Electronics > Projects, Designs, and Technical Stuff
Make a car alternator voltage regulator
Zeyneb:
Hi there!
I’d be interested to see if I can make a voltage regulator for a car alternator. I already bought a replacement alternator at a junkyard and I was able to install new bearings in the alternator.
Also I was able to bench test the alternator by connecting it to a car battery. The car battery provides current to the rotor field winding so it will become an electromagnet. Then I spin the rotor around with an electric drill. When a certain speed is reached the voltage increases from 12.6V to 14.5V and the battery gets charging current.
I know a voltage regulator can become really complex to deal with various electric loads, temperature compensation and engine RPM variations. For now I just want to see if I can get a stable charging voltage of 14.5V in the bench test.
I believe the first thing to focus on is to provide a stable field current to the rotor. Maybe it is best to make a new setup without the car battery but with an lab power supply, so when I end up with an oscillating stator voltage it can do no harm.
My first approach is the following coil driver circuit with an opamp and a mosfet. I know the actual rotor field inductance is 6.3 mH (measured). But that doesn’t matter to understand the principle right?
Ok about the circuit, I know the current needs some time to develop so Vf cannot keep up with Vi, I also see the gate voltage being maxed out so the opamp is a bit of an hyper control freak in this circuit. This is a stability concern right? My question is how can I expose this problem in a simulation?
I also did an ac analysis. But I’m wondering if inputting sinewaves at a range of frequencies (the way I understand ac analysis) will expose all the possible stability issues in this circuit. You know in transient analysis Vi during the pulse is 500 mV and inductor current gets to 1A. But in ac analysis Vi are 1V sinewaves but the inductor current only gets to 6 mA, however there is a peak at 700 kHz of 14 mA. Do you know some application notes or internet search terms to learn about ensuring stability in circuits like this?
Thanks for your time!
Edit: How can I sort the pictures attached here?
duak:
Older regulators used PWM to vary the field current. This was first done with voltage sensitive relays and then with transistors and zener diodes in a hybrid. Motorola/Onsemi came out with a chip that does much the same thing: https://www.onsemi.com/pub/Collateral/CS3341-D.PDF This data sheet might might give some ideas about controlling an alternator.
Zeyneb:
Thanks for that IC. I also found some of these ICs dedicated for alternator use. They all seem to operate with PWM. I'd be interested to see if I can make an analog one. The reason is that I'm aiming to minimize voltage ripple.
I hope someone is willing to give me advice about the shown circuit regarding stability.
elektrolitr:
There is no point to make linear field regulator.
First, with PWM you control field voltage, but what actually produces magnetic field is current. Make PWM frequency high enough, and inductance of rotor will filter all the ripple out
Second, the highest ripple in car alternator comes from rectifier. It will easily overpower any ripple from field current variations.
Large synchronous machines in power plants use the same technique - with static excitation system the output of 6-pulse thyristor bridge is routed directly to rotor winding. You look at field voltage- it's terrible. Look at the current- it's smooth enough (and they really care about power quality!)
amyk:
Being a mass-produced part, they are quite simple:
http://www.xs650.com/attachments/delco-10si-jpg.94744/
http://www.xs650.com/attachments/delco-12si-06-jpg.94743/
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