Regulating a dynamo (DC generator) output to charge a battery

Hi,

I have a dynamo (DC generator) that is connected to an old regulator which are used to charge a 24V 100A lead acid battery.

I have no experience at working with dynamos. I need to replace the old electromechanical regulator with a new electronic one. From several readings on the internet I understood that (and please correct if I am wrong) the dynamo has 3 connections:

1. "D" which is the output voltage/current of the dynamo.
2. "F" which is the input field current.
3. Ground.

The "F" connection should be used by the regulator to increase/decrease the field current which then increases/decreases the output at the "D" connection and keep the dynamo output voltage suitable to charge the battery and operate the system safely. Let's say this output should be around 26V.

Can you suggest a modern electronic regulator circuit to do this job? are there any ICs that can do this regulation?

P.S. I am not sure if this is the most suitable forum section to post on. Feel free to move it where I can get the most help!

3Deye said:

I need to replace the old electromechanical regulator with a new electronic one. From several readings on the internet I understood that (and please correct if I am wrong) the dynamo has 3 connections...

Can you suggest a modern electronic regulator circuit to do this job? are there any ICs that can do this regulation?

Click to expand...

Yes those are the main parts of it. I do not know how to implement electronic regulation but I think is good to know what type of DC generator you have since there are several options. Here are those options:

The "F" connection should be used by the regulator to increase/decrease the field current which then increases/decreases the output at the "D" connection and keep the dynamo output voltage suitable to charge the battery and operate the system safely. Let's say this output should be around 26V

Click to expand...

This sounds like a separately wounded mode...

Both dynamo and motors look very similar but they perform opposite functions.

Both have two sets of coils- one on the rotor and the other on the stator. Many small dynamos have the rotor made of permanent magnet and this is also common for small motors.

For motors the windings on the rotor and stator are combined in (i) series (ii) parallel (iii) series parallel. This mostly determines the torque output characteristics of the motor.

For alternators that uses slip rings instead of commutators, the rotation determines the frequency. For DC generators, the rotation determines the voltage. The current in the field coils are regulated to regulate the output voltage.

The boilerplate will have some vital statistics about the dynamo: power rating, voltage, etc. How the dynamo is driven? Diesel engine?

3Deye said:

Can you suggest a modern electronic regulator circuit to do this job? are there any ICs that can do this regulation?

Click to expand...

The original electromechanical voltage regulator just had a relay that pulled in at just over 28v and drops out just below around 28v.

The idea is that when the relay released power was fed into the field winding, and that increased the output power, and output voltage.
When the relay pulls in, power was cut to the field winding, and the output power starts do decrease.

The relay sparks and buzzes away merrily controlling the output voltage. That is the general idea.

A modern electronic voltage regulator works exactly the same way, but the switching on and off of the field winding is done electronically.
But the basic mode of operation remains exactly the same.

Many very early model cars had generators and vibrating contact voltage regulators. Guys that restore these often pull the now non functional guts out of the original voltage regulator and fit the electronic regulator from a modern alternator. But that is for a twelve volt system.

Twenty four volts makes it more difficult, but large trucks and busses often have 24v systems, so a suitable electronic regulator may be tracked down.

My honest opinion is that it is probably not worth doing, as a brand new regulator IC will probably be worth about the same as a second hand 100 Amp 24v alternator off a truck.

Its certainly possible to build your own circuit, but then there is the risk of making an error that might be rather destructive unless you know exactly what you are doing.

A second hand truck alternator will be guaranteed to work, and work reliably for a very long time. Shop around, there are bound to be some really good deals out there if you are patient.

Thank you guys for your replies. I just want to learn how to do it by myself. I would appreciate if you can help with a circuit or give me a starting point.

A bit of poking around on the web turned up this:

This circuit (as drawn) is for about 14 volts, but with very few changes it could be made to work at 28 volts.

Anyhow, its pretty typical for this type of application.

c_mitra said:

Both have two sets of coils- one on the rotor and the other on the stator. Many small dynamos have the rotor made of permanent magnet and this is also common for small motors.

Click to expand...

Yes is true, but a 28 V is considered small enough to have permanent magnetic field produced by stator?

3Deye, I think is useful to tell what kind of DC generator you have...what kind of excitation if it has no permanent magnets...

Having permanent magnets I do not think you can regulate EMF unless you play with speed.

@Tony, thank you for the circuit. Will simulate it and see how it goes.

@CataM, Unfortunately I don't have a part number for the dynamo and would much appreciate your help if you can tell me how to measure different dynamo parameters. In general, what are the specifications of the dynamo that should be clear to me to be able to design the compatible regulator for it? The specs I have are its output voltage (28V), output current (~107A) and that's it. I think The missing specifications are the field winding polarity, voltage and current. How can I measure them? Is there any other missing specs?

Car and truck generators rely on a little permenance of the magnetic field to get the process going. This is why you have too flash the windings across the battery if you want to change polarity. Warpspeeds circuit with some modifications should work OK.
Frank

I am trying now to modify the circuit suggested by Warpspeeds to work at 24V. I changed the zener to 25V. But my question now is how to model the generator (dynamo) in Orcad spice?

Circuit provided by Warpspeeds is for a shunt wounded (figure C, post #2). Your motor/generator ~3kW so I think it is clear that has not permanent magnets for magnetic field.

To variate output of the generator in shunt wounded you have 3 options:
1) Vary speed
2) Vary magnetic field
3) Electric circuitry to change output which is what warspeed has done I think

To model in Orcad I think you could place instead of the generator a dependant voltage source. This dependant voltage source depends on the current through inductor "F -" (in the picture on post #6). This would work for a determinated speed.

Consider this : EMF generated = kE · n · phi
kE= depends on the physical construction of the machine
n= rotation speed in rpm
phi= magnetic flux from the stator (inductor "F -")

I doubt if you can model the generator without first knowing its characteristics.

The first thing to do might be to run it up to working speed and try exiting the field winding manually from a variable dc power supply, and measuring the output voltage and current under various load conditions.

That will give you a pretty good idea of the required voltage and current required for field excitation to reach full rated output. That will enable the value of R2 on the above circuit to be selected.

Resistor R5 is for current limit, and should drop roughly about 650mV at around 100 Amps.

The zener will be selected to give P1 a suitable range of output voltage adjustment. The rest of the circuit should work pretty much as it is.
TR2 will need a suitably large heat sink, but most of the power will probably be dissipated by R2.

Warpspeed said:

I doubt if you can model the generator without first knowing its characteristics.

The first thing to do might be to run it up to working speed and try exiting the field winding manually from a variable dc power supply, and measuring the output voltage and current under various load conditions.

Click to expand...

So you are saying that it is a separately excited field not a shunt wounded ? If is going to work with the circuit provided, then should check it's characteristics in shunt mode because that way is going to work with it.

Separate independently excited field winding is the usual way these machines are made.

Without that feature, there would be no way to regulate the output.

Warpspeed said:

Separate independently excited field winding is the usual way these machines are made.

Without that feature, there would be no way to regulate the output.

Click to expand...

I disagree with both statements.

1) The field connection you can do it yourself the way you want, you just need access to the filed windings.
2) It is possible to regulate output with 2 different ways:
First way: speed
Second way: place a resistor in series with the field windings.



SOURCE: Electric Machinery Fundamentals - 5th edition, Chapman - page 534

Well yes...

But do you seriously believe changing the speed is a practical method ?

These days we are much more likely use electronics with voltage feedback instead of a manually controlled rheostat.

Second way: place a resistor in series with the field windings.

Click to expand...

A shunt wound generator with the option to connect a series resistor is in fact a separately excited generator. I wonder why you keep on harping about the difference...

Varying the speed is more a theoretical option for most generator applications.

Warpspeed said:

But do you seriously believe changing the speed is a practical method ?

These days we are much more likely use electronics with voltage feedback instead of a manually controlled rheostat.

Click to expand...

Yes I know, but I just wanted to point out how it works a generator , I mean, its concept.
Of course nowadays is easier to use electronic circuitry instead of rheostat. It is the same as using a PLC instead of ladder logic for automation of a plant.

A shunt wound generator with the option to connect a series resistor is in fact a separately excited generator. I wonder why you keep on harping about the difference...

Click to expand...

Yes, you are right.