Odd box that connects to alternator and produces 120 volt DC....

[klook]

First off, not an engineer, just invading your forum as no one seems to be able to answer this problem.
I have an ambulance/fire marshals vehicle, a 1992 Ford E350 with 7.3 IDI diesel. Pre Powerstroke. It came with a red box in one of the compartments that has no labels or identification. It is heavy. It has 3 lugs for wires coming from the 160 amp alternator that state AC taps. It has 2 15 amp glass fuses. It has 2 standard 120 vac plugs that are separate, not a duplex, and they are labeled 120 volts DC. Is there an explanation for what it is and maybe what it is for? Who needs 120 vdc in a vehicle?

 

[Baluncore]

Welcome to PF.

I expect it is an inverter, used to produce the higher voltage that was once needed to run vacuum-tube radios. Maybe it drove one or more sirens, with 120V universal motors.
Does it operate continuously, or does it cycle on and off every 30 seconds?

Early inverters had vibrating metal contacts and a transformer. They were replaced by transistorised inverters in the 1960s.
It is also possibly a rotary converter, a 12V motor, driving a DC generator.
Now you will have to look inside the box.

 

[berkeman]

I have an ambulance/fire marshals vehicle, a 1992 Ford E350 with 7.3 IDI diesel.

Are you dealing with this vehicle as part of the EMS or Fire service, or because you acquired it on your own and are salvaging it. I hope you know the difference...

 

[klook]

Thanks for the replies. I am an RV repair tech, years in the custom home building industry. I have picked up an education along the way, lacking as it is. But I know the difference between AC and DC in general terms. I am using it as a work platform. There was an inverter in the unit but it was removed. I am changing things around to suit my needs. It did not drive sirens, they are still on it and ran off 12vdc. Here is a pic of the unit, if that helps.

 

[klook]

I am old enough to remember vacuum tube stuff, this is a 1992 vehicle....Its never seen a tube....

 

[Baluncore]

Its never seen a tube....

It may be unlikely, but there is the possibility that some legacy equipment was transferred from the previous vehicle to the new in 1992, then partially removed later.
The unit is unusual, so positive identification will not be decided by statistics.

Maybe it provided portable power to an available 120V ECG or defibrillator.
We could keep guessing, or you could open the box and look inside.

 

[DaveE]

Clearly this is either mislabeled or built by amateurs. No real EE would use a standard household AC connector for a DC source. It's remarkably stupid and just not done... ever. So, whatever it's for isn't likely to be normal in any way.

 

[Rive]

It has 3 lugs for wires coming from the 160 amp alternator

What voltage did that alternator produced?

My guess is, that there will be a three phase transformer inside, with a rectifier: so the output would be valid DC 120V, but I have no idea what medical or other equipment would need it.

Especially that the remark from @DaveE is correct, no way this was anything approved.

Ps.: On second thought, I think @Baluncore is also correct that it might be needed for a legacy equipment of some kind... 120V DC vintage seems to exists...

 

[klook]

Thanks again for the responses. Here are the opened up pics. This sucker is heavy. Now I know why.

 

[klook]

The glass fuses are not one each for the outlets. The white wire and black wire on each outlet is fused separately but connected together. This is not a homemade project. Even the lettering on the face is neat and commercial looking. Now I need to find out what equipment used 120vdc. By the way, I tested it and it is indeed putting out 120 DC.

 

[Baluncore]

It looks like a three-phase transformer, with bridge rectifiers to produce the DC. There appear to be no filter capacitors, so there will be some alternator ripple on the DC.

The 3PH from the alternator will have a higher frequency than normal mains power. So the transformer should have thinner than normal laminations, as used for the 400 Hz power on aircraft.

 

[klook]

A friend from an RV forum has stated that he had one of these back in the day. They have been replaced by inverters. He said they would run motors that had brushes......cutting tools, etc. Any opinions about this possibility?

 

[Baluncore]

Any opinions about this possibility?

They would run 120 V lights, and "Universal motors" with brushes, such as most 120V corded hand-held power tools. They will not run induction motors.
https://en.wikipedia.org/wiki/Universal_motor

 

[Rive]

Any opinions about this possibility?

120V, 15A => around 1200W, with some margin for reliability
Feels a bit thin.
On the other hand, this could explain be an explanation for the connectors.

 

[sophiecentaur]

The transformer would need to operate over a range of frequencies as the engine speed varies. Wouldn't that be a serious problem at tickover without using a lot of iron? Or is it made clear that there's an inverter involved?

 

[Baluncore]

The transformer would need to operate over a range of frequencies as the engine speed varies.

The 3PH from the alternator will have a higher frequency than normal mains power. So the transformer should have thinner than normal laminations, as used for the 400 Hz power on aircraft.

Because the 3PH is rectified, the universal motors do not operate at the higher AC frequency.

Only the transformer must operate at the higher frequency. Thinner laminations are needed than are normally used for 60 Hz, but with the same total weight of iron.

The laminations in the transformer need be no different, in mass or thickness, than those used for the stator of the alternator.

 

[sophiecentaur]

Only the transformer must operate at the higher frequency.

I was referring to the low frequency condition - at tickover speeds. But, comparing this imagined situation with the design of an aircraft electrics is questionable. Aircraft engines run over a smaller range of speeds so the magnetic design of their alternators (and any transformers) is less demanding than that of a vehicle alternator which has to operate over a frequency range of, perhaps 6:1. I doubt that a transformer would do this job.

 

[Rive]

I was referring to the low frequency condition - at tickover speeds

I believe they are designed with those speeds in mind: basic operation of the vehicle and battery charging should be available already at the lowest operational RPM...

 

[sophiecentaur]

I believe they are designed with those speeds in mind: basic operation of the vehicle and battery charging should be available already at the lowest operational RPM...

Yes, that would make sense but normal vehicles don't use step up transformers. An extra 120V alternator could be handy in some circumstances but 120V DC distribution around the vehicle could be risky / scary and of little general use; you'd still need an inverter which would have ben designed specially for the job.

I reckon an small petrol generator would be cheapest and easy to get hold of (camper vans use them, I think).

 

[Baluncore]

I was referring to the low frequency condition - at tickover speeds.

The problem with low alternator speeds, is the higher field current needed to supply the battery charge voltage. Alternators are not designed to operate at low-idle because they are then very inefficient. A vehicle, designed for alternator operation while stationary, will have a hand-throttle or a high-idle switch. The internal light would brighten slightly, when the voltage regulator began to operate, to charge the battery.

Aircraft alternators were designed to produce a steady 400 Hz for the avionics. The avionics included synchro-resolvers that used 3PH 400 Hz sine waves. 400 Hz gave a fast instrument response, with lower weight magnetics.

The vehicle alternator frequency is greater than typical mains frequency at idle because it is a multipole alternator and has a smaller driven pulley. If the vehicle alternator produces 50 Hz at 500 RPM, then it would produce 500 Hz at 5000 RPM.

Laminated audio output transformers operated over an even wider range of frequencies, so why should the alternator core, or the transformer core, pose a problem?

 

[Rive]

Yes, that would make sense but normal vehicles don't use step up transformers.

I meant that those alternators (thus: properly paired transformers) should be able to handle the frequency, and nothing more.

Then, that 'more' could be, for example, an installed speed controller (called 'governor', maybe?) to provide elevated/stable engine speed while additional load is engaged... Modifying a vehicle for special purposes is often tricky, and I do not know what a 'fire marshals vehicle' of this kind is prepared for.

 

[sophiecentaur]

Aircraft alternators were designed to produce a steady 400 Hz for the avionics.

Exactly. Doesn't the speed of the alternator scale directly with the engine revs? Or is there some extra mechanism? Either way, the notion of an alternator for vehicle AC supply is basically a nonsense(?); there's no correspondence between the two situations.
But nowadays, there are efficient inverters so why not start with 12V DC in the vehicle? A replacement inverted for the OP's purposes would make things simple and reliable (but not free, of course0.

 

[Baluncore]

Either way, the notion of an alternator for vehicle AC supply is basically a nonsense(?); there's no correspondence between the two situations.

It was never supplying 120V AC for use in the vehicle.

30 years ago, it was dedicated to supplying 120V DC, to run a couple of power tools, fitted with universal motors. It rectified the high frequency AC because a universal motor could not work efficiently on the higher frequency AC, since the motor laminations were designed to work at a maximum of 60 Hz.

No one is advocating using that solution today.

If the vehicle alternator produces 50 Hz at 500 RPM, then it would produce 500 Hz at 5000 RPM.

Exactly. Doesn't the speed of the alternator scale directly with the engine revs? Or is there some extra mechanism?

YES.