The Power Supply needs only to supply 2/3 of the rated motor current
An exchange of posts on the Gecko Yahoo forum in the last few days:
Mariss: Here's an easy way of looking at the question. 50VDC at 7A is 350 Watts. Divided four ways, it portions out to 87.5W per axis. Very few NEMA-23 step motors will deliver more than 75 Watts to a load. Rarer still is having all 4 axis loaded to the limit and turning at a speed high enough to deliver that amount of power simultaneously. If you are interested enough, set your multimeter to the 10A DC current range and put it in series with your power supply. You will be surprised how little current is drawn from the supply as you exercise your machine. Four axis worth of 3.5A motors will have 19.8A sum of total current flowing in their windings. The power supply current will be under 5A while that happens. Power supply current is always much less than the sum of the motor winding currents. Gerald: Mariss, I have done that measurement and confirm that the actual current drawn off the power supply is to the order of well below one third the sum of the motor currents in typical CNC routers. However, the advice for sizing the supply is to go for 67% of the sum of the currents. Can you give some background on why you suggest the 67% figure? Mariss: The purpose of the "67% rule" is to size a power supply to a limit that will not ever be exceeded by the drive under any condition. It (67%) is measured data taken from many dozen motors of various sizes and ratings. Not emphasized enough is under what conditions this was measured. The test motors were run at moderate to high speeds (800 RPM to 3,000 RPM) and were then loaded to stall. The supply current was measured at the instant before stalling. The rule is very conservative because maximum supply current draw occurs at the 100% load point at moderate to high speeds. It is less sensible in a 4-axis system because the likelihood all 4 axis being operated in this fashion simultaneously is very low. This current is independent of the supply voltage. This is expected because motor power output increases proportionally with supply voltage. Assuming constant motor efficiency, power supply Watts required increases proportionally with voltage as well. This assumption requires the current to be independent of voltage. Gerald Thanks for the reply Mariss. |
All times are GMT -6. The time now is 06:11. |
Powered by vBulletin® Version 3.8.3
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.