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
1.26 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
