Homebuilt Electric Motors

Frequently Asked Questions


How do I identify the North and South poles of magnets?
When installing magnets in a bell, you don't necessarily have to know where the North and South poles are - you just have to be able to tell them apart. A good method is to take a stack of magnets and mark the top of the first magnet on the stack with a permanent marking pen. Then take the marked magnet off and put it aside. Now, mark the top of the next magnet as well and put it aside too. Mark all the magnets in the stack this way, taking care to make the marks on the same side of all the magnets. This way all the North (or South) poles are marked.
How do I change the direction of rotation of a brushless motor?
Just swap any two (of the three) leads between the brushless motor and the controller. Never swap the positive and negative battery leads, as is done for a DC motor.
Why does my motor misfire / stop running or sometimes doesn't even want to start ?
The most common cause of a motor going out-of-sync with the controller is:
a) Bad solder connections on the connectors.
b) The magnets may be too weak. The ring magnet that comes standard in a CD-ROM motor has to be replaced. Installing Neodymium magnets proved to be the single best method of getting reliable motor performance in my experience. If the motor ran ok before and recently 'developed' this problem, it could be that the magnets got 'fried' (demagnetized due to excessive heat). Replace the magnets or if you have a similar motor that works fine, use the bell with magnets from the other motor to see if it solves the problem.
c) Other causes are winding-to-stator shorts, broken/intermittent wire leads or poor star connection joints. If the motor doesn't run at all and just buzzes, one phase may be damaged or burnt. Check the continuity of all 3 phases.
d) The settings on the ESC are also very important. Changing the timing setting, sometimes make a huge difference when it comes to running CD-ROM motors. The ESC's (speed controller) timing may not be set for 'high pole-count' motors. Refer to the instruction to set the correct timing for outrunners. Keep in mind that some ESC's that are intended for inrunner motors (usually 2-pole), just wont run CD-ROM/outrunner motors properly.
e) The battery leads may be too long. The maximum length should be about 150mm between battery and ESC. If longer leads are required, rather lengthen the 3 motor leads.
f) The propeller you're using may be too big. Try a smaller one.
I rewound a motor and it performs ok, but gets very hot.
The primary cause of heat in a motor is copper loss. Make sure that you completely fill the slots with copper (wire). If there is still space left for another turn of wire after you've wound the required number of turns for a certain Kv, then you've used wire that's too thin. It's easy to calculate the correct thickness of wire needed, but it's just as easy to ignore this important step. Remember, the more copper you put in the slots, the lower the copper loss will be (less heat) and the result will be a motor with a higher efficiency rating.
What glue should be used for magnets?
It is generally a good idea to use high temperature epoxy, i.e. UHU Endfest 300. In smaller motors like CD-ROM motors, I've had success with CA (super glue), but be aware that CA will burn and disintegrate when it gets too hot, so take special care when selecting a propeller, so that it doesn't overload the motor and cause excessive heat. Another option is CA-type metal adhesives, e.g. Loctite 480 or Loctite 380 'Black Max'.
Why do some motors have more cogging than others?
Cogging is not an indication of the power of a motor. It is a result of the geometry, shape and number of the slots and poles. Whereas hard cogging is usully not a problem with motors, with wind generators, this is very undesirable. But we can do much to get rid of cogging. Careful selection of the number, shape and size of slots and poles, as well as the airgap size and even magnet strength can all be used to minimize cogging in permanent magnet generators while still retaining a high degree of efficiency.
How is the Kv of a motor measured?
Kv (RPM per volt) can easily be measured in a home workshop. For brushless motors, a special formula is used to calculated Kv from measured values. Refer to this page for an explanation and a useful online calculator.
Websites often specify the Rm of hobby motors. What is Rm used for?
Rm is the resistance of one phase of an electric motor (measured in Ohms). It is an indication of the quality of the windings on a motor. Motors with better windings will have lower resistance values (less Ohms). However, when comparing Rm values of different motors, the size and Kv of a motor should be taken into account as well. Rm is also used in motor simulation programs. Refer to this page for directions on measuring the Rm of any hobby motor.

Motor Constants & Efficiency

I have a motor which is designed/rated for 3S. Can I run it on more cells?
Firstly, motors are not rated for any number of volts or cells. If you see that in the specifications, it is usually just a recommendation and will almost aways be accompanied by propeller sizes. Whenever you see a voltage and prop suggestion, the bigger prop goes with the lower voltage, and vice versa.
Bottom line: The insulation in motors can handle several hundred volts. It is current or RPM that will kill it. Motors should be (and usually are) rated for current, and not volts. Use a Wattmeter and don't exceed the current rating.
How will the current be affected if I add one more battery cell?
Motor current is proportional to voltage squared.
If you change from 3S to 4S, theoretically the voltage will change from 11.1v to 14.8v, (1.33 times higher) and thus the current will be 1.33² = 1.8 times higher. For example: If the current draw is 10A on 3S, it will be 18A on 4S. Time to change to a smaller propeller.
Doubling voltage will give an increase in current by factor of 2 = 4.
What effect does higher Kv have on motor current?
Motor current is proportional to Kv cubed.
If you change motors from 1000Kv to 1200Kv, the Kv is increased 1.2 times. So the current increase will be 1.2³ = 1.73.
Example: If the current draw is 10A with the 1000Kv motor, it will be 17A when Kv = 1200.
Will a motor get damaged if I use an ESC with too high amp rating?
An ESC doesn't 'push' amps to the motor. The motor draws only the amount of current (amps) that it requires to turn the propeller.
It may even be beneficial to use a higer rated ESC, because bigger ESC's have lower internal resistance. This will result in lower ESC losses and increase system efficiency.


Why do I have to rewind a CD-ROM motor? Can't I just use the standard windings that's on there already?
The Kv (RPM per volt factor) of a standard CD-ROM motor is usually too low and therefore the prop will turn fast enough to fly a model satisfactorily. That means that there are too many turns (usually 30+) of wire on the stator. Rewinding with less turns of thicker wire will result in a higher Kv and therefore the motor will turn faster while drawing more amps. The Kv of a motor is inversely proportional to the number of turns on the stator. A good starting point for a typical CD-ROM motor is say, 15 - 18 turns of 0.35mm wire (or thinner if it doesn't fit) with a Star (Wye) connection.
I don't know if I should use Delta or Wye (Star) termination for a motor. What is the difference?
With both Delta and Wye (Star), it's possible to achieve practically the same results with any 3-phase motor. With WYE (Star) termination, 1.73 less turns need to be wound to get the same Kv as with Delta. Wye is a popular termination for CD-ROM motors while Delta is more popular for bigger outrunners. Performance-wise, there is no difference between Delta and Wye, as long as the Kv is the same. How many winds, and how those winds were connected to make up the finished motor is of no practical concern.
Why is it necessary to replace the standard ring magnet in a CD-ROM motor?
In most cases (if not all), a CD-ROM motor will perform better if you install Neodymium magnets. Not only will the motor have more power, but it will also run cooler. I have noticed that starting/stalling problems also go away after installing magnets.
How do I remove the magnet ring from a CD-ROM flux ring (bell)?
Heating it gently with a heat gun or butane flame (pencil torch), will soften the adhesive and allow you to pry out the magnet ring. A sharp screwdriver tip may be needed to break out a piece of the magnet before removing the rest. Don't go overboard with the heat - when a little smoke is coming from the bell, it's normally a sign that the adhesive is burning and you can start removing the magnet. Some guys soak the bell and magnet in acetone (instead of using heat), but I haven't tried that.
How much air gap is required between the stator and the magnets?
I normally aim for about 0.2mm on small (CD-ROM) motors. But unless one turns a custom flux ring or bell, you pretty much have to stick to standard magnet sizes. Most 20mm and 22mm (stator dia.) motors work well with 1mm thick magnets. Keep in mind that the smaller the air gap, the more power can be extracted fom a CD-ROM motor, but it should never touch. With larger air gaps, say 0.8mm and bigger, you start to lose efficiency and power.