You apparently are planning on using a power supply that is somewhere in the vicinity of 40v on motors that are rated for 3A; I've already calculated that 6.9v is the maximum voltage to apply if you are not going to use any form of current limiting. If you try to use several times the 6.9v with no current control, you will burn the motors up very quickly, or damage the power supply.
is 6.9v the max volt the motor can handle ? " who u calculate it ? " , when i buy this motor , the Seller say to me that this motor can handle 3A max and 40V max .
so i suggest use 40v.
Now if i used 6.9V 9A power supply so i don't need current limiting controller ?
As high as you can go without exceeding the 40v limit. However, this means that you have to use a chopper driver. I don't think that you understand how a chopper driver works.
i found some resource about chopper circuit: "Current limiting is used when a stepping motor is driven
at a voltage that is higher than the motor’s rated
voltage. There are several advantages to driving a
motor at high voltage, namely, the torque and speed
characteristics of the motor are improved. These
parameters are improved because the current in the
motor windings is more responsive to changes made
by the controller."
from http://ww1.microchip.com/downloads/en/AppNotes/00906a.pdf
i found some resource about chopper circuit: "Current limiting is used when a stepping motor is driven
at a voltage that is higher than the motor’s rated
voltage. There are several advantages to driving a
motor at high voltage, namely, the torque and speed
characteristics of the motor are improved. These
parameters are improved because the current in the
motor windings is more responsive to changes made
by the controller."
from http://ww1.microchip.com/downloads/en/AppNotes/00906a.pdf
If you want to operate those motors with a supply voltage above 6.9v, you will have to use a current limiting circuit, and a chopper driver is the best way to do that.
If you don't mind losing torque and speed, you can go with the simple schematics of just the MOSFETs - but you'll need separate inputs for each motor.
The L298 by itself does not have any chopper driver controls in it; it needs the L297 to provide that function.
I'm sorry, but it's late and time for me to retire for the evening.