Capacitor-savvy electricians needed to translate!

[jeremyshaw]

Forgive the longish post, and be warned this is of little interest to anyone who isn’t savvy about electrics, but I hope someone can help me with this.

Some months ago the capacitor on my Bauer Junior II compressor melted. Bauer’s UK agent supplied me with a new one at the handsome price of £110. I fitted it and the compressor ran for a few seconds then packed up. Now it won’t even try to start. I have tried to check the capacitor as my multimeter has this function and I think it is OK but am not certain.

I emailed Bauer UK to ask them if they had perhaps supplied the wrong capacitor as the spec was different.

To which I eventually got a reply from Bauer Germany as follows
======================================
Today I have opened the W- Motor to look into and control the dates of capacitors.

The Capacitor- start motor N 27170 has got MF130 230 VAC Type Facon.
The operating capacitor has got 50 MF.
It is always with problems when a generator is not strong enough to give the power for running the unit. If the unit is also used with a long cable than the Motor get quickly overheated.

When the compressor runes than is it important to measure the power supply Ampere.
If you measure more than 14 A at final pressure than you can be nearly sure that the motor get very hot, sometimes to hot.
In this I recommend to change the motor pulley in a smaller dimension.(80 or 90mm instead of
100mm) You will loose a little bit delivery capacity but the electric power trop to approximately =12 A.
==========================================================

This leaves me pretty baffled. My genset can produce 30amps so I don’t feel there is a problem with it not being “strong enough”. Also I have filled dozens of bottles over the last few years so I don’t believe there is a problem with the installation, although the failure did occur after filling about 8 tanks in a row, which is probably the first time I’d done that.

So my questions are:

1. Am I right in thinking that he is saying that if the compressor draws more than 14 amps as the tanks are nearly full I should change the pulley to alter the ratios regardless of my supply capability i.e. there is a design fault? (it usually draws about 10amps on startup but I can’t say I have checked it when I’m actually filling, though I can do that).

2. The original capacitor was labelled 40 MF 450V 50Hz, the replacement BHC 100MF 260V 50/60Hz. Would that be a problem or is it OK to have an oversized capacitor as that appears to me? Is there a relationship between microfarads and voltage so that 40MF at 450V is roughly the same as 80MF at 220V? I can buy an off the shelf capacitor for less than £20 so I’d much rather do that than try again with a ludicrously overpriced Bauer one, but am not quite sure how close I have to be to the spec, especially as it looks like I might have to get two as there seem to be two capacitors in this unit, which was news to me.

3. I’ve also been told the problem might be with the cutoff switch, and asked Bauer about that but they have not commented on that or how I can check the switch operation – so any thoughts on that much appreciated.

The problem is the boat is in Venezuela and I have no chance of getting a good engineer there. I’ll be going out in April and moving it to Curacao where I can probably get help, but I fancied doing a fair bit of diving along the way, and so it would be great if I can fix this thing up myself.

Many thanks

 

[savageseadog]

Doubling the capacitor size would double the current through it and the motor winding. The voltage rating of the capacitor is the maximum it can handle as long as it's a high enough voltage it will do the job. I think the 250 volt rating isn't high enough for 250Volt operation you need to be looking at 300 volt plus. I think they've sent you the capacitor for 115Volt USA operation The price you were charged was very high but not quite extortionate. I have some capactors of near enough the right value for say £25 (400+ volt)

 

[Evadne]

I'm not familiar with the specific equipment but hopefully can give a little useful general advice. A capacitor will have two quoted values:
1) Capacity in Farads (or milliFarads in this case).
2) maximum rated voltage.
The frequency is not important.
When checking the capacitor, measure its (DC) resistance as well as capacitance. If the resistance is low, it is bugered.
There is no direct relationship between capacitance and voltage, except that the price will increase if both go up. If you apply 450V to a 230V cap., it will go pop.
In terms of mains applications, 275V will be used across the mains in Europe, 450V is for 3-phase or other higher voltages. As you say he appears to be saying that there are two capacitors, one associated with the "start motor" (MF130=130mF? an odd value if it is,) and one "operating" which is the one I assume you are talking about.

The tolerance in an electrolytic capacitor is usually pretty poor, so 40mF to 50mF won't make a lot of difference. 40mF to 100mF will, but used for smoothing, higher capacitance is usually better. I wouldn't increase it unnecessarily, if you're not sure which capacitor he's talking about, stick with the specified values.

Hope this helps.

 

[npf1]

Can't answer all your questions but I had a capacitor problem recently with my generator, so some of what I learnt just might be relevent, although I'm no expert in this area.

The capacitors are used to compensate for the high current draw at motor startup. They do give up eventually. There's a basic test that can be done with a multimeter (checking for continuity) although does not reveals all faults. An earth leakage meter (like an electrician would use to test your house wiring) can add some juice to the capacitors so you can see if they are accumulating some charge. They will then self discharge over a few seconds/minutes or you can put a screwdriver across them to discharge them. If they don't accumulate any charge, they are donald ducked.

There's nothing special about these types of capacitors - they are standard fit in many larger electric motors used in industrial applications. They are also used for phase shifting/compensating to improve efficiency. So, as long as the specs are similar, the £20 one will do the same as the £110 - sounds to me like the marine label price hike in action again. Sorry I don't know if a 40 mF at 450V is the same as 80mF at 220V. I can't see that 50Hz or 60Hz rating will make a difference

 

[Nickcf]

confusingly some capacitors are marked with the AC voltage ie 250v ac. Others are marked with the DC rating so worth checking on the cap which it is. If it is marked as AC voltage then there will be a little sine wave after the voltage. If not it will be a DC (peak) voltage value.
The DC rating for 250v AC is about 340v (1.4 x rms value). Still not a 400v cap though so looks odd if that is what they supplied...?

 

[Mal113]

Are we talking about a Generator set or an air compressor? within the text there is mentioned of producing 30 amps and filling 8 bottles? This the motor a single phase machine?

 

[LadyInBed]

Mains electric motor belt driving an air compressor used for filling diving cylinders.

 

[LadyInBed]

Jeremy the people at www.divex.co.uk are very helpful, you could try asking them or maybe post on the www.bsacforum.co.uk

 

[Mal113]

You did not mention if it a single phase motor but I assume it is, as you do not need a capacitor to start a 3-phase motor. The capacitor is there to provide a phase difference between the start and run windings of the motor, without this phase difference there will be no rotating magnetic field and hence the rotor will not turn. On some motors not all there is a switch at the end of the rotor (non drive end) this switch must be closed for start and opens by centrifugal force as the motor speed increases, once it is open the start winding and capacitor are disconnected.

So what checks to make: -

1. Will the motor start if the compressor belts are removed? Motor running light.
2. If no to the above, be careful and spin the rotor shaft and see if the motor will run up to speed.
3. If the motor starts in 2 above then either the switch on the rotor is seized open or the capacitor is faulty.
4. Once you can get the motor to start and have the belts on you must ensure that the start winding is disconnected by the switch if not then the motor will get very hot and the capacitor damaged as it is not for continuous uses. so you must be sure the motor speed is high enought to open the switch, that may be why the manufacture suggested a different size pulley.

 

[misterg]

How many capacitors are there on the motor? The reply from Bauer suggests there are 2 - 50MF running, & 130MF starting.

Voltage ratings are irrelevant as long as they're higher than your peak supply voltage (400V or so for 240V AC).

Your post suggests you've changed a 40MF for a 100MF - have you put a starting capacitor (100MF) in place of the running capacitor (40MF)? As already stated, the tolerances on these things are huge, so I wouldn't worry about 40 vs 50, or 100 vs 130, but 40 for 100 could cause problems, and possibly burn the motor out (you'd know about it from smell).

Tests suggested by Mal113 will tell you if the motor is still alive, but I'm not sure what the effect would be of the running capacitor being u/s.

(BTW. my interpretation of the original advice is that they thought you were overloading the motor on the compressor, and you should change the pulley to reduce the gearing)

Andy

 

[William_H]

Firstly to expllain why they have a capacitor. A single phase electric motor is a little like a bicycle with the pedals vertical. If you push down on the pedals there is no rotational force and in fact the motor doesn't know which way to turn. Once the motor is rotating near normal speed there is no problem. As stated a 3 phase motor has a rotating field equivalen to a bicycle with 3 pedals you can't go wrong.
OK on single phase motor they have an extra coil on the motor which is fed via a capacitor to provide a turning incentive in the correct direction. Like a bike with extra pedals for starting to ensure rotation even with main pedals vertical. Some motors use an extra winding but no capacitor.
In larger motors it is necessary to disconnect this start winding once the motr is up to speed. The start winding often uses a lot of current and would overheat if powered continually. Many motors have a speed switch mounted on the end of the shaft to disconnect the coil/capacitor above about 1000RPM.
So you need to ensure this switch is operating correctly. (if there is one fitted) They are usually reliable and often you can hear them click as the motro starts. As you can imagine starting a motor under load of a compressor can slow down the acceleration hence the comments about smaller pulley and of course if the generator is not powerful the motor will not quickly reach operating speed.

The value of the capacitor is quite important to get the correct phase change at the start coil. The capacitor is not an electrolytic type as it must handle AC no DC. It will be accurate to 10%. As stated the voltage rating is a bit confusing. Capacitors for general electronics are rated at a DC. Capacitors for AC work are rated at an AC voltage rating which takes into account the internal heating from the AC current as well of course as the actual breakdown voltage(DC). Hence common electronics type capacitor will not suit.

So yes you can get a replacement capacitor. It is usually built in a metal can and must be the same capacitance and be rated for the same or more AC voltage as the original.

Lastly if all else fails in South America you can often start the electric motor without the start circuit by applying power then spinning or turning the motor in the correct direction and providing the comporessor load is not too much it will come up to speed. If we use the bicycle analogy again the feet pushing the pedals go up and down at 50 herts which relates to 3000 RPM.
At initial start when the pedals are stopped there is a huge mismatch of speed and so power is not transfered very efficiently. As the motor (pedals) increase in speed you get to about 2850 RPM where there is a difference of speed of the AC to the motor enough to provide the torque to maintain speed and drive the compressor. A little less gives more torque a little more speed gives less torque but a lot lower motor speed doesn't give so much acceleration torque. So you might consider a way to reduce compressor load on start up until motor is up to speed. regards olewill

 

[DaveS]

I'm sticking my toe in late, and I wouldn't quarrel with most of the responses. One small but important point, however, which might matter if you're buying capacitors direct from electronics (rather than motor etc.) suppliers: for this application I would understand "MFD" to mean "micro farad" (10^-6) rather than "milli farad" (10^-3) i.e. u[but with a leading tail: its a Greek mu - does anyone know how to do this?]F rather than mF. (Only a factor of 1000 out, but hey, who's counting?)

This is not just a pedantic point (aka smart-arsery). A few years ago an error would have been corrected automatically. The farad was an incredibly large unit: uF was the normal unit used to describe capacity and even big electrolytic capacitors tended to be described as 1000's or 10000's of uF rather than 1's or 10's of mF. Now, however, thanks to a remarkable technical breakthrough, you can buy a 1F capacitor which easily fits in the palm of your hand...

 

[philip_stevens]

µµµµ character code 00B5/micro - copy from word.

 

[jeremyshaw]

Well, thanks to all for such a wonderful range of informed advice. That's what makes these forums so great, being able to access the experience of people who really know what they are talking about. And for once no arguments ensued!

So I feel much better informed and able to tackle the problem, and at least to understand it if I can't fix it!

Thanks again

Jeremy

 

[irisrei08]

A single phase electric motor needs to stay and run at a constant pace without any ups and downs. For this, the electric motor uses start capacitor and run capacitor. A start capacitor produces required torque by creating a high phase shift for starting up, and the run capacitor creates a rotating magnetic field to give an auxiliary current to the electric motor.

 

[RichardS]

A single phase electric motor needs to stay and run at a constant pace without any ups and downs. For this, the electric motor uses start capacitor and run capacitor. A start capacitor produces required torque by creating a high phase shift for starting up, and the run capacitor creates a rotating magnetic field to give an auxiliary current to the electric motor.

Welcome to the forum.

That's a very old thread though.

Richard

 

[JohnGC]

Welcome to the forum.

That's a very old thread though.

Richard

Yes, welcome.

Missing Nigel's old thread policing though.

Are you going to take over Richard?

 

[RichardS]

Yes, welcome.

Missing Nigel's old thread policing though.

Are you going to take over Richard?

I'd not thought of the connection with poor Nigel but that's made me feel sad now. I'm so sorry that he's gone.

Richard

 

[William_H]

Just one further comment here. Capacitors used for AC as in motor start are not electrolytic capacitors but rather what used to be called paper capacitors. The difference is that electrolytic capacitors use a kind of electroplating of insulation to get a very thin insulating layer to get very large capacitance in a small size. but this means they are polarity sensitive. And often very low voltage rating. (always have a +ve and a -ve connection) The exception being non polarised electrolytics used in loud speaker crossover networks.
Capacitors for AC use plastic or similar insulation so are not polarity sensitive but much less capacitance for their size. (usually ina steel can with mounting bracket). 40 microfarrad being typical size for AC motor start. As said needs to be a voltage rating a lot more than 250v for safety and in some cases a capacitor will get hot from AC current so a higher voltage rating indicates bigger more robust.
A dive tank compressor is an extreme case where start loads my be very high if the tank you are trying to top us is over 2000psi. Hence the makers comments re pulley size etc. Pulley size being a compromise between fast filling and achieving high final pressure. Lets hope OP is happily diving witha usable compressor by now. olewill