Putting on the Turns on a electric motor
What wire and how thick?
We use single strand enamelled copper wire for our motors. Some people like to call it magnet wire. There are other types of wire that can be used from square shaped copper strands to mutli-stranded litz wire. As the expected power handling capability of a motor rises, the need for better wire becomes more important.
One way of handling the problem is to use parallel strands instead of a single strand of thicker wire. The reason for this is two-fold. Thick wire becomes troublesome to wind and there is the issue of skin effect.
Just briefly, skin effect is a term used when the electrons flowing through a copper wire tend to travel only in the outer circumference of the wire. The centre of the wire is then just unwanted weight.
This starts happening when the switching frequency of the current rises. The skin effect issue has been debated for some time as to whether it is a concern in our motors, since the switching frequencies in our speed controllers are not very high. Some swear by it, and others not. For our lower power motors though, we will not be concerned about it.
To get any suitable amount of power out of a motor, we want the lowest resistance possible. Resistance in our windings is what we use to calculate copper losses which reduce the overall efficiency of our power system. So the incentive is to use the thickest wire we can — but too thick wire will mean too few turns and unless you are designing a very high speed motor, we will want more turns to create useable torque.
Too thin wire results in high resistance and then you cannot get enough current through the motor. If we raised the operating voltage of our motor, Ohm’s Law says more current will flow through. Yes, but the copper losses will cause heat build-up and destroy the motor. We’ve got to use the appropriate wire for the intended power.
For CD rom type motors, the range of wire used is from about 0,4mm to 0,6mm diameter.
The 0,4mm wire will obviously allow more turns, but have higher resistance than the 0,6mm wire. The trick in this case becomes an issue of how you connect your wire ends.
There are two methods used, one called Star and the other Delta, but we will discuss this further in a little while.
As for determining the resistance of your motor, trying to measure resistances of less than 1 ohm is not easy unless you have sophisticated equipment. Conventional digital multimeters are not up to it as they do not take the instrument’s leads into consideration.