Experts talk about technology | RAMDA algorithm, the core motor control solution of Renesas Electronics

Author: Renesas Electronics (China) Co., Ltd. Beijing Li Tiegang Gu Chenyu

As green environmental protection, energy conservation and emission reduction gradually become the theme of the Chinese market and society, the government's regulatory efforts have been strengthened year by year, and the low-energy consumption requirements for various products have become increasingly stringent. From small household appliances such as shavers and hair dryers to industrial automation production lines, a wide variety of motors play a core role in various consumer and industrial products. In order to achieve the goal of energy conservation and environmental protection, motor control technology has become particularly important. For the Chinese market, Renesas has actively promoted high-performance, low-power RX microcontrollers while also developing an advanced motor control solution RAMDA algorithm with completely independent intellectual property rights to build high-performance, high-reliability permanent magnet synchronous motor drive solutions to help customers develop a new generation of green and environmentally friendly new products.

The number of household appliances is huge, and the total energy consumption is also considerable, which is the focus of national attention. In particular, the energy efficiency requirements for air conditioners, which are large energy consumers among household appliances, are increasing year by year, and the driving technology of air conditioner compressors is the key technology to improve energy efficiency.

By combining the RAMDA algorithm on the RX MCU, customers can quickly develop high-performance, high-reliability air-conditioning outdoor unit systems.

The following figure is the RAMDA control logic block diagram:

A-PAM technology has the following advantages:
• Variable DC bus voltage
• Adjustable overall efficiency of frequency conversion system

Based on A-PAM, users can accurately control the output value of the DC bus voltage and provide sufficient DC bus voltage while ensuring the overall efficiency of the system. Compared with traditional PFC, A-PAM can limit the output value of the DC bus voltage, and the PFC circuit efficiency is also relatively high.

By limiting the DC bus voltage, customers can choose filter capacitors with lower withstand voltage ratings to achieve lower system costs. For example, in a single-phase 220V power supply system, the output DC voltage of a traditional PFC is generally between 350[V] and 390[V], and system designers will choose filter capacitors with a withstand voltage of 450[V]. With the A-PAM technology, the DC bus voltage can be limited to a maximum of 340[V]±1[V] in real time according to load changes, and system designers can choose filter capacitors with a withstand voltage of 400[V], reducing the hardware cost of the system.

In addition, the A-PAM technology does not require hardware circuits to detect the zero crossing point or amplitude of the AC voltage, further reducing system costs.

The RAMDA algorithm provides basic permanent magnet synchronous motor drive functions, and enables the motor to operate efficiently and stably based on the basic parameters of the motor (number of poles, winding resistance, synchronous inductance, maximum magnetic flux, and moment of inertia).

What makes the RAMDA algorithm unique is its original torque control algorithm.

Single-rotor motors are commonly used in household appliances, such as air conditioning compressors. Driven by ordinary algorithms, single-rotor motors will cause system vibration. This vibration is harmful. Taking the air conditioning system as an example, at low speeds (less than 30 [rps]), system resonance will occur, system noise will increase, and in severe cases, stress concentration points of the air conditioning system will be damaged. During high-speed operation, the peak value of the motor phase current is high, the harmonics are large, the high-frequency noise is high, and the output capacity of the inverter is limited. In response to such market requirements, the RAMDA algorithm includes the following two torque fluctuation compensation technologies:

Under the action of G-type torque control, the system vibration is minimized, thus ensuring the system to operate smoothly and normally at low speeds, and expanding the low-frequency range of the actual operation of the system. As can be seen from the figure below, G-type torque control can significantly reduce system vibration when it is below 30 [rps].

Comparison of system vibration amplitude at various speeds under various torque control effects.

Under the M-type torque control, the inverter outputs a constant torque, the peak value of the motor phase current waveform decreases, reducing harmonics and power loss, and reducing system noise. The following figure compares the motor phase current waveform under M-type torque control with the motor phase current without torque control.

Phase current waveform without torque control

Phase current waveform during M-Type torque control

In conjunction with A-PAM technology, the utilization rate of DC bus voltage is improved. Overmodulation will cause harmonic distortion. In order to overcome this defect, the RAMDA algorithm limits the harmonic distortion to less than 10%. By accurately calculating the relationship between the modulation index and the ratio of the output voltage to the DC bus voltage, the RAMDA algorithm has developed an original overmodulation algorithm. As shown in the figure below, if the overmodulation technology is not applied, the maximum modulation index can only reach 1.15.

Linear Modulation

While ensuring that the harmonic distortion is less than 10%, the overmodulation technology of the RAMDA algorithm is applied, and the maximum modulation index can reach 2.

Overmodulation

Try to avoid output voltage saturation to ensure that the output torque meets the requirements under limited DC bus voltage.

As the speed increases, the amplitude of the induced voltage in the winding becomes higher and higher. When the DC bus voltage is constant, the amplitude of the voltage at the winding end is limited. In this way, if the voltage command is still sent according to the MTPA algorithm, the winding voltage is even lower than the voltage of the induced electromotive force, which will lead to insufficient current in the winding and the inverter cannot deliver enough power to the motor. In order to solve this problem, a negative current is intentionally added to the d-axis to make the phase of the winding voltage lead the induced electromotive force to maintain the power output to the motor. This is the weak magnetic control. For IPM motors, we define three "realms" of weak magnetic 1) MTPA algorithm that fully utilizes permanent magnet torque and reluctance torque. 2) "Shallow weak magnetic" to keep the output torque constant under the condition of constant DC bus voltage 3) "Deep weak magnetic" to maintain high speed and constant output power under the condition of constant DC bus voltage.

Basic principles of field weakening control