Richards

Wed 02 February 2011, 18:32

We've all been using the formula:

32 X SQRT( Inductance ) = MAXIMUM voltage as the basis for selecting a power supply. In reality, that formula tells how high of a voltage we can use before the insulation melts in the motor; it doesn't really tell us what the best power supply is.

I just completed some interesting tests where I played with various motors and various power supplies. After running those test, I decided to find out what the minimum voltage would be to still give me 750 RPM (shaft speed, before a belt-drive or gearbox) with a PK299-F4.5 stepper (2.5 mH inductance) and a PK296B2A-SG3.6 stepper (1.5 mH inductance).

The PK299-F4.5 ran at over 2,000 RPM with a 47VDC power supply computed as near maximum using Mariss's formula. It sounded great. It made a great impression. But, in reality, everything over 750 RPM was just for show. At speeds higher than 750 RPM, the torque chart shows too little torque to be of any practical value.

I set Mach 3 to 45,000 pps, with 2,000 steps per inch and 750 inches per minute as the top speed, then I used my Variac to turn down the voltage until the motor could not hit 750 RPM. I went all the way down to 24VDC, and the motor was still running perfectly. 24VDC was the lower limit because my test bench has G202 stepper drivers that require a minimum of 24VDC to work properly.

The PK296B2A-SG3.6 motor did just as well. I limited it's top speed to 1,500 RPM so that the gearbox wouldn't be damaged. At 47VDC, it hit that speed with no problem. Then, I changed the parameters in Mach 3 and limited it to 750 RPM. At 24VDC, it was still running perfectly at 750 RPM.

My conclusion is that I don't have to worry about getting a perfect match between the motor and the power supply on a CNC router. With the motors that have been suggested for years (which is basically any motor that has an inductance of 2.5mH or lower), a 24v to 35v power supply will probably work just fine.

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I selected 2,000 steps per inch in Mach 3 as a simple way to figure RPM. A Gecko stepper driver requires 2,000 steps per revolution, so with 2,000 set as the number of steps per unit (inch), I could just dial in the number of inches per minute that I wanted and know that I had also told the program what RPM to use.

32 X SQRT( Inductance ) = MAXIMUM voltage as the basis for selecting a power supply. In reality, that formula tells how high of a voltage we can use before the insulation melts in the motor; it doesn't really tell us what the best power supply is.

I just completed some interesting tests where I played with various motors and various power supplies. After running those test, I decided to find out what the minimum voltage would be to still give me 750 RPM (shaft speed, before a belt-drive or gearbox) with a PK299-F4.5 stepper (2.5 mH inductance) and a PK296B2A-SG3.6 stepper (1.5 mH inductance).

The PK299-F4.5 ran at over 2,000 RPM with a 47VDC power supply computed as near maximum using Mariss's formula. It sounded great. It made a great impression. But, in reality, everything over 750 RPM was just for show. At speeds higher than 750 RPM, the torque chart shows too little torque to be of any practical value.

I set Mach 3 to 45,000 pps, with 2,000 steps per inch and 750 inches per minute as the top speed, then I used my Variac to turn down the voltage until the motor could not hit 750 RPM. I went all the way down to 24VDC, and the motor was still running perfectly. 24VDC was the lower limit because my test bench has G202 stepper drivers that require a minimum of 24VDC to work properly.

The PK296B2A-SG3.6 motor did just as well. I limited it's top speed to 1,500 RPM so that the gearbox wouldn't be damaged. At 47VDC, it hit that speed with no problem. Then, I changed the parameters in Mach 3 and limited it to 750 RPM. At 24VDC, it was still running perfectly at 750 RPM.

My conclusion is that I don't have to worry about getting a perfect match between the motor and the power supply on a CNC router. With the motors that have been suggested for years (which is basically any motor that has an inductance of 2.5mH or lower), a 24v to 35v power supply will probably work just fine.

-----

I selected 2,000 steps per inch in Mach 3 as a simple way to figure RPM. A Gecko stepper driver requires 2,000 steps per revolution, so with 2,000 set as the number of steps per unit (inch), I could just dial in the number of inches per minute that I wanted and know that I had also told the program what RPM to use.