Need assistance with designing a reversible Variable speed DC motor

[jparcjr]

HI everyone,

I am currently in the design phase of an electric car project and ran into a snag with the drive circuitry. I am trying to design a system that I can vary the speed and have both a forward and reverse setting. I am wanting to use power relays and use a three position switch to select fwd and reverse with the mid position as neutral. I also want it to have the capability to control the speed of the motor with a variable speed controller. I want to keep it as close to aviation in nature as that is what I know the best but I will use what will work.

I have the accessory system already designed as it was the easiest to do but I also will be coming to you guys with more questions about tying in multiple charging systems from two generators (I am considering alternators as well), several Ram Air Turbines as well as Solar panels and if I would need charge controlling devices.

Addition information, for the drive system there are at the present time 4 batteries of Deep cycle marine orbital gel cell batteries in two sets in series and one battery to power the accessories which all need to have their charge maintained. I should add I don't accept the word impossible or it can't be done as I believe that anything can be accomplished.

 

[alec_t]

There are many circuits on the web for DC motor control. Go googling and pick one which meets your power needs (which you haven't specified).

 

[Mithrandir2008]

Hey jparcjr,

About your drive circuit. Are you sure you want to use power relays? I am almost sure that an IGBT/MOSFET would better suit your requirements since you need variable speed.

What is the power rating and the motor type? If you need both reverse and forward motoring, a Full bridge DC-DC converter is probably your only choice(assuming your source is DC). As for speed control, you would need a speed sensor and some form of PI controller. Are you planning a digital implementation or an analogue(i.e., Opamp) implementation?

Whichever implementation you choose, get ready for some frequency domain modelling if you want closed loop control

 

[jparcjr]

Mithrandir2008,

No I am not sure I want to use power relays for the drive motors, its that I know that layout the best. Please keep in mind that I don't have a background in electrical engineering so I don't know what IGBT/MOSFET or anything else means. I want to keep the system as simple as possible so I am planning more analog then digital.

Right now I am looking at 24VDC motors with a relatively high initial current load at start. As I said I am still in the design phase and My goal is to keep it as simple as possible.

I know my limitations so I know when to ask questions like these, as I find that is the only way to learn how to do thing in electrical that work as I want them too. My background is aviation maintenance where we don't play with many wires or electrical wizbangs all that much as the boomatrons tend to get the best of us most times.

Philippe

 

[Mithrandir2008]

Well this is the thing.

If you are using power relays, it's a simple ON-OFF switch. So is a MOSFET/IGBT except that it can do it at a higher frequency.

If you need a variable speed motor, it means that you should be able to change the input voltage smoothly. The ON-OFF timings(or duty cycle) controls the amount of input voltage being applied. If however this switching is done by a relay, you would need a rather large filer(simple LC) to average out the voltage. Conversely if a MOSFET with relatively high switching frequencies are used(50KHz should suffice), the filter size becomes small(typically around 1mH inductor(ferrite core) and 50uF capacitor should be more than enough) . So if you need smooth variations in speed a MOSFET/IGBT switch is preferred.

For your particular motor voltage rating, a MOSFET(IRF540 should do nicely) is better(IGBT's are a bit expensive). You should look up the full bridge topology(which allows for negative voltages to be impressed for reverse motoring). You will need a simple PWM driver(SG3524/3525 floating voltage for top 2 switches, I think I saw the circuit in some other thread here).

As for the actual design, how much accuracy in speed do you need? Depending on that maybe you can avoid the more complex modeling.