Analyzing the Current, Power, and Voltage of Brushless DC Motors.
DC brushless motors are composed of permanent magnet rotors, multipole winding stators, position sensors, etc. They have the advantages of high reliability, no commutation sparks, and low mechanical noise, and are widely used in floor cleaning robots E-bike、 New industrial fields such as smart tables and chairs, smart dishwashers, smart stir fry machines, and garbage disposers. Today, we will use the application of DC brushless motors in the field of drones as an example to introduce their current, power, and voltage.
1、 Maximum current and maximum power
Maximum current: The maximum current that the motor can withstand and operate safely
Maximum power: The maximum power that the motor can withstand and operate safely. Power=Voltage * Current
Each motor has its own upper limit of power, and the maximum power is this limit. If the working condition exceeds this maximum power, it will cause the motor to burn out due to high temperature. Of course, this maximum power is also determined based on the specified operating voltage. If it is at a higher operating voltage, the reasonable maximum power will also increase.
This is because: Q=I2R, the heating of a conductor is directly proportional to the square of the current. At higher voltages, if the same power is used, the current will decrease, resulting in a decrease in heating and an increase in maximum power. This also explains why a large number of 22.2V or even 30V batteries are used to drive multi axis aircraft on professional aerial vehicles. Brushless motors under high voltage have lower current, less heat generation, and higher efficiency.
2、 The relationship between power and efficiency of brushless motors
People often ask: What is the difference between 2208 1000KV and 2216 1000KV? They are both 1000KV, aren't they the same? The difference is actually quite significant.
In the case where the motor diameter and KV value are the same, a higher motor has a higher natural power, and a higher power motor can naturally drive a larger load. For example, if a man weighs 100 pounds or 160 pounds, and you ask them to carry a bag of 50 pounds of rice, a man weighing 100 pounds may find it a bit difficult but can still carry it, while a man weighing 160 pounds may consider it a small dish. But what if they were to carry two bags of rice? A man weighing 160 pounds gritted his teeth and lifted his back, while a man weighing 100 pounds may not even be able to straighten his waist. This is their difference. For motors, the easier the work, the higher the efficiency. Using the previous theory, iron and copper consumption are also low.
Remember this formula: torque is proportional to the square of current
As the work of DC brushless motors becomes increasingly tiring, their efficiency will rapidly decrease. So when choosing a multi axis motor, it is necessary to choose a suitable power motor and a propeller that matches it, so that the motor can work in a relatively easy state. Generally speaking, when hovering, the working power should be between 30-45% of the maximum power. You cannot pull a big cart with a small cow, nor can you pull a small cart with a large cow.
3、 The Relationship between Voltage and Efficiency of Brushless Motor
Let's start with two formulas:
1. Power=Voltage * Current
2. Heat generation=square of current * resistance
Two conclusions can be drawn from the formula: at the same power, the higher the voltage, the smaller the current, and it can be inferred that at the same power, the higher the voltage, the smaller the heat generated. The final conclusion is that for the same aircraft, the higher the voltage used, the smaller the current and the less heat generated, resulting in higher efficiency.
Now you know why high-voltage power lines need to be subjected to high voltages of 100KV, even 220KV, 550KV (which is kilovolts).
Of course, aircraft require batteries for propulsion, specifically lithium batteries. The number of lithium batteries naturally depends on the size of the battery, and the larger the battery, the higher the voltage it can produce. So in terms of voltage, there isn't really much we can do because many batteries on the market are serialized. For example, for models like the 450, you can look for 6S batteries for the 450 helicopter, but the price is high and the required power adjustment price is also higher.
So in terms of voltage, what we should do is to avoid using low-voltage batteries for large models as much as possible, which will result in relatively high operating current and higher copper consumption. At the same time, it is also necessary to avoid using high-voltage batteries for small aircraft, as the weight of the batteries is too large.
