[Vol.005] The Flux-weakening Phasor Diagram of the Brushless Surface-Magnet PM Motor

Summary

This video explains the phasor diagram of the brushless AC surface-magnet PM motor, focusing on the principle of flux-weakening, crucial for high-speed motor operation. It discusses how the motor’s EMF increases with speed, eventually exceeding the available inverter voltage, and how negative d-axis current is used to reduce the net flux. The video also explores the impact of speed on torque, mechanical power, and power-factor angle, while addressing the importance of voltage drops in the stator and armature. It concludes with considerations on the limits of superposition in a saturated magnetic circuit and the challenges at high speeds.

Takeaways

• 😀 The phasor diagram is an essential tool for understanding brushless AC surface-magnet permanent-magnet motors, particularly in high-speed operation.
• 😀 Flux-weakening is a key principle in motor design, where a negative d-axis current reduces the net flux when the motor’s EMF exceeds the inverter’s available voltage.
• 😀 The surface-magnet motor, being rotationally symmetrical, has identical synchronous reactances in the d- and q-axes, simplifying its analysis compared to interior permanent-magnet motors.
• 😀 In the phasor diagram, the relationship between EMF, current, and voltage components (resistive and reactive voltage drops) can be clearly observed.
• 😀 The d-axis is associated with the open-circuit flux of the magnet, and its alignment with the real axis in the complex plane is fundamental to the motor’s operation.
• 😀 The angle between the current phasor and the EMF phasor plays a critical role in determining the torque of the motor, with a typical phase difference of around 11.09°.
• 😀 The motor’s terminal voltage is determined by the sum of the resistive and reactive voltage drops, and it must stay within the inverter's voltage limit for stable operation.
• 😀 At higher speeds, the EMF grows, and a larger negative d-axis current is required to maintain operation, leading to a reduction in torque while mechanical power may increase.
• 😀 Power-factor analysis is important in motor control as it determines the required size of the inverter, influencing both cost and efficiency.
• 😀 Saturation effects complicate the application of the phasor diagram, as superposition may not be valid under nonlinear magnetic conditions, especially in salient-pole machines.

Q & A

What is the main purpose of the phasor diagram in the context of brushless AC surface-magnet PM motors?

The phasor diagram is used to explain the principle of flux-weakening, which is essential for high-speed operation of brushless PM motors. It helps in visualizing the relationship between the motor's control and design aspects.

Why is flux-weakening important for high-speed operation of brushless PM motors?

Flux-weakening is important because at high speeds, the EMF generated by the magnets exceeds the available voltage from the inverter. By driving a negative d-axis component of current, the net flux is reduced, allowing the motor to operate at higher speeds without exceeding the inverter's voltage.

What does the phasor diagram represent in relation to the surface-magnet and interior permanent-magnet (IPM) motors?

The phasor diagram is nearly the same for both surface-magnet and IPM motors. The key difference lies in the values of the reactances and the resulting control laws for the d- and q-axis components of the current.

How does the inverter's voltage limit affect motor performance at high speeds?

At high speeds, the EMF generated by the motor may exceed the inverter's voltage capacity. The inverter must use a negative d-axis current to reduce the net flux, keeping the motor within the available voltage limit and preventing performance issues.

What does the 'per-unit' scale mean in the context of this motor's operation?

The 'per-unit' scale is a normalized form used to express the motor's current, voltage, and other electrical parameters. It allows for a uniform representation of these values regardless of the motor's actual size or rating.

How is the torque related to the current and EMF in a surface-magnet brushless motor?

In a surface-magnet brushless motor, the torque is proportional to the product of the EMF and the q-axis component of the current. The current is typically controlled to be in phase with the EMF to maximize torque.

Why does the current lead the EMF by a specific angle in this example?

In this example, the current leads the EMF by 11.09° to demonstrate the phase shift between the two. This phase shift helps in resolving the current into its d- and q-axis components, which are essential for analyzing motor performance.

What happens to the current and torque as the motor speed increases?

As the motor speed doubles, the EMF doubles and the reactances increase. To maintain the same current magnitude, the d-axis current becomes more negative, reducing the q-axis current and therefore the torque. While speed increases, torque decreases.

What is the role of the voltage-drop components in the phasor diagram?

The voltage-drop components, including resistive (ER) and reactive (RV) drops, are represented in the phasor diagram to show how they contribute to the terminal voltage. These components are crucial for understanding the overall voltage behavior in the motor.

How does the power factor change with increasing motor speed, and why is this important?

The power factor improves with increasing motor speed, as indicated by the decreasing angle between the terminal voltage and current phasors. A higher power factor means more efficient use of the inverter's power capacity, which is important for minimizing inverter size and cost.