Thread: parralell port voltage.
and yes jonathon has talked me through toroidal psu's... not going the pre commercial route... deep end and both feet me...hahaha
at the end of the day, i was looking at the single driver tb6580.? i think... then i was pushed toward m542. (eyup budget change).lol i want a machine that i can learn from atthe min.) was even gonna go the route of threaded rod as the lead screw... thats changed.. now looking into sprockets, rack n pinion. ive got time no were to build it yet. or put it.. workshop is only just starting...
hey deebee you wanna say sumat.. you aint spoke since 2010.... lol. summat weired on ere...
Steppers get there speed/torque from voltage/amps. So if your drives or PSU limit you below the motors ideal operating voltage/amps then you'll get far less speed/torque before the motors ideal corner speed is reached and torque drops away and stalls the motor.
So say at with 48V they will only reach approx 650-700rpm when motor stalls.
With 65v this changes to approx 1000-1100rpm. These figures are lose figures but you get the drift it could be 25-30% less performance than what the motors are capable of giving with correct size drives/psu.
Look around and some where on here Irving did a spread sheet that shows you the differences at different voltages.
All I'll say is your a braver man than me "Gunga Din" if you try to run these drives any were near them voltages. Give them 65V put the kettle on and see how long they last.? Bet you £10 the drives boil before the Kettle.!!
Last edited by JAZZCNC; 14-04-2012 at 01:57 AM.
BTW I am also a Wiganer but living down in London, where in Wigan are you? My mum still lives near Atherton. Cheers.
Last edited by crossleymarko; 14-04-2012 at 11:53 PM.
I'd bet more than £10 on that!
For a fuller explanation see my post here:
However you've got to consider that although you may find motors with a lower inductance, if their torque rating is much less you'll still have less torque at the corner speed.
Lets look at Zapp's offering:
The 1.85Nm motor inductance in Bipolar parallel is 1.6mH (compared with 3.2mH for the 3Nm). The corner speed can be approximated by 191*V/(IL), so on 48V that's 191*48/(4*1.6)=1432rpm and similarly 682rpm for the 3Nm motors.
Now we want to know which motor has the highest torque at the corner speed of the 1.85Nm motor, i.e. 1432rpm. This is a bit more difficult...
When operating at or below the corner speed the motor output torque is approximately the rated torque divided by square root of 2, so for the 1.85Nm motors it's 1.85/1.41=1.31Nm.
When operating above the corner speed the torque is calculated from the motor's rated power, which is calculated from; P=[torque at or below corner speed]*[corner speed]/9.55, so for the 3Nm motor the torque below corner speed is 3/1.41=2.12Nm. Substitute that into the formula for motor power and you get, P=2.12*682/9.55=151.5W. Now we can find the torque at 1432rpm, which is given by 9.55*P/rpm=9.55*151.5/1432=1.01Nm.
So at the corner speed for the 1.85Nm motor you will get about 30% more torque from the 1.85Nm motor than the 1Nm motor, however below 1100rpm the 3Nm motor will develop more toque than the 1.85Nm motor.
This demonstrates why it's a bad idea to just go and buy the biggest motor you can find, especially with Nema 34 motors, since the bigger motor may well actually be worse. But...
Thing to do now is do the calculation again, but this time take into account having pulleys to change the ratio since I'm confident you will find the 3Nm motors on 48V will perform better than the 1.85Nm on the same voltage if you gear them up - larger pulley on motor.
(This is essentially the same method of calculation as Irving's spreadsheet. I've just explained it step by step to hopefully make how the system interacts clearer.)
Oh and if you want to know the recommended minimum voltage for a stepper motor it's 32*L^0.5, where L is the inductance. So for the 3Nm it's 32*3.2*2^0.5=57V, and similarly 40V for the 1.85Nm motors.
If I were you I'd wait until you've got much further with the design and know what ballscrews (silly to use anything other that ballscrews since most of the motor power is lost via friction with single start leadscrews or threaded rod) you'll be using and the mass of the gantry etc.
The Following User Says Thank You to Jonathan For This Useful Post:
Hard facts of life here, no calculations but based on building over 120 cnc machines or conversions. The X3 cnc kit sold by ARC sold 58 units for a start and I'm not counting any of the Sieg CNC machines that are made by Sieg.
Hard fact is on any decent CNC stepper build the weak point is always the stepper drive, motors hardly ever go wrong unless you do something seriously bad to them.
Speed is voltage and the average stepper motor needs to run at 20 to 25 times it's nameplate voltage, as most are between 3 and 6 volts that's 75 to 150 volts. As most available stepper driver max out at 80 volts that normally determines the max. However the popular 80 volt Leadshine drivers also max out at 7.2 to 8 amps [ beware the 9.2's released , they were not reliable ]
When people read these specs they look well over spec'd for a 3.1Nm motor, which they are but as I said earlier the drivers are ALWAYS the weak link. now if you spend on these more expensive drivers and down rate you get various advantages, one is longer life because they are never stressed and always operation well with limits, never get hot, again for the same reason and you always have the correct driver for if you ever upgrade.
Do the sums, 3 expensive drivers that are the best part of bomb proof or buying cheaper ones and replacing then 2 and even three times ?
Out of those 58 X3 kits which drove 2.1Nm triple stack motors, we had 3 bad drivers and this one was from a guy who lives right next door to a power station and regularly has to replace televisions, computers etc because of spikes, as do his neighbours.
So discount this and it's a 100% success rate on machine used by users with different skill and experience levels.
Compare this to say a Route-out machine, I have often seen posts on here wanting new boards.
End of the day you get what you pay for.John S -
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