enough volts available with the alternator cranked up to its maximum?

btw, is your second sentence the right way around? Or is it a fan that you are driving?

3. Feb 12, 2010

What do you mean by "take the voltage sense signal to the regulator from the remote load rather than directly (internally) from the regulator output" ( sorry i am novice to all this)

Also is the voltage sensing circuit going to be from pos to one of the field terminals?
The alternator will be running at a constant speed of 1120 so i need to improves its low end performance and cutting in speed. (in an ideal world)

4. Feb 12, 2010

### sophiecentaur

I'm assuming that it is an automotive type, with a regulator. This senses the output voltage and maintains it at a fixed value by adjusting the field current. If what you need is to have a good 12V at the remote end then it may be a good idea to detect this voltage (a wire taken back to the alternator 'sense' terminal.

If 1120 is a bit slow for the alternator, can't this be sorted out using different pulleys / gearing?

These non-standard arrangements can be difficult to get right and really need some local expertise, which is why installations can be so expensive when done professionally.

5. Feb 14, 2010

Changing the gear ratio will add more starting torque from stand still, so changing the gear ratio is really the very last resort.

When you say "remote end", what do you mean by this?

6. Feb 14, 2010

### sophiecentaur

Two points to make.
If your turbine can't get the alternator started then you couldn't expect to get much current from the alternator anyway. Is it ever feasible to put in a centrifugal (or perhaps an eletromechanical) clutch which will only engage when the turbine is actually spinning?
By "remote" I mean at the load, 60m away from the alternator. It's the volts here that need to be at 12V, surely.