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

Dear Carl,


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

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