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Inrush Current


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#1 Carl

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Posted 09 December 2006 - 03:03 PM

Hello Gents,
I’m at a stage of developing the circuit art work for an AC squirrel cage motor. I’ve just noticed that I have no inrush limiter.
The route I’ve chose is a resistor in parallel with a normally open relay.

1 I have 3x 450v 330uF Capacitors in parallel for the bus
2 I’m targeting a 1Kw three phase 220v star connected motor
3 I have a single phase input with a 10A fuse.
4 I’m going to be monitoring the bus voltage via a resistor voltage divide network.
5 Can’t remember the input bridge rating, but have an idea its about 15 amps

My concern at the moment is the actual resistance value and wattage rating of the inrush resistor. I don’t want to stress the bridge and obviously I don’t want to be blowing fuses. I plan to switch the bypass relay on with a uProcessor after the bus voltage has reached a value of ??

I’m not concerned about the target motor size at this point; I just want to see the thing come alive. I’ll most likely load it up with some light bulbs.

q/ what should the resistor be rated at?
q/ what would the be the voltage point to switch the bypass relay in ?
q/ Is there anything wrong with this design approach
q/ Are there any other point that I should be considering.

Thanks in advance


#2 mariomaggi

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Posted 09 December 2006 - 10:04 PM

Dear Carl,
QUOTE
q/ what should the resistor be rated at?
q/ what would the be the voltage point to switch the bypass relay in ?
q/ Is there anything wrong with this design approach
q/ Are there any other point that I should be considering.


Shortly,
- the resistor must be in position to accept the full voltage when the relay will open for overvoltage or for other reasons. Probably it is interesting to add a PTC in series. In this case a resistor around 500 ohms 10 W shall be used safely.

- this is not so critical. 60% or 70% should be a good choice.

- capacitor capacity probably could be reduced

- select a bridge with an high repetitive peak current, avalanche controlled

Regards
Mario

Mario Maggi - Italy - http://www.evlist.ithttps://www.axu.it


#3 Carl

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Posted 11 December 2006 - 09:07 AM

Hello Mario,
Appreciate your input Mario. It’s nice to bounce some ideas off, especially when you’re doing something in isolation. I’ll have a closer look at the capacitor rating. At this stage it’s a prototype and I’m sure that I’m going to find loads of things wrong with it, when its built.

Thanks
Carl





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