# Key points for selecting BLDC motors (2)

**03 Select the Key Performance Parameters of the Motor**

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

**Torque: The load Capacity of the Motor**

Motor torque, also known as motor torque, is simply the size of the motor's rotating force, which can reflect the motor's load capacity. **The greater the torque, the greater the load the motor can withstand, and the better the motor performance.**

However, in actual applications, we need to consider the maximum torque required by the application. The **maximum torque** can be obtained by adding the motor torque, moment of inertia and friction. In addition, there are some additional factors that affect the maximum required torque, such as the resistance of the air gap, which requires at least 20% torque margin. In addition to the maximum required torque, the square root torque (RMS) of the motor must also be considered. The square root torque can be approximately considered as the continuous output torque required in actual applications. It is determined by many factors: maximum torque, load torque, moment of inertia, acceleration, deceleration, and running time.

**Speed: How Many Revolutions per Minute can it Reach?**

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

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

**RPM=KV*V(power supply voltage)**

According to the formula, the speed and voltage of the brushless motor are theoretically linear. The KV value is the amount by which the motor speed increases for every volt increase in voltage. If the voltage increases, the motor will indeed speed up. Generally, the speed of an electric bicycle is a few hundred revolutions, and that of an electric car will be higher, reaching several thousand or even tens of thousands of revolutions. If a very low output speed is required, it is necessary to consider whether to match a reduction gearbox and what type of reduction gearbox to match.

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

**Power: How Many Watts is Required at Most**

The motor power should be selected according to the power required by the production machinery, and the motor should be operated under the rated load as much as possible.

The following two points should be noted when selecting:

If the motor power is selected too small, the "small horse pulling a big cart" phenomenon will occur, causing the motor to be overloaded for a long time, and the motor will heat up, vibrate, reduce the speed, make abnormal sounds, etc. The motor will even be burned when the load is serious.

If the motor power is selected too large, the "big horse pulling a small cart" phenomenon will occur, and its output mechanical power cannot be fully utilized, and the power factor and efficiency are not high, which is not only unfavorable to users and the power grid, but also causes waste of electric energy.

Therefore, it is very important to choose a motor with reasonable power according to the application needs. We can reasonably choose the motor through the calculation formula of the three parameter values of motor power, torque, and speed:

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

For example, if you want to choose a 120W motor, how to choose it? First, you can confirm the speed, and then confirm how much torque the load requires. If the speed is confirmed to be 3000RPM and the torque is only 0.3NM, according to the above formula, W=RPM*NM/9.55=90. It is confirmed that the maximum power is 90W, so a 100W motor should be enough. However, if the application environment is harsh or the continuous operation time is long, then 125W can be selected. It will be safer to increase the margin slightly.

In addition to the above three main parameters, depending on the application requirements, you may also need to consider **motor voltage, current, operating temperature range, waterproof level, reliability, flexibility and other performance**.

**Voltage**

The rated voltage of a DC motor indicates the most efficient voltage when running. You need to ensure that the voltage applied by the application is appropriate to the rated voltage of the motor. If too little voltage is applied, the motor will not work, while too much voltage applied will short-circuit the windings, causing power loss or complete damage to the motor. Currently, BLDC motors on the market are available in 24V, 36V, 48V, 60V, etc.

**Current**

Current is what drives the motor, and too much current can damage the motor. For DC motors, operating current and stall current are important. Operating current is the average current the motor is expected to consume at typical torque. Stall current applies enough torque to keep the motor running at stall or 0RPM.

**Operating Temperature Range**

Generally speaking, the operating temperature range of brushless motors is between -20 and 100 degrees, more strictly between -40 and 140 degrees.

**Waterproof Level**

The protection level of the brushless motor is represented by IP plus a number. The larger the number, the higher the waterproof level.

**Reliability**

Trust in the motor field means reliability and robustness. Your motor controller will not be damaged in a fault and will be able to operate normally again when the fault condition is eliminated.

**Summarize**

Due to the large number of factors to consider, BLDC selection is a comprehensive system analysis process. It is necessary to balance the selection of various aspects of the motor according to the application requirements and finally arrive at the optimal solution.