Diode selection help

[bgolash]

Hi
I would greatly appreciate some help selecting a
digikey diode to work in conjunction with my gecko
fusing. I`m adding a diode to each motor fuse to
protect the geckos from voltage spikes if the fuse
blows
My DC supply is 41 volts. The motors draw
7 amps maximum. I`ve looked at the specs on some
of the diodes but am not knowledgeable enough to understand
what rating would be correct. I did find a part # that looked
like it was in the ballpark FR606DICT-ND
http://www.digikey.com/

Thanks and regards,
Barry

[Al_The_Man]

How are you connecting the diode as there are other ways of using diode protection for voltage spikes etc, it is not good practice to place a component across a fuse, if the diode was to short out, you would not know it and you would have no protection.
Al.

[bgolash]

Hi Al

I was going to place the diode acrossed the fuse. You make
a excellect point indicating this is bad practice. How should
I design the diode placement.
Thanks and regards ,
Barry

[Al_The_Man]

First check if the Gecko's already have reverse emf diodes, if you add your own they could be connected reverse biased from after the fuse, if the fuse is in the DC line, to ground.
The voltage rating or PIV of the diode would rated at higher than your DC supply and the current rating would be the anticipated reverse emf current which is probabally an unknown, but cost of diode volt/amps are cheap, so you can always overkill with very little cost.
Al.

[Mariss Freimanis]

The purpose of the rectifier is to return the motor's stored inductive energy back to the power supply should the fuse blow. Otherwise you have a "the operation was a success but the patient died" situation. The fuse blows and disconnects the supply from the drive. The collapsing magnetic field in the motor returns the stored inductive energy which now has no place to go now that the fuse has done its job. The voltage on the drive rises until it reaches breakdown voltage (about 116 to 118 VDC) and releases the magic smoke.

Putting a rectifier (3A at 200V is plenty) across the fuse, cathode (banded end)towards the power supply, gives this energy pulse a non-destructive path. Now it has a place to go.

Mariss

[Al_The_Man]

I have never seen this method or practice used in any commercial system and I still think it is a not only a bad idea, but not neccessary to place any other component across a fuse.
Al.

[gar]

050802-1914 EST USA

Mariss:

Al is correct. The stored energy in the inductors does not have to be transferred back to the power supply. A diode across the fuse is a bad idea.

In any inductive circuit there is energy stored in the inductor when a current is flowing. A characteristic of an inductor is that you can not instantaineously change this current. Thus, when a switch to an inductive circuit is opened and there is no alternative path the inductor will produce a voltage large enough to maintain the current that existed just before opening the switch. This causes an arc across the switch or some where else to maintain a closed circuit for the current.

One needs to know more about the exact circuit in the driver to best determine how to protect components in the event a fuse blows. Maybe the circuit is self protecting, at least if power remains on the control circuitry,

Basic to a switching power supply is a short pulse of current to an inductor and some means to provide a current path when the switch opens.

In a chopper drive to a stepping motor you will find a path for this current, it may be a diode or another switch. There is always the internal resistance of the inductor to dissipate energy, but also there may be external resistors and other devices that dissipate energy,

.

[Mariss Freimanis]

You may be confusing a fuse in the primary circuit with a secondary protection fuse. Nothing should ever be paralled with the primary fuse. Its function is to absolutely and decisively disconnect the mains from the failing equipment it powers. This is for all sorts of safety reasons and is meant to prevent a catastrophic failure. This stands even if the fuse activating causes further economic damage to the equipment.

Secondary function fuses are optionally present to prevent economic damage to what they protect; personal safety is already addressed by the primary fusing. Common examples of "economic" fuses are semiconductor resetable fuses and short-circuit protection circuits in drives for that matter.

The fact that a fuse used for a secondary protection function resembles or even is the same component as used in primary fusing is coincidental. Here I consider it just another electrical component I can use as I please, diode included.

Hope that helps.

Mariss

[Mariss Freimanis]

GAR,

It doesn't much matter if you disagree with me; I don't make the rules, nature does. If you like, perform the following experiment but please don't use a bipolar drive you want to keep afterwards.

Use a NEMA 34 motor with a bipolar drive and connect the power supply via rectifier diode to the drive. It will simulate a blown fuse by not permitting current to return to the supply. Use a supply voltage less than the drive's rated voltage of course.

If you have a scope, place it across the drive's power supply inputs (Gnd and cathode end of the rectifier).

Run the motor up to 3,000 RPM, preferably to 6,000 RPM. Most unloaded NEMA-34 motors can reach that speed with a 40VDC supply if accelerated very carefully. A large NEMA-34 motor will store about 35 Joules of mechanical energy at 6,000 RPM, simulating a larger real-life inertial load at lower speeds.

Now abruptly stop sending step pulses to de-synchronize and stall the motor. Be watching the scope when you do. I simplified when I said inductive energy; at 7A, 2mH motor coil stores only 0.049 Joules per coil.

About two-thirds of the stored 35J or 23 Joules will be returned as electrical energy, the balance being dissipated frictionally.

The results may be spectacular.

Mariss

[Al_The_Man]

It seems to me the logical conclusion to this pointless discussion is to not not fuse the DC output of the supply as is common commercial practice and applies to all of the NEMA 34 drives and motors I have installed.
Al.

[Mariss Freimanis]

Absolutely correct Al, just like you can watch that unfused drive smoke as a consequence of a momentary short on a motor lead, reversed power supply polarity, etc. etc. Totally pointless if you are perfect. I will not bother you with my opinions on the matter; I'm just a dumb EE that designs motor drives for a living.

Mariss

[gar]

050802-2102 EST USA

Mariss:

How about back to back Zeners across the motor, or SCR crowbars, or other appropriate transient limiters.

.