EBME Forums Forums Maintenance Spare Parts Drager Incubator -8000 NC

Seems you are indeed being a bit thick there, Huw. Either that or your memory is not as good as it once was.

In the past people have been roundly vilified on this site for merely hinting at “modifying” equipment!

Ah well, having lived in the middle east, I'm sure you'll appreciate the need for 'free speech', which is why I let people get away with so much on this forum.

Not sure if there's any difference in the selection of new components to cover obsolescence in medical equipment to other other electronic equipment, but in my previous employment this would be too small a task for a designer and would often get done by either junior hardware engineer or technician.

Mainly a paper work exercise, calculate heat produced, max current flowing in critical failure, half a dozen other parameters and then pop it in see if it works, then half a dozen various tests including temperature and emc checks to see if the new component mucks anything up.

On the internet there use to be websites where you could enter the characteristics of your existing component and it would return equivalents with similar and higher ratings, but its been a long time since I've had to use one.

Chris

It seems these capacitors failing are a common problem. Does that mean these calculations are not fool-proof and even manufacturers get it wrong?

I am sure at a previous place I worked (a good few years ago) we found a source of these capacitors. I will see if I can find out where they came from. They must be available fro somewhere, surely they are not specially made for Draeger.

Robert

I don't know what the type of motor is in the 8000NC but, assuming it's an ac motor with a start/run capacitor, capacitors used in start/run synchronous induction (ac) motor applications have to have characteristics suited to being continuously connected to the motor.

Depending upon how the starter winding is connected, in relation to the motor winding, then the capacitor is either passing current in series into the starter winding, and depending upon the configuration of the motor windings, may also have the peak supply voltage across it continuously.

I guess selection of a capacitor depends heavily upon knowledge and understanding of the characteristics of the motor if a reliable alternative replacement capacitor is to be selected.

The size of the task is irrelevant - what matters is that the individual selecting the component has the skill and the information available to do it and ensure it's reliable, i.e. the design's been validated or checked by the manufacturer.

My mistake. The bipolar capacitor was in the overhead heater of an open cot - I cannot remeber its function but there was no motor in there.
Are bipolar electrolytic capacitors prone to early failure? Draeger do seem to have a problem with them.
Robert

I believe the phase is sh*t happens sooner or later with age something will fail, if nothing failed we wouldn't be in a job.

Then again if the capacitor choosen was not the correct choice it might take years while the capacitor is stressed. Or that stress could come from voltage spikes on mains or external heat.

Failure in the way PeteM mentions, i.e. a capacitor case swelling (indicating it's being stressed), in 5 months compared to 16 years is stretching the "sh*t happens over time" scenario a bit I think. I suppose some of the questions the designer must ask are how's the component expected to fail and what are the consequences of it failing? after they've done all the sums.

In this case when the parallel capacitor fails O/C or low capacitance. i.e. high impedance, the series ones certainly would be stressed even more(even if there was still sufficient starting torque). Eventually something would go pop I reckon.

Consideration should be given to how long the component's likely to last based on factoring in reliability based on prior experience or the component manufacturers data and the conditions the component's operated under. If it's stressed in any way then it's likely to fail much sooner rather than later. Again it's up to a designer to work this out.

Does it have the potential to fail with with a pop, bang or fizzle after a few months or just go low capacitance and/or the motor fail to startup with no other consequences after 16 years? Personally I leave this to the designer who should carry the can for poor design.

The whole point of changing like for like is that you avoid adversely influence the reliability of the device you're working on and/or how the replacement component's going to fail, by avoiding making the wrong choices of what alternatives to replace with, based on limited skill or information.

Hi Richard,
I hope i'm not teaching 'Granny to suck eggs' but here goes.
The motor is a standard 24vAC 3 phase star connected unit. The 2 wire AC supply is connected to 2 of the 3 motor windings with the capacitor connected between the other winding and the supply to generate a phase shift to provide starting torque.

After installation of the capacitor, the incubator was soak tested for 120 hours under normal temperature settings. The capacitor showed no heating or other adverse effects to this trial.
The incubator was put back into service but after detection of the swelling, it was withdrawn and capacitor supply problems forced the decision to de-commission.
regards
Pete

Yes, ignoring the details of whether the starter is series or in parallel with the motor windings, a single phase induction motor with starter winding is effectively a 2-phase motor isn't it? - effectively the same as a 3 phase induction motor, where 1 phase is unused. As I said I didn't know what motor is used in the 8000NC (whether mains or lower voltage).

The idea is to use the phase shift in the LC circuit (starter winding and capacitor) to provide a starter torque that's up to 90 degrees out of phase relative to the motor winding current - otherwise the motor would not start since the net torque at power-up with single phase induction would tend to approach zero until and the motor is given a "push" in a particular direction by an external force.

I'm really not criticising what you posted Pete, just indicating what my "hangups" are about not replacing like-for-like and giving my reasons for it. It's totally down to individuals what they do RE: repairs or policy concerning repair and their own opinion - I've not seen anything laid down in stone besides guidance from the MHRA.