Re: contactor contact sizing help needed

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "oldpayphones" <larry@...> wrote:

>
> Hello,
> I am planning a CNC servo power supply and I need a little help with
> contactor (relay) contact sizing. My plan is to use some type of
> contactor that has 2 NO contacts to power the AC side of the 3000VA
> 220V transformer and 2 NC contacts to dump the capacitors into
> resistor for an e-stop / table limit situation. My question is this: I
> have calculated the DC output of the PS and it will be around 120V @
> 35A so what size (rated in ACV / Amps) contacts should the relay have
> for the DC dump circuit. I have been told to use a 20W 7 ohm resistor
> for the load and I presume this is large enough. Please help as I am a
> little confused when it comes to the DC. ANy help is always
> appreciated.
>
> Larry Olson

The DC voltage across the filter cap is 120V. Using a 7 ohm resistor
and 120VDC that says that 17Amps will flow through the resistor at
turn off and will tail off at discharge rate of the voltage. Bottom
line is the "average current" over the several seconds is a factor of
the inital voltage charge and the time constant of the CAP and the 7
ohm resistor. The result is non-linear. What all of that means is
that your average power dissipation over 5 seconds (just a rough
number to get the cap discharged to 10%) would proabably be around 5A
and the power dissipation would average about 175W. If you are pumping
any back EMF from motors that average current could be higher. The
peak current at the instant of turn-off is over 2,000 watts!! Me
thinks a larger value resistor (both ohms and watts) might be in order
If you get less aggressive with the discharge by using a 20 ohm
resistor it will take longer but the peak drops to 576W and that will
average less current over a longer discharge time but current is a
square factor so your power dissipation is less.

The current rating for the contactor should be around the peak value
or the 17A number (if you stick with the 7 ohm) but since it's making
instead of breaking a 10A rating would proabably survive.

We use an active load in our DCM-100 module that dissipates the power
across a resistor and a power FET on a heatsink. It self limits the
load to a constant current to not allow the huge initial turn on and
to more rapidly discharge the cap since the load stays the same.
Unfortunately your application is outside our voltage range of 100VDC max.

>

Do you really think your motors are going to pull 35A at 120VDC? That
is a lot of power (something like 5.63 HP) Those must be some killer
motors (:-). I have 1/2 HP servos (120V rated) on my big mill and
they are belt reduced 4:1 and with the advantage of the screw they
will twist the cape off Superman.