Brushless Motor Selection parameters
There are two main criterion's
to consider when choosing a motor:
1-Electrical characteristics
2-Physical characteristics
2-Physical characteristics
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Electrical characteristics:
1. Motor Kv:
The no-load RPM per volt of
the motor. For example, if a motor is rated at Kv = 3000 and is being run on a
10 cell (12 volt) battery, then the motor will spin at 36,000 rpm
Given two motors which are
identical except for the Kv, the motor with the higher Kv will be more powerful
and use more current, and the motor with the lower Kv will be weaker but use
less current.
The no-load current rating
for a motor. This is the electrical current used by the motor when the motor is
running at full speed with no mechanical load on the rotor.
The motor's Io is one factor
which will help you determine the motor's efficient operating range.
Better quality motors usually
have a lower Io because they are more efficient and expend less power.
High-power motors usually
have a higher Io because they are designed to run efficiently at high loads and
therefore run much less efficiently at low loads.
3. Motor efficiency:
Motor efficiency is important
because the power not used to fly the UAV is wasted as heat. For example, you
might compare two motors, one with 80% and 90% efficiency. The difference
appears to be only 10%.
In terms of heat generation, the 80% efficient motor will generate almost twice as much heat as the 90% efficient motor. If a helicopter requires 150 watts to hover, then the 90% efficient motor will use 166.7 watts of power and waste 16.7 watts as heat.
The 80% efficient motor will use 187.5 watts of power, and waste 27.5 watts as heat. This generates 65% more heat than the 90% efficient motor. So, motor efficiency is very important because efficient motors run much cooler.
In terms of heat generation, the 80% efficient motor will generate almost twice as much heat as the 90% efficient motor. If a helicopter requires 150 watts to hover, then the 90% efficient motor will use 166.7 watts of power and waste 16.7 watts as heat.
The 80% efficient motor will use 187.5 watts of power, and waste 27.5 watts as heat. This generates 65% more heat than the 90% efficient motor. So, motor efficiency is very important because efficient motors run much cooler.
Some motors are rated for a
very low maximum RPM, such as:
JETI motors have a maximum rating of 15k-20k
RPM
Older Kontronik Tangos have a maximum rating
of 25k RPM
If you use the motor past its
rated RPM, then motor failure will probably occur (most likely the magnets will
loosen from the rotor).
2. Physical characteristics
Some motors are too large to
fit in the some helicopters. For example, the C50-13L is too large to fit in
the ECO 8.
A quick guide to suitable in
runner motors is to allocate 10-15% of the helicopter's AUW to the motor so,
the in runner motors suitable for an ECO 8 (AUW ~1600 grams) will be between
160-240 grams in weight.
Out runner motors can be
lighter because they are more efficient at dissipating heat. For out runner
motors, it is only necessary to allocate about 7-10% of the helicopter's AUW
for the motor.
If you are not experienced with selecting motors, then you should select a motor in the 13-15% of AUW range. Motors in the 10-12% AUW are somewhat small and must be selected carefully to fly the helicopter well and avoid overheating.
If you are not experienced with selecting motors, then you should select a motor in the 13-15% of AUW range. Motors in the 10-12% AUW are somewhat small and must be selected carefully to fly the helicopter well and avoid overheating.
3. Motor mounting holes:
Some motors are unsuitable
for multirotors use because the motor mounting holes are nonstandard.
For indoor flight we have to
use motors with fan or according to the temperature.
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