Hi everyone,
Is it possible to use VDC directly from the alternator without having a battery with the circuit?
Say for example.....
Gas Motor -> 12v Alternator -> 24v Step up -> 24v motor

The alternator is 12v 100amp.
24v Step up (10amp max)
24v motor (10amp max)

Will the current from the alternator fry my Step Up?

 

The current will be what ever the load demands.
You need a initial excitation voltage for the field.
After that it should be self excited.
Max.

 

I had a feeling it does have to depend on the load, but I just wanted to make sure.
I have two self curious questions:
1. So then, if the 24v Step Up Regulator max input is 10amp, it's not going to get blasted by 100amp is it?
2. Can you somewhat look at the Alternator like a battery where the voltage and max out amp is alway fluctuating?

 

Supply Current will always fluctuate according to load variations.
Max.

 

If your 24 volt motor decided it needs/wants more than 10 amps of current(stalled or overloaded) then your 12 to 24 converter may develop a bad smoking habit.

This is why fuses were invented. A sacrifice to the gods of magic smoke that allows expensive circuits to survive over loads.

 

You might be ahead on things to just modify they alternators voltage regulator circuit to make it put out 24 volts rather than 12.

 

Just keep in mind a typical alternator with built in on board regulator will output about 14 VDC (slightly over) and you are going to 24 VDC @ 10 Amps. You are looking at a fairly expensive DC to DC converter. Twenty-four VDC @ 10 Amps is a 240 Watt load and will be 12 VDC @ 20 Amps on the input side so allow for some excess power and fuse your load accordingly. For example Locked Rotor current to the motor.

You also may want to consider tcmtech's suggestion.

Ron

 

Ahh. Great thoughts. I've completely forgot about the regulator on the alternator.
So with this all and said, I don't need a battery, correct? I've been seeing many designs with it and I'm just curious.

 

You just need it for initial excitation to get the alternator to output, a simple regulator can be made from a comparator, LM311 etc.
Max.

 

The current will be what ever the load demands.
You need a initial excitation voltage for the field.
After that it should be self excited.
Max.

With some motorcycles, there was often enough residual magnetisation in the core to get things going - a life saver if you had to bump start it with a dead flat battery.

No idea whether car alternators do that - and I wouldn't count on it if it did.

I'd go for a small SLA battery that serves no other purpose then getting the field winding started.

 

I'm fairly sure that in a typical automotive system that the battery also provides smoothing (spikes/transients,etc..).
A modern car certainly is not happy if you were to disconnect the battery while running..

 

.

No idea whether car alternators do that - and I wouldn't count on it if it did.

The one I set up on the bench would not.
So nothing to bank on.
Max.

 

Will the current from the alternator fry my Step Up?

The local power generating station can produce over a million amps of 120 volts for my house. I have a light bulb in the ceiling. Will the million amps demand to go through my light bulb?

An example so extreme that it sounds stupid, but it's the thought that counts. Amps do not demand to flow. If the voltage is right, the load will decide how many amps will flow. Pay attention to the volts. That's where you start the math.

 

Thanks for all of the informations. It's been very helpful.

 

I'm fairly sure that in a typical automotive system that the battery also provides smoothing (spikes/transients,etc..).
A modern car certainly is not happy if you were to disconnect the battery while running..

The TS appears to be constructing some sort of generating set.

Most of the crap on automotive electrics is from the ignition and various relays etc. alternators are certainly a lot quieter than the old dynamos.

The simplest route is to use a SLA battery - it only needs to get the field winding started, then the generator output can take over. The battery needs charging when the generator is going, and isolating when its not.

Some motorcycles were stripped down for track use, which meant losing the battery - sometimes the alternator poles didn't coincide with the ignition timing so an electrolytic capacitor was added. If the alternator phases didn't co operate with the field winding - the same cure would apply.

 

The advantage of a 3ph rectifier it has very low ripple, <5%, also the ripple is 6x the output frequency.
Max.

 

The advantage of a 3ph rectifier it has very low ripple, <5%, also the ripple is 6x the output frequency.
Max.

Most motorcycle alternators have the same number of poles - the ripple can still cause problems sometimes.

 

For some reason that I'm not really sure of, most automotive alternators burn out if the battery is disconnected while they're spinning even though they're still connected to the other loads on the vehicle. I don't know if it's the rectifier diodes and/or the regulator that dies. But if you take the regulator off an automotive alternator, connect the field winding (the rotor winding connected via the brushes on the slip rings) to 12v & measure the dc output on the diodes it's probably over 24v

 

I have often started a car with a dead battery by putting a good battery in, starting the car, and removing the good battery while the engine was running, and reinstalling the dead battery to charge. Never had an alternator or generator fail from doing that.
It is also one way I load test an alternator, by disconnecting a cable from the battery while it is running, then turning on lights and accessories to ensure the alternator was able to carry the load. If the car died while doing the test, it meant the alternator was weak.

But in every case, a battery was needed to start the alternator field, otherwise it would not charge anything.

 

Also the capacity ratings of alternators are very misleading, just look at the unit and components used and guess just how long it would last supplying 100 to 150 amps as listed!
Max.