S-T Characteristics and I-T Characteristics of Brushless Motors
2025.09.08
I will explain the S-T characteristics and I-T characteristics of the three states of brushless motors shown in the previous article.
Contents of Episode 6
- ・Brushless Motor Output Characteristics: Torque and Rotation Rate
- ・Brushless Motor Output Characteristics: Driven States of Actual Motors
- ・S-T Characteristics and I-T Characteristics of Brushless Motors
- ・Brushless Motor Specification Changes and Accompanying Changes in Characteristics
S-T Characteristics and I-T Characteristics of Brushless Motors
The S-T characteristics of a motor can be represented by a graph with the motor speed on the horizontal axis and the torque on the vertical axis. The I-T characteristics are represented by a graph with the motor winding current or power supply current on the horizontal axis and the torque on the vertical axis.
S-T characteristics and motor driven states
The S-T characteristic curve varies depending on the type of motor and the applied voltage. Here are the characteristics when the motor duty is 100%, 50%, and 25%. In the previous article, I explained that there are three driven states for motors. The operating range of the motor in these driven states is shown in the S-T characteristic graph.
・No-load state: This is the area where torque is zero. The rotational speed at this time is called the “no-load rotational speed.” This rotational speed generally changes in proportion to the applied voltage (horizontal axis).
・Locked state: This is the condition in which the rotational speed is zero. The torque at this time is called the “starting torque,” which refers to the torque when the motor starts rotating. This torque also varies in proportion to the applied voltage.
・Loaded state: This is when the motor is rotating under a load torque, and is the triangular area in the figure above.
Unless there is interference from external forces, the motor cannot be driven outside of these areas. Incidentally, if the motor rotates excessively due to external forces, it will enter an electricity-generating state (regeneration). In addition, in the reverse rotation state, a torque greater than the starting torque may be generated.
I-T characteristics of brushless motors
The I-T characteristic is a graph with the motor winding current or power supply current on the horizontal axis and torque on the vertical axis. The slope of this curve varies depending on the type of motor. Additionally, when the duty is changed, the slope of the winding current curve remains unchanged, but the slope of the power supply current curve changes. So, when checking the I-T characteristic, you must confirm whether the horizontal axis represents the winding current or the power supply current.
What is clear from S-T and I-T characteristics
From these two graphs, you can determine whether the motor can be used under the operating conditions. For example, when deciding whether to adopt a motor for a device, if the required rotational speed and torque fall within the triangular range of the S-T characteristic in the figure, you can conclude that the motor can be used in accordance with its specifications.
If the required conditions fall outside the triangular range, it may be necessary to change the power supply voltage or the motor winding specifications. However, changing the power supply voltage may affect the voltage rating of the IC, and changing the winding specifications may alter the I-T characteristics, thereby affecting the driving current of the IC. It is important to understand that such changes may also affect the motor driver.
Brushless Motor Specification Changes and Accompanying Changes in Characteristics
Motor characteristics change depending on the power supply voltage and motor winding specifications. They also vary depending on the motor size, winding specifications (winding wire diameter and number of turns), and the magnetic force of the permanent magnets. The S-T characteristics and I-T characteristics that change with these modifications are shown in the graph.
All of the examples below can improve motor efficiency, but they also increase costs.
So far, I have explained motor torque, rotation speed, driven states, and S-T and I-T characteristics. These constitute basic knowledge that is necessary for controlling motors, so please make sure you understand them thoroughly.
Next time, I will explain motor output torque in more detail.
Key points of this article
・The S-T characteristics of a motor represent the relationship between the motor speed and torque.
・The I-T characteristics represent the relationship between the motor winding current or power supply current and torque.
・The characteristics of motors vary depending on changes in the power supply voltage and motor winding specifications.
・You can decide whether or not to use a motor by checking the S-T and I-T characteristics.
Teacher Sugiken’s Motor Library
Teacher Sugiken’s Motor Driver Dojo
- [Episode 1] I Can See Them! Motor Fairies
- [Episode 2] Sugiken appears! The first step to becoming a super engineer
- [Episode 3] All of Sudden, a Rival Appears for Ichinose Manabu!?
- [Episode 4] A Sudden Closeness?! New Things the Two Have in Common
- [Episode 5] Passion! Which Thoughts Did Ichinose Sense from Ninomiya?
- [Episode 6] Test showdown! A serious battle between Ichinose and Ninomiya!
- [Episode 7] This Is Just the Beginning! Ichinose and Friends’ Motor Driver Dojo
- [Episode 8] The First Meeting! Lessons Learned in a Real Setting
- [Episode 9] A Shortcut to Becoming a Super Engineer!? Learning from the User’s Perspective
- [Episode 10] Beyond the Questions! What Engineer Ichinose Learns
- [Episode 11] Learning And Growing: It’s Not Just About Turning the Motor!
- [Episode 12] To the Next Stage! The Door To Becoming a Super Engineer Opens
An Introduction to Motors
Brushless Mortor Driver
Motor Q&A