SINGLE PHASE SANDER REPAIRED MODEL: BLACK AND DECKER KA 273 250 W, 240V
My friend gave me this Sanding machine and told me that it was not working. He said it just stopped working as he was sanding his wood work.
My first step was to check power supply outlet in his workshop. It was okay 240V AC was available. Then I checked the fuse fitted in the plug. The fuse can be accessed by taking out the small rectangular white plastic carrier.
Then you can get it out for easy measurement as shown below. I set my digital multimeter on continuity test and after measuring the fuse a continuous reading (almost Zero Ohms) indicating that it was okay.
To measure the continuity of the power cable I had to open the outer cover of the machine. This is done by unscrewing six screws and pull the cover out.
Side cover removed showing a switch, the stator, rotor and brush assembly
The power cables continuity to the terminal inside the machine was also okay. The next step was to check the operation of the switch. To my surprise it did not require de-soldering to remove the switch. I just pulled it out from the board! I removed a small circuit board to see the connection of the switch on the under side of the board.
I realized that the machine was worked on sometime back. You can see the scratches and the soldering. This soldering did not have strong contact with the switch terminals. There was dry joints and cold soldering. This makes the switch contacts to move about and eventually causing intermittent transmission of power to the other part of the circuit. I also discovered that the scratch had removed the copper on the board and there was no continuity from the switch to the inductor. I removed the old solder and re-soldered the switch back. I also re-soldered all the other points on the board to control the dry joints and cold solder.
I tested the continuity and the resistance of the coils they showed 11 ohms each on my meter. I verified the tension of the brushes and their condition. After ensuring that everything was okay I put back the cover. I plugged the machine and powered it ON, Glory to God It worked well!!!
Conclusion: This is a single phase AC machine that uses a Universal motor principal (it can be used on both AC and DC supplies). When the machine is connected to the AC power supply, current flows to the stator/field windings. The same current flows to the rotor winding through the brushes in contact with the commutators. The alternating magnetic flux created by the AC current flowing in the field interacts with the flux produced in the rotor winding. The interaction of the two fluxes produces torque. Since the alternating fluxes from both the field and the rotor change at the same time the rotor runs in the same direction.
I don’t believe what my friend said that the machine stopped when he was using it. Perhaps at the beginning it stopped while using it but the time he was giving me he had tried to work on it (he is a daring man who tries everything) and made all the scratches.
We need to be cautious with what the clients say about the fault of their appliances and be aware of the repair attempts made before we start working on them. This will reduce the time spent during fault finding.
Luciano Francisco Thomas Khware (Malawian) studied Electrical and Electronics at Comboni Technical College and at Polytechnic (University of Malawi). Currently, he is a student at Tangaza University in Nairobi Kenya.
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