What makes them so loud? Circular saws, pumps, blenders, etc all seem to be fairly loud, but some louder than others. What makes some louder than others and what is actually making the sound?
Many motors have fan blades attached to the rotor for cooling purposes. They contribute a lot of the noise from a high rpm motor.
Some motors have brushes which conduct electricity to the rotor. These can clack horribly as they rub against the uneven surface of the rotor.
Bearings or sleeves that enclose the spinning shaft often squeel, rumble or scream.
Electric motors can be very quiet, even very large ones.
Things that make the noise, partly the way it is mounted, some mountings resonate and transmit more vibration than others.
Next up, most electric motors will have some form of cooling fan fixed to the shaft, these can be extremely noisy.
If the machine has a gearbox in it, like a drill, blender, saw of any description, these will make plenty of noise.
Portable electric motors such as those in drills, will have carbon rod type of contact brushes, and these can be noisy, the smaller the motor the worse they are, and on big DC motors they can be very quiet.
Recprocating action devices, such as jigsaws have noisy gearboxes in particular, due to the many changes of direction in the output tool.
Crap cases, light, cheap and plastic make wonderful sounding platforms and thses will transmit any vibration through them which is usually worse on cheap little motors where the balance of the rotor is not usually all that good.
Bearings can be noisy, the smaller the louder in propotion to their size, right down to the plain bush bearing you find in portable appliances.
Blenders have a particular problem, they have those lightweight blades that flap about, they have gearboxes with lots of free play in them(backlash) they have cheap cases, the jug is a great resonator, and add all this to them often being mounted upon three feet maybe half an inch from the top of the resting surface, and you have another good soundchamber underneath them.
This caught me as odd. If that is the case, how come my circular saw is considerably louder than the Dremel tool and how come a ceiling fan is quiet as a whisper?
Your circular saw almost certainly has a gearbox, and quite possibly a drive belt, and lots of inherant backlash, plus the large cooling fan.
Your Dremel will have none of that.
Ceiling fans are very lightly loaded, run extremely slowly for electric motors.
Not only the above, but power saws, vacuum cleaners and other devices needing a small, powerful motor nearly always use a universal wound motor–which means it can run on either AC or DC. But the primary advantage is small size compared to power. Unfortunately, as a group, these motors tend towards the loud side.
I thinks that it is because the cooling ports in the cans on the small motors have the same physics as a simple siren. I know that the reason leaf blowers are so loud is because of this principle. The basic idea is this… Imagine a flat plate of steel with holes drilled around the outside edge, so if it were laying flat on a table it would look like a shower head. (except it only has the holes that are nearest to the outside edge) This plate is mounted in a housing so that it can spin like a wheel. The housing has the same hole design as the plate on the front and back of it. When the plate inside spins the holes alternate between lining up (so you could see through the housing and plate) and obstructing each other. The holes produce a quick alteration between high and low pressure in the air and make the siren sound, the fast you spin it the higher the pitch.
This is how the old mechanical sirens work and I think that this why small motor make that whining sound.
It is just a guess though.
Gears, bearings, brushes, cheapness and fan air are all important. But don’t forget that the winding of a lot of wire around metal plates cause a lot of vibrations. Transformers can hum quite loudly and they aren’t moving. Throw “transformer hum” into a rapidly spinning device and you’ve got real noise. Even little DC motors get humming because of the rapidly switched current.
Nobody would buy ceiling fans if they sounded like a vaccuum cleaner. So they do things to dampen/prevent noise. Adds to the cost of the device. They also have an intrinsic advantage that they aren’t being spun as fast as a lot of AC motors. But the big difference between the $40 and the $100 ceiling fan at Home Depot is that the $100 is quieter.