Re: Currect size relay for E-STOP

OK, fair enough, but:

If you look at the A ratings listed on the side of the relay you get;

35A @ 277 VAC
15A @ 600 VAC

Calculated out very close mathematical similarity using OHM's law
(about 9000W)... This is what led me to believe I could extrapolate
the contact rating for 70VDC based on the listed rating of 28VDC @
25A

From what you said, it appears that the voltage rating is more
important than the amps. Can I then assume that the rating of 28VDC
is considered an upper limit for DC voltages?

I have been looking and have not seen any DC rated relays above
28VDC that aren't HUGE package sizes.

Perhaps the following line of reasoning will allow me to use the
above mentioned relay:

The relay will be wired NC with the resistor in the loop normally.
The intention is to open the circuit when the AC power is supplied
(controller turned on and e-stop open). So under normal on-off
switching, the resistor is removed from the circuit when the power
is turned on. When the estop is pressed, a seperate relay (SSR)
kills the AC to the PS and the relay in question loses it's control
voltage and clamps the resistor. Since in estop mode it will not be
trying to "pull apart" the contacts, am I OK? Upon powerup the
motors are not running so the only current draw is from the resistor
itself until the relay opens.


--- In CAD_CAM_EDM_DRO@yahoogroups.com, Andy Wander <awander@v...>
wrote:

> This is not correct, because you are using a "power" equation on a

device

> that doesn't use power.
>
> The equations you cite are used for calculating the way max

voltage and max

> current are related for stuff that DOES dissipate power, like

resistors,

> etc.
>
> An ideal relay has either infinite(contact open) or zero(contact

closed)

> resistance, so it will not use or dissipate ANY power. Even "Real"

relays

> have very low contact resistance, and so we can approximate their

behavior

> by assuming their resistance is zero.
>
> This means that a relay either has "some" voltage and zero current

across

> it(when the contacts are open), or zero voltage and "some" current

across

> it(when the contacts are closed.
>
> Relay current ratings are the maximum that the contacts can handle

for

> sustained current flow, AND still be able to "break" the current

flow

> without excessive arcing, which could cause the contacts to become

welded

> shut. They are rated as "Xamps at Y volts, because the contacts

can only

> reliably break that current if the voltage causing that current is

within

> the spec. For example, most relays have a much higher voltage spec

for AC

> than they do for DC, because with DC it's harder to extinguish the

arc.

>
> Relay contact ratings are the maximum that they can handle,

without being

> able to mathematically compute the other quantities using Ohm's

law.

>
> Sorry.
>
> So in your example, the relay rated for 25A at 28VDC wikk NOT be a

good

> choice for use at a higher voltage, because even at rather low

current

> levels, you may experience arcing when you open the contacts,

which can burn

> the contacts, or even weld them together.
>
> Andy Wander
> Verrex Corporation
>
>
> -----Original Message-----
> From: Michael Gamber [mailto:mgamber@c...]
> Sent: Thursday, April 01, 2004 2:31 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Currect size relay for E-STOP
>
>
> I intend to clamp a wire-wound resistor across the filter cap on

my

> 68.7v DC PS.
>
> I have 3 75v 2A servos running. Assume the purpose of this is to
> absorb energy when the e-stop is pressed and the AC is cut to the

PS.

>
> If I use a relay rated at 25A @ 28v DC does this mean that it is
> good for 10A @ 70v DC by using OHM's law?
>
> P=E*I
> 28v * 25A = 700W
>
> so
>
> I=P/E
>
> 700W / 70v = 10A
>
> Currect?
>
> And is this enough, or do I need more?
>
>
>
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