Originally Posted by
Geof
The title started as "battery assisted" but it transformed into battery powered. This is an idea that has popped into my head a couple of times reading posts by people who have set up rotary phase converters for CNC machines. It is necessary to size the rpc for the largest expected load and
normally with a CNC this is during spindle acceleration. For instance a Haas VF2 with a 10000 rpm spindle will pull 50 amps at 208V 3 phase (18kva) for around 300 - 400 milliseconds when accelerating but this drops back to around 40 amps (14kva) when it is roughing at a spindle load of about
120%, is down to around 10 amps or less (4kva) for finishing cuts and down to around 5 amps (2kva) when the machine is being reloaded. This means the full capacity of the electric service is being used for only a small fraction of the total operating time; averaged over time the machine needs about 5kva. The hour meters on the machine support this conclusion; Feed Cutting Time is approximately half the Cycle Time which itself is less than half the Power On Time so this gives the rough calculation that the average power needed is about one quarter of the full load power or about 4.5kva.
By now anyone who has done any reading on hybrid automobiles may have an idea where I am headed: Many CNC machines rectify the incoming power onto a DC buss which is used to synthesize variable frequency 3 phase AC or supply servo amps for the DC drives. A small amount of single phase AC is used to run cooling fans, operate solenoid valves and relays and run coolant or hydraulic pumps but this rarely adds up to more than the equivalent of 2 hp. On the DC buss there is a very small amount of averaging of power draw because there is a small amount of capacitative smoothing of the ripple coming out of the rectifiers but this represents only a few milliseconds of maximum power draw. However a bank of lead acid batteries on the buss could give several minutes of maximum power draw. Using the Haas system as an example; the DC voltage can range from 320 to 340V so acceleration draws 55 amps off the buss for about 400 milliseconds, roughing 34 amps and finishing 8 amps all plus or minus a bit. There would have to be twenty eight 12 volt batteries in the bank each about the size of a small automobile battery. These batteries would experience the same type of use as automobile batteries; brief surges of moderate to high current draw followed by a recharge period. Even a small automobile battery can provide 55 amps for several minutes.
Just slapping all the batteries in series across the DC buss might not be the brightest idea; having 340 volts available with almost no limit on the short
circuit current, and DC at that, could make a spectacular bang if something went wrong and be pretty hazardous for maintenance. The batteries would
have to be separated by fuses and some form of power-off isolation would have to be engineered. Also the simple bridge on the incoming supply would have to be replaced by a proper charging circuit so there would be a limit to the recharge current going into severely discharged battery and protection against overcharging when the machine was idle for a prolonged time. All of this is standard stuff and could probably be assembled almost off the shelf and all operate from a 30 amp 240 volt single phase (7.2kva) supply which would also have enough capacity to run the single phase AC requirements mentioned above. But if you threw into the mix a 2kva inverter creating 120V and 240V AC from the DC buss you would have a CNC running off its own uninterruptible power supply totally immune to incoming voltage fluctuations and able to be shut down in an orderly manner in the case of power outages; or you would be able to run for a long enough period of time to start and warm-up a 7500 watt diesel genset which could be useful in some situations
So has it been done?
