Current Limiting Fuse

[jpl5085]

I was trouble-shooting a control panel that had 3 small motor starters in it. Whenever I had two starters energized and brought in the third, the contactor would chatter or drop out all three contactors. The problem turned out to be that the 100VA transformer had a 1 amp KLD-R current limiting secondary fuse. And it did what it was supposed to - it limited the current and thus causing the contactors to drop out or chatter (during the initial inductive inrush). By the way the control voltage is 24VAC therefore the fuse should be around 4-6 amps.

OK now the question ... How does this type of current limiting fuse work? It has three characteristics: Current limiting, Short Circuit, slo-blo. I was just curious on the physical design of the fuse.

 

[jraef]

It is not the fuse that is limiting the current. A "current limiting fuse" is called that because it will BLOW in time to limit the inrush current to the advertised levels. It has NO EFFECT on limiting current on a continuous basis.

Most likely your transformer is undersized for the load and it was unable to maintain it's output under the slight voltage drop caused by the inrush of the motor on the 3rd starter, plus the coil inrush of the 3rd starter itself.

If the problem went away when you relaced the fuse, a secondary problem may have been in a bad connection on one of the fuses causing just enough additional resistance in the circuit to exacerbate the above issues.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jpl5085]

I would agree with you to yesterday. The transformer was properly sized (i rechecked it according to the transformer mfg's formula). A 75VA would be sufficient with the inrush and seal-in characteristics of the coils. I actually put in a 150VA for your reasoning above. Same problem occured. When I replaced the fuse with a piece of wire the problem went away. There was also a audible difference in the intensity of the coil pulling in. Maybe the fuse doesn't actually limit it - it is hard to believe a piece of wire would. That is why I am asking how or why it appears to limit the current.

 

[buzzp]

They do make current limiting devices but they are not called fuses. I have seen them used on PCB's and such but never in a coil circuit. Do you look up the fuse part number to get a spec sheet? Maybe the connections were corroded on the fuse block or something as previously mentioned.
Maybe the fuse is bad, try replacing it with another of the same type.

 

[jpl5085]

Buzzp

brand new everything in the panel. Checked and tightened all connections. Eventually pulled the fuse and replaced with wire. Worked like a champ. Put fuse back in and same ol' problem. I replaced the fuse with a larger one and it works fine. I would think that if a fuse was bad it would be "open". Like I mentioned above, it is hard to believe a piece of wire would choke the current. Unfortunately our current clamp-on is not sensitive enough to measure this small current.

 

[AFab]

Fuses do have a specific amount of resistance.That is how
they work.When the current exceeds the fuse rating the fuse
produces enough heat to "blow".Having said this the fuse is in the control circuit in series with the coils,ohms law
states will be a voltage drop across this fuse.The higher the resistance the greater the voltage drop.Thefore in a series circuit the sum of the voltage drops is equal to the
applied.
See where I am going with this? If you put in a chunk of copper wire or an oversized fuse with less resistance in the fuse holder the voltage drop across the fuse/wire is less thus maintaining the correct (24vac) to your coils

 

[REDDOG]

Replace with a time delay fuse

 

[mc5w]

Things that you can try:

1. Clean all of the wire strands in your wiring connections. Include the neutral connections.

2. Bring in 120 volts off of your 120Y208 volt 3 phase or 120/240 volt single phase electrical system. I know somebody who had to abandon a 1 KVA control transformer and run a 20 amp 120 volt circuit off of the building power for 120 volt outlets and lighting.

The building 120 volts that supplies outlets and lighting is likely to have better voltage regulation than any control transformer. I have made it standard practice to run 120 volts from the outlet circuit that is on the machine to the control box in case the control transformer dies.

Mike Cole mc5w at earthlink dot net

 

[advidana]

looks like you have a voltage drop problem, increase the wire sizes to the motor from the panel board source.increase control fuse size if code allows it for your installation. why are you using a current limiting fuse in the first place. the transformer will choke any fault currents to the control circuit on the 24 volt side.

 

[CarlPugh]

Try different brand fuse or fuse rated for a lower voltage.

Many years ago had circuit with fuse. Fuse was supposed to open when ourput was shorted. Fuse was glass type where you could see the fuse element. Imagine my surprise when the short was applied and the fuse element turned red, but didn't open. Fuse could withstand 10 volt across itself without opening.
Changed fuse manufacture and fuse opened like it was supposed to.

 

[ScottyUK]

Carl,

I saw something like this when I was a kid running PA systems for local bands as a college sideline. The small monitor loudspeakers that one band used had fuses which were externally accessible, and there were several occasions when they 'lit up' like little tubular lamps when one of the big amps was working them hard. It may be that the fuses really were lamps - it was not my kit, so I didn't really care - but the memory has stayed with me.



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