How to select a relay for short, very high currents?

[Rob Storey]

(my first post to this group)

I have a magnetizer project that needs to discharge 3x4700uF at 300V
into a coil via an SCR, resulting in a current of around 2000A for about
5ms. The complication is that the direction of current through the coil
needs to be selectable, which implies the use of a DPDT style relay.

Arcing is not a problem here, as the relay will never switch while
discharge is in progress, nor when voltage is present. The prime issue
is the very short term current carrying capacity of the closed contacts.

My problem is selecting a relay with adequate contact ratings for this
job. Relay specs typically only give ratings for continuous AC and DC,
and for various load types (resistive, inductive). The ratings also
appear to be specified for conditions where voltage is present on make,
and current is flowing on break.

Where I have found some short term current figures (minutes and
seconds), extrapolation down to 5ms leads me to conclude that a
contactor rated at 50A or above would handle the job. But these cost
hundreds of dollars, and being triple-pole/single throw, I would need
two of them, and also need to ensure only one is active at a time.

The unit I'm building is to replace an existing system (where the
reversal is done via a large manual switch), and in that unit the wiring
used to carry this large current appears to be nothing special, just 20A
automotive type wire.

So if that wire doesn't turn into a strip of smoke, surely I can get by
with a modest relay costing $50-$100?

Can anyone suggest a type of relay that would be adequate for this
project?
Also, since there would never be any current flow during switching,
would contact maintenance be an issue?

Any help or suggestions will be greatly appreciated.

Thanks in advance,
Rob Storey

 

[Spehro Pefhany]

(my first post to this group)

I have a magnetizer project that needs to discharge 3x4700uF at 300V
into a coil via an SCR, resulting in a current of around 2000A for about
5ms. The complication is that the direction of current through the coil
needs to be selectable, which implies the use of a DPDT style relay.

Arcing is not a problem here, as the relay will never switch while
discharge is in progress, nor when voltage is present. The prime issue
is the very short term current carrying capacity of the closed contacts.

My problem is selecting a relay with adequate contact ratings for this
job. Relay specs typically only give ratings for continuous AC and DC,
and for various load types (resistive, inductive). The ratings also
appear to be specified for conditions where voltage is present on make,
and current is flowing on break.

Where I have found some short term current figures (minutes and
seconds), extrapolation down to 5ms leads me to conclude that a
contactor rated at 50A or above would handle the job. But these cost
hundreds of dollars, and being triple-pole/single throw, I would need
two of them, and also need to ensure only one is active at a time.

A 50A 3-phase contactor is only about $110 new. 40A for < $50. Check
surplus, you can probably get by for $10-25.00.

The unit I'm building is to replace an existing system (where the
reversal is done via a large manual switch), and in that unit the wiring
used to carry this large current appears to be nothing special, just 20A
automotive type wire.

So if that wire doesn't turn into a strip of smoke, surely I can get by
with a modest relay costing $50-$100?

IMHO, a relay type construction is asking for trouble.

Can anyone suggest a type of relay that would be adequate for this
project?
Also, since there would never be any current flow during switching,
would contact maintenance be an issue?

Any help or suggestions will be greatly appreciated.

Thanks in advance,
Rob Storey

Best regards,
Spehro Pefhany

 

[Fred Bloggs]

The complication is that the direction of current through the coil
needs to be selectable, which implies the use of a DPDT style relay.

It does? Have you heard of something called an H-bridge?

 

[Rob Storey]

It does? Have you heard of something called an H-bridge?

Duh, yes.
At a pinch, an DPDT anything wired for current reversal can be
considered an H-bridge.
But what is the point of your comment? Are you suggesting four relays,
or four SCR's, or what?

Rob Storey

 

[Rob Storey]

A 50A 3-phase contactor is only about $110 new. 40A for < $50. Check
surplus, you can probably get by for $10-25.00.

Here in .AU, I haven't seen new units for prices that low, plus I'd
still need two.
I don't know of any surplus source here, (and used gear is not
acceptable, if that's what you meant)
I'll try looking for other suppliers, and sourcing from overseas.

IMHO, a relay type construction is asking for trouble.

That's my gut feeling too, but the piddling wire used in the existing
system makes me wonder if big expensive contactors would just be
overkill.

Thanks for replying,
Rob Storey

 

[John Smith]

(my first post to this group)

I have a magnetizer project that needs to discharge 3x4700uF at 300V
into a coil via an SCR, resulting in a current of around 2000A for about
5ms. The complication is that the direction of current through the coil
needs to be selectable, which implies the use of a DPDT style relay.

Arcing is not a problem here, as the relay will never switch while
discharge is in progress, nor when voltage is present. The prime issue
is the very short term current carrying capacity of the closed contacts.

My problem is selecting a relay with adequate contact ratings for this
job. Relay specs typically only give ratings for continuous AC and DC,
and for various load types (resistive, inductive). The ratings also
appear to be specified for conditions where voltage is present on make,
and current is flowing on break.

Where I have found some short term current figures (minutes and
seconds), extrapolation down to 5ms leads me to conclude that a
contactor rated at 50A or above would handle the job. But these cost
hundreds of dollars, and being triple-pole/single throw, I would need
two of them, and also need to ensure only one is active at a time.

The unit I'm building is to replace an existing system (where the
reversal is done via a large manual switch), and in that unit the wiring
used to carry this large current appears to be nothing special, just 20A
automotive type wire.

So if that wire doesn't turn into a strip of smoke, surely I can get by
with a modest relay costing $50-$100?

Can anyone suggest a type of relay that would be adequate for this
project?
Also, since there would never be any current flow during switching,
would contact maintenance be an issue?

Any help or suggestions will be greatly appreciated.

Thanks in advance,
Rob Storey

Hi, Rob -

You should be able to get a definite-purpose contactor as used in air
conditioners. Of course, I don't know if it will work in your application,
but it's probably cheap.

Good luck on your project.

John

 

[mike]

(my first post to this group)

I have a magnetizer project that needs to discharge 3x4700uF at 300V
into a coil via an SCR, resulting in a current of around 2000A for about
5ms. The complication is that the direction of current through the coil
needs to be selectable, which implies the use of a DPDT style relay.

Sounds like a capacitive discharge spot welder. Might look into
welding goodies.
Doesn't this thing ring like crazy? I guess there's a good reason
that the initial current direction has to be reversible.
4 scrs in a bridge might be cheaper.
mike


--
Return address is VALID.
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
Compaq Aero floppy,ram,battery.
MINT HP-41CV, 2-METER AMPS, 200CH SCANNER
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Rene Tschaggelar]

I'd consider a 50A relay to be undersized, unless a very limited
lifetime is acceptable. Beside a 200A relay, I'd consider a wench
and make the connection with nut and bolt.

Rene

 

[BFoelsch]

Why don't you just use 4 SCRs?

 

[Frank Bemelman]

Duh, yes.
At a pinch, an DPDT anything wired for current reversal can be
considered an H-bridge.
But what is the point of your comment? Are you suggesting four relays,
or four SCR's, or what?

Mosfets? Perhaps a bunch of smaller ones, to reduce the
on resistance further, with your 0.15 ohm coil.

I don't like the idea of 2000A switched by mechanical relay,
unless twice a year.

 

[Spehro Pefhany]

Here in .AU, I haven't seen new units for prices that low, plus I'd
still need two.

If you use two SCRs you could use two relays with single form A
(SPST-NO ) shorting bar type contacts. There are some offshore 100A
latching relays which are not very expensive (a few dollars US in
moderate quantity), Zettler sells one type of them. I'm not sure if
their internal construction is suitable, but it wouldn't be very
costly to buy a few and rip one apart.

I don't know of any surplus source here, (and used gear is not
acceptable, if that's what you meant)

No, I meant surplus new. These things are used in HVAC and other
industrial/commercial products.

I'll try looking for other suppliers, and sourcing from overseas.

That's my gut feeling too, but the piddling wire used in the existing
system makes me wonder if big expensive contactors would just be
overkill.

I would not fool with anything that isn't shorting bar.

Remember too, anything over a few tens of m-ohms is going to reduce
your current.

Best regards,
Spehro Pefhany

 

[Paul Burke]

I'd consider a wench
and make the connection with nut and bolt.

I've never considered making THAT connection with a nut and bolt...

Paul Burke

 

[Fred Bloggs]

Duh, yes.
At a pinch, an DPDT anything wired for current reversal can be
considered an H-bridge.
But what is the point of your comment? Are you suggesting four relays,
or four SCR's, or what?

Rob Storey

Something like this: turn the SCR's on first to setup output polarity,
then trigger either anti-parallel SCR's or TRIAC to apply current to
coil. The 300V/5ms is only 0.06V/us and is really no big deal, and the
I^2T< 7K A^2-sec is also no big deal. Use some cheap isolated supplies
for SCR trigger ckt drives. RTO is a turn-on trickle current for the
SCRs, a compromise between loading your PS and ensuring selected SCR
junctions are fully on steady state prior to TRIAC trigger.
View in a fixed-width font such as Courier.

..
..
.. c2 c1
.. o o
.. / \
.. +---|<|-----+-----|>|---+
.. | | | TRIG
.. | | | o
.. | +------+ | \
.. | +-+-+ |+ | |>|
.. +----+ |PS | ----- +-----+----| |-----------+
.. | | +---+ ----- | | |<| |
.. | | | |- | | |
.. | | +------+ | | |
.. | | | | | |
.. | | | | / R |
.. | +---|>|-----+-----|<|---+ \ TO |
.. | / \ / )
.. | o o \ )
.. | c3 c4 | C.U.T.)
.. | | )
.. | | )
.. | | |
.. | | |
.. +----------------------------------+------------------+
..

 

[Winfield Hill]

Fred Bloggs wrote...

It does? Have you heard of something called an H-bridge?

My 8000A 5ms 200V pulse generator used four hockey-puck SCRs in a
modified H-bridge to control the pulse length and to reverse the
current direction mid pulse and rapidly shut it off.

 

[R Adsett]

(my first post to this group)

I have a magnetizer project that needs to discharge 3x4700uF at 300V
into a coil via an SCR, resulting in a current of around 2000A for about
5ms. The complication is that the direction of current through the coil
needs to be selectable, which implies the use of a DPDT style relay.

Arcing is not a problem here, as the relay will never switch while
discharge is in progress, nor when voltage is present. The prime issue
is the very short term current carrying capacity of the closed contacts.

So if that wire doesn't turn into a strip of smoke, surely I can get by
with a modest relay costing $50-$100?

Can anyone suggest a type of relay that would be adequate for this
project?
Also, since there would never be any current flow during switching,
would contact maintenance be an issue?

Any help or suggestions will be greatly appreciated.

Thanks in advance,
Rob Storey

You could go with a reversing contactor set like the Albright SW64

http://www.bridex-ets.com.au/albright_sw64_series_dc_contactors.htm

Full ratings

http://www.bridex-
ets.com.au/albright_miniature_contactor_performance_data.htm

I believe these are the smallest that they make. They are used a lot in
small electric vehicles and should be relatively inexpensive. I'd be
wary of using them to hold off large voltages but they might be worth a
look at for your application. I rather suspect they'd last longer than
your capacitor.

Robert

 

[Fred Bloggs]

Fred Bloggs wrote...



My 8000A 5ms 200V pulse generator used four hockey-puck SCRs in a
modified H-bridge to control the pulse length and to reverse the
current direction mid pulse and rapidly shut it off.

YIKES!!!!! How do you keep the EMP from destroying everything in the lab

 

[Fred Bloggs]

Something like this: turn the SCR's on first to setup output polarity,
then trigger either anti-parallel SCR's or TRIAC to apply current to
coil. The 300V/5ms is only 0.06V/us and is really no big deal, and the
I^2T< 7K A^2-sec is also no big deal. Use some cheap isolated supplies
for SCR trigger ckt drives. RTO is a turn-on trickle current for the
SCRs, a compromise between loading your PS and ensuring selected SCR
junctions are fully on steady state prior to TRIAC trigger.
View in a fixed-width font such as Courier.

.
.
. c2 c1
. o o
. / \
. +---|<|-----+-----|>|---+
. | | | TRIG
. | | | o
. | +------+ | \
. | +-+-+ |+ | |>|
. +----+ |PS | ----- +-----+----| |-----------+
. | | +---+ ----- | | |<| |
. | | | |- | | |
. | | +------+ | | |
. | | | | | |
. | | | | / R |
. | +---|>|-----+-----|<|---+ \ TO |
. | / \ / )
. | o o \ )
. | c3 c4 | C.U.T.)
. | | )
. | | )
. | | |
. | | |
. +----------------------------------+------------------+
.

Maybe a little more explanation. If you want to use a relay, then in
series with your charging PS output is the place to put it. To set this
thing up for a discharge, you turn the PS off for a few 10's
milliseconds to allow all SCRs to go off and voltage across RTO is zero.
Then apply DC gate drive to selected SCRs, either 2-4 or 1-3. Turn on PS
and allow voltage output across RTO to ramp up slowly, the DC SCR gate
drive reduces the SCR holding current to nearly zero so that there is a
uniform cross-sectional current through the junction. When RTO voltage
magnitude reaches the required 300V, or whatever, then fire the TRIAC
into the coil and turn the PS off.

 

[Rene Tschaggelar]

I've never considered making THAT connection with a nut and bolt...

There are nuts and bolts as well as cable in 250mm^2 size and above.
The parts tend to become heavy ... but good for 2000A continous.

Rene

 

[Spehro Pefhany]

There are nuts and bolts as well as cable in 250mm^2 size and above.
The parts tend to become heavy ... but good for 2000A continous.

Rene

Wrench:
http://www.mcli.dist.maricopa.edu/show/interact/images/wrench.gif

Wench:
http://www.totallycracker.clara.net/images/wench.jpg



Best regards,
Spehro Pefhany

 

[Rene Tschaggelar]

Wrench:
http://www.mcli.dist.maricopa.edu/show/interact/images/wrench.gif

Wench:
http://www.totallycracker.clara.net/images/wench.jpg

Thanks.
The finer details of english spelling ...

Rene