Mixer Grinder: 'Just Change the Brushes'
A 750 W, 230 V, three-speed mixer grinder, four years old, comes to a service centre. The customer reports sparks visible through the ventilation slots near the brushes, a sharp fall in speed when wet masala is ground, loud humming under load, and the 6 A MCB tripping after 10–15 s of loaded running. On no load the mixer runs at almost normal speed. On opening: one brush is worn to 5 mm (replacement limit 8 mm) while the other measures 11 mm; the commutator is blackened with one burnt segment; the suppressor capacitor across the supply reads open circuit on the multimeter's resistance range; each field coil half reads 4.2 Ω with no sign of burning; insulation from the plug pins to the metal body is above 50 MΩ. The customer says, 'Just change the brushes and give it back, I have a function at home tomorrow.'
1. Using the evidence given, explain why replacing only the brushes will not cure the fault.
Read the worked answer
Brush wear by itself does not burn a commutator segment, and the brushes are not equally worn - 5 mm against 11 mm. That difference means one brush has been arcing for a long time, not simply worn out.
The burnt segment is the cause, not the result. When an armature coil or its riser short-circuits, that segment carries a heavy circulating current, overheats and blackens, and the brush running on it sparks and wears rapidly. The load behaviour fits this: on no load the back-EMF holds the current down, but under load the current rises and the shorted turns take a disproportionate share, so the 6 A MCB trips after 10–15 s while the no-load speed still looks nearly normal.
Fitting new brushes would buy a few quieter days, then the same arcing returns, destroys the new brushes and may score the commutator beyond recovery. The armature must be tested and almost certainly replaced or rewound before the machine is returned.
2. Describe how you would confirm an armature fault using only an ordinary multimeter in a small workshop.
Read the worked answer
- Visual: lift the brushes and inspect. One burnt or pitted segment, or a burned riser, is strong evidence on its own.
- Segment-to-segment resistance: on the lowest resistance range, rotate the armature step by step and measure across each pair of adjacent segments, probing clean copper rather than carbon dust. In a healthy 750 W mixer every pair reads nearly the same, a few ohms. A pair reading noticeably lower or zero = shorted coil; a pair reading open = broken winding or bad riser joint. Clean the slots between segments first, otherwise copper dust gives false low readings.
- Segment-to-shaft / segment-to-core insulation: any reading other than infinity means the armature is earthed and must be replaced.
- Confirm the rest: field coils 4.2 Ω + 4.2 Ω with no burning, capacitor open circuit (good), brush length and spring pressure, and free rotation of the bearing. If all these pass, the armature is the only suspect left, and the test in step 2 has already named it.
3. State what you would tell the customer and what you would write on the estimate.
Read the worked answer
Be direct: this is not a brush job, and no honest repair will have it running by tomorrow evening. Explain the findings in plain language - one burnt commutator segment, uneven brush wear, and a shorted armature winding confirmed by the segment test - and that new brushes would fail within days.
Then offer priced options with turn-around times: (i) replace the armature and both brushes, clean or skim the commutator, and give a 3-month warranty on labour and the armature; (ii) if the armature plus labour exceeds about half the price of a new 750 W mixer, recommend buying new and charge only the inspection fee.
On the estimate write parts + labour + GST, the warranty terms, the collection date, and a line stating that the field coils and capacitor tested good and are not being replaced. Never release an unsafe machine to meet a deadline; offer a loan unit or a referral if the function is urgent.