Limiting inrush current

One of our larger 100A/200A 29V chargers has a single-phase variant that has an inrush problem. The simplest way around it is to either use a resistor with shorting contacts, or you can use a choke in series with one leg of the primary.

I think if the choke design is just so, you wouldn't need to worry about shorting it out once running. Peter

-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk

The motor (which will start off load) >could< be spun up to speed

Our multispindle lathes have 20 hp direct start motors.These motors at start up are spinning all the gearbox and six 40 mm diameter bars

3000mm long at 1280 rpm. They are fused at 30 amps (FL 28 amps) and never blow a fuse. Mark.

option

But Mark, this 20 HP will be running through a phase converter which steps 230v single phase up to 415 so there will be proportionally more current. I am not worried about fuses blowing as it is only a very short term effect and thermal inertia in the fuse takes care of that. No, I'm trying to avoid the 'oh blimmey Andrew's at it again' comments when the lights in the whole street flicker

AWEM

A simple solution is a negative temperature coefficient resistor (a power thermistor) in series with the supply. You will need more than one of them at your power level but they can be paralleled without problem.

See

Jim

I was about to suggest the same thing,

It's a lot cheaper than lots of clever electronics

-Mark

We used to bang a 250 hp slip ring motor of and on every few minutes.We dimmed the whole scheme,never mind the street. Mark.

Oo - ar ! Clever devices - I didn't know that the NTC ones came in such large current ratings - certainly worth bench testing a few I think - thanks.

AWEM

The magnetising current itself is not the problem, it's the short circuit current that flows when the core saturates because you switch the mains on to the primary at the wrong point in the cycle, that's why you sometimes get a surge when switching a large transformer and sometimes don't, it just depends where you were in the cycle at the time. You can solve this problem with a triac but not by ramping, just switching on at the peak of the mains voltage which is also the zero point of the magnetising current, so that way the current at switch on is zero just as it should be at that point in the cycle.

As to the motor, well you definitely have to spin it up slowly by some means as you have to put a lot of energy into it, spinning up quickly would draw that energy in a short time so inevitably cause a dip.

Greg

A simple solution is a negative temperature coefficient resistor (a power thermistor) in series with the supply. You will need more than one of them at your power level but they can be paralleled without problem.

See

Jim

PS Thinking further about the problem I realised that there could be a problem with current hogging with paralleled thermistors - all the current going to the shortest time constant component. Probably worth checking current division precautions with wecc.

what about using smaller start capacitors.. Mine starts without you being aware of any switching or coming up to speed with 200uf, it starts ok on 100uf but your aware of the caps swithing and the motor[7.5kw] takes maybe a couple of seconds to spin up..