Running a Stepper on Certain Rating

[baberjaved]

I interfaced a two-phase Bipolar Stepper motor (5.7V - 1.6A Rating) to a L297/L298 circuit and i am powering up the motor with a 5V power supply that can handle upto 2A of current but when i measure the current currently used up by the motor using a Ampere meter it comes out to be only 0.2A. How can i increase the current used up by the motor. Also What effect would it make if i increase the 5v applied to the motor.

 

[Nigel Goodwin]

The current draw depends on the load, on a light load it will take less power.

However, it's normal to run steppers off a MUCH higher voltage, and limit the current - the usual way is via large wirewound resistors, although you can use PWM if you wish.

 

[gramo]

The L297 and L298 combo incorporates a chopper circuit. The sense lines sample the current driving the windings, and ensure the current doesn't exceed your "set level" via the choice of resistors on the sense lines, i.e.

**broken link removed**

 

[gramo]

Just in regards to the circuit;

One pulse from the PIC will result in a 1/2 step on the motor, and it will hold there awaiting the next step. Note C1 and R1 - 3, they form the chopper circuit. This is a form of current control for the motor, to allow the input voltage to be greater than that of what the motor is rated for. To find out how much current the stepper can handle, read the datasheet, or divide its nominal voltage by the resistance of the coil. The ammeter in this circuit was used to ascertain the values of R1 and R2, as the supply voltage was 24VDC and the stepper nominal voltage is 12V. Without chopper control the motor will draw too much current, so R1 and R2 are increased in value until the current being consumed by the motor is the same to that as if it were only connected to 12VDC (~.05A).

 

[baberjaved]

Yes, I calculated the sense resistors to be around 0.5ohms in order to deliver 1A current to the motor but the motor still only uses 0.2A
Do i need to adjust the values of those resistors in order to increase the current level? Or would just increasing the voltage increase the current?

 

[Nigel Goodwin]

Like I said above, the current it draws will depend on the load - if it's going round OK, and only drawing 0.2A, then that's probably all it needs?. Bear in mind that you lose a LOT of volts through the driver chip, so the motor won't be getting 5V - so increasing the supply is probably a good idea anyway.

 

[baberjaved]

my bad.. Maybe i am not being clear enough. What i actually want is to increase the RPM of the motor... would increasing the voltage do that? Coz its moving quite slowly like 30 rounds per minute only...

 

[Nigel Goodwin]

my bad.. Maybe i am not being clear enough. What i actually want is to increase the RPM of the motor... would increasing the voltage do that? Coz its moving quite slowly like 30 rounds per minute only...

Yes it will, using the higher voltage is used to increase speed and torque - by running it substantially under voltage you're really slowing it down.

 

[baberjaved]

Ok i got this current thing. One more thing, If i have a 1.8 degree stepper and i run it on Half-Stepping.. one step would move it 0.9 degrees right?
But what if i want to move it exactly One degrees? Any possible way to do this except using Microstepping?

 

[gramo]

Use gearing

 

[baberjaved]

lol Cool... Thanks! Why didnt i think of that =)