How to Connect an AC Inverter to an Electric Motor

Connecting an AC inverter (VFD) to an electric motor is a critical step that directly affects motor performance, efficiency, safety, and lifespan. Incorrect wiring or poor parameter setup can cause overheating, nuisance trips, torque loss, or even permanent damage to the inverter or motor.

This guide provides a step-by-step, practical explanation of how to correctly wire, configure, and test an AC inverter with an electric motor. It also covers control wiring, communication signals, advanced setup parameters, real equations, and hands-on project examples, making it suitable for beginners and technicians alike.

Table of Contents

1. Pre-Installation Checks (Do Not Skip)

Before touching any wires, verify the following:

1.1 Match Inverter and Motor Ratings

Always compare the motor nameplate with the inverter specifications.

Check:

Motor voltage (V)
Motor current (A)
Motor frequency (Hz)
Motor power (kW or HP)
Motor speed (RPM)

Rule:

The inverter current rating must be greater than or equal to the motor rated current.

Example:
If motor rated current = 8.2 A, inverter output current should be ≥ 8.2 A (preferably with margin).

1.2 Power Supply Compatibility

Verify:

Single-phase or three-phase input
Input voltage range (e.g., 220–240 V or 380–415 V)
Proper grounding system

⚠️ Never connect a three-phase inverter to a single-phase motor unless explicitly supported.

2. Power Wiring: Inverter to Motor

2.1 Main Power Terminals

Most AC inverters have the following terminals:

Terminal	Description
R / L1	Input phase
S / L2	Input phase
T / L3	Input phase
U	Motor output
V	Motor output
W	Motor output
PE / ⏚	Ground

2.2 Input Power Wiring

Connect the mains supply to the inverter input terminals:

Single-phase: L → R/L1, N → S/L2
Three-phase: R → L1, S → L2, T → L3

Always install:

MCCB or MCB
Contactor (optional)
Input fuse (recommended)

2.3 Motor Output Wiring

Connect the motor directly to:

U → Motor terminal 1
V → Motor terminal 2
W → Motor terminal 3

⚠️ Do NOT place a contactor between inverter and motor while the inverter is running.

2.4 Grounding (Very Important)

Ground inverter body to earth
Ground motor frame to earth
Use short, thick ground cables

Proper grounding:

Reduces electrical noise
Prevents inverter faults
Protects personnel

3. Motor Terminal Connection (Star / Delta)

Motor terminal configuration depends on voltage.

Example Motor Nameplate:

If inverter output = 230 V → Delta (Δ)
If inverter output = 400 V → Star (Y)

Incorrect connection will cause:

Overcurrent
Low torque
Motor overheating

4. Control Wiring Basics

Control wiring allows you to start, stop, change speed, and direction without using the keypad.

4.1 Digital Inputs (DI)

Typical terminals:

DI1: Start
DI2: Stop
DI3: Forward / Reverse
COM: Common

Example:

Push button between DI1 and COM → Start motor

4.2 Analog Speed Control (0–10V / 4–20mA)

Most inverters support analog speed reference.

Terminals:

AI: Analog input
+10V: Reference supply
GND / COM

Potentiometer Wiring (10kΩ):

One end → +10V
Middle → AI
Other end → GND

Speed equation:

Example:

AI = 5 V
Max frequency = 50 Hz
Output = 25 Hz

5. AC Inverter Control Wiring

5.1. Buttons Control + Speed Control by Inverter Keypad

Application

Training panels
Commissioning and testing
Simple machines

Wiring Concept

Start / Stop via external buttons
Speed reference from inverter keypad

Control Wiring

Start button → DI1 to COM
Stop button → DI2 to COM

Parameter Settings

Command source = Terminal (DI)
Speed reference = Keypad
Forward/Reverse = Optional DI or keypad

Operation Logic

Operator presses Start button
Inverter runs
Speed is adjusted using ▲ / ▼ keys on keypad

Advantages

Very simple
No analog wiring
Good for beginners

Limitations

Operator must access inverter keypad
Not suitable for remote control

5.2. Buttons Control + Speed Control by Internal Potentiometer (Keypad Pot)

Application

Small machines
Local manual speed adjustment

Wiring Concept

Start/ Stop→ External buttons
Speed → Built-in inverter potentiometer

Control Wiring

Start→ STF
Stop→ STR
PC→ Common

(No analog wiring required)

Parameter Settings

Run command = Terminal
Speed source = Internal potentiometer

Operation Logic

Press Start button
Rotate inverter knob to increase/decrease speed

Advantages

No extra components
Fast setup

Limitations

Manual control only
Limited precision

5.3. Buttons Control + Speed Control by External Potentiometer

Application

Conveyors
Mixers
Packaging machines

Wiring Concept

Start / Stop → Push buttons
Speed → External 10kΩ potentiometer

Wiring Diagram Logic

Pot terminal 1 → +10V
Pot terminal 2 (wiper) → AI
Pot terminal 3 → GND

Control Wiring

Start → DI1
Stop → DI2
COM → Common

Parameter Settings

Command source = Terminal
Speed reference = Analog input (0–10 V)
Max frequency = 50 or 60 Hz

Speed Equation

Advantages

Remote speed control
Smooth adjustment

Limitations

Analog noise possible
Cable shielding required for long distances

5.4. Buttons Control + Speed Control by Multi-Speed (Preset Frequencies)

Application

Sorting conveyors
Indexing tables
Machines with fixed speeds

Wiring Concept

Start / Stop → Push buttons
Speed → Multiple digital inputs

Example Wiring

Input	Function
DI1	Start
DI2	Stop
DI3	Speed 1
DI4	Speed 2
DI5	Speed 3

Parameter Settings

Speed mode = Multi-step / Preset speed
Preset frequency 1 = 15 Hz
Preset frequency 2 = 30 Hz
Preset frequency 3 = 50 Hz

Logic Table Example

DI3	DI4	Speed
0	0	Speed 1
1	0	Speed 2
0	1	Speed 3

Advantages

No analog signals
High repeatability
Immune to noise

Limitations

Fixed speeds only
Requires more digital inputs

5.5. Buttons Control + Speed Control by Modbus Communication

Application

PLC-controlled systems
SCADA integration
Centralized control panels

Wiring Concept

Start / Stop → Push buttons (backup/manual)
Speed → PLC via Modbus

Communication Wiring

RS-485 D+ → Inverter D+
RS-485 D− → Inverter D−
Shield grounded at one end

Parameter Settings

Command source = Communication
Speed reference = Communication
Modbus address = Unique
Baud rate = 9600 / 19200

Data Flow

PLC writes frequency command
PLC reads inverter status & faults

Advantages

Full automation
Remote monitoring
Scalable system

Limitations

Requires PLC programming
Communication configuration needed

5.6. Buttons Control + Speed Control by Profibus Communication

Application

Siemens PLC systems
Large industrial plants
High-speed deterministic control

Wiring Concept

Start / Stop → Hardwired buttons (safety/manual)
Speed → PLC via Profibus

Profibus Wiring

Shielded Profibus cable
Termination resistors at both ends
Proper grounding

Parameter Settings

Control mode = Fieldbus
Speed reference = Profibus
GSD file installed in PLC

Data Exchanged

Control word (Start/Stop/Reset)
Speed setpoint
Status word
Fault codes

Advantages

High reliability
Fast communication
Industry standard

Limitations

Higher cost
More complex setup

5.7. Hybrid Control (Buttons + Communication Backup)

Typical Use Case

Normal operation via PLC
Emergency or manual mode via buttons

Logic

Selector switch chooses control mode
DI enables terminal or communication

Benefits

Redundancy
Maintenance-friendly
Safer operation

5.8. Comparison Table of Control Methods

Method	Wiring	Accuracy	Noise Immunity	Typical Use
Keypad	Very Low	Medium	High	Testing
Internal Pot	Low	Medium	High	Small machines
External Pot	Medium	High	Medium	Conveyors
Multi-Speed	Medium	Very High	Very High	Fixed speeds
Modbus	Medium	Very High	High	Automation
Profibus	High	Very High	Very High	Industrial plants

5.9. Practical Recommendation

Training / Testing → Keypad or internal pot
Manual machines → External potentiometer
Repeatable speeds → Multi-speed
Automation systems → Modbus
Siemens environments → Profibus

6. Basic Motor Parameters (Must Be Set)

After wiring, program these mandatory parameters.

6.1 Motor Rated Voltage

Set according to motor nameplate.

Example:

6.2 Motor Rated Current

Used for overload protection.

Example:

6.3 Motor Rated Frequency

Usually:

50 Hz
60 Hz

6.4 Motor Rated Speed or Poles

Used for slip compensation and torque control.

Equation:

Example:

50 Hz
4 poles

7. Acceleration and Deceleration Setup

7.1 Acceleration Time

Time to reach target speed.

Too short → Overcurrent fault
Too long → Slow response

Example:

7.2 Deceleration Time

Time to stop motor.

If too short:

DC bus overvoltage
Need braking resistor

8. Advanced Motor Control Parameters

8.1 V/f Curve Adjustment

Used to optimize torque.

Linear V/f → Standard loads
Quadratic V/f → Fans and pumps
Custom V/f → Heavy starting loads

8.2 Torque Boost

Increases voltage at low frequency.

Useful for:

Conveyors
Crushers
Mixers

Example:

Too high → Motor overheating

8.3 Slip Compensation

Maintains speed under load.

Equation:

Higher slip compensation improves torque but increases current.

9. Direction Control and Interlocks

Forward / Reverse is controlled by:

Digital input
Keypad
Communication command

⚠️ Always stop motor before reversing direction.

10. Communication Between Inverter and Control Systems

10.1 Common Communication Protocols

Modbus RTU (RS-485)
Modbus TCP
Profibus
CANopen
Ethernet/IP

10.2 RS-485 Wiring (Modbus RTU)

Terminals:

D+ (A)
D− (B)
GND (optional)

Rules:

Use twisted pair cable
Shielded cable preferred
Termination resistor (120Ω) at last device

10.3 Communication Parameters

Set:

Slave address
Baud rate (e.g., 9600 / 19200)
Parity
Stop bits

Example:

10.4 Data Exchanged via Communication

You can:

Start / stop motor
Set frequency
Read current, voltage, faults
Monitor temperature

11. Protection and Safety Settings

11.1 Overload Protection

Set motor thermal protection:

11.2 Overvoltage and Undervoltage

Protects inverter during:

Regeneration
Supply fluctuation

11.3 Stall Protection

Stops motor if:

Speed = 0
Current is high

Useful for jammed machines.

12. Testing and Commissioning Procedure

Step-by-Step Test

Disconnect motor
Power inverter
Check no fault
Connect motor
Run at 5–10 Hz
Check rotation direction
Increase speed gradually
Monitor current and temperature

13. Practical Mini Project

Project: Conveyor Belt with Speed Control

Components:

1.5 kW motor
AC inverter
Potentiometer
Start/Stop push buttons

Steps:

Wire motor to U-V-W
Wire pot to analog input
Set motor parameters
Acceleration = 15 sec
Max frequency = 50 Hz

Result:

Smooth start
Adjustable speed
Energy efficient operation

14. Common Wiring Mistakes to Avoid

Switching motor output while inverter is running
Incorrect motor star/delta connection
Poor grounding
Using contactor on inverter output
Incorrect motor current setting

15. Best Practices for Reliable Operation

Always follow motor nameplate
Keep control and power cables separate
Use shielded cables
Save inverter parameters after setup
Perform regular inspections

Conclusion

Correctly connecting an AC inverter to an electric motor requires more than just wiring three cables. Proper setup involves accurate parameter configuration, control wiring, communication integration, and protection settings. When done correctly, the system delivers smooth operation, precise speed control, lower energy consumption, and extended equipment life.

This guide provides a complete, practical foundation you can confidently apply to real industrial projects or educational setups.