VFD Wiring Guide: Main Circuit (R/S/T, U/V/W) & Control Terminals | SHZK

Correct wiring is the foundation of a reliable VFD installation. This guide covers the SHZK 300N main circuit — supply, motor, braking and ground — and the control terminals for start/stop, speed reference, relay outputs and RS485. Follow it in order and you will avoid the faults that bring most new installations back to the panel: swapped input/output terminals, missing grounds, and the wrong Sink/Source setting.

Main circuit terminals

• R / S / T — AC supply input (single-phase models use R & T, S is empty). Connect mains here only.
• U / V / W — output to the motor. Connect the 3 motor leads here.
• P+ / PB — braking resistor terminals (external braking unit on larger drives).
• (⊥) / PE — protective earth; connect to a solid, dedicated ground.

Critical rule: never swap R/S/T with U/V/W. Connecting mains voltage to the output terminals destroys the IGBT modules the moment power is applied. This is not covered by warranty, so double-check before energising. The motor must rotate forward (U–V–W); if it runs backward, swap any two of U/V/W — do not change the input side.

Main-circuit wiring best practice

• Size cables and the input breaker/fuses to the rated input current of the drive, not the motor — a 0.75–2.2 kW single-phase drive draws more input current than the motor FLC suggests.
• Fit an AC input reactor or EMC filter where the supply is shared with other converters or power quality is poor; it also protects against surge.
• Use a contactor on the input for emergency-stop isolation, but do not start/stop the motor by cycling the contactor — use the control terminals; repeated power-on charging stresses the capacitors.
• Keep motor cable separate from control and communication cables; cross them at 90° if they must meet. Use shielded motor cable for EMC compliance.
• Ground the drive with the shortest possible PE connection; on a multi-drive panel do not daisy-chain the grounds — star them to a common earth bar.

Control terminals

DI1–DI5	Digital inputs 1–5 (opto-isolated)
COM	Common terminal for digital inputs
AI1	Analog input 1, 0–10V (voltage / current selectable)
AI2	Analog input 2, 0–10V or 0/4–20mA
GND	Analog common (0V reference)
+10V	Stable +10V supply for analog potentiometer
AO1/AO2	Analog outputs 1/2, 0–10V or 0/4–20mA
HDO	High-speed pulse output / open collector
CME	Open-collector / relay output common
T1A/T1B/T1C	Relay output: T1A–T1B normally closed, T1B–T1C normally open (contact 3A/250VAC)
A+ / B-	RS485 Modbus communication terminals
P/+24	24V supply / external input control terminals

Two-wire run command (most common)

For a simple run/stop using a maintained switch, use DI1 as forward run and COM. Close the contact to run, open it to stop. Set the run-command source to terminals (P0-02 = 1). For forward/reverse, use DI1 = forward and DI2 = reverse, with a three-position selector so both cannot be closed together.

Sink vs. Source — set it or the DI will not respond

The digital inputs support two wiring logics, selected by the jumper (P/+24) on the control board:

• Sink mode (NPN): the DI switches to COM; internal +24 V sources the input. This is the typical factory setting.
• Source mode (PNP): the DI switches to an external +24 V; the return goes through COM. Use this when wiring to a PLC with PNP outputs.

If a start command appears dead — the keypad shows the terminal input LED not changing — the jumper/logic is almost always mismatched to the PLC output type. Confirm both sides use the same logic and share a common 0 V.

Speed reference wiring

• Potentiometer (manual speed knob): wiper to AI1, one end to +10V, the other to GND. Set AI1 for 0–10V and the frequency source to AI1 (P0-03 = 2).
• 4–20 mA signal (PLC / sensor): signal positive to AI2, return to GND; select current mode on AI2 and set the 4 mA = 0 % / 20 mA = 100 % range.
• Always use shielded twisted-pair for analog signals with the shield grounded at one end only, and run them well clear of the motor cable — analog noise shows up as speed hunting and can even cause false trips.

RS485 Modbus and relay outputs

For Modbus RTU connect the host A to A+ and B to B-, using shielded twisted pair in a bus (daisy-chain, not star) with 120 Ω termination at both ends; set the drive address, baud and data format in the Pd group. The relay terminals T1A/T1B/T1C give a volt-free contact (up to 3 A at 250 VAC) — configure what it signals, such as “running” or “fault”, via parameter. Use it to drive a panel lamp or send a fault signal to the SCADA system.

From our test bench

Before we energise any panel build, we do a simple two-step check: measure continuity from each PE to the earth bar, and confirm with a meter that mains voltage is present on R/S/T and absent on U/V/W. Those thirty seconds catch the two wiring errors that cause the most — and most expensive — failures: a bad ground and swapped power/output terminals.