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Soft start for shore power


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I wonder if anyone has installed a soft-start on their shore power line. If I randomly connect on the wrong part of the mains phase the isolation transformer trips the shore-side MCB, which is a nuisance. I'm thinking along the lines of a big resistor at a few tens of ohms with a shorting relay and a simple time delay circuit to let the transformer field build up.

Martin/

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1 minute ago, Batavia said:

That's one way of doing it, but a better approach is to install an MCB that can handle the transformer's inrush current - typically a Type C or Type D.

Chris G

You cant usually change what the mooring provider fits but you can add a soft start to your TX as suggested, 150W bulb in series

 

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10 minutes ago, AndrewIC said:

You can get varistors intended to go in series with the primary for this purpose. Ametherm MS35 5R025, for example. They run pretty hot though.

NTC thermistor would be a cheap and nasty solution, and might do the job (assuming that 5 ohm is sufficient to avoid tripping the marina's MCB, which it might not, as it's rated at only 6A and seems quite sensitive. It's a class B so moves fairly fast. Last time I looked 6A type C MCBs were as rare and expensive as unicorn bits, even if the marina would let me put one in.)

As I've got all of the bits I'll probably assemble something like figure 1 in this: http://www.edn.com/design/analog/4349191/Time-delay-relay-reduces-inrush-current

I'd wondered what other people use, if anything.

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11 minutes ago, Onewheeler said:

I'd wondered what other people use, if anything.

A Galvanic Isolator?

 

Anyway, I presume you are plugging the IT into the shore bollard first, then plugging the IT into the boat or otherwise switching on the boat's supply (e.g. at the consumer unit) - if there is no load on the transformer output then obviously the peak current will be minimised.

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36 minutes ago, nicknorman said:

Anyway, I presume you are plugging the IT into the shore bollard first, then plugging the IT into the boat or otherwise switching on the boat's supply (e.g. at the consumer unit) - if there is no load on the transformer output then obviously the peak current will be minimised.

Not obvious at all. If you happen to make the connection at zero degrees phase the current into the transformer will be maximum. The minimal load usually connected is of minor significance.

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1 hour ago, Onewheeler said:

NTC thermistor would be a cheap and nasty solution, and might do the job (assuming that 5 ohm is sufficient to avoid tripping the marina's MCB, which it might not, as it's rated at only 6A and seems quite sensitive. It's a class B so moves fairly fast. Last time I looked 6A type C MCBs were as rare and expensive as unicorn bits, even if the marina would let me put one in.)

As I've got all of the bits I'll probably assemble something like figure 1 in this: http://www.edn.com/design/analog/4349191/Time-delay-relay-reduces-inrush-current

I'd wondered what other people use, if anything.

I have a feeling Loddon did something like this

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22 minutes ago, Onewheeler said:

Not obvious at all. If you happen to make the connection at zero degrees phase the current into the transformer will be maximum. The minimal load usually connected is of minor significance.

I feel you are contradicting yourself a bit - you are refuting that removal of secondary load minimises the inrush, whilst also agreeing that it has some minor significance. I agree that the phase at the point of connection will be the primary issue but removing all secondary load will, as I said, minimise the maximum inrush current. Whether it would make enough difference to be significant in your case is of course another matter.

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I would be tempted to add a solid state relay in line with the transformer.

most of them have zero crossing switching (meaning they switch on / off when there is no voltage on the AC connections)
a 25A @ 480v one costs around £30 so I would imagine one rated for 250v (to allow for surges) would be cheaper (even more so if you only wanted a 10A capability)

example http://www.ebay.co.uk/itm/222536785873

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20 minutes ago, Jess-- said:

I would be tempted to add a solid state relay in line with the transformer.

most of them have zero crossing switching (meaning they switch on / off when there is no voltage on the AC connections)
a 25A @ 480v one costs around £30 so I would imagine one rated for 250v (to allow for surges) would be cheaper (even more so if you only wanted a 10A capability)

example http://www.ebay.co.uk/itm/222536785873

Counter-intuitively, the inrush current is worst when the switch on occurs at zero voltage in the cycle.

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2 hours ago, ditchcrawler said:

I have a feeling Loddon did something like this

Talked about it worked it out but never got a round tuit. Its a similar soft start circuit to the ones used in high power audio power amps 

I have changed the 16amp mcb in the shed to a K type and the 32amp mcb in the distribution point to a D type.

A 3.6kva toroidal with no load or softstart can take out a 32amp B type mcb on startup..........

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1 hour ago, WotEver said:

Of course, if you buy the popular (and comparatively cheap) Airlink IT then it comes with a surge limiter built in. 

https://airlinktransformers.com/product/boating-transformer-bt3231

i fitted a varistor to my Airlink (didn't used to be an option), with a switch wired across it. Open the switch before plugging in, close the switch afterwards. Works a treat, used to work ok on a 2A(!) bollard breaker.

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1 hour ago, WotEver said:

Of course, if you buy the popular (and comparatively cheap) Airlink IT then it comes with a surge limiter built in. 

https://airlinktransformers.com/product/boating-transformer-bt3231

Eek! I only paid a few tens of quids for my 1 kVA isolating tfmr. One doesn't see those prices on Ebay any more!

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4 minutes ago, Onewheeler said:

Eek! I only paid a few tens of quids for my 1 kVA isolating tfmr. One doesn't see those prices on Ebay any more!

The price keeps going up cos they have to pay WotEver 15% every time he mentions them:)

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