I am looking at building a change over panel to change over between 4
different 240v 1 phase AC supplies on a barge - 3 supplies are max 20A
(shore, alternator and inverter) and the fourth is max 48 Amp
Had looked at using control relays with 2 NO and 2NC poles but have
been advised that they are not suitable for changing over between 2
different power supplies as the change over is very short and there is
no mechanical interlock.
Now looking at using power contactors and reckon way ahead is to use 3
pole NO contactors in pairs, using only 2 of the poles, and a
mechanical interlock in each pair to prevent connecting 2 power
sources at the same time. As I understand it with the mechanical
interlock, when contactor A is made, contactor B cannot close even
with its coil energised until contactor A is de energised and the
contacts open.
Some of the conatctors need 240v AC coils and some 24v DC coils, but
this does not seem to be a problem.
Have RCDs etc downstream in the fuse panel.
Grateful for any thoughts.
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Consider using 3 sets of mechanically interlocked contactors. For this discussion the sources are A, B, C, or D.
Set 1 selects A or B to a circuit I will call Z, Set 2 selects C or Z to a circuit I will call X, Set 3 selects D or X to the output circuit.
This way it is not possible to connect more than one source to the outout circuit. You may need some control relays to get the intended connection priority.
Mechanically interlocked contactors are readily available in the USA for up to 60A ratings.
Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA
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Bill Kaszeta / Photovoltaic Resources
Thanks for this. I had worked out a priority sequence of supplies, starting with the shore supply, and I have sourced conatcters with interlocks to cover the current range. But you also mention using control relays as well - I would appreciate an explanation of the role these would play.
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There are two related problems that control relays (or a low end Programmable Logic Controller - PLC) solve:
1. With interlocked contactors, the first section activated holds out the other section until power on the coil of the first section is released.
2. When you have more than one source available, such as when your shore supply becomes available while the alternator is still active and was supplying the load, a control relay can be used to disconnect the coil on the alternator section of the interlocked contactors and apply power to the coil of the shore power section.
A form 'C' (SPDT) contact set on a control relay that is powered from the shore power can do this.
The same basic idea is repeated for each interlocked contactor pair. Additional form C contact sets or auxillary switches on the contactors can be used to totally unpower the control relays and contactors that are further down the chain from the primary source of power.
A PLC can add delays in connecting or disconnecting power sources. This is common with industrial controls. However, the PLC will need a continuous source of power such as 24 volts dc. Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA
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Bill Kaszeta / Photovoltaic Resources
Thanks for your simple explanation. The shipping list gets longer - 10 contactors and 5 control relays to go with them!! Are they essential or nice to have. When changing over supplies I do not envisage leave both supplies connected to a contactor pair for very long until one of the coils is released and the mechanical interlock can unlock it. Is there an advisable time limit for this operation - seconds or minutes???
On a related topic, I have identified contactors with typically a thermal current Ith of say 50A and AC3 of say 35A. Am I correct in assuming that AC1 equates to Ith - on some spec sheets it seeme to.
Much appreciate your assistance - would you be prepared to look at my AC contactor system if I emailed it to you off line.
Colin Stone
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