Why is the electric motor in kitchen mixer/grinders so noisy compared to the motor in a ceiling fan or the motors in an electric car?
In: Engineering
Ok, firstly the ceiling fan has an induction motor that has no physically touching parts, the fan has permanent magnets that work with the electro magnets in the fan to produce motion, as a fact they are low torque high speed units.
In your food processor, you need high torque and noise is not a problem because it wont be working for very long, the motor in the food processor has brushes, and the spindle has not got permanent magnets but electro magnets, when the electro magnets work against each other they produce way more torque but as a result you can hear the brushes feeding on the electricity as they produce friction. More expensive motors have brushless action but you wont find this in cheap electronics in the home.
Both those items have gearboxes that are designed to reduce the speed of the motor, and consequently increase the torque (or turning force) so they can do what they can do.
Gearboxes are noisy devices, basically. They have meshing mechanical parts, and many many more bearings each of which will make a little noise itself. It all adds up.
Small tolerances in the gear system make less noise, and also makes the transfer of energy more efficient. Same thing applies to bearings.
Now there isn’t much demand for high precission machined gears and bearings in your kitchen equipment. Granted some high end machines are quieter than their more affordable counterparts.
Electric cars don’t actually need a gear box, they can attach the motor directly to the wheels. I haven’t kept up with electric car designs, but I think this is the common solution nowadays for electric cars, while hybrids use a gearbox.
Now since the sound you hear is poor power transmission, caused by friction and poor contact, in a system which calls for high efficiency you want to avoid this. Better fitting and smaller tolerances produce less noise and achieve greater efficiency and speeds.
In short is because a consumer wants to buy a powerful blender but a silent fan, and that drives the engineering efforts. Due to high torques (more power) involved and inter exchangeable parts (think loose connectors vibrating) they will be noisier almost by default. That said they could be engineered to be much more silent, but that would cost more and bring almost no benefit since they are only used for a short amount of time and all the slicing they do is already very noisy.
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