Electric motor basics

An internal combustion engine like a Zenoah (shown) develops peak torque and power at similar rpm. 

A dyno chart for an internal Zenoah 23 with pipe combustion motor is shown at left.  With a torque peak just below maximum

rpm the power peak defines the effective maximum rpm.

Electric motors are somewhat different. The chart below shos max torque at 0 rpm i.e. when they are stalled;   Max power is at 50% of peak rpm; Maximum efficiency at approximately 75% of peak rpm – 90% for a good brushless and additional curves for amps and current

Where as a petrol motor runs at peak power i.e. peak rpm an electric runs at peak efficiency where power is less than at the peak.

When the load on a petrol motor increases and rpms drop, torque also falls. For an electric motor torque increases.

How does an electric motor work?

Simple magnetic fields push against one another and turn the motor shaft.

To create the opposing magnetic fields a current is passed through a wire. This generates a magnetic field around the wire. By bundling the wire around steel laminations the field strength increases.

By arranging the bundles adjacent to magnets – the fields generated by the coils interacts with that of the magnets. If the poles of the two fields are the same they will repel each other and tend to move apart. By holding either the magnet or coil still and mounting the other on a shaft which can rotate the magnetic repulsion is captured as rotation.  

The force with which it turns is called torque. The amount of torque is always proportional to the current or amps run through the wiring.  That means in a given motor 1 amp of current always generates X units of torque and 50 amps always generate 50X units of torque. This characteristic is constant for a motor regardless of the voltage level and is known as the torque constant or Kt and expressed as in/oz/amp or mm/N/amp.

The speed at which the motor rotates is related to the voltage used. However voltage is only potential energy and the actual rpm a voltage will create is determined by how many turns of wire there are in each coil and magnet strength.   More turns and stronger magnets mean fewer rpm’s per volt. Weaker fields mean more rpm’s per volt.  This characteristic is the voltage constant or Kv. It is expressed as the rpm/v of the motor. 

As you can see the parts of the motor which determine the torque – the windings and magnets also affect how fast the motor turns.   In a perfect motor they would have no affect and 100% of the energy put in would be put out at the shaft. However losses due to electrical resistance in the wire, the steel in the motor reacting against changes in magnetic fields and parasitic drag in the moving parts mean a motor is less than 100% efficient.

The efficiency of a motor is known at ‘eta’ and is expressed as a percentage eg 90% when 90% of the input is converted to power.

Together the inputs: voltage and current, and the efficiency, determine the power output of a motor which is expressed as watts.  Power is calculated by the formula Volts x Amps x motor efficiency = 12v*10a* .90 = 108 watts output (losses = 12w)

The energy which isn’t converted into motion becomes heat. This heat, or rather the amount of heat relative to the motors ability to cool itself (surface area and mass), determines how much power the motor can be made without causing damage. 

This is because if the motor becomes too hot the electrical resistance increases, heating increases further and magnets loose their strength and motor wires melt.  Also a motor remaining at high temps may fail because the modern adhesives use to construct it suffer heat induced fatigue and the magnets separate from the casing or shaft. At high speed the collision of magnets and coils short the wiring leading to a catastrophic failure of the speed controller and battery.

Electric motors in radio control setups

The effect of too much heat means it’s important to set electric motors up to run at maximum efficiency – not only to get the most output for the input but also so motor heating is minimised.  This is why the efficiency curve is as important as the power curve in the dyno chart for an electric motor.

This means when using an electric motor we aim for a system which loads the motor so it can run at about 90% of its free running rpm. The good thing is there are two indicators the motor is running at max efficiency – first the rpm and second the current draw.  If the motor has very high efficiency we can load it a little more and draw a little more current. This will give extra output power with only a slight and sustainable reduction in efficiency.

However as well as the power advantage this trade off also illustrates why if you have more voltage turning the motor harder you cannot use the same size prop. More voltage turns the motor faster – in effect it increases the load on it just as the larger prop does for the same voltage. It also has the same result more amp draw. However voltage jumps are usually 1.2-6 volts and this large a rise can load the motor to the point where efficiency falls and heating rises to dangerous levels. So remember if you increase the voltage to your motor – reduce the prop diameter.

The safe heat level is determined in the first instance by the neodymium magnets which are vulnerable to heat damage. Typically a safe motor temp is up to 85 degrees Celsius or 185 degrees Fahrenheit at the magnets.  However if a motor is too hot to hold onto indefinitely – its definitely hot enough!

In some disciplines such as boat racing electrical systems are often loaded to a point where the motor can not dissipate enough heat to remain reliable. To avoid damage water cooling is added to keep heating to a level below the point of damage.  An alternative once common was to cool the motor before competition. Cooling after competition remains popular. Usually by the time additional cooling is being used it’s also a good idea to keep a fan handy to keep cooling the motor batteries and esc after a run as the heat remaining in these components can be quite high once the air or water flow around them is removed.

Andrew Gilchrist; July 2007