DC Generator polarity reversel problem.

[mark001]

I have a 60KW dc generator that has reversed its polarity (positive is now negative). It appears that the poles have reversed and according to my publican friends there is a way to correct this without changing over the cables. Can any electrical genius shed some light on this problem? Any help would be very much appreciated.

 

[john_morris_uk]

It depends on the design of the generator. Traditional car dynamos relied on the residual magnetism of the stator to 'start' in the correct polarity. One could reverse the polarity of the dynamo by putting 12v across the field a few times. (If you didn't do this successfully, you welded the contacts in the regulator together the first time they closed...)

Assuming your generator is a similar design, you need to flash the field a few times with the correct polarity to get it to start up the 'correct way round'.

Hope that this helps.

Be careful!

 

[pampas]

As J.M. says apply a POSITIVE voltage to the field connection, on a car this would be the small connector, It sounds as though yours is a bit bigger and that you are not conversant with the procedure. dissconnect the positive feed (isolate negative at battery before starting) and the field feed (Smaller wire) connect battery NEG, take a 12 volt test lamp connect one end to the main Positive the other TOUCH the field connection and remove slowly, the idea is to make it ark. Repeat a couple of times Isolate the Battery Neg, and replace all connections, start engine Hey- presto you should have cured your problem. BE very careful that no loose ends touch any metal earth or the Negative in an all insulated system . Thats the procedure Ive used many times but I suppose some whag will disagree it even works on 750 Kw generators as well The lamp voltage MUST be the same as your system voltage.

 

[William_H]

Just confirm that your generator is an old style gen with brushes and segmented commutator like a motor. If the brushes are on a slip ring ie continuous copper ring then it is in fact an alternator with rectifiers. As in a modern car alternator. If it is the old style with segmented commutator then do as suggested. For my opinion a quick touch with 12 volt battery or more if it is a higher voltage generator without the lamp. I do not understand or agree with the idea of starting an arc. Just one contact short burst of current should produce the permanent magnetic field you need for starting the generator.
The problem is that it needs to generate some power to provide current for the field winding which provides the magnetic field for the armature to rotate in to generate current. Usually there is some residual magnetic field enough to generate a little voiltage to augment the residual and so lift itself up into operation. If that residual has become the wrong way around the output will be negative (or opposite). It could become reversed by long periods of inactivity in a certain orientation of the earths field. Once it is run and goes backwards it will have consolidated the residual to backwards.

good luck olewill pS is it really 60Kw is it off an old ship or similar and what voltage?

 

[mark001]

Many thanks for all the replies; I am now getting some ideal of what is involved.
I should be able to get down to the vessel again on Tuesday. I will then try your suggestions and let you all know what happens. Again, thanks for the help.

William-H – Yes it is 60KW and came from a 40 year old vessel scrapped about two years ago. It’s a large piece of kit, giving a voltage of 220Vdc and runs everything with plenty of spare power.
I have not opened the unit, but I can see brushes and a slip ring through the air vent. There are four terminals, one small for the field and two with the output 220v on. I don’t know what the fourth is for, but will investigate.

 

[Bergman]

I wonder what exactly it does run, can't be many things that work off 220V DC nowadays

Certainly flashing the field with 12V was sort of standard practice on car dynamos but how will you get 220v to flash this thing, will 12V be sufficient to reset the field.

Also wonder what sort of engine drives it - around 80 - 100 HP I guess, sounds to be a fine piece of machinery. Do you know its original application.

 

[William_H]

I think it is worth trying 12volt to flash the field. Certainly won't need a lamp in series. Almost certainly the field coiul will be designed to operate with a voltage of near 220volts. The regulator would reduce this somewhat with a resistor in actual running. but try 12volts or better 2x 12 volt batteries in series.
You mention slip rings. I don'think slip rings are used in normal DC generators. Just to ensure you understand what I mean by slip ring. This is a ring of copper which is continuous around the shaft of the armature on which brush bears. In a car alternator there are 2 slip rings and 2 brushes.
A commutator on the other hand which is more likely is a ring of copper segments each insulated from one another (possibly 24 segments) 2 brushes 180 degrees separated) bear on the commutator such that the brush contacts connect to the winding connected to the 2 segments. When the armature rotates by a few degrees it brings in connection to a different pair of segments and so different coil. This process has the effect of making the AC which is inherently generated in a rotating magnetic field into DC by effectively connecting the load only to the coil when it is generating positive then disconnecting from that coil as it starts to change and connecting to the adjacent coil to receive that current only while it is +. It is a complicated system requiring much maintenance especially as all the load current passes through the brush to commutator connection which is constantly moving and sparking. No wonder the modern car alternator is so much more efficient and reliable. Here brushes contact only a smooth slipring and the current carried is only the field current about 5% of actual load current. good luck olewill

 

[mark001]

I’ve attended the vessel again (I don’t get much time with her these days) and yes, problem solved. Flashing the field coil the opposite way (-ve applied to + ve) eventually changed the polarity.

The first time I attempted this I used 24Vdc supply from batteries. I ran-up the alt and measured nothing on the supply terminals. I again applied the battery voltage and achieved success. When run-up, the volts moved a little in the positive direction, being viewed from the switchboard meter. It then came right up to 220Vdc.

I played around with the unit a bit and found that the field can be changed using as little as 12Vdc. I also view the stator, rotor and brushes. William was quite correct when he said a commutator was more likely; this is what I have. The unit has four sets of brushes and is self exciting. The field coil is fed from the main output; it’s a basic as that. During experimentation, I did find that drawing an arc helped, because as the magnetic field reversed the arc became less (from this I would think that the bulb would do the same job). The only problem is it burns the jump cable clamps (I used my Wife’s car jump cables, so no real problem yet).

General information for those interested:-
The Alternator is a Lavid Mc Clure and was made in Stockport, England. It is driven by a Cat 3208 of 210HP, running at 2800 full load. It supplies all the dc circuits (some of the lights, and the galley). I also have some small hydraulic cranes which run from this unit and a dc motor, which drives an ac alternator to supply the ac board. This board can also be supplied by another ac alt, driven by a small air cooled Lister.

Many thanks to every one that helped, it was very much appreciated.