RE: [CAD_CAM_EDM_DRO] Re: Currect size relay for E-STOP

Michael:

Your questions, as good questions always should, made me start thinking even
more about this-and as usual, when I think, I realize just how little I
really know!

I did a little further research, and found that at least one manufacturer,
Leach International www.leachintl.com
Recommends the following in their Application note No 101, says that your
method of using Ohms Law to use a relay at a higher than rated voltage can
be valid.

"DERATING OF CONTACTS FOR DC VOLTAGES
ABOVE NOMINAL RATING
To establish a standard for the derating of relay contacts is, at best, a
subjective practice. Limitations are governed by the type
of relay, contact gap, maximum voltage capabilities of the relay contact
system, and the contact material.
The most common method is to derate the contacts by use of the Power
Formula, using the known current and voltage.
This method is valid only for Resistive Loads, and is an approximation only;
keeping in mind the limitations mentioned above.
Power = IE (Current x Voltage)
I2 E2 = 2/3 I1E1
Example:
A designer is working with a 55 volt DC system and has a relay rated at 10
amps resistive at 28 volts DC.
What is the maximum current that can be switched at 55 Vdc.
I1 = 10 Amperes
E1 = 28 VDC
E2 = 55 VDC
I2 = ? (Current ratings at 55 VDC Resistive)
I2 E2 = 2 I1 E1/3
I2 = 2 I1 E1/E23
= 2 (10 x 28)/55 x 3
= 560/165
I2 = 3.4 Amperes at 55VDC
In addition, the user should always be concerned about the following:
1. Derating contacts that are rated for less than 10 Amperes at nominal
voltage.
2. Derating contacts for use in system voltages above 130 Volts DC"

So maybe it would work just fine. You might look at those P&B App notes I
steered you toward in the last message, where they talk about methods of
suppressing the arc.

Also, given the costs of higher-voltage rated relays, it might be worth just
trying-I would think that at worst, you would lose a relay-can't think of
how it would damage anything else, even if the relay welded shut and the
resistor was always in circuit


Andy Wander
Verrex Corporation


-----Original Message-----
From: Michael Gamber [mailto:mgamber@...]
Sent: Thursday, April 01, 2004 4:19 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] 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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