1. Introduction Variable frequency speed control of single-phase AC induction motors is widely used in many consumer products (fans, washing machines, etc.) and industrial products (blowers, pumps, etc.). While general-purpose single-phase frequency converters can be used, they are expensive, complex in structure, and bulky, requiring external installation, which increases inconvenience and wastes resources due to excessive settings. Therefore, this paper designs a small intelligent variable frequency control circuit for single-phase motors. It requires only one IC controller (ML4421), one IPM power amplifier driver module, some capacitors and resistors, requires no software development, has a simple structure, small size, and can be integrated into a general induction motor, achieving both electromechanical integration of the frequency converter and increased ease of installation and use. 2. Control Circuit Design The control circuit uses the ML4421, a dedicated chip for single-phase motor frequency control manufactured by Micro Linear Technology. It is a simple external connection and comprehensive PWM intelligent control chip. It has a built-in control program with two waveform generators, a PWM modulator, and slip control, all 90° out of phase. It operates in a true linear proportional, integral, and derivative (PID) feedback loop, providing a 10:1 speed control range and a constant, programmable V/F ratio. Because it eliminates the need for software programming, it saves on development costs. The wiring diagram for the ML4421 is shown in Figure 1. The following is an explanation: [align=center] Figure 1 Wiring of ml4421[/align] (1) Pins 1 and 2 are connected to the motor current feedback input from the current transformer. isa and isb together form the differential input, which detects the current, voltage and phase difference of phase a, and then determines the given phase angle together with the front-end voltage feedback (only effective for precision slip control); (2) Pin 3 is the slip angle setting (only effective for precision slip control); (3) Pin 4 is used to finely adjust the amplitude of the sine wave during precision slip control. When this pin is grounded, precision slip control is not performed; (4) The amplitude and frequency of the sine wave can be controlled by adjusting the input voltage of pin 6. Let c0 be the starting capacitor of the 0° sine wave generator, then the frequency of the 0° sine modulation wave f = (r6/r5)/(2×c0×0.356), and similarly, the frequency of the 90° sine modulation wave can be obtained; (5) Select different resistors r8 on pin 8 to set the dead time t, t = r8/(5.2×104); (6) Connect the starting capacitors c0 and c1 of the sine wave generator to pins 9 and 10, set the reset time on pin 11, and set the PWM carrier frequency on pin 12. The carrier (triangular wave) frequency f = vref/(rref×7.2×c PWM); (7) Pin 13 is for forward and reverse rotation selection. When driving a two-phase motor, controlling its high and low levels can achieve forward/reverse rotation; (8) Pin 14 is for braking time selection, and pin 15 is for overcurrent protection input from the current converter. The current limiting protection resistor r15 = 0.5V/imax; pins 16 and 25 are for sine wave test points; (9) Pins 18 to 23 drive 6-channel PWM waveform output, pins 18 to 20 low-end output, pins 21 to 23 high-end output; (10) Pins 26 to 28 are connected to voltage feedback input, which refers to the average value of the PWM output voltage of the two phases of the motor after voltage division. The above capacitor, resistor and other parameters can be selected according to the actual application to meet different control effects [1]. 3 Main circuit and drive circuit The power drive and inverter circuit of the traditional frequency converter consists of two modules. Now these two parts can be completed by a small IPM power module. The IPM consists of a high-speed, low-power IGBT, a preferred gate-level driver and protection circuit. Among them, the IGBT is a combination of GTR and MOSFET. The MOSFET drives the GTR. Therefore, the IPM has the advantages of high current density, low saturation voltage, high withstand voltage of GTR, high input impedance of MOSFET, high switching frequency and low drive power. The protection functions include control power supply undervoltage lockout protection, overheat protection, overcurrent protection and short circuit protection. When any of the protection functions is activated. The ipm will output a fault signal fo. Therefore, the ipm power module is powerful and can simplify circuit design. In order to ensure that the main and auxiliary windings of the single-phase motor are strictly 90° apart during frequency conversion speed regulation, a three-phase full-bridge inverter is selected for the main circuit. Each winding is equivalent to an independent full-bridge inverter, eliminating the phase-splitting capacitor, overcoming the disadvantage of frequency instability caused by the phase-splitting capacitor, and having a high DC bus voltage utilization rate [2]. Therefore, a 6-unit Mitsubishi PM20CSJ060 (20A/600V) is selected to drive a 0.75kW single-phase AC induction motor [3]. The ipm wiring is shown in Figure 2. The n and p pins are connected to the DC power supply. The DC power supply signal is obtained through a single-phase rectifier circuit or DC bus. To prevent external signal interference, an opto-isolation circuit was added between the ML4421 and the IPM. A high-speed optocoupler HCPL4504 was used for the PWM waveform input, while a low-speed optocoupler PC817 was used for the fault output and brake input. An isolation power supply M57140L was selected, which can provide four sets of +15V isolated power supplies. Up, VP, WP, UN, VN, and WN are connected to the six PWM waveforms (HA, HC, HB, La, LC, LB) output from the ML4421 via high-speed optocouplers. U and V are connected to two phase windings of the motor, and a bypass capacitor of 0.01µF is used. [align=center] Figure 2 Wiring of the IPM[/align] 4 Conclusion This paper utilizes the ML4421, IPM power module, and some capacitors and resistors to design a low-power single-phase frequency converter. It requires no software development, has low hardware cost, simple structure, high reliability, strong protection capability, and shortens system development time. It provides an idea for the author to manufacture low-cost, small-size single-phase frequency converters.