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Inrush Current


Carl

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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

 

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Dear Carl,

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.it - https://www.axu.it

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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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