control4it
Member
Can anyone explain why a relay would burn-up that is rated for 24Vac and 24Vdc is applied. Other than the obvious that the wrong voltage was applied, but why or how did this happen?
Vdc-Vac
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quogueelectric
Senior Member
- Location
- new york
the ac coil is designed for cooling in between zero cycles. the dc voltage never cools off until failure point.
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
I'm with rattus on this one.
If you have 1A RMS flowing through the coil, you will have essentially the same heating, AC or DC.
But if you apply 24V DC to a coil designed for 24V AC, then you will get much higher current flow. The reason is that the coil has substantial inductance, and so has a much higher AC impedance than DC resistance.
-Jon
If you have 1A RMS flowing through the coil, you will have essentially the same heating, AC or DC.
But if you apply 24V DC to a coil designed for 24V AC, then you will get much higher current flow. The reason is that the coil has substantial inductance, and so has a much higher AC impedance than DC resistance.
-Jon
control4it
Member
The power supply was humming but it blew a 5A fuse but did not blow a 10A fuse. The power supply is rated for 12A @ 24Vdc at it's output. I used a 24Vac pwr. supply and it works as as expected. Just trying to figure out how to explain why the dc pwr. supply would not work. Must be that it don't have that cooling cycle as ac passes through zero. There was significant voltage drop as I had multiple dc relays firing ac relays.The plc outputs would be on, but the dc relays' coil would not pull in and the voltage dropped to 4Vdc.
gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
080507-0728 EST
control4it:
There is quite a difference between an AC and a DC relay of otherwise the same design.
If you measure the DC resistance of the two coils you will see a very great difference. In general for a relay like the Potter & Brumfield KUP you can use a 120 V AC unit in a 24 V DC application.
In a DC relay there is no difference in the current flow relative to armature position and there must be enough current when the armature is open to provide enough force to close the relay. This current determines the heating of the coil. If the armature is held open the same power is dissipated as when closed. The steady-state current is defined by the applied voltage and coil resistance.
In an AC relay a large part of the impedance once the armature is closed is determined by the coil inductance and the AC frequency. When the armature is open this inductance is much lower and therefore the impedance is much lower. This is an advantage of an AC relay so long as you do not have the armature stick in the open position. Force is a function of current and the magnetic circuit. When an AC relay is first energized a large current flows providing high pull-in force, and when the armature is closed because the inductance increases the current drops substantially. Even though the current drops the holding force is very good because the magnetic circuit has much less leakage flux.
I believe you will find that the AC coil will run cooler than the DC coil for a similar relay because you can get high initial closing force because of the initial current and have the current drop way down when closed.
On the subject of heating and AC vs DC. A resistance converts a current, AC or DC, to heat. If you use RMS as the measure then equal values of AC and DC thru a resistor of value R produce the same amount of heat.
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control4it:
There is quite a difference between an AC and a DC relay of otherwise the same design.
If you measure the DC resistance of the two coils you will see a very great difference. In general for a relay like the Potter & Brumfield KUP you can use a 120 V AC unit in a 24 V DC application.
In a DC relay there is no difference in the current flow relative to armature position and there must be enough current when the armature is open to provide enough force to close the relay. This current determines the heating of the coil. If the armature is held open the same power is dissipated as when closed. The steady-state current is defined by the applied voltage and coil resistance.
In an AC relay a large part of the impedance once the armature is closed is determined by the coil inductance and the AC frequency. When the armature is open this inductance is much lower and therefore the impedance is much lower. This is an advantage of an AC relay so long as you do not have the armature stick in the open position. Force is a function of current and the magnetic circuit. When an AC relay is first energized a large current flows providing high pull-in force, and when the armature is closed because the inductance increases the current drops substantially. Even though the current drops the holding force is very good because the magnetic circuit has much less leakage flux.
I believe you will find that the AC coil will run cooler than the DC coil for a similar relay because you can get high initial closing force because of the initial current and have the current drop way down when closed.
On the subject of heating and AC vs DC. A resistance converts a current, AC or DC, to heat. If you use RMS as the measure then equal values of AC and DC thru a resistor of value R produce the same amount of heat.
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L
Lxnxjxhx
Guest
Just for my own info, how long did the relay hold out before dying?
Doubling the coil temp in degrees C should halve the lifetime.
Doubling the coil temp in degrees C should halve the lifetime.
L
Lxnxjxhx
Guest
Make that
Make that
each 10 C rise in temp. halves coil lifetime.
Make that
each 10 C rise in temp. halves coil lifetime.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080507-2057 EST
Some specific information on the Potter & Brunfield KUP relay.
24 V DC 1.2 W 472 ohms 51 MA
24 V AC 2 VA 85 ohms 84 MA AC, power 85*0.085^2 = 0.61 W excludes power in shading coil and AC losses.
Operated on 24 V DC power is 24*24/85 = 6.8 W, somewhat less because the wire gets hot.
The 24 V AC coil temperature rise when operated on 24 V DC would be about 5 times that of the DC coil relay. But failure will occur before the rise gets that much.
.
Some specific information on the Potter & Brunfield KUP relay.
24 V DC 1.2 W 472 ohms 51 MA
24 V AC 2 VA 85 ohms 84 MA AC, power 85*0.085^2 = 0.61 W excludes power in shading coil and AC losses.
Operated on 24 V DC power is 24*24/85 = 6.8 W, somewhat less because the wire gets hot.
The 24 V AC coil temperature rise when operated on 24 V DC would be about 5 times that of the DC coil relay. But failure will occur before the rise gets that much.
.
L
Lxnxjxhx
Guest
0.61 W for a, say, 10 C rise above ambient gives a thermal resistance of 0.06 w/deg C. 6.8 W should then give ~110 C rise above ambient.
One stinky relay.
One stinky relay.
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