Jan. 22, 2025
Electric motors are commonly found in various tools and vehicles. In drones and electric aircraft, motor speed is typically regulated through a throttle controller and an ESC (Electronic Speed Controller).
When you increase the throttle on your controller, you are essentially boosting the amount of electrical power drawn from the power source (such as a battery) and delivered to the motor via the ESC. To slow down the motor, you decrease the power; to speed it up, you increase the power.
But how does this process of controlling the motor speed actually work?
In this article, we will explore:
How to increase or decrease the speed of an electric motor
The formula for calculating motor speed
The relationship between motor speed, PWM duty cycle, and ESC output voltage
How to calculate motor RPM based on the motor's KV rating
Controlling the speed of an electric motor essentially boils down to regulating the amount of electrical power supplied to the motor. In this discussion, we will focus on a steady-state scenario where there is no acceleration happening.
More specifically, for a given mechanical load (or resistance), it is an increase in voltage that results in a higher motor speed. Alternatively, reducing the load on the motor can also lead to an increase in speed.
To understand how motor speed is related to voltage, let’s take a look at the formula for angular velocity:
ω = (V ⋅ Kₑ) / (R + Kₜ)
Where:
ω = angular velocity (motor speed, or RPM)
V = voltage
T = torque
Kₜ = motor torque coefficient
R = resistance / load
Kₑ = back EMF constant of the motor
From this equation, we can see that the angular velocity (or RPM) is directly proportional to the voltage and inversely proportional to the torque.
Therefore, to increase the motor's speed, you have two options:
Increase the voltage supplied to the motor
Decrease the torque (though changing the load or design may not always be practical)
It’s also important to note that at a constant voltage, an increase in torque leads to a decrease in RPM.
While the ESC introduces small losses, meaning the voltage delivered to the motor is slightly less than the input voltage, this difference is typically minimal, often only a few percent.
Another way to understand the process of controlling motor speed is by looking at the ESC (Electronic Speed Controller) protocols, particularly how PWM (Pulse Width Modulation) signals are used to regulate power delivery.
With PWM, the signal sent from the controller (measured in microseconds, µs) represents a percentage of the maximum voltage that can be supplied to both the ESC and the motor.
The PWM signal in µs typically ranges from 1000 (no throttle) to 2000 (full throttle). Values in between correspond to a duty cycle between 0% and 100%.
At 0% duty cycle, no power is sent to the ESC or motor.
At 100% duty cycle, the ESC receives continuous power, delivering it constantly to the motor.
By adjusting the PWM signal, you essentially control the average voltage and, therefore, the speed of the motor.
The implementation of motor speed control can vary among different ESC (Electronic Speed Controller) manufacturers. However, the relationship between the duty cycle and motor speed typically reflects the average output voltage of the ESC.
Most ESCs generate an approximate sine wave output on each phase. At 50% ESC output, the amplitude of the sine wave will be roughly half of that at 100% ESC output. It’s important to note, however, that the electronic drivers in most ESCs operate in an on-off state, so the actual signal is not as smooth as a perfect sine wave.
Because different motor and ESC manufacturers have varying designs, and electronics can introduce non-linearities, it is often advisable to characterize the motor and ESC combination using a dynamometer to obtain accurate performance data.
Digital protocols such as Dshot and Oneshot also use the concept of duty cycle but with faster signal delivery and unique signal patterns. These protocols allow for more precise control and faster response times.
In some cases, the duty cycle may be represented as a value between 0 and 1024, which stems from the 8-bit system used in early controllers. In such a system, there are two states (on or off), and with an 8-bit system, you have 1024 possible values.
This system can also be used to calculate the ESC’s output voltage. For instance, a value of 796 corresponds to about 78% of the maximum output, relative to the 100% value.
In a no-load condition, you can use the motor's KV rating to estimate its speed based on the voltage supplied to the motor.
As explained in our previous article on how to calculate motor KV, we can substitute the back EMF with the input voltage to estimate the motor's rotation speed.
Let's take MAD Components' M50C35 EEE 9 KV motor as an example. This motor has a maximum voltage of 400 V and a KV rating of 9 KV. Plugging these values into the formula, we get the following calculation for its maximum speed:
RPM = KV × Voltage
RPM = 9 × 400 = 3600 RPM
So, at full throttle (400 V), this motor will rotate at 3600 RPM. If we apply only 75% throttle, corresponding to 300 V, the motor will rotate at:
RPM = 9 × 300 = 2700 RPM
As demonstrated in this article, there are a few different methods to increase motor speed.
The most practical approach is to increase the voltage supplied to the motor by increasing the throttle. While it is also possible to decrease the torque to increase speed, this is typically more disruptive to the design or functionality of the UAV or system.
WingFlying is commited to supplying precise and easy-opertional drone propulsion testing tools and technical support for drone companies, university labs, UAV education, and drone communities.
E-mail: sandy@wing-flying.com
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