I have an application (*) where I need to two pole switch about 12 amps DC
at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to
use a contactor rated at that voltage, but will presumably have to derate
it's current carrying capacity when switching DC. What factor should I use?
Manufacturers give loads of ratings but only tend to specify for either AC
or DC but not both in the same contactor!

(*) the application is a DC injection brake. The three phase motor is
driven via a contactor from 415v AC. When the contactor is de-energised,
after a fraction of a second delay, a second contactor (the one in question)
will squirt about 12 amps from a 24v DC source into one winding for a
maximum of ten seconds then itself de-energise. (The delay between the first
contactor dropping out and the second one pulling in is obviously a self
preservation exercise to avoid a big bang and blown up 24v power supply -
they will also be interlinked to prevent dual energisation)

AWEM

(An alternative approach would be to use the braking inherent in a suitable
inverter drive or commercial DC braking units, but the three motors in
question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three inverters
or DC brakes of suitable beefiness available for a song they are ruled out
of my budget!)

Andrew Mawson wrote:
I have an application (*) where I need to two pole switch about 12 amps DC
at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to
use a contactor rated at that voltage, but will presumably have to derate
it's current carrying capacity when switching DC. What factor should I use?
Manufacturers give loads of ratings but only tend to specify for either AC
or DC but not both in the same contactor!

(*) the application is a DC injection brake. The three phase motor is
driven via a contactor from 415v AC. When the contactor is de-energised,
after a fraction of a second delay, a second contactor (the one in
question)
will squirt about 12 amps from a 24v DC source into one winding for a
maximum of ten seconds then itself de-energise. (The delay between the
first
contactor dropping out and the second one pulling in is obviously a self
preservation exercise to avoid a big bang and blown up 24v power supply -
they will also be interlinked to prevent dual energisation)

AWEM

(An alternative approach would be to use the braking inherent in a suitable
inverter drive or commercial DC braking units, but the three motors in
question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three inverters
or DC brakes of suitable beefiness available for a song they are ruled out
of my budget!)

Andrew, I think I'm right in thinking that the main requirement for
increased performance of DC contactor on an inductive load is the the
high voltage/current combination when the contacts open.
If you arrange for your brake current to be low/zero at the end of the
braking cycle and then open the contactor, there should be little or no
need for higher spec contacts.
You could ensure this happens by having a current controlled relay coil
in series with the DC braking circuit and interlock this with the DC
contactor.

The brake sequence could be:-
remove AC power
connect output of DC power supply
switch on mains to DC supply
Wait for appropriate time
Switch off mains to DC supply
When current sense relay has dropped out, disconnect output of braking
supply
reset everything for next braking cycle.

hth

Bob

On 28/01/2012 13:22, Bob Minchin wrote:

[...]
If you arrange for your brake current to be low/zero at the end of the
braking cycle and then open the contactor, there should be little or no
need for higher spec contacts.

Which of course is easily done by means of a suitable shunt diode across
the input terminals of the DC feed contactor. The polarity must be such
that the diode is reverse-biased when the DC is on. When the DC source
is switched off the current keeps flowing in the same direction,
forward-biasing the diode, thus limiting the voltage swing to the supply
voltage plus the forward voltage drop of the diode. The current then
falls roughly exponentially toward zero with time constant of L/R,
safely dissipating the stored energy (mostly in the resistance of the
winding (R) and partially in the diode). Choose a diode with a
continuous forward current rating of the full 12 A, to be safe, but it
doesn't need to have a particularly high reverse voltage rating - 50 0r
100 V should be OK.

--
Andy

"Andy Wade" wrote in message ...

On 28/01/2012 13:22, Bob Minchin wrote:

[...]
If you arrange for your brake current to be low/zero at the end of the
braking cycle and then open the contactor, there should be little or no
need for higher spec contacts.

Which of course is easily done by means of a suitable shunt diode across
the input terminals of the DC feed contactor. The polarity must be such
that the diode is reverse-biased when the DC is on. When the DC source is
switched off the current keeps flowing in the same direction,
forward-biasing the diode, thus limiting the voltage swing to the supply
voltage plus the forward voltage drop of the diode. The current then falls
roughly exponentially toward zero with time constant of L/R, safely
dissipating the stored energy (mostly in the resistance of the winding (R)
and partially in the diode). Choose a diode with a continuous forward
current rating of the full 12 A, to be safe, but it doesn't need to have a
particularly high reverse voltage rating - 50 0r 100 V should be OK.


I think Andy that the shunt diode would need to be on the output side of the
contactor rather than the input terminals, ( ie across the inductance of the
motor coil) to be effective, and of course next time the motor starts up the
diode would vaporise as it would have 415v AC across it. I'd like to be
shown that I'm wrong, but I don't think so.

AWEM

On Jan 28, 12:35*pm, "Andrew Mawson"
wrote:
I have an application (*) where I need to two pole switch about 12 amps DC
at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to
use a contactor rated at that voltage, but will presumably have to derate
it's current carrying capacity when switching DC. What factor should I use?
Manufacturers give loads of ratings but only tend to specify for either AC
or DC but not both in the same contactor!

(*) the application is a DC injection brake. The three phase motor is
driven via a contactor from 415v AC. When the contactor is de-energised,
after a fraction of a second delay, a second contactor (the one in question)
will squirt about 12 amps from a 24v DC source into one winding for a
maximum of ten seconds then itself de-energise. (The delay between the first
contactor dropping out and the second one pulling in is obviously a self
preservation exercise to avoid a big bang and blown up 24v power supply -
they will also be interlinked to prevent dual energisation)

AWEM

(An alternative approach would be to use the braking inherent in a suitable
inverter drive or commercial DC braking units, but the three motors in
question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three inverters
or DC brakes of suitable beefiness available for a song they are ruled out
of my budget!)

Use a car starter motor solenoid? You would needa 12vdc supply.
It would mean a scrapyard job these days of pre-engaged starter
motors.

DC current effects can be migitated by using a capacitor wired across
the switch contacts. (Think ignition system points on a car in days of
yore).
You need quite a big one.

"harry" wrote in message
...

On Jan 28, 12:35 pm, "Andrew Mawson"
wrote:
I have an application (*) where I need to two pole switch about 12 amps
DC
at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to
use a contactor rated at that voltage, but will presumably have to derate
it's current carrying capacity when switching DC. What factor should I
use?
Manufacturers give loads of ratings but only tend to specify for either
AC
or DC but not both in the same contactor!

(*) the application is a DC injection brake. The three phase motor is
driven via a contactor from 415v AC. When the contactor is de-energised,
after a fraction of a second delay, a second contactor (the one in
question)
will squirt about 12 amps from a 24v DC source into one winding for a
maximum of ten seconds then itself de-energise. (The delay between the
first
contactor dropping out and the second one pulling in is obviously a self
preservation exercise to avoid a big bang and blown up 24v power supply -
they will also be interlinked to prevent dual energisation)

AWEM

(An alternative approach would be to use the braking inherent in a
suitable
inverter drive or commercial DC braking units, but the three motors in
question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three
inverters
or DC brakes of suitable beefiness available for a song they are ruled
out
of my budget!)

Use a car starter motor solenoid? You would needa 12vdc supply.
It would mean a scrapyard job these days of pre-engaged starter
motors.

DC current effects can be migitated by using a capacitor wired across
the switch contacts. (Think ignition system points on a car in days of
yore).
You need quite a big one.


I'm not aware of any car starter motor solenoid that I'd be happy having
415v AC on it in the open circuit mode !

AWEM

On 28/01/2012 15:20, Andrew Mawson wrote:

I think Andy that the shunt diode would need to be on the output side of
the contactor rather than the input terminals, ( ie across the
inductance of the motor coil) to be effective, and of course next time
the motor starts up the diode would vaporise as it would have 415v AC
across it. I'd like to be shown that I'm wrong, but I don't think so.

The diode will still be effective if the DC contactor is used to isolate
the DC PSU under AC motor-run conditions, but not for DC load switching,
as Bob suggested.

The braking sequence would be:

- AC off (open AC contactor)
- pause
- close DC contactor (with DC PSU off)
- energise DC PSU, by switching its mains supply, or otherwise
- hold for set braking period
- shut down DC PSU (DC contactor remains closed, so diode is in-circuit)
- pause while winding inductance discharges via the diode
- open DC contactor
- pause
- ready to re-apply AC.

--
Andy

On Jan 28, 6:25*pm, "Andrew Mawson"
wrote:
"harry" *wrote in message

...





On Jan 28, 12:35 pm, "Andrew Mawson"
wrote:
I have an application (*) where I need to two pole switch about 12 amps
DC
at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to
use a contactor rated at that voltage, but will presumably have to derate
it's current carrying capacity when switching DC. What factor should I
use?
Manufacturers give loads of ratings but only tend to specify for either
AC
or DC but not both in the same contactor!

(*) the application is a DC injection brake. The three phase motor is
driven via a contactor from 415v AC. When the contactor is de-energised,
after a fraction of a second delay, a second contactor (the one in
question)
will squirt about 12 amps from a 24v DC source into one winding for a
maximum of ten seconds then itself de-energise. (The delay between the
first
contactor dropping out and the second one pulling in is obviously a self
preservation exercise to avoid a big bang and blown up 24v power supply -
they will also be interlinked to prevent dual energisation)

AWEM

(An alternative approach would be to use the braking inherent in a
suitable
inverter drive or commercial DC braking units, but the three motors in
question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three
inverters
or DC brakes of suitable beefiness available for a song they are ruled
out
of my budget!)

Use a car starter motor solenoid? *You would needa 12vdc supply.
It would mean a scrapyard job these *days of pre-engaged starter
motors.

DC current effects can be migitated by using a capacitor wired across
the switch contacts. (Think ignition system points on a car in days of
yore).
You need quite a big one.

I'm not aware of any car starter motor solenoid that I'd be happy having
415v AC on it in the open circuit mode !

AWEM

And RC snubber, not C. C only kills switch contacts by putting a heavy
current surge through them when they close.


NT

"Andy Wade" wrote in message ...

On 28/01/2012 15:20, Andrew Mawson wrote:

I think Andy that the shunt diode would need to be on the output side of
the contactor rather than the input terminals, ( ie across the
inductance of the motor coil) to be effective, and of course next time
the motor starts up the diode would vaporise as it would have 415v AC
across it. I'd like to be shown that I'm wrong, but I don't think so.

The diode will still be effective if the DC contactor is used to isolate
the DC PSU under AC motor-run conditions, but not for DC load switching, as
Bob suggested.

The braking sequence would be:

- AC off (open AC contactor)
- pause
- close DC contactor (with DC PSU off)
- energise DC PSU, by switching its mains supply, or otherwise
- hold for set braking period
- shut down DC PSU (DC contactor remains closed, so diode is in-circuit)
- pause while winding inductance discharges via the diode
- open DC contactor
- pause
- ready to re-apply AC.



Nice one. I hadn't thought of switching the DC psu at it's mains. Ok it adds
a couple more delay circuits but it's neat.

AWEM

Andrew Mawson wrote:
"Andy Wade" wrote in message ...

On 28/01/2012 15:20, Andrew Mawson wrote:

I think Andy that the shunt diode would need to be on the output side of
the contactor rather than the input terminals, ( ie across the
inductance of the motor coil) to be effective, and of course next time
the motor starts up the diode would vaporise as it would have 415v AC
across it. I'd like to be shown that I'm wrong, but I don't think so.

The diode will still be effective if the DC contactor is used to
isolate the DC PSU under AC motor-run conditions, but not for DC load
switching, as Bob suggested.

The braking sequence would be:

- AC off (open AC contactor)
- pause
- close DC contactor (with DC PSU off)
- energise DC PSU, by switching its mains supply, or otherwise
- hold for set braking period
- shut down DC PSU (DC contactor remains closed, so diode is in-circuit)
- pause while winding inductance discharges via the diode
- open DC contactor
- pause
- ready to re-apply AC.



Nice one. I hadn't thought of switching the DC psu at it's mains. Ok it
adds a couple more delay circuits but it's neat.

AWEM
but that was the basis of my earlier reply.......

On 28/01/2012 22:14, Bob Minchin wrote:

but that was the basis of my earlier reply.......

I did _try_ to give you the credit...

--
Andy

On Jan 28, 6:25*pm, "Andrew Mawson"
wrote:
"harry" *wrote in message

...







On Jan 28, 12:35 pm, "Andrew Mawson"
wrote:
I have an application (*) where I need to two pole switch about 12 amps
DC
at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to
use a contactor rated at that voltage, but will presumably have to derate
it's current carrying capacity when switching DC. What factor should I
use?
Manufacturers give loads of ratings but only tend to specify for either
AC
or DC but not both in the same contactor!

(*) the application is a DC injection brake. The three phase motor is
driven via a contactor from 415v AC. When the contactor is de-energised,
after a fraction of a second delay, a second contactor (the one in
question)
will squirt about 12 amps from a 24v DC source into one winding for a
maximum of ten seconds then itself de-energise. (The delay between the
first
contactor dropping out and the second one pulling in is obviously a self
preservation exercise to avoid a big bang and blown up 24v power supply -
they will also be interlinked to prevent dual energisation)

AWEM

(An alternative approach would be to use the braking inherent in a
suitable
inverter drive or commercial DC braking units, but the three motors in
question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three
inverters
or DC brakes of suitable beefiness available for a song they are ruled
out
of my budget!)

Use a car starter motor solenoid? *You would needa 12vdc supply.
It would mean a scrapyard job these *days of pre-engaged starter
motors.

DC current effects can be migitated by using a capacitor wired across
the switch contacts. (Think ignition system points on a car in days of
yore).
You need quite a big one.

I'm not aware of any car starter motor solenoid that I'd be happy having
415v AC on it in the open circuit mode !

It would stand it. Plus you could put in additional insulation if you
were worried.

Andy Wade wrote:
On 28/01/2012 22:14, Bob Minchin wrote:

but that was the basis of my earlier reply.......

I did _try_ to give you the credit...

Yes you did indeed Andy. Thank you! My comment was more directed at
Andrew (someone who I know) for not seeming to pick up my comment before
you reinforced it.

Bob