UC3843 Current control

[zuq]

Hi guys

I am trying to build a motor speed controller, How would I set the reference current for inner loop when using UC3843? I want to control the current to be able to control the output torque.

Thanks

 

[randell_xtian]

Connect the Isense (current sense PIN 3) of this IC to the Input Voltage of the motor.

IS(MAX) = 1.0V / RS

To correct the output, connect a voltage divider to motor and the output of the divider should connect to the feedback of the IC (Vfb).

 

[zuq]

Hi Randell,
Thanks for your response.
I am still a little bit confused, should I be connecting the current sense pin straight to the input of DC motor??? Won’t that simply destroy the IC?? I was thinking more like using a current sensor such as LEM HS150P, it is a fairly large motor about 3kW.
I just need the throttle to be able to control the current reference.
Cheers

 

[mtwieg]

Do you need to run current in both directions? If so I don't think the UC3843 will work, unless you use some novel modification to the circuit.

 

[zuq]

Hi mtweig,
When you say running current in both directions, are you trying to say if i will be reversing the motor ?? If so, No motor only needs to go forward.

I had an idea about using UC3843 as a current controller, please some correct me if I am wrong.

Since the output of the error amplifier controls the duty cycle and sets the reference for output current,
how about using a the Vfb pin as providing current reference using a potentiometer or something, and using a Current sensor for current feedback? Would that work??
Keeping with in the 1V limit of the input of current comparator offcourse.

Aim is to control the current goin to motor to control the torque.

Cheers guys

 

[vimalkhanna]

Actually , for motor control ,you need to current control in biphasic mode .ie:;current could lead when the voltages go down.
As such using a 3525 would be a better bet

 

[randell_xtian]

No, I gave you a formula to calculate the Rsense. The max current that will pass thru or going to the motor is equivalent to the 1.0V over Rsense. The Rsense sholud be connected in series to the supply voltage of the motor and current sense PIN 3.

 

[zuq]

Thanks Randell

I see what you mean, i can have a fixed value of Rsense which would determine max motor current, but because I want to directly control the current to be able to vary the output torque would I need a variable Rsense?
Thanks for your patience

P.s this speed controller is for a kart

 

[mtwieg]

Hi mtweig,
When you say running current in both directions, are you trying to say if i will be reversing the motor ?? If so, No motor only needs to go forward.

Just because the motor is only spinning in one direction, doesn't mean the current will only need to flow in one way all the time. Do you need to exert torque in both directions? If so you will need bidirectional current control.

 

[zuq]

Thanks mtweig,
Yes u are right, current will flow in reverse direction at some point when motor emf becomes larger than average voltage applied. How would I go about bi directional control? I am using a single leg chopper. Can it be done with uc3843?

Thanks

 

[mtwieg]

With just a DC chopper circuit you can't force current in the reverse direction. That's just a limitation of the drive circuit, not the controller. So you can't control torque in the opposite direction; the only thing that could apply reverse torque to the motor in the reverse direction would be from mechanical braking.

 

[zuq]

Mtweig,
Motor only needs to go in forward direction, othrwise I would have used full bridge. What about variable currrent control now??

 

[mtwieg]

With a chopper circuit you're limited to controlling current and torque in one direction. Doing so with that control IC shouldn't be hard. Set up the error amp as an inverting amplifier, and feed it a variable voltage to control the setpoint on the current comparator. For a 3KW motor you'll probably want to sense current with a DC current transformer, but you could also use a shunt resistor on the source of the FET if you are careful with the layout.

 

[zuq]

Thanks mtiweg,
The thing with Uc3843 is that they have internal fixed voltage refrence which is compared to the output voltage through some voltage divider. In this case you can only have fixed output vlltage? How would I vary the setpoint then?

Any idea about the compensating the Error amplifier?? Would that be the PI controler in this case?? What would I need to consider when choosing values for compensator.

Much appreciate your help

 

[zuq]

Hi Mtwieg

Regarding UC3843, would it make sense to control the output current by varying the voltage on the inverting input of the Error Amplifier? since Voltage on non inverting input is fixed.

Cheers