Very remote stop

Is the control loop AC or DC? Related discussion and links in thread237-83120

Hello AfricanElectric

It sounds like you m ay be using an elecronic input for you remote stop input (PLC or something?)
You will have leakage and capacitance on a long run, and the most reliable way is to ensure that energy is fed down the circuit.
I would suggest that you use a fairly large relay at the receive end so that there is a reasonable current flowing. I would also use DC to reduced capacitive effects.
If you wish to have a fail safe system, then a two wire circuit has problems. If you use the circuit with the relay powered for a healthy circuit, then a short on the circuit will defeat your remote stop. If you use the circuit with the relay power for a stop, then an open circuit will defeat the system.
It would be more reliable to use two relays and a three wire circuit with a change over contact at the remote end. The two relays would be arranged such that one and only one is powered at any time. Relay A is powered for run and relay B is powered for stop. The contacts from the relays are wired in series, NO for relay A and NC for relay B.
This will determine an open circuit, or a short circuit and give you a more fail safe circuit.

Best regards,

Mark Empson

If you're using ac control power, then this circuit is way too long and you are seeing classic problem of capacitance in the control circuit.

You can use interposing relays with dc control power, probably using the same wires you now have in place.

As Marke mentions, if this is truly an Emergency Stop function with life safety implications, you will need to address fail-safe issues.

Hello AfricanElectric,

What you describe is classic in the coal mining industry. We used to use the same setup with the same problems. The coal mining industry (at least in the USA and Australia) has moved to using a Dupline system. Dupline is a product family manufactured by Carlo Garvazzi, and can be found at It can be designed to be failsafe and it has a range of several kilometers. The only drawback in your application is the type of cable spanning between the E-Stop button and the motor controls. Dupline is a communications bus technology and works over a single twisted pair cable. If you are using single (untwisted) conductors in a cable or conduit then it won't work reliably.

If you want, I can send you more detailed information.

Cheers,
John

Hello again

Thanks a lot for the help, blokes.

Given me quite a bit to think about.

Its a brilliant website.

Andrew

Hello AfricanElectric,

One way to do your emergency stop circuit is to use a 24 volt direct current power supply such as a power brick at the emergency stop end of the circuit. The emergency stop button would need to use double pole double throw ( form C ) contacts and at the motor end you would need a 24 volt DC relay preferably of the wide voltage range that can tolerate the 20 volt end voltage of a 24 volt battery. The normally closed contacts of the emergency stop button should interrupt both sides of the 24 volts from the power supply. The normally open contacts should be connected in parallel with each other and connected downstream of the normally closed contacts so as to short the line and discharge its capacitance when the emergency stop botton is pushed. The relay at the far end might need to be a 12 volts DC relay that uses a 7812 voltage regulator with some 1 microfarad capacitors for noise reduction and stabilization. Either voltage relay also needs a diode connected across the coil so that the diode is biased off when the DC is applied so that the diode will carry the inductive kick of the coil when turned off - this will increase the dropout time but only a little. The emergency stop button should be of the maintained contact type so that no harm will be done by a slow dropout time due to wiring capacitance.

An alternate way to do a failsafe emergency stop circuit is to use a UL Listed fire alarm or burglar alarm box at your ski lift motor controller.

Failsafe circuits for fire alarms and burglar alarms use the Wheatstone Bridge principle to detect normal, shorted, and open circuit conditions. Three of the bridge resistors, usually 10,000 ohms each, are located in the alarm panel. The fourth resistor is located at the far end of the circuit.

In a normally closed circuit, what is used for lead tape on windows and so forth, an open circuit condition triggers the alarm and s short circuit triggers the fault alert. A circuit resistance of 10,000 ohms indicates normal conditions.

For a normally open circuit the shorted condition is the alarm condition and an open circuit is the fault condition. For 4 wire circuits that supply smoke detectors and motion detectors there is a relay at the far end of the circuit that opens the resistor upon failure of the control power. A more sophistocated relay would be an interval relay that altenates between 10,000 ohms and open circuit so that a bad relay can be detected.

For either method you would need to use direct burial rated telephone cable that is filled with a waterproofing gel. 19 gauge telephone cable would be a good investment because that is the largest gauge telephone cable next to open wires. You can get direct burial cable that is 19 gauge with as few as 3 pairs of wire and a layer of steel to deter gophers and tree roots from breaking the cable.

You will need 2 kilometers of cable, maybe more. When direct burying small telephone cables you need to pay out 1.5 to 2 feet of cable for each foot of trench. (1.5 to 2 meters of cable per meter of trench for metric.) This is so that the cable can conform to the bottom of the trench when backfilling. If you are running the cable overhead using a steel messenger cable for support you will need to leave lots of slack when stringing it during the summer and leave a generous slack loop at each support pole.

You will also need a telephone protector block at each end. What you can do with the extra wire pairs is to put in a dedicated telephone circuit and maybe a buzzer or other telegraph circuit. You also need to ground one end or the other of each circuit.

Your resistor at the far end of the fire alarm or burglar alarm circuit might need to be a variable resistor to compensate for wire resistance. Wire resistance will be lower during the winter but if you can bury the cable 1 to 1.5 meters ( 3 to 5 feet ) deep you will have less temperature swing and fewer other problems.

Mike Cole, mc5w@earthlink.net

Hello African Electric,
Yes we have seen this same type of problem with long control cables in the underground mining industry. In our case, it was due to the capacitance in the cable. Some of our E-stop circuits extends for 3 miles. It has been solved in several ways including:
a) use low voltage DC rather than AC;
b) Use a relay type that looks for a terminating diode (Service Machine Co. C54-004 or equivalent); or
c) load the line sufficiently with a resistance at the sensing device (PLC input, etc.) that the capacitance will not allow the input to be active, but the loop resistance operates properly.
I am sure that there are other ways, but these have all worked properly in the past.
Considering it is an E-stop circuit, be careful with line loading to make certain that the load resistance does not open circuit during normal operation.
Good luck,
Dave

A possible quick-and-dirty brute-force solution:

If the point where you are installing the emergency-stop button is near the electrical supply lines to the tow, just have the button trip a nearby contactor, interrupting power to the tow. ( I assume you already have a way to deal with loss-of-power to the tow not creating another emergency )

Its rough, but nearly foolproof.

To change the control to another scheme (eg DC voltage), it needs quite a lot of modification.

Maybe we can try to increase the current consumption of the contactor coils to reduce the effect of capacitances. This can be done in 2 ways:
1. Change to a larger contactor (but there may have a space constraint in the motor starter board)
2. Add additional shunt coil/transformer across the existing contactor coil, just to consume/drain the current for nothing useful.

I think this method can be done easily and almost immediately.

In our plant, the AC-controlled start-stop control switches located beside the motor needs 4 wires from the motor starters. This include a neutral wire from the contactor coil. When we turn the switch to Stop, additional contact is used to short across the contactor coil. This will discharge the capacitance or stored energy in the coil.

If the control cable to your emergency stop button has 3 cores or more, use can apply the same principle, ie, on pressing the emerg stop button, one contact to disconnect the AC voltage to contactor coil, another contact to apply a short across the contactor coil.

Hi digitrex,

Relying upon a contact closure to ensure operation of an ESD is not very good practice, is it? If the contact fails to close, the circuit remains energised through the cable capacitance.

The idea of loading the circuit sounds more feasible, although the burden must be secure and not prone to disconnection or failure, i.e. don't use a lamp or similar device as a burden.

Conversion to DC operation appears to be the best technical solution, but there are no doubt other factors we aren't yet aware of.




----------------------------------

If we learn from our mistakes,
I'm getting a great education!

ScottyUK,
I agree with you on the ESD that need to be fail safe. I was thinking adding a NO contact just to improve the reliability.

Anyway, loading up the ESD circuit by adding more burden(additional coils/transformer) can be easily achievable. What AfricanElectric can do is to run the motor and operate the ESD button. While the ESD button is pressed and motor is still running, measure the ac voltage on the contactor coil and compare this voltage with the contactor drop-off voltage(need separate off-line test using a variac). Then add some load to parallel with the contactor coil until the contactor drops off. Add another 50% more(or 10V below drop-off voltage) as safety margin.

Raisinbran,

I did a web search for the Service Machine Company C54-004 ground check relay and could not find it. What is their web site?

I am familiar with how a ground check circuit with a terminating diode works. I am also familiar with rigging a ground check circuit to provide start and stop buttons. Source is a book titled Collier Electrician that is published by the British National Coal Board. In 1980 and electrical engineer for BICC gave me this book.

Mike Cole, mc5w@earthlink.net

The two problems that arise are the “series impedance” and the “shunt capacitance” of the control long wires.
The series impedance effect will result in excessive voltage drop, this drop is normally limited to 5% as to insure satisfactory operation of the circuit.
The distributed capacitance affects when the circuit is opened. A large enough capacitance will prevent the contactor from dropping out when the circuit is open.
The solutions depend on the critical factor, “series impedance” or “shunt capacitance”. Some solutions are ( probably you could develop much more) : a) limiting the length and size of the control wires, b) For capacitance problems, locate the power source adjacent to the Stop Button (control device), c) For series impedance problems, Interpose a control relay of lower burden than the contactor coil.
As a generic guideline, the series impedance problem is serious with low voltage (V< 48 VAC) and the shunt capacitance problem when the voltage is high (V >48 VAC).

Have you considered going to an RF device, eliminating the problems with wires broken, impediance, etc. Look at

One method used with gondola control I have seen uses a single wire and a ground return 30mA DC current loop. The loop is arranged for stop signals to be less than 12 mA, slow speed 15 to 22, and fast speed 25 to 30. The idea is a positive monitored signal, with loss of signal equaling stop. System has been running for 10 years, biggest problem is drying out of earth in summer affects the earth resistance