Look at this document, these calculations work.
Im entering a new phase in my cnc project now and yesterday I recived my 3 4.5 Nm Biopolar steppers that will have parallell wiring.
The Torque/speed curve shows 75v so I guess that the voltage I´m aming for. The specs for the motor says the rated current is 6.3 Amps when wired parallell.
According to Gecko "Step motor Basics" you should count with 2/3 of the rated current when parallell wired --> 4,2 A / motor x 3 = 12,6 A
Im using 3 Gecko 203V and they are good for 80 V, but I want to run at 75 V. I found this in another forum "To calculate the desired transformer voltage divide the desired power supply voltage by 1.4. "
If this is right I get: 75/1.4=53,57143 V
To calculate the transforemer I use: V*A -->53.57143*12,6=675 VA
Capasitor calculation: C= (100 000( for 50 Hz) * I) / V. I dont know i should use 53.57 for V or 75V but if I use 53.57 from above I get:--> C=(100 000*12,6 ) / 53,57143 = 23 520 uF
Is there anyone that could help me to check if this is right berofre I go and spend my money on parts?
Before deciding on 75V for the power supply, what is the motor's parallel inductance? A very useful formula that Mariss at Gecko has provided is MAX voltage = 1000 * SQRT(inductance). For example, an Oriental Motor PK299-F4.5A motor wired parallel has an inductance of 2.5mH, so 1000 X SQRT(0.0025) = 50V. The data sheet shows a torque curve with a 60V power supply. I've tried 60V and could smell unpleasant odors coming from the motor. Now, I run that motor with a 35V power supply to keep from frying the skin off my finger tips when I touch the motor.
I always use the full current rating for square type motors instead of the 2/3rd's rating. When I use old round motors, I use the 2/3rd's rating.
If you can find the proper caps, 20,000uF to 30,000uF should handle most of the A.C. ripple on this kind of power supply. I use 20,000uF on a 70V power supply that has four 3A motors connected to it and have about 2 to 3 volts ripple, depending on how the motors are being used. Typically, I like to see less than 5% ripple when I'm using a power supply that produces 70 - 75% of the maximum computed voltage for a motor.