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Using AC contactor coils with DC.

imported_brian_m

Cast Iron
Joined
Jun 25, 2006
Location
Oregon
Greetings.

I am rebuilding the control system on a DSG lathe. I changed the original 15HP motor which had a 2-step time-delay start system to a 10HP standard motor so this allows considerable simplification of the control panel.

The original control panel used 24V AC control power but I am integrating some solid state components and I would like to use 24V DC for control throughout the system. However, some of the control panel functions would be simpler to implement using the original contactors and I would like to re-use some of these so I will have a mixed system of SSRs and mechanical contactors.

What do you think of simply switching the power supplies from AC to DC? I plan on adding anti-surge diodes to the coils but are there any other problems with this idea?

Thanks, Brian
 
A direct substitution will not work. To use DC on AC designed contactor requires a special coil and aux contact or the equivalent to allow for enough coil current to pull in the contacts and then reduce the holding current. The difference is how the magnetics work. To further complicate things, the aux contact that switches from start to run is a special design late break. Modern control have kits for this, but if the control is at all old, finding one or more may be difficult.

Tom
 
I am using a 24Vac with a mercury wetted relay that has a 24Vdc coil. I made a small circuit board that contains a full wave bridge and a two wire connection block. The Philips screw points are the standard connections for 24Vdc. The system works because the threshold for the DC coil is well below 24Vdc.
 

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I am using a 24Vac with a mercury wetted relay that has a 24Vdc coil. I made a small circuit board that contains a full wave bridge and a two wire connection block. The Philips screw points are the standard connections for 24Vdc. The system works because the threshold for the DC coil is well below 24Vdc.

SO all the OP needs to do is build some boards with some of them anti-diodes on them.....?
 
Thanks Guys.

Actually the quick back-up solution is to add a suitable small AC transformer to the system. Fortunately the circuit design will make mixed operation quite simple because we are only talking AC for the contactor coils, the rest of the system will run on DC with no problems.

Thanks for the replies, I had a feeling that the idea could not be that simple!

I'm tempted to replace the relay logic with a microprocessor, with suitable voltage level translation it looks as though this may be quite easy.

.
 
As I said, check the VA requirements. Physical size alone is not enough. The OP has not said what size contactors he is dealing with. !5 hp could require a size 3 starter that takes a 200VA transformer.

Tom
 
As I mentioned the original motor had 2-stage, soft-start, windings, this motor has been replaced. The original control panel had five contactors that are now reduced to two with SSRs doing the rest of the work so the contactor coil control current is presently less than half of what it was with the actual control system current remaining constant. I have an 8 amp DC power supply providing control current which is negligible apart from the electric brake clutch at 1.3 amps and a number of indicator bulbs which will be changed to LEDs.

The motor power contactor has been replaced by a high current SSR but I may need to beef-up this item as a result of experience. The FLA of the motor is 25amps and I am going to try it with a 30 amp SSR (I know the SSR should probably theoretically be about 100 amps to cover start-up surge but the 3-phase 30 amp item is on my parts shelf now). The motor does not start up under load (clutch) so lets see how this goes.

The wiring does provide fail-safe operation because failure of any power shuts down the control power supply until a physical reset is performed.

.
 
For safety, make sure those ssr's are backed up with a mechanical switch. Using ssr's for motors will require a heavy duty one that handle the start time as roughly 6x fla.

Tom
 
You *should* be able to measure the coil current under proper conditions, then determine the equivalent DC voltage that gives that by measuring the resistance.

You may need a starting "blip" of higher voltage to get it to pull in, but I doubt it.
 
I found some random guy on youtube talking about the difference between the ac and dc coils for the same hvac reversing valve; ac coil was 10 ohms, dc was 100 ohms for 24 volts, they were about the same volume of copper.


A resistor in series will be needed to keep the current consumption down to avoid burn out unless you reduce the voltage, such as run them on at 12 volts instead of 24. if you place a good sized capacitor across the resistor you will obtain the momentary full voltage in the time it takes the capacitor to charge up. 5000uf 16 volt capacitor is probably appropriate for a coil that consumes on the order of 1 amp at 12 volts and you need a 10-15 ohm resistor to drop the voltage from 24 down to 12.
 
I found some random guy on youtube talking about the difference between the ac and dc coils for the same hvac reversing valve; ac coil was 10 ohms, dc was 100 ohms for 24 volts, they were about the same volume of copper.


A resistor in series will be needed to keep the current consumption down to avoid burn out unless you reduce the voltage, such as run them on at 12 volts instead of 24. if you place a good sized capacitor across the resistor you will obtain the momentary full voltage in the time it takes the capacitor to charge up. 5000uf 16 volt capacitor is probably appropriate for a coil that consumes on the order of 1 amp at 12 volts and you need a 10-15 ohm resistor to drop the voltage from 24 down to 12.

I think you have those numbers backwards. DC resistance will always be lower than AC impedance.

The capacitor idea is excellent, but make sure you have a bleeder that can discharge the cap in the minimum off time the contactor will see, otherwise the effect will "wear out" with high switching speeds.
 
I think you have those numbers backwards. DC resistance will always be lower than AC impedance.

The capacitor idea is excellent, but make sure you have a bleeder that can discharge the cap in the minimum off time the contactor will see, otherwise the effect will "wear out" with high switching speeds.

should not be a problem if the capacitor is sized only for the 1/20th of a second you need or want full voltage, as the capacitor is connected across the inline resistor resistor. note that you don't really want a diode directly connected across the coil unless you need it. instead a zener or a mov or some other diode clamp that discharges the inductor into a much higher voltage than the 1 volt offered by a diode will dump the energy much quicker. this is probably not a problem for relays but it is for fuel injection solenoids (of which some vehicles use the (avalanche rated) mosfet as the clamp)

anyhow, the man was reading the dc ohms of the coils, not the impedance.
 








 
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