solenoids and relay coils

[kinarfi]

Some relays and solenoid have heavy gauge wire and some have a lot smaller wire for the coil, my experience with this is that the heavy gauge wired devices get hot and there for have lower duty cycles, where the devices with the finer wire can run forever and not get warm.
My thoughts for the heavy gauged wired devices is so they will slam in hard to make fast contact, for starter motors and winches and the likes. Fine wired relays don't need the speed or force of closure and have wiping contacts to keep the contact surface clean and stay closed for extended period.
Does any one have other thought about this?
Thanks, Jeff

 

[alec_t]

As a general rule the greater the contact current rating the greater the power needed to drive the relay coil. For a given coil power, the choice is between high voltage and low current, or low voltage and high current. The first uses lots of turns of thin wire and the second uses fewer turns of thick wire.

 

[Les Jones]

The coils with thicker wire are for lower voltage. Thicker wire and less turns will have less resistance than fine wire with more turns. The strength of the magnetic field produced is in proportion to the product of the number of turns multiplied by the current. So if you half the diameter of the wire it will have one quarter of the cross sectional area so it will have 4 times the resitance for any given length. If you thing about winding the thiner wire into the same space you will get about 4 times the number of turns so the resistance will be 16 times the one with the thicker wire. This coil would give the same magnetic field with 4 times the voltage that was applied to the original coil with the thicker wire. As the resistance is 16 time the original and the voltage is 4 times the original the current will be one quarter but as there is 4 times the number of turns the amper turns will be the same. The power consumed will also be the same as the original.

Les.

 

[MaxHeadRoom78]

There is also a distinction between AC and DC coils, AC are typically wound with fewer turn and larger gauge than it DC counterpart for the same voltage.
This is due partly because AC solenoids not only have high inrush current but pass heavier current during the time the armature moves over.
The current is then restricted by the value of the coils inductive reactance once the armature has moved over.
This is also why if you happen to push over the armature of solenoid when it is energized, often result in the burn out of the coil.
Max.

 

[MikeMl]

My Cessna has a 12Vdc "master relay" rated for a current-carrying capacity of 400A (because the starting motor current runs through it), but the coil resistance is such that the coil draws about 1A (12W @ 12V), so it can be left on continuously while flying. Note that this relay is not rated to ever break the circuit while the starter motor is cranking the engine. Its break rating is only 60A.

In-series with the master relay is the 12Vdc "starter relay". They look nearly identical **broken link removed** However, the coil current required to pull-in the starter relay is about three times higher than the master relay; 36W instead of 12W. The coil can only be powered for 30s every 3min without overheating it (same restriction on the starter motor, itself). The contacts in this relay are also rated for 400A, but it is also rated to break an inductive circuit carrying 400A. So what is the difference?

The spring in the starting relay is much stronger in the starting relay. It is the spring that separates the contacts when the coil is de-energized. The starting relay needs a much heavier spring to move the contacts points apart more rapidly (to quench the arc), and also to prevent the contacts from "welding". The strong spring requires a stronger magnetic pull, hence a much lower resistance (power hungry) coil. The downside is that it is not rated for continuous use...

 

[Reloadron]

I worked with a very old High Voltage power supply. The High Voltage was about 200 VDC and the current was 100 Amps. Old design with a 480 Volt 3 Phase supply and a large transformer and 3 Phase Variac. Big stuff and the output was switched by a large contactor. Just as a side note when does it quit being a relay and become a contactor? Anyway, opne of several task this was used for was heating the stator for a test of a very unique 3 phase motor, we ran DC phase to phase on two phases. One problem we had was occasionally the control rod technicians woule start to disconnect a stator under power. These stators would have about 170 VDC and 70 amps running on them and the connectors were male with silver plated pins. Imagine a very, very expensive connector. Disconnect under load would cause arcing and roach the pins.

So we developed a stator disconnect protect circuit where a prox switch was mounted on the connectors. They needed to disconnect the prox switch which would kill the power supply contactor. Perspective, the contacts on the output contactor were about a 1.0" gap. When I began implementing the prox switches they were applied to several power supplies and the idea worked out quite well. However, the one big DC supply was eating my prox switches. They were rated at 120 VAC 1.0 Amp. This was a my bad as I never really considered that contactor coil current. The coil current was 1.5 Amp. No surprise I was eating up my prox switches. I added a slave relay which in turn powered the contactor and that worked fine. While I have seen some large high voltage switch gears that contactor had one serious coil and contacts. The entire power supply was a 1960s design and the thing was a beast. Almost a 200 watt contactor coil, go figure.

Ron

 

[kinarfi]

Did I forget to mention I was dealing with DC, My bad. DC coils on AC do tend to chatter, so they have a shunt ring in the armature to square up the hysteresis loop. I think Mike hit the nail on the head, high wattage for bigger springs needed to move the armature fast at opening, but, couldn't the coil with the large spring be made of more turns of smaller wire, enough so as to pull the moving piece in any way, or would that create large counter EMF and slow the speed of the moving piece, or would that just move the half way there and not close the circuit? - - - Strong Spring, more wattage, more heat, smaller duty cycle or larger case for heat dissipation.

Ron, were they trying to heat the armature of a 3 phase motor while running of before running it, and I would guess this is a Y connected motor. I've applied dc to 3 phase motors to brake the rotor, rolling cranes, 12VDC on a 208/240 VAC motor creates a lot of drag, but is was interlocked for one or the other.

 

[Reloadron]

Ron, were they trying to heat the armature of a 3 phase motor while running of before running it, and I would guess this is a Y connected motor. I've applied dc to 3 phase motors to brake the rotor, rolling cranes, 12VDC on a 208/240 VAC motor creates a lot of drag, but is was interlocked for one or the other.

Sort of. These were large three phase motor stator assemblies fitted with their water jackets. The water jackets were filled with helium and while cooking were sniff tested as part of a helium leak test. The best wat to get the stators hot was simply to run DC across two phases. They ran like that for a little over 24 hours.

Ron

 

[MaxHeadRoom78]

Did I forget to mention I was dealing with DC, My bad. DC coils on AC do tend to chatter, so they have a shunt ring in the armature to square up the hysteresis loop. .

The reason for the shading ring is it causes a phase shift in that part of the winding, the current of which carries the coil through the zero point of the applied wave form and reduces chatter.
Max.

 

[MikeMl]

Kin,
are you asking how to calculate the wire gauge/number of turns? If so, here is a calculator.

For the two relays I mentioned above, the wire is wound on a bobbin. The bobbin is the same for both relays. The bobbin has a fixed length and fixed height. If you select different wire gauges, the thing that changes is how many turns n fit on the bobbin.

The magnetic force that attracts the armature to close the relay is nI, where n is the number of turns as above, and the coil current I.

The voltage applied to the coil is the same for both relays, 12V, so I=12/R

R is a function of the length of wire (at any given gauge, see the resistance vs length in the wire table).

So I gave you enough info to design your coils...