Generator / power supply switching circuit

[kohansey]

Hello Everyone,

I have more questions for this project then I have answers. I was hoping that the great people of Electro Tech Forums would be able to give some insight into what I will need and what I will be able to do. Without farther ado:

First the project:

I am looking to build a small circuit that will switch a load from a power supply to a generator when the generator's voltage reaches that of the power supply.

Specs:

Generator:
Brushless Motor
3-phase
0 - 35 V peak

Power Supply:
18.5 V
25 Amps

Load:
17 Amps nominal, 25 amp peak (@ 18.5 V)

Design:

The circuit need to be able to convert the AC signals from the 3-phase generator and regulate the voltage to 18.5 V. When the regulated voltage matches that of the power supply, the load needs to switch from the power supply to the generator. If the generator's voltage drops below 18.5 V, we need to switch back to power supply.

Ideas:
Take the 3-phase AC signals and feed them into 2 single phase bridge rectifiers, rated 25 Amps. Converting AC to DC voltage.
Feed the rectified DC voltage it a capacitor to stabilize.
Feed stabilized voltage through voltage regulator rated at 18.5 V (unknown is possible).
Do some kind of hardware magic and switch between the power supply and the regulated generator voltage.

Now the questions:
1. Is it possible to measure the amperes generated from a generator directly? Or do you need to provide a load and measure the amps supplied to the load?

2. If it is possible to measure the amperes directly, how would one do so with a regular voltmeter?

3. What would be a good size for the capacitor to stabilize the incoming DC voltage?

4. How would you be able to switch the load from power supply to generator?

5. How do you "sense" if an incoming voltage drops below a threshold?

6. Is it possible to regulate voltage to 18.5 V using a SMT or thru-hole chip, or do I need some kind of resistor-diode-capacitor combo to regulate the voltage?

7. How would you minimalize power loss in this type of circuit?

Any help would be greatly appreciated.

 

[Hero999]

]You can buy three phase rectifiers that use six diodes instead of four to convert AC to DC or you could build your own.

1. Obviously current can't flow without a load.
2. You can measure current using a: shunt resistor, hall effect sensor; look them up on Google or in a text book if you don't understand.
3. There's a formula to calculate it; again look it up! There again if you design your regulator circuit well enough you might not even need a capacitor at all a the output of a three-phase rectifier never drops to zero!
4. You could use MOSFETs or a relay but an active current or voltage sensing circuit is required or if you're not bothered about the voltage loss use a diode or gate.
5. Comparator and voltage reference.
6. You could use a linear regulator but at 25A you probably can't stand the power loss so a switching regulator is the only practical solution. Linear and Maxim both make switching regulator ICs that can do the or alternatively you might be able to buy a module that does it but it'll be expensive.
7. Overall good design, i.e using a highly efficient regulator and a switch with a low on resistance.

Going by your questions it looks as though you've been thrown into the deep end a bit. Have you attended all the classes? Has your lecturer being explaining things properly? Perhaps you need to ask him/her more questions

 

[ssembo]

Where are you going to use that? Have worked on something similar.

 

[tcmtech]

You are about 4 years late for the party.

 

[ssembo]

You are about 4 years late for the party.

You guys am learning alot from you. Its a funny one. The forum is quite interesting.

 

[kohansey]

I'm surprised this thread is still here. This project wasn't for a class, it was for something I wanted to do at home. Having no electrical engineering background, or studies, I would hoping someone with some background could point be to a circuit or schematic that I would be able to copy. However, it has been 4 years, and with no expertise, the project went by the wayside. Thanks again for you Hero999

 

[MikeMl]

So, have you learned something from the forums in the past four years?