Why some low speed electrical drives employ a high-speed motor and some form of mechanical speed reduction rather than direct drive motor. Is it because of size?
Answer
Power is the product of torque and speed. If you want high power at low speed, then you need high torque.
In an electric motor, the torque is basically a function of the radius of the rotor, its surface area, and the magnetic field intensity between the rotor and the stator.
You can play with all three of these variables, but there are definite limits on how much field intensity you can get with reasonable materials. Therefore, you end up needing either a long motor or a large-diameter motor, both of which require lots of extra (expensive) material to produce.
This is why it usually works out best to go back to the first equation and raise the speed of the motor by means of a gearbox. This gives you the required output speed and torque while allowing the motor speed to be higher and its torque to be lower in order to get the required power.
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