Bridgeport Z Axis

[kewl_cat]

My Z axis blew a fuse after it hit a M2 instruction. I replaced the fuse but the axis has not worked too great since. It will no longer jog but it will work in very slow feedrates under 5.0 ipm. I checked the current on the axis and it was around 12A. I lowered it via the ACC board but it did not change anything. I checked the voltage in the back cabinent and the z is pulling more volts. All the voltages at the T1 and T2 and they are fine. When I take out the fuse for the Z, all voltages return to normal. I thouroughly checked for stray bolts and washers around the SMD boards and everything looks good. I have checked everywhere I can think of for shorts, bare wires and like things. I changed the ACC board and swapped SMD boards and nothing seems to help. I have always had problems with the Z since I bought the machine. It has a new motor, boards and such but the Z is always a struggle. Oh, and it it not the quill because it does the same thing with the belt off. Any ideas where or how to trace down this problem? Thanks in advance.

[machintek]

There are 2 Z axis fuses. One for the DC side of the bridge rectifier and one for the AC side. You should be pushing 8 amps through the Z axis DC fuse.
Put it in step and monitor the base voltage as it turns on and off for each transistor. Monitor the output of each transistor as you step the axis.
Make sure all transistors are good, bridge rectifier is good, and no screws or washers on or in the terminal strip with the motor wires.
Do you have the screw in fuse holders? Or snap in?
Check the SMS board if so equipped and the resistor on the saturable core reactor.
T2 should be no less than 54 VAC and no more than 63 VAC.

George

[NC Cams]

If you have high current draw, this means either mechanical binding or current is going someplace it shouldn't via path it shouldn't be taking.

Mechanical binding should be easily traceable.

Current draw is either due to "leak" in wire someplace OR it could be "shooting through" the amplifier.

Shoot thru occurs if the amp is DC and it is using an H bridge drive scheme.

SHoot thru means that one of the transistors from DC+ to motor is conducting current around the motor to ground directly thru the transistor from Motor to DC- on the same side of the H bridge.

Cause: either partially shorted/conducting drive transistor or drive circutry that is turning on both top and bottom of right or left side of H bridge simultaneously - thus current is "shooting thru" the transistors which are normally NEVER on simulaneously.

If you disconnect the Z axis motor, you shouldn't draw current when a drive command is give. If you STILL draw current with motor disconnected, the Z axis amp is probably leaking electrons in an improper manner and needs service or the wire from amp to motor is shorting current to ground.

Mosfet based servo amps are not that hard to fix. As a matter of course, you replace any/all diodes in the output stage as these could also be leaking. After you replace diodes, check. THis simple and easy and inexpensive fix may solve the problem. IF not, then do the mosfets.

Most drives have pretth robust fet drive circuits and either fet or diodes are the cause of current leakage.

Hint: once you find the part number of the diode, replace with similar type or one of same series but with higher amperage/wattage rating. IF you replace the fets, simply replace with the same make and model or equivalent from another vendor. For the minor cost involved, be safe and replace all of them .

If you have amps with Darlingtons, these can be hard to get and some are getting REAL expensive. Reason: darlingtons are old technology and fets have pretty much replaced them for high current/high frequency switching applications.

Good luck

[kewl_cat]

It is the DC fuse for the Z-axis that was pulling 12Amps.
Per one of your previous posts I changed the fuse blocks to snap-in.
T2 is 56V.
The bottom transistor does not switch like the others so I replaced it. No change. The others would switch from 3V to about 1.5V. The bottom only goes 1.5 to 2. It is wire 90 which I think goes to the SMD. I switched out the SMD and no change. I also changed out the ACC and no change.
All the transistors and bridge rectifier checked out good.
No wayward screws or washers anywhere that I could find.
The voltage on the SMS board for the Z was 6V and the others were about 8.
I think this has indicated a problem but what to do next? Thanks, -rob

[NC Cams]

Are there flyback diodes protecting the H bridge transistors??? If so, these can short or zener. These are not the same as rectifier diodes as they bypass flyback current generated from motor around the switching transistors.

I've learned that it is easier to replace them than "check" them. Why? Diodes can start to zener when there is current flow potential yet still check fine with a voltmeter. You specifically did not mention checking or identifying these.

i don't think you know yet where the current is going nor how it is getting there. Yes it is flowing but until you find the actual leakage PATH to ground, you're going to be stumped. This will result in random parts changing until/unless you luck into something.

You have to move down stream of the fuses and home in on the leakage path.

Does the current still flow WITHOUT the motor being connected??? Yes or no. Yes, its not motor, no, it could be motor

Does it flow without the motor and without the output wire from amp being connnected??? yes or no. no it is probably harness short, yes it is looking more and more like amp centered leakage

Remove amp transistors from motor to DC- bus, one leg at time. if you stop current flow, it is in the circutry of the transistor you removed, if not, try another.

Now remove transistors from motor to DC+ in same fashion - use same logic. Find where current is going.

NOw the hard part. If all this checks good or you find transistors to be ok, look to see if the drive current/voltage is appropriate, logic wise. Voltmeter or scope time.

You'll need to determine if the transistor is an NPN or PNP type for this. Semiconductor manual will help here as will manual for the amp.

Look to see if you have gate or base drive voltage to switching transistor. Recall that applying voltage to NPN gate/base causes the transistor to turn ON. Applying gate voltage/current to PNP causes it to turn OFF.

Make sure voltage is appropriate to turn all transistors off when they should be off and vice versa. If voltage is wrond or leaking, it may be time to have amp serviced. You did say the leakage followed the amp, didn't you??? If so, that says VOLUMES.

A short in the drive circuit could/would improperly holding an NPN transistor ON thus shoot thru. A latched driver (shorted off) could allow a PNP to be staying ON instead of turning it off thus shoot thru.

If you use a logical method of finding where the current is improperly flowing and WHY, you'll find your problem and identify a solution all at the same time....

This root cause identification process worked for me and thus led me to a pinched wire. The 'world authority' on my machine indicated "you have a defective amp - send $1300 for a replacement" - DUH. I didn't think so and the problem solving logic proved correct.

When challenged with the logic, the "expert" issued credit for his bill, too, no questions asked...

Once you find out where the current is going via leakage, you're in much better shape in regards to fixing the problem.

The report that one transistor seemed funky suggests that there still may be something wrong in amp or H bridge. Semi shorted transistor or a shorted transistor driver or zenering flyback diode will result in "shoot thru".