How do I read a BDLC motor spec sheet
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Authors: Julian Ho, Cass Wang
BLDC motor stands for Brushless DC motor, as their name implies, brushless DC motors do not use brushes. With brushed motors, the brushes deliver current through the commutator into the coils on the rotor.
With a BLDC motor, it is the permanent magnet that rotates; rotation is achieved by changing the direction of the magnetic fields generated by the surrounding stationary coils. To control the rotation, you adjust the magnitude and direction of the current into these coils.
By switching on/off each pairs of stators really quickly, the BLDC motor can achieve a high rotational speed.
This is a simple table comparing a brushed DC motor, AC induction motor, and a BLDC motor:
BLDC motors are commonly found in drones, electric cars, even robots!
There are a couple different types of BLDC motor on the market for different applications. Here are some examples,
<150g weight
<5cm diameter
11.1-22.2v operational voltage
~0.3 NM torque
application: small drones
400-1000g weight
5-10cm diameter
22.2-44.4v operational voltage
~4 NM torque
application: RC cars, electric skateboard, robot actuator
~400g weight
5-8cm diameter
11.1-22.2v operational voltage
~0.9 NM torque
application: 3D printers, servos
When shopping for a BLDC motor, there are a couple motor specific terms to consider.
Max RPM (KV - RPM per volt)
2200KV @ 10v = KV x V = 22,000 RPM max speed
1 NM = able to lift 1 KG weight attached to the end of a 1 meter long stick
Max Power (W - Watts)
835w @ 10v = W/V = 83.5Amp max power draw
Motor Efficiency (%)
90% efficiency = 90% of theoretical power
Input Volt (S - li-ion Cells)
6S = S x 3.7V = 22.2v
Max Current (A - Amps)
50A @ 10v = A x V = 500W max power
Motor poles (N-P)
24N40P = 24 stator poles, 40 permanent magnet poles
Outrunner/Inrunner
Outrunner = motor body spin with output shaft
Inrunner = only output shaft will spin, body is stationary
Motor numbering
6355 = 63mm diameter, 55mm length
To drive a BLDC motor, you need a dedicated speed controller (ESC) to control it. Here are different types of ESC for different applications. These ESCs (like the motors above) are considered hobbyist-use, but they are quite sufficient for building small/mid-size robots.
very light ~9g
very small footprint (size of a dollar coin)
1-6S input voltage
~40A max continuous current
cheap
application: small drone, small fighter robot, RC helicopter
downside: cannot handle big motors, heat up very quickly, only simple motor control algorithms available
3-12S input voltage
~50A max continuous current
can handle medium size motors
have active cooling
affordable
application: RC car, RC boat, electric skateboard
downside: limited control protocol (PWM only), only simple motor control algorithms available
commonly used in robotic arm, actuator control
more expensive
~120A max continuous current
can handle large motors
offer fine positional control of motor
offer programmatic control (serial/USB/CANbus)
application: robot, robotic arm
downside: quite pricey, not plug-and-play, need to tune the motor manually before use
There are 2 most common motor control algorithm used in hobbyist ESCs.
Sensorless BLDC Motor Control
Advantage: No need for dedicated encoder on the motor
Downside: Weak low speed control, less speed less torque
Field Oriented Control (FOC)
Advantage: Full torque at any speed
Downside: Require fine motor tuning (PID), and dedicated encoder
Max Torque (NM - )
