60-70v unregulated power supply recommendations

[m_c]

Only the input to the transformer should be fused.

Fusing the output is more likely to blow up your drivers, than protect anything.

[Agathon]

You have gone to town on that. Have you fired it up to see if you are getting around 68V DC if so fine. Are you going to mount the tx on a ali back plate? Re the fuse if you are going to make a control box use din rail fuses and fuse to each drive

I'm getting 72v DC. I wanted to mount the whole lot on an ali or steel plate, but just couldn't find anything suitable in my stock. Do you think the transformer needs to be on a metal plate? It has a rubber washer bottom and top that would frustrate any heat transfer. Most of the old Swiss machines that I deal in for a living have low voltage control systems (low current-draw of course) and they usually use Paxolin to mount all the electrics. I did think about using this as I have a large sheet, but it was just easier to use the MDF.

Only the input to the transformer should be fused.

Fusing the output is more likely to blow up your drivers, than protect anything.

Thanks, that saves me a lot of trouble.

[JoeHarris]

Only the input to the transformer should be fused.

Fusing the output is more likely to blow up your drivers, than protect anything.

Is this true!? If so I may be heading in the wrong direction! I already have fuse holders for my drives - could put Neutral Links into them I suppose and convert them into switches! or just take them off... I have seen both sides of this argument and haven't got a clue which is right...

Also when selecting fuses for the VFD and the 24v supply I'm not sure what I should be getting in terms of current or speed. Apologies - clueless.

[m_c]

When modern drivers decelerate motors, the extra energy gets returned back into the power supply. With an unregulated power supply, the energy gets absorbed by the smoothing capacitor(s), which under even hard deceleration of your typical machine during normal use, might lead to a few volts increase.

If you add a fuse, and that fuse blows for any reason while the motor is moving, there is now nothing to absorb the return energy, which leads to a voltage spike on the drivers input, which can be more than enough to cause the drive to go overvoltage and release magic smoke.

Plus fuses are not likely to protect the drivers. By the time enough current has flown to blow a fuse, the chances are the electronics in the drive have already failed. Fuse blowing times are measured in milliseconds upwards. Electronic failure will happen in microseconds.

Fuse selection is generally best to see what manufacturers recommend. For anything with a high start up surge, you really want to use time delay fuses.

[Agathon]

When modern drivers decelerate motors, the extra energy gets returned back into the power supply. With an unregulated power supply, the energy gets absorbed by the smoothing capacitor(s), which under even hard deceleration of your typical machine during normal use, might lead to a few volts increase.

If you add a fuse, and that fuse blows for any reason while the motor is moving, there is now nothing to absorb the return energy, which leads to a voltage spike on the drivers input, which can be more than enough to cause the drive to go overvoltage and release magic smoke.

Plus fuses are not likely to protect the drivers. By the time enough current has flown to blow a fuse, the chances are the electronics in the drive have already failed. Fuse blowing times are measured in milliseconds upwards. Electronic failure will happen in microseconds.

Fuse selection is generally best to see what manufacturers recommend. For anything with a high start up surge, you really want to use time delay fuses.

Great explanation - it's obvious once you think about the BEMF issue.

[john swift]

you can have fuses in the DC suppy wiring to stepper drivers provided you have diodes wired across the fuse holder to maintain the path for the returned energy




John

PS

the Gecko G540 has an internal 10A fuse in the positive supply line !

[Lee Roberts]

John,

What's the formula to workout what size Diodes should be used ?

I find this one intresting because surly the diodes need to be rated such that they dont negate the use of a fuse in the first instance but can also still handle what may get thrown at them?

Any clarification is appreciated.

[Clive S]

John,

What's the formula to workout what size Diodes should be used ?

I find this one intresting because surly the diodes need to be rated such that they dont negate the use of a fuse in the first instance but can also still handle what may get thrown at them?

Any clarification is appreciated.

Lee the diodes will not conduct in normal use but if the fuse does blow and the motors is decelerating it would conduct to dump the BEMF into the caps and it would only happen for a few m/secs a diode handling 1-2A would probably be ok