F.A.Q.

How does a motor run?

An electric motor is a device that can produce a force when electricity is applied. The force is usually the attraction or repulsion between magnets. Usually the magnets are arranged so that the force is tangential to a fixed axis called a shaft. This produces the usual rotary motion, but it can be arranged to work in a straight line hence "linear motors". This force then produces torque, ( a distance times a radius) , measured in ft-lbs, oz-ins, gm-cms., or some other unit like newton-meters.( If your arm were 2 feet long and you held a 6 ounce apple in your hand; the torque your arm muscles would have produce, to cancel the weight of the apple, would be about one newton-meter).


What controls the speed of a motor?

 The speed at which the motor runs depends on the load that is put on the shaft trying to slow it down, and also on the design and type of motor. There is no lower limit but as the speed increases some motors will not work because of mechanical problems, for example brushed motors are not much use over 15,000 RPM. At very high speeds over 100,000 RPM bearings start to be a problem and air resistance enters the picture. A vacuum is one way to get rid of the friction but then the motor has difficulty in getting rid of it's heat. By using magnetic bearings and a vacuum speeds to 1 million RPM are possible. Each type of motor has a different method of controlling the speed. This can be just varying the voltage for a DC motor or universal AC/DC motor to complex electronic circuits often using micro-controllers for the brushless motor.


What's the difference between an AC and a DC motor?

All practical motors are really AC in that the magnetic field has to keep changing direction so that the moving part is attracted or repelled by the stationary part. In a DC motor the voltage applied is made into a kind of AC by a device called a commutator in a brushed motor or by electronic switches in a brushless motor.


How can I reverse the direction of a motor? 

Unless it is a shaded pole motor it can probably be reversed. If it's an AC motor then reversing the wires going to either the start winding OR the main winding will reverse the direction, if it's a 3 phase motor then reversing any pair of the main winding will also reverse the rotation. If it's a DC motor reversing either the field OR the armature windings will reverse it. If it is a permananet magnet motor then just reversing the wires to the motor will work for a brushed motor and for a PM brushless motor the controller should have a reversing switch or jumper. ( Check with the controller manufactuer )
BTW: Some motors have auxialliary windings and this scheme may not be OK. Always have a fuse in the circuit so it can blow and NOT your motor!.




Can I use a three phase motor on my single phase home supply?

The answer is yes and no! You can but you'll have to make the motor think it's getting a three phase supply. This can be done in several ways, you can read about it in. Phase conversion or using a 3 phase motor on single phase supply


Why do magnets attract each other?

Although it is easy to see the effect of two magnets attracting each other there is no clear understanding of what causes the attraction. A magnet sticks to the fridge door because it produces a magnetic field in the steel door of the opposite polarity, an effect called induction. The force per unit area produced by a magnet is proportional to the square of the flux density and the area of the magnet.

How easy is it to design a motor?

Efficient electric motors are very difficult to design because there are so many interactive parameters that must come together to give the result. Also most of the parameters have a non-linear relationship with each other so to solve the problem requires more than simple algebra. Computer simulations of magnet field equations help with some of the parameter but to get it all together takes a combination of electrical and mechanical engineering, and to control the motor an electronics and feedback control systems specialist will round out the team.