Mixer-grinder that hums on speed 1 and trips the 6 A MCB on speed 3
A customer brings a 750 W, 230 V, three-speed mixer-grinder to your shop. Complaint: on speed 1 it only hums and does not rotate; on speed 3 it runs slowly, sparks visibly at the brush caps, and the 6 A MCB in the plug point trips after about 20 seconds. With the jar removed the motor shaft turns freely by hand. Both field coils measure 9.0 ohm and 9.2 ohm. Brush inspection shows one brush worn down to 6 mm (new brushes are 14 mm) with a chipped face; the commutator is blackened with arcing marks. Armature resistance measured between adjacent commutator segments is 2.1-2.4 ohm everywhere except one pair, which reads 0.6 ohm. The speed-selector contacts look pitted. The customer mentions that last week he ran dry masala at speed 3 for ten minutes, pressing down on the jar. He wants only the brush changed because 'that is the cheap part'.
1. Interpret the readings and identify the confirmed fault. Explain the humming on speed 1, the sparking and the delayed MCB trip on speed 3.
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Field coils: 9.0 and 9.2 ohm, closely matched, so the field is probably sound. A shorted field would read noticeably low and would draw heavy current on every speed.
Armature: readings between adjacent commutator segments should be roughly equal, because each pair measures one coil. One pair at 0.6 ohm against 2.1-2.4 ohm elsewhere means a coil with shorted turns. That short is the confirmed fault.
Symptoms explained:
- The shorted coil carries a large circulating current, overheats and distorts commutation, so the brushes arc and the commutator blackens.
- Torque falls because part of the armature is not producing useful flux. On speed 1 the lowest tap voltage cannot overcome friction and load, so the rotor stalls and hums - stalled current is high but there is no rotation.
- On speed 3 the combination of high speed, arcing and the shorted coil pushes line current above 6 A, so the MCB trips in about 20 seconds, in its thermal (overload) region rather than instantly.
Dry grinding explains the history: sustained overload at low speed with high pressure overheated the armature insulation until turns shorted.
Brush: at 6 mm it is worn out and must be replaced anyway, but it is a symptom and a contributor, not the root cause. Fitting only a brush will produce the same sparking and tripping within days, and the new brush will be ruined.
2. Decide between repair and replacement, and set out the parts, the work and the post-repair tests. A comparable new 750 W mixer sells for about Rs 3,200; your prices are armature Rs 520, brush pair Rs 90, speed switch Rs 140, labour Rs 350.
Read the worked answer
Economics: 520 + 90 + 140 + 350 = Rs 1,100, about 34 percent of a new machine. Repair is clearly justified, and the customer keeps a familiar jar set.
Work
- Replace the armature with a genuine one of the same rating and shaft/bearing fit.
- Replace both brushes as a pair, not just the worn one, so they bed in evenly. Check brush springs and caps.
- Clean and, if the commutator is being reused anywhere, polish it; never undercut or skim a commutator that will be replaced.
- Replace the pitted speed switch - the pitting shows it has been arcing and it will cause intermittent speeds and volt drop.
- Inspect the field coil for discolouration, check its insulation, check the bearings, fan and jar couplers, and inspect the cord and plug for heat damage.
Post-repair tests
- Insulation resistance at 500 V, live parts to exposed metal: at least 1 Mohm (a Class II mixer normally shows far higher).
- No-load run of five minutes on each speed: current must settle comfortably below the rated 3.3 A, with no more than a faint blue spark line at the brushes.
- Loaded run for ten minutes with a jar of dal and water on speed 2 and speed 3: no tripping, no abnormal heating or smell, speed selection crisp.
- Record the readings and tell the customer that if the field coil has been thermally stressed, a repeat failure is possible - state this before taking the job.
3. The customer says the 6 A MCB is faulty and wants a 16 A MCB fitted instead. How do you answer, and what does the pitted speed switch tell you?
Read the worked answer
Refuse the MCB upgrade. The mixer is rated 3.3 A, so a 6 A device is correct. The MCB tripped because the machine was drawing more than 6 A - it was protecting the cord, the socket and the windings. Fitting a 16 A MCB would let a faulty armature pull current until the armature or the plug top burned out, and it would leave the 0.75 mm2 flexible cord and the 6 A socket unprotected. Explain that an MCB that trips is giving information, not causing trouble; the repair is in the motor, not the board.
What the pitted switch tells you: the contacts have been arcing repeatedly. Two likely causes:
- The shorted armature draws more current than the switch was designed to break, so every switch-off draws an arc.
- Normal wear with age, which raises contact resistance, causes a volt drop and local heating, and makes speed 1 intermittent or dead.
So the pitting is evidence of the fault current history and a second fault in its own right - replace the switch rather than clean the contacts. After replacement, re-check the plug top and socket for discolouration, confirm the cord is undamaged, and demonstrate to the customer that the machine now runs on all three speeds with the original 6 A MCB in place.