Introduction to VFD Internal Components and Terminal Wiring

Introduction to VFD Internal Components and Terminal Wiring

Introduction

A variable frequency drive (VFD) is an electrical control device that adjusts the speed of an AC induction motor by changing the frequency of the power supplied to the motor. It is widely used in fans, pumps, conveyors, machine tools and many other industrial applications. The VFD provides stepless speed control and energy savings, and offers soft start and overload protection for the motor. To achieve safe and reliable variable-speed operation, engineers need to understand both the internal power and electronic components of the drive and the correct wiring of the main-circuit and control terminals. This article introduces the main internal components of a VFD, the function of each terminal group and the wiring requirements, and provides industry-standard operating recommendations.

1 Internal Components of a VFD

A VFD generally consists of four main parts: the rectifier, the DC intermediate circuit (DC bus), the inverter and the control circuit, together with auxiliary components for cooling, filtering, pre-charging and protection.

1.1 Power (Main) Circuit

1.1.1 Rectifier

The rectifier converts the incoming fixed-frequency AC power into DC. General-purpose VFDs typically use a three-phase bridge rectifier. The three-phase AC input is usually introduced to the rectifier bridge through a network of snubber capacitors and varistors, which absorb high-frequency harmonics and surge overvoltages from the AC mains to protect the drive. For a three-phase 380V supply, the maximum reverse voltage of the rectifier devices is generally 1200–1600V, and the maximum rectified current is about twice the rated current of the drive.

1.1.2 DC Intermediate Circuit (DC Bus)

The DC intermediate circuit filters and smooths the pulsating DC produced by the rectifier. Its core is a bank of large electrolytic capacitors that store and smooth energy, providing a steady DC voltage to the inverter. Some drives are equipped with a DC reactor (connected through the P1 terminal) to improve power factor and reduce harmonics. For applications requiring rapid braking or frequent start/stop, a braking unit and braking resistor can be connected to the DC bus to dissipate the regenerative energy produced during motor deceleration and prevent the DC bus voltage from rising too high.

1.1.3 Inverter

The inverter is the core circuit of the drive. It typically consists of six power switching devices (modern drives mostly use IGBTs, Insulated-Gate Bipolar Transistors) arranged in a three-phase bridge. Under the control of the control circuit, the switches open and close in a defined sequence to convert the DC power back into AC output with adjustable frequency and voltage. Small and medium-capacity drives often use integrated intelligent power modules (IPMs) that combine rectifier, inverter, driver, sensing and protection functions, with a typical switching frequency up to 20kHz.

1.2 Control Circuit

The control circuit receives external command and speed references and coordinates the operation of the main circuit. It mainly includes the following parts:

Control board: based on a microprocessor (CPU/DSP), handling frequency reference, control algorithms, parameter management and fault-protection logic;

Gate driver board: amplifies the control signals and reliably switches the IGBTs on and off;

Switching power supply: converts the DC bus voltage into low-voltage regulated supplies for the control board and sensors;

Current/voltage sensors: usually Hall-effect sensors that measure the output current (typically two phases) and the DC bus voltage for current closed-loop control and overcurrent/overvoltage protection;

Keypad/display: used for parameter setting, frequency monitoring and fault indication.

1.3 Auxiliary and Protection Components

Cooling system: because the IGBT modules and the rectifier bridge generate significant heat, the drive is equipped with a cooling fan and heat sink to keep the devices within the permitted temperature range;

EMC filter and reactors: suppress electromagnetic interference (EMI) and improve the input current waveform;

Pre-charge / inrush-limiting circuit: the DC bus capacitors are charged through a limiting resistor on first power-up, and a relay bypasses the resistor after charging to limit inrush current;

Braking unit: works with the braking resistor to dissipate regenerative energy for rapid braking.

2 Terminal Wiring of a VFD

VFD terminals are divided into main-circuit terminals and control terminals. Correctly identifying each terminal and wiring it according to the specifications is a prerequisite for safe and stable operation.

2.1 Main-Circuit Terminals

Common main-circuit terminals and their functions are shown in the table below:

Main-circuit wiring requirements:

Connect the input power only to the R, S, T (three-phase) or L, N (single-phase) terminals, and the motor only to the U, V, W output terminals. Never connect the AC input to U/V/W, otherwise the internal devices of the drive will be damaged.

The grounding terminal must be reliably grounded. The ground conductor should be short, thick and of low impedance, ideally sized the same as the power conductors.

The DC bus (P+, P-) and braking-resistor terminals carry dangerous high voltage. Only qualified personnel should touch them, and never while energized.

2.2 Control Terminals

Control terminals receive external commands such as start, stop, forward/reverse and multi-speed selection (digital inputs, DI), or speed/frequency reference signals (analog inputs, AI, e.g. 0–10V voltage or 4–20mA current), and output running status and fault signals to a PLC or a host system (relay/transistor outputs, RO). A +24V supply, a common terminal COM/0V and a +10V reference terminal are also usually provided.

2.2.1 Digital Input (DI) Wiring

Digital inputs are usually controlled by dry contacts. The NPN (sink) or PNP (source) wiring mode is selected by parameter. When an external 24V supply is used, the negative terminal of the external supply should be connected to the 0V/COM terminal of the drive to establish a common potential.

2.2.2 Analog Input (AI) Wiring

Voltage signals (0–10V) and current signals (0–20mA or 4–20mA) usually share the same terminals; the input type is selected by DIP switch and parameters. Analog signals should use shielded twisted-pair cable with the shield grounded at one end to improve noise immunity.

2.2.3 Relay/Transistor Output (RO) Wiring

Relay outputs provide signals such as running, reaching speed and fault. Typical contact ratings are on the order of 250VAC/3A or 30VDC/1A; refer to the manual of the specific model.

2.3 Wiring Practices and Precautions

Reliable grounding: establish a direct ground connection between the drive and the motor to avoid electromagnetic interference (EMI) and high-frequency current crosstalk caused by poor grounding.

Shielded cable: shielded cable is preferred for the input and motor output cables, with the shield properly grounded.

Separation of power and control wiring: control and communication cables must not be routed close to and parallel with power cables (above 50V). If parallel routing is unavoidable, keep as much spacing as possible (recommended about 50mm or more), and cross at 90° where they must intersect.

Separate input and output wiring: keep the drive input cables separate from the motor output cables to prevent PWM high-frequency interference from coupling into the control signals.

Safety: switch off the input power and wait for the DC bus capacitors to discharge before wiring or maintenance.

3 Frequently Asked Questions (FAQ)

Q1: Can the AC input be connected to the U, V, W terminals? No. R/S/T (or L/N) are the power input terminals and U/V/W are the motor output terminals; reversing them will damage the drive.

Q2: What are the DC bus terminals P+ and P- for? P+/P- are the positive and negative DC bus terminals for connecting an external braking unit; a braking resistor can be connected between P+ and PB.

Q3: Should the analog reference use 0–10V or 4–20mA? Both are acceptable; the choice depends on the field instrumentation and control requirements, and the corresponding input type and range must be set in the parameters.

Q4: Why is shielded cable required for the VFD? The motor output is high-frequency PWM voltage; shielded cable suppresses electromagnetic interference (EMI) and prevents it from disturbing the control signals and other equipment.

Q5: Why must one wait for discharge before wiring? The DC bus electrolytic capacitors store high-voltage energy and remain charged after power-off; waiting for discharge is necessary to avoid electric shock.

4 Conclusion

A VFD consists of four main parts — the rectifier, the DC intermediate circuit, the inverter and the control circuit — together with auxiliary components for cooling, filtering, pre-charging and protection. Correctly identifying the main-circuit terminals (R/S/T, U/V/W, P+/P-, PB, P1, G) and the control terminals (DI, AI, RO, +24V/COM, etc.) and following wiring practices such as reliable grounding, shielding and separation of power and control wiring are the keys to safe, stable and long-life operation of the drive.

5 References

[1] Elecfans, "Four Main Components and Working Principle of a VFD", http://www.elecfans.com/d/1864805.html

[2] Shanghai Jinxin, "Detailed Explanation of the Main Circuit Structure of a VFD", https://www.jinxinservice.com/maintainance/2518.html

[3] Shanghai Jinxin, "How to Use the Terminals of a VFD", http://www.jinxinservice.com/maintainance/2421.html

[4] Siemens Industry Support Center, "MM430 Wiring Diagram" and "Digital Input Wiring Method", https://www.ad.siemens.com.cn

[5] Eaton, Application Note AP040076EN "VFD Wiring Best Practices", 2014

[6] ABB, Technical Note 138 "North America packaged drive wiring practices"

[7] Phase Technologies, "VFD Anatomy of a Drive", https://www.phasetechnologies.com/support/educational

[8] Various VFD user manuals (main-circuit terminal definitions: R/S/T, U/V/W, P+/P-, PB, P1, G)