4472
« on: May 25, 2018, 04:05:11 AM »
Hi,
I've been doing some more calculations and have come to the conclusion that 800W is more likely peak power rather than continuous.
My reasoning goes like this:
The voltage feedback constant is 7V/1000 rpm.
The back EMF at rated speed is 7/1000 x 3500 =24.5V
The back EMF is in effect the electrical reaction to the motor delivering rated power to a mechanical load, therefore the current is:
power = voltage x current
800W =24.5V x current
current= 32.6A
The total terminal voltage when producing 800W into a mechanical load will be the sum of the back EMF and the resistive loss of the armature.
Vtot = 24.5 + (32.6 X 1.3Ohm)
=66.9V
The total electrical power delivered to the servo is:
Ptot=66.9 x 32.6
=2180 W
Of which 800W is deliverd to the load so the remaining power is dissipated in the resistance of the armature:
Pwaste=2180-800
=1381W
Your servo would cook up BIGTIME if this were continuous. My guess is that the torque figure quoted is peak torque not continuous rated torque.
As peak torque is typically three to four times rated I would guess the true CONTINOUS RATED torque is 0.5 ft.lb and rated power is about 200W.
That would bring the dissipation in the servo down to 79W. That is a lot more realistic.
If my reasoning is correct then 200W servos could be used to good advantage on your machine and that may bring the price down somewhat.
I have heard CNCers say that very large and heavy and fast machines might have axis motors of 1 hp or 750W. It seems more likely that a smaller machine
like yours would have axis motors somewhat less than that, 200W is an estimate.
The power figures I quoted earlier from the DMM website are continuous power. It makes sense to compare your existing servos continuous rating
to the continuous rating of DMM's AC servos.
If you follow the proposal to replace the DC servos with AC servos then the biggest expense in the retrofit will be the servos and drives themselves. As you have seen
the DMM 400W 200V servos and drives are about $400 a set, or $1200 for the three axes. A 200W 200V servo saves you only $24 per axis, it makes sense then to get
the 400W units.
You may note that DMM list both 60V and 200V servos and drives in these power ranges. The 60V servos and drives are cheaper however you would then require high current
power supplies. The 200V units however are fed with 230V AC line voltage, no power supplies required. All up its a wash, they are both near identical in price given the power
supply. The 200V units require much less current and therefore are going to run cooler at elevated loads so the higher voltage units are preferred.
This proposal would cost you $1200 for the servos and drives and about $695 for Mach4, an ESS, two BoBs and a Lattepanda. This assumes you have a monitor and keyboard that you
could re-purpose. Not far off $2000.
Is this within your means? What would your machine be worth once its done?
Craig
Guest
Show Posts