How does a mixer grinder work?

A mixer grinder, the "mixie" in most Indian kitchens, has two halves. The heavy base holds an electric motor standing on its end. The jar sits on top, with the blades fixed in its floor.

The motor is a universal motor: copper field coils on the outside, a spinning armature in the middle, and two carbon brushes that rub on a ring of copper bars called the commutator. A cooling fan on the bottom of the shaft pulls air through the base.

The top of the shaft ends in a rubber coupler. The jar has a matching coupler under it, so the blades lock onto the motor when you set the jar down. A bushing and seal let the blade shaft turn without the jar leaking.

The speed switch has 1, 2, 3 and P for pulse. A thermal overload protector cuts the power if the motor gets too hot, and vacuum feet grip the counter.

A mixer needs a motor that is small, light and very fast: around 18,000 to 22,000 rpm with nothing in the jar. That is the job of the universal motor.

Current flows through the field coils, which turn the iron into a magnet. Then it flows through a carbon brush, into the armature coils, and out through the other brush. The magnet pushes on the current in the armature wires, and the armature turns. As it turns, the commutator keeps swapping which coils are connected, so the push always goes the same way round.

Here is the clever part. Mains electricity flips direction 100 times a second. But the field coils and the armature are wired in a line, in series, so the same current flows through both. When it flips, both the magnet and the armature current flip, and two flips cancel out. The push stays the same way. That's why it's called universal: it runs on AC or DC.

Load it up and it slows down. A slower armature makes less back-voltage, so more current flows, giving more push, but also more heat.

Mixer blades are only 6 to 8 cm across, but they spin about 300 times a second. The speed of a blade's tip is v = ω × r: how fast it turns times how far the tip is from the centre. At 18,000 rpm a tip 3 cm out moves at about 57 m/s, roughly 200 km/h, as fast as a racing car.

At that speed the blade doesn't really slice. It smashes bits of food and shears them between the edge and the rest of the load. Each hit breaks bits into smaller bits, so the size falls quickly at first and then more and more slowly. Engineers describe this with Rittinger's law: making bits twice as fine needs about twice the energy again.

Each jar has its own blade. The dry jar has flat, sharp blades that mill spices and dal into powder. The chutney jar has tips bent up, to churn wet food and throw it up the wall. The liquidiser has four wings, two up and two down, to stir a whole jar of liquid.

Spin a blade in a jar and it drags the food round with it. Soon the whole load is swirling, pressed against the wall, with a dip in the middle: a vortex. The blade just slips through food that is already moving with it, and bits at the top never come down to be cut.

That's why jars have ribs on the inside wall, called flow breakers. They stop the swirl. The blade flings food outwards, it hits the ribs and climbs the wall, rolls back across the top and falls down the middle into the blades again. Every bit gets many turns at the blades.

Thick food makes it worse. A dry chutney sticks to the wall and the blades dig an air pocket under it. A splash of water lets it flow back. Pulse mode helps too: each stop lets the load slump back onto the blades.

Every watt a mixer draws ends up as heat. Some warms the motor: current through the copper windings heats them, just like a heater. The rest does work on the food, and that work turns into heat in the food. So Q = P × t: 200 W into a jar of spices for one minute is 12,000 joules, enough to make 100 g of cumin and its steel jar almost 30 °C hotter.

That's why dry grinding in short bursts keeps spices tasting fresh: their flavour lives in oils that escape when they get hot. It's also why batter warms up in a mixie.

A heavy load slows the motor and makes it draw more current, and the heat in the windings grows with current squared. Mixers are made for short runs: manuals say to grind in bursts and let the motor rest, and many say never more than a few minutes at a time. Here we try 30 seconds on, one minute off. Run one too long and the windings could burn, so a thermal overload switch sits against them. Inside is a bimetal strip: two metals that expand by different amounts, so it bends as it warms and snaps a contact open. Once it cools, you press the reset button under the base.

A spinning blade has to be balanced. Stick a lump on one wing and, as it whirls round, it pulls the whole machine towards itself, 300 times a second. The force is F = m × r × ω². Because ω is squared, speed makes it huge: just 1 gram at the blade tip at 17,000 rpm pulls with about 110 newtons, more than three times the mixer's own weight.

That shaking makes the mixer walk across the counter. Its vacuum feet are soft rubber cups: press the mixer down and they suck onto a smooth counter, adding grip.

Mixers are loud, often 85 to 95 dB up close, louder than a busy road. The noise climbs steeply with speed, and a shaking jar rattles louder still.

Some rules: keep the lid on (some mixers have a lid interlock that won't start without it). Don't fill past the max line: the load climbs the wall when it spins and pushes the lid. And don't run it empty: the motor races to top speed with nothing to cushion the blades, which wears the bushing and coupler.