Fuse blown but there's still voltage measured

[grasscut]

Hi, i have a question regarding why voltage can be measured even after a fuse blown. The application i have gives an synchro output L1, L2 and L3 with reference voltage as Reference High(RH) and Reference Low(RL) of 115Vac.
The receiving end failed to receive any signal and one of my fuse across RH has blown. There are one fuse each placed across RH and RL to protect the application. However, my question is, even after the fuse got blown, I am still able to measure voltage across my RH and RL, and between the L1 L2 and L3 end. My reading for my RH and RL were measured around 50-60V. So may i ask, why is the voltage still present even after a fuse was blown?
thanks!

 

[scottdave]

Some induced EMF perhaps. Do you have a diagram?

 

[grasscut]

Some induced EMF perhaps. Do you have a diagram?

thanks for the reply! I am sorry but i do not have the diagram.
i can briefly explain what is this application. This application transmit data of ship's heading using synchro signal.
Example of the synchro signal: SYNCHRO SIGNALS.— A single-speed synchro transmitter outputs three waveforms that indicate the angular position of the rotor in the transmitting synchro, for example a control transmitter (CX). Waveforms are induced in the stator coils by the magnetic field of the rotor coil. The two rotor connections of the CX (R1 and R2) are fed from a 115-volt ac supply voltage (also called the reference voltage) (reference: http://electronicstechnician.tpub.com/14091/css/Synchro-Signals-318.htm)
So one of my fuse was blown at the reference voltage at RH, however, when i measured the voltage across RH and RL, there still voltage present. Thus, I am wondering why was the voltage still present, but at a lower voltage around 50-60V, when the fuse has already blow.

 

[scottdave]

Are you measuring the voltage between the two terminals, or between the terminal and "ground"? Ships don't have neutral tied to ground, like you see in land based electrical distribution systems. So rather than reading 0volts on the "neutral" and 115v on the "hot", the voltages may float. You still get 115 volts, across the terminals, but the voltage to "ground" (ships chassis) is not constant. I recall one electrician showing me a coffee pot, plugged into an outlet, and the metal warmer plate read 90 volts.

You may find the Module 15 NEETS module (Synchros,Servos and Gyros) helpful (it does have a diagram for typical synchro systems). Here is a link to all of the Navy NEETS modules. http://www.phy.davidson.edu/instrumentation/NEETS.htm then just click on Module 15 to download a PDF.

Or, after studying the diagram on page 81 of the PDF, the following seems like a possibility. Can you see that perhaps if reference voltage is still present at the receiving synchro, that voltage could be induced in the stator windings, which current could still flow back to the transmitter, and induce voltage in the reference winding of the transmitter?

 

[grasscut]

I'll try to study the reference you shared, its really useful! thank you so much for that. Anyway, I am measuring the voltage between two terminals, RH and RL, the reference voltage. I was told that this measured voltage is the so called Ghost/Phantom Voltage. the reference voltage is generated by a rotary converter, and from what i know it should be a single phase supply. The synchro signal distributed is actually from a Digital-to-Synchro card which converts digital signals into synchro data.
Regarding whether the reference voltage is still present at the receiving synchro, yes the reference voltage is still present even after the fuse blow. However, the reading is no longer 115 Vac, which is theoretically correct, since the link is now broken. But it still measures a voltage of around 50-60V between the two terminals of RH and RL.

 

[scottdave]

Is there any way to turn off the signal from the Digital-Syncrho card? Maybe the AC signal, which it is generating, is inducing a voltage into the reference coil.

 

[scottdave]

I'll try to study the reference you shared, its really useful! thank you so much for that.

I think those NEETS modules are well written. I am glad that Davidson college decided to host them. I have seen them other places on the web, as well.
Have you tried replacing the fuse? Does it blow again?

Just curious... what kind of ship is it, and what type of information is being represented by the synchro?

 

[scottdave]

Another thing that I thought of... try pulling both fuses and then read the voltage.

 

[grasscut]

Is there any way to turn off the signal from the Digital-Syncrho card? Maybe the AC signal, which it is generating, is inducing a voltage into the reference coil.

I wasnt able to do so and the data from the card is been transmitted to this application i talking about. and the reference voltage is supplied by an external source.
Its a naval vessel. the synchro signal is transmitting the angular position of the ship such as ship heading which i shared the previous reference link. Replacing the fuse did solve the issue and the point between the two terminals of reference voltage was back to 115V. So I am just curious why was there voltage present when fuse has already blow.

 

[scottdave]

I went back and read the posts. I could not find this: are you measuring at receiving end? Is there a transmitting synchro, or is is just fed by the digal synchro card?

 

[grasscut]

I went back and read the posts. I could not find this: are you measuring at receiving end? Is there a transmitting synchro, or is is just fed by the digal synchro card?

yes i am measuring the receiving end and the signal is fed by the digital synchro card, but the reference voltage is from external power supply

 

[scottdave]

i am measuring the receiving end and the signal is fed by the digital synchro card, but the reference voltage is from external power supply

This is what sounds like is happening.
So the digital card is supplying ac voltage to the 3 stator coils of the receiver. These now act as a primary coil of a transformer. A voltage is induced I to the reference coil, which is acting as a transformer secondary winding.

 

[scottdave]

i am measuring the receiving end and the signal is fed by the digital synchro card, but the reference voltage is from external power supply

This is what I think is happening, based on how I understand it.
The digital card is supplying ac signal voltage to the 3 stator coils of the receiver. These now act as a primary coils of a transformer. A voltage is induced into the reference coil, which is acting as a transformer secondary winding.

 

[scottdave]

I came across this link, which gives some troubleshooting and maintenance tips for synchros. It may be useful to you.
http://www.tpub.com/gunners/136.htm
Perhaps there is something with the blown-fuse indicator lamp circuits which is giving you some voltage readings on the reference coil. Without specs or diagrams, everything I'm giving you is speculation. But perhaps some information that you can use in your diagnostics.

 

[grasscut]

I came across this link, which gives some troubleshooting and maintenance tips for synchros. It may be useful to you.
http://www.tpub.com/gunners/136.htm
Perhaps there is something with the blown-fuse indicator lamp circuits which is giving you some voltage readings on the reference coil. Without specs or diagrams, everything I'm giving you is speculation. But perhaps some information that you can use in your diagnostics.

Thank you for the reference! i'll try to study the links that you provide :)