Help sizing fuses for Gecko 320x drivers

[Jackal66]

Hello,

I have (3) Gecko 320x drives. The servo motors are rated 90 VDC, 12 amp continuous and 20 amp peak.

The power supply is putting out 77 volts DC.

What size fuses do I need for these?

I have a diagram that came from somewhere, but another site said the 320x's didn't need fuses.

Here is the diagram. It always makes me feel better to run fuses.



Help set me straight on this.

JAckal

[morestuff]

Using fuses to protect a device based on semiconductors may
make you feel good, but it will seldom accomplish what is
intended. The reaction time of fuses is just too slow to actually
protect the semiconductors in most cases. When there is an
overload rather than an outright short, and the semiconductors
were designed with sufficient margin, it can work; but more often
the event that we are trying to protect against is indeed a
short circuit and the semiconductor is killed before the fuse
has time to react. For this reason, many systems that have
semiconductors are designed with fast reacting internal electronic
circuits to protect them against short circuits. The G203V stepper
motor driver and the G540 driver box are examples of this approach.

This does not mean that there is no need for fuses. They can
help by preventing total melt down or flame out even if they
do not save the semiconductors. This makes it more likely that
the unit will be repairable by replacing the semiconductors. Fuses
also serve to protect items like wiring and transformers that do not
fail faster than the fuse can blow.

With this in mind the question becomes where should the
fuse be located. My personal choice is to put the fuse in
the primary side of the power supply. This will protect the
supply and the items that it powers in most cases and only
requires a single fuse. True, it may allow one driver to draw
more current than it is rated to withstand, but if this is going
to happen with a G320X, the G320X has already failed. The
same can be said of most stepper and servo motor drivers because
their normal operation includes controlling the maximum current
into the motors.

If you do choose to put fuses between the power supply and
the G320X drivers, I would recommend that you also put a diode
across the fuse that allows current to flow from the G320X back
into the power supply even if the fuse is blown. This will help
protect the G320X from over-voltage after the fuse is blown.
Motor windings are inductors that will produce voltage spikes
when current is suddenly interrupted by the fuse blowing. The
diode provides a path to shunt this voltage spike. Also, a servo
motor that is being mechanically driven and operating as a generator
may apply excess voltage to the G320X if there is nowhere for
the current to go after the fuse is blown.

A fuse placed on the primary side of the power supply should be
sized according to the ratings of the power supply, assuming that
the supply is not grossly over sized.

Regards,
Steve Stallings
PMDX.COM - Products for CNC and motion control applications

[Jackal66]

Thanks Steve,

That was well explained.

What size fuse should I use for the transformer?


It is 1600 Volt Amps, 60 Volts, 30 amp rating.

After removing a few turns, it is putting out 54 VAC before the rectifier.

Thanks,

JAckal

[morestuff]

For a fuse rating, you can divide the VoltAmp rating
of the transformer by the line voltage. For example
1600 VA divided by 120 VAC yields a 13.3 ampere
fuse. In this case I would probably recommend a
12 ampere fuse since 15 amperes would be the next
common size and it is more that I feel is needed.

You may also want to consider a slow blow fuse.

Regards,
Steve Stallings
PMDX.COM - Products for CNC and motion control applications

[Jackal66]

Steve,

Thanks for all of your help,

JAckal

[Larken]

If the motors are rated at 12 amps continous, use a 12amp slow blow fuse for each drive.

Its the expensive motor your trying to protect, not the drive.

The powersupply fuse is a different story, like said above its to protect the transformer.

Larry K

[Jackal66]

Larry,


Thanks for the info.

Jack