How To Choose A Suitable BLDC Motor

Brushless DC motors (BLDC) are widely used in industrial automation, automotive, medical equipment, artificial intelligence, and other industries because of their long life, low noise, and high torque.BLDCs come in a wide range of sizes and types, and in practice, we need to pick out the right BLDC motor to meet the performance requirements of the application. In this blog, SIT will introduce how to choose the right BLDC motor for your application.

 

1. Select the correct BLDC motor type 

According to the structure, brushless DC motors can be divided into two types: Inrunner type and outrunner type. Fundamentally, they work on the same principle but have different advantages and limitations. Inrunner BLDC motors place the stator (fixed element) outside and the rotor (rotating element) inside, with the rotor rotating inside the stator. Outrunner BLDC motors have the opposite design: the stator is placed inside, the rotor is outside, and the external rotor rotates around the stator in the center. Comparatively speaking, inrunner brushless motors are smaller in diameter and faster, making them more suitable for high-speed applications, while outrunner brushless motors have a larger diameter and better heat dissipation, which facilitates the use of low-speed torque and is suitable for low-speed applications.

 

 

According to whether having the sensors or not, BLDC motors are also divided into sensored BLDC motors and sensorless BLDC motors. The choice between them usually depends on cost. Sensorless motors are increasingly being used in applications such as low-cost variable-speed motor applications such as fans, refrigerator compressors, air conditioners, and garden tools. However, applications that require high torque at start-up, such as electric tools like e-bikes and pedelecs, require the use of motors with sensors.

 

In addition, in terms of materials, we give priority to ferrite BLDC motors with high efficiency, low price, and low-temperature rise. Alnico BLDC motors or rare-earth BLDC motors should be considered only in the case of strict performance requirements, small size, and high ambient temperature.

 

2. Key performance parameters for motor selection

When selecting a brushless DC motor, parameters are one of the most important selection factors. Among them, the three most important performance parameters are torque, speed, and power.

 

2.1 Torque

Motor torque, in simple terms, is the size of the motor's rotational force, which reflects the motor's load capacity - the higher the torque, the greater the load the motor can withstand, and the better the motor's performance will be in comparison.

 

In practice, however, we need to consider the maximum torque required for the application, which can be obtained by adding the motor torque, moment of inertia, and friction. In addition, there are some additional factors affecting the maximum demand torque, e.g. the resistance of the air gap air, etc., which require a torque margin of at least 20%.

 

In addition to the maximum demand torque, the root square torque (RMS) of the motor must also be considered. The RMS can be approximated as the continuous output torque required for the actual application. It is determined by a number of factors: maximum torque, load torque, moment of inertia, acceleration, deceleration, and run time.

 

2.2 Rotation speed

The rotational speed of the motor is closely related to the application, for example, the rotational speed of the motor will directly determine the flight speed of the drone. Therefore, when selecting a motor, RPM is an indispensable consideration.

 

Motor speed is represented by the KV value, KV - revolutions per volt, or RPM/V, which indicates how many times the motor will rotate per volt. The maximum speed (RPM) of a motor can be determined by simply multiplying the KV value by the battery voltage.

 

RPM = KV * V (supply voltage)

 

According to the formula, the speed of a brushless motor is theoretically linearly related to the voltage. the KV value is the amount by which the motor's RPM increases for every volt increase in voltage. If the voltage increases, the motor does spin faster.

 

Typical e-bikes rotate at a few hundred RPMs, and e-vehicles go a little higher, up to a few thousand RPMs or even tens of thousands of RPMs. If a very low output speed is required, it is necessary to consider whether to pair it with a reduction gearbox and with what kind of gearbox.

 

In addition, under some application conditions, we also need to consider the speed adjustment range. For example, the speed requirement of hair dryer is that the difference between the maximum speed and the average speed is not big, that is to say, the speed range of the motor is small; while in some point-to-point positioning systems such as conveyor belts and robotic arm systems, motors with large speed range are required.

 

2.3 Power

The power of the motor should be selected according to the power required by the application machinery and try to run the motor at rated load. The following two points should be noted when selecting

 

If the motor power is selected too small, it will cause the motor to be overloaded for a long period of time, which will lead to the phenomena of motor heating, vibration, speed drop, abnormal sound, etc. In serious cases, the motor will even be burnt.

 

If the motor power is selected too large, its output mechanical power can not be fully utilized, and the power factor and efficiency are not high, not only for the user and the grid is not good, but also causes a waste of power.

 

Therefore, it is very important to choose a motor with reasonable power according to the application needs. We can choose a motor by using the formula for calculating the values of three parameters: power, torque, and speed.

 

Power (W) = Speed (RPM) * Torque (Nm) / 9.55

 

For example, if you want to select a 120W motor, how do you do it? Firstly, you can check the RPM, and then you can check how much torque the load requires. For example, if the speed is 3000RPM and the torque is only 0.3NM, according to the above formula, W=RPM*NM/9.55=90, i.e., the maximum power is confirmed to be 90W, then choosing a 100W motor should be fine. However, if the application environment is harsh or the continuous running time is longer, then you can choose 125W, the margin will be slightly larger and safer.

 

In addition to the above three main parameters, depending on the application requirements, it may also be necessary to consider the motor voltage, current, waterproof rating, etc.

 

2.4 Voltage

 

The rated voltage of a DC motor indicates the most efficient voltage for operation. It is necessary to make sure that the voltage applied by the application is appropriate to the rated voltage of the motor; if the voltage is too small, the motor will not work, while too much voltage applied will short-circuit the windings, resulting in power loss or complete damage to the motor.

 

2.5 current

Current is the power to drive the motor, and too much current can damage the motor. For DC motors, operating current and stall current are important. The operating current is the average current the motor is expected to draw at typical torque. The stall current will apply enough torque to run the motor at a stall or 0 RPM.

 

2.6 Waterproof Rating

 

The protection rating of a brushless motor is indicated by IP plus a number, the higher the number, the higher the waterproof rating.

 

3. Define application objectives

 

For different applications, brushless DC motors will have different designs and drive methods, making selection very complex. To simplify motor selection, we need to define the core requirements based on motor usage.

 

No	Main Application Field	Application Examples	Requirements
1	Industrial automation	Industrial robots, industrial large ceiling fans, industrial large water pumps	High starting torque, simple structure, low price, high reliability, easy maintenance, etc.
2	Car	Drive motor, electric seat, electric windows, electric mirrors, water pump, fuel pump, etc	Ultra-high speed, small weight, wide adjustment
3	Electric tool	Drilling machines, circular saws, grinding machines, pendulums	High power, high speed, etc
4	Medical rehabilitation equipment	Ventilators, medical beds, electric wheelchairs, massage chairs, etc	Wide adjustment range, small size, long life, low noise, etc.
5	Household appliance	Refrigerator, washer, dryer, vacuum cleaner, ceiling fan, etc	low power, etc
6	Personal care products	Hairdryers, beauty devices, curling irons. etc	High insulation level, low noise, good heat dissipation, etc
7	Garden Tools	lawnmowers, leaf blowers, electric pruners. etc	High power and long duration, etc
8	other	drones, pedicabs, and video surveillance machines. etc	Depends on the situation

 

BLDC selection is a process of comprehensive system analysis, which requires balancing all aspects of motor selection according to the application requirements. SIT is one of the BLDC motor suppliers in China. We have an experienced engineering team, which can help you find the best solution for your applications. If you have any requirements for the motor, please feel free to contact us.