amp rating question

[HSS]

Would anyone happen to know how many amps a 12vdc 30 amp relay will carry at 90vdc? TIA

Patrick

[MaxHeadRoom]

Depends on resistive or inductive load, if you intend switching a 30amp inductive load you will need a relay rated as such and preferably magnetic arc-blow-out.
P&B make them.
Max.

[HSS]

Thanks Max, it's a Bridgeport motor and it is rated 4.6 amps at 90 vdc. The switches in the control box are only rated at 1/4 amp at 28vdc IIRC and I need to switch the 4.6 amps.

Patrick

[MaxHeadRoom]

I would still get the PRD version relay from P&B, you may get away without the blow-out feature for that current.
What controller is this? one of the KB/Baldor SCR types?
If so and are switching the motor side, the drive can be disabled first or simultaneously with the I1 I2 pins, and these should be used anyway if using reversing, and also eliminate the need for any braking resistor.
Max.

[Black_Moons]

Thanks Max, it's a Bridgeport motor and it is rated 4.6 amps at 90 vdc. The switches in the control box are only rated at 1/4 amp at 28vdc IIRC and I need to switch the 4.6 amps.

Patrick

Are you sure those switchs switch the motor? Its much more common to use switchs to power relays or circuity as then you can use small cheap switchs.

[HSS]

Max,the controller is a Danfoss S1000 board and controls the speed on the motor but will not reverse the motor, for that I have to use relays. The switches were used to reverse the motor on the original board, but the switches did not carry the current of the motor, the original board handled that. Now, I have to use relays because the new board doesn't have a reverse capabillity. The switches smoked a little when I tried to run the motor thru them.
Black Moons the switches are for control circuitry only. That's what the original board handled. I have 3 original Bridgy boards and none of them work. They were all used, so I went to the S1000 board. I now have two 40 amp 2 pole 24v contactors and a 40va 24v transformer and I am going to wire the switches to the contactors to reverse the motor. What I didn't know was would 40 amp contacts handle the 5 amp dc load at 90vdc. Hey, thanks for all the help, I really appreciate it.

Pat

[HSS]

Got everything back together and put back on the mill and it works great. Thanks fo all the help.

Patrick

[Paul Alciatore]

OK, first be sure that the Voltage rating you are talking about is for the contacts and not the coil. Both of these will have a Voltage rating and it can be easy to confuse them. I say this because a 30 Volt rating on the contacts is fairly low for any relay that is not in the sub-miniature league. One of the main concerns with a Voltage rating is arching when the circuit is broken and an inductive load (as in a motor) will increase this. The contacts must physically separate by a sufficient distance to break this arc quickly when the relay opens.

Most normal sized relays will have a fairly large gap between the contacts when they are open and Voltage ratings of 100, 200, 300 Volts or more are realized. Only when the relay is very small and the gap is correspondingly small is that Voltage rating that low. I strongly suspect that these relays will work for a long time.

Another consideration is that you will probably not use the relay to reverse motor direction while it is under power. I don't think that would be a good idea with any motor. So if the relay is only operated, the contacts are only opened while there is no power applied to the motor, then the relays will last almost to forever. They can certainly carry the 4.6 Amp. current with a 40 Amp. rating, even considering start-up surges.

[ken572]

Would anyone happen to know how many amps a 12vdc 30 amp relay will carry at 90vdc? TIA

Patrick

Your question is to generalized.
Your 12vdc is the (control or coil side) of the relay.
Your 30 amp is your working contact rating of the relay.
You will need to add up all your control loads from switchs etc.,
and total them up.
You will need to find your Motor and or working load Voltage.
You will need to add up your total working load amps.
With this information you can click on the links below and find
what you need to know and purchase.

http://www.newark.com/jsp/search/browse.jsp?N=422+202204+2203&Ntk=gensearch&Ntt=12+ vdc+30+amp&Ntx=mode+matchallpartial

http://relays.te.com/pnb.asp

http://relays.te.com/

http://us.idec.com/Product/Relays_And_Sockets.aspx

I hope this is helpfull.
Ken.

[J. R. Williams]

You have another potential problem. Many control units do not like to have the output switched under load. One brand uses a three pole manual switch with a center off detent that shuts off the power to the unit during the reversing time.