I have an Electrical Question for Hardcore retrofitters

[springlakecnc]

Hello electronic engineer,

Ok, you know about sizing servomotors to drives, and so do I (kinda).

If a servomotor is a nema42, 12 amps continuous, 35 amps peak, a 40-amp drive will work nice.

If you wire this same servomotor to a 20-amp drive, the servomotor will probably take off like a bat out of hell, but when you go to stop it, POW! THERE GOES THE POWER MOSFETS!

Whenever I do a board level retrofit, when I’m ready to test the system, I always put way undersized fuses in everything, for start-up. I did notice, sometimes you can run 3, 12 amp, 40 amp peak, nema42 motors, in a 3 axis milling machine, with a 6 amp fuse going to the power supply for all 3 axis. Meaning that most of the time, you are not drawing anywhere near the power the system could put out, maybe only for acceleration or when you crash into something.

Rutex has some interesting reading on their web site (or they used to) telling you the calculation of how to put an "in line" resistor, in one of the power lines, that will protect the drive against oversized motors and not sacrifice as much power than you may think. Rutex does say you can get away without doing anything special, if you can guaranty you will not decelerate the motor any faster than you accelerate it. But, if you’re motor goes into oscillation while tuning, or a heavy load on the mill table pushes to hard when you are stopping, or you simply crash an axis into something, BAM! Drive gone!

My question is, will this work??

Say you have that 35-amp peak servomotor, and the 20-amp drive. You wire the two power wires as you normally would, with exception of: The Armature + line, you put a 20 amp fuse, thus anything over 20 amps will blow the fuse. Now you run a second line in parallel around the fuse with the correct resistor in it. The theory behind this is, the electron flow will take the easy path through the fuse, and just bypass the resistor allowing the drive to put out 100% performance. If there is a power surge from the servomotor, the fuse blows and the resistor helps the drive sop up the braking amps, thus sparing the life of the drive! (I e mailled this to a couple of drive manufactures, and got no responce)

Any thoughts?

If I don’t get any input, someday I’m going to take a 45 amp electrocraft servomotor like on a Hurco machine, and hook it to a Gecko drive, with the 20 amp fuse & resistor, and see what happens!

This is kind of important, as MANY of the people that are building a cnc control, either don’t have the money to do it right, OR, they just don’t know the difference between “peak” and “continuous” amp ratings. I just answered a man’s questions; he bought a Hurco with a dead cnc, and was going to run the servomotors that were on the machine, with Geckos! He thought 20 amps were enough to power the motor, but he obviously doesn’t know what stops a servomotor!

Talking about undersized, I hooked my 18 volt drill motor battery to a Cincinnati 20hc2000 100”x40” travel, Ghetty’s servomotor, to jog the axis out of the way, and it worked!

Thanks All.

[Al_The_Man]

Originally Posted by springlakecnc If you wire this same servomotor to a 20-amp drive, the servomotor will probably take off like a bat out of hell, but when you go to stop it, POW! THERE GOES THE POWER MOSFETS! .

Using a lower current rating of a drive does not run a motor any faster, as a generalization, current is torque, voltage is speed.
Using a larger motor within reason, will prevent obtaining the torque capability of the motor.
I am not familiar with the drives you quoted, but all the ones I have used have adjustable current limit feature, I have fitted larger motors to smaller drives in a pinch until I can obtain the correct sized one.
The loading has to be watched or the current limit will kick in and shut the drive output off, I have never lost a drive to this yet.

Originally Posted by springlakecnc Rutex has some interesting reading on their web site (or they used to) telling you the calculation of how to put an "in line" resistor, in one of the power lines, that will protect the drive against oversized motors .

If the drive is designed right, there is no need for series resistors etc.
I always fuse the AC input to the drive PS rather than individual drives on the DC side, this can sometimes blow the drive.
Also if you stall a servo motor, it is going to offer a load that imitates a MUCH larger motor, obviously it should not blow the drive if this happens.

Originally Posted by springlakecnc Talking about undersized, I hooked my 18 volt drill motor battery to a Cincinnati 20hc2000 100”x40” travel, Ghetty’s servomotor, to jog the axis out of the way, and it worked! l.

Done that many a time with an automotive battery.

Al.

[neilw20]

If you have as motor running and it regenerates that energy has to go somewhere. If it is going through a fuse, and the fuse blows, now you end up with spare energy being turned into a higher voltage. While the fuse is blowing the plasma in the arc is quite low impedance and any extra voltage is sent back to the supply. If this exceeds the ratings (either voltage or current) of the MOSFET the fuse really hasn't protected anything yet as the plasma is still allowing current to flow.
Putting a resistor across the fuse can allow the plasma to extinguish earlier and the resistor may reduce the current to a safe value.
The voltage across the plasma may be in the order of 10-80v.

If the fuse is big enough, then any excessive current has to go somewhere or become a higher voltage. This is normally absorbed by your DC filter capacitor. Naturally if you pump up your supply to above the design rating of the drive circuitry it is a potential (no pun intended) risk.

The extra energy needs to go somewhere and if the avalanche capabilities of the MOSFETS can't handle it, then you need a DYNAMIC SNUBBER.This can take the form of a MOSFET in series with a low value power resistor that gets switched across the supply during the spike. But it not a simple as that, because you don't want the supply powering the resistor too.
This is the mechanics of it. The solution is beyond the space available (on my screen).

[CarbideBob]

"Hardcore retrofitters"?
Would that be people who build strange machines for XXX movies??

I don't think your idea will work so well as it's most likely the volts not the amps that is taking out your drives. Besides what good is it if you blow a fuse every time you decel from rapid?
These amps don't have overvoltage and overcurrent protection ???

See the engineering notes on AMC's website. Also see the section on shunt regulators.
Never use a power supply voltage of more than 75% of the amps full voltage rating.

I've driven many motors with amps rated at 25% of the motors rating and I've never toasted an amp.
Bob