AC-DC-AC Circuit Design

[dharrington86]

Hi all,

I am currently a studying Electrical Engineering and for my final year project I am studying the way in which an electrical tram works (light rail system). These systems distribute electricity in DC and convert it to AC on the tram to feed the motors.
For this project i will need to build a circuit using a software package (PSpice, Yenka or something similar), i will also be building a physical model once i have a working version figured out using a software package. My degree so far has been largely based on electric Power systems rather than electronics. i was wondering if anybody could advise me on a three phase converter system that i could build to change an three phase AC input to DC and then back to AC again. I have been trying it with IGBT's and diodes but with little luck. Any help would be greatly appreciated.

Thanks in advance
Derek

 

[ronv]

You don't say how big (current and voltage) you motor is but it sounds big.

You should be able to rectify the 3 phase comming in - for safety it would be nice to isolate it but motor size may prohibit that.

The motor is probably a brushless motor? In which case brushless motor chips are easy to find. Then maybe your big IGBTs to acually drive the windings.

 

[dharrington86]

Hi ronv,

The circuit will be a scaled down version of actual circuit. I will be building it in the University lab so it will be made up of much smaller components than are actually used. my rig will be used to show how AC is changed to DC and back again. Designing it using a software package is what im finding really tricky, esp trying to find a way to create a firing circuit.

 

[tcmtech]

Why bother building a VFD unit when you can use off the shelf units that can be had for far less and will work far better than any homemade design?

 

[ronv]

Much easier that way.
But you still have no specs.
You can make it real easy by using say a PC fan motor. You could then use a small transformer to drop the line voltage to a safe 12 volts. There is a neat speed controller posted here under articles by Alec T.
For software look at LTSpice - It's free.

 

[alec_t]

You can make it real easy by using say a PC fan motor. You could then use a small transformer to drop the line voltage to a safe 12 volts. There is a neat speed controller posted here under articles by Alec T.

Unfortunately no. PC fan motors generally, and the speed controller referred to, are 2-phase. The controller depends on commutation within the motor where current falls close to zero twice or more per rev. That wouldn't necessarily happen with a 3-phase motor.

 

[ronv]

Yes a 2 pole motor, but I think they are quite common and meet the requirement for "AC" motor that I think he needs. Correct me if I'm wrong but the 3 phase was just for example of a high current / high voltage motor that might be used in actual operation.

 

[tcmtech]

Well if its scaled down 3 thee phase VFD's and motors you need that's the realm of off the shelf RC toy motors and drive systems.

 

[ronv]

There you go! You could probably even find gears and stuff to make a tram. The big question is can you use "off the shelf" things or do you have to build all the circuits?

 

[alec_t]

Come to think of it, aren't VCR capstan motors and disk drive motors 3-phase BLDC? Might be worth cannibalising if the OP does need a low-wattage 3-phase motor.

 

[Mr RB]

Yep, they are 3 phase delta connected and will run quite well with good torques at low speeds. Their chips are hard to use, it's probably easier to build 3 half bridges and drive the 3 coil wires directly.

It was common for electric trams/locos etc to use only AC->DC and DC motors, this allowed the use of triac phase angle control onboard and simplified all the electronics. That would also be a much easier project.

 

[dharrington86]

View attachment 69051
Would any of you guys know what size components i would need to use to build the circuit attached (if this circuit is suitable) if i was to use a three phase 230V AC supply and then use the new AC supply to feed a three phase 0.5 kw 230V motor.

 

[dharrington86]

Thanks for the advice and I agree that it would be much easier but i have based my project on specifically on a new tram system which is being built in Edinburgh and these trams use AC motors. DC motors would have made this project much more managable!

 

[alec_t]

Have you checked out this?
https://www.google.co.uk/url?sa=t&r...kpC72r51A&sig2=eAgCuFNmD7qtBVVNKVywqg&cad=rja

 

[dharrington86]

Hi alec, thanks for that link, paper looks really helpful!

 

[dominiz]

Am a student of ElectricalElectronic, 100 level, I want materials on circuit reading and building.

 

[alec_t]

Dominiz, you have hijacked someone else's thread. Go back to the Forum tab on the home page of this site, choose an appropriate forum and create your own thread there by clicking the 'Create new thread' button.