VFD Training: A Practical Guide to Variable Frequency Drives

VFD training teaches you to select, wire, set up and troubleshoot variable frequency drives, the electronic units that control an AC motor's speed by changing the frequency and voltage it receives. The core skills are nameplate parameters, command and speed-reference sources, PLC control, safe stopping and fault finding. You can learn them free, with no drive needed.

In this lesson by ATO Automation, the question is how to choose a VFD for a given motor. It sits in the "What a VFD Is and Which One to Buy" module of the free VFD course, whose practice task is to choose drives for a 5.5 kW fan and a 1.5 kW conveyor and justify the control method for each. The sections below give you the background to do that task.

How a VFD works

A typical low-voltage drive has three stages:

1. Rectifier: converts the AC supply to DC.
2. DC bus: capacitors smooth and store that DC.
3. Inverter: transistors (IGBTs) switch the DC on and off thousands of times a second, using pulse-width modulation (PWM) to build an AC output whose frequency and voltage the drive controls.

The motor's synchronous speed in rpm is 120 times the frequency divided by the number of poles. A four-pole motor on 50 Hz runs at just under 1,500 rpm; at 25 Hz it runs at about half that. Changing frequency is how the drive changes speed.

V/f and vector control

• V/f (volts per hertz) control keeps voltage in proportion to frequency, so the motor's magnetic flux stays roughly constant. A 400 V, 50 Hz motor sees 8 V per hertz. It is simple, robust and suits fans and pumps.
• Sensorless vector control uses a motor model to control flux and torque separately, giving much better torque at low speed and steadier speed under changing load.
• Closed-loop vector control adds an encoder for precise speed and torque, as on hoists and winders.

A drive is not the only answer. A soft starter only ramps the voltage at start and stop, which suits pumps and fans that run at full speed, and older plants still use the star-delta starter for the same job. Our soft starter vs VFD comparison covers cost, energy and harmonics. A servo drive controls position, for axes that must stop at exact points; servo drives and motion control covers that side. Knowing which one a job needs is one of the first things VFD training should teach.

Parameters you set on every drive

Menus differ between Schneider Altivar, Allen-Bradley PowerFlex, Siemens SINAMICS, ABB and others, but the same settings appear on every drive:

Our VFD start-up and parameter setup walkthrough follows this order on a real drive. Two notes. Motor identification (autotune) measures the motor's electrical characteristics and matters most for vector control. Ramps that are too short cause overcurrent faults on acceleration and overvoltage faults on deceleration, the two faults beginners see most.

Controlling a VFD from a PLC

There are four common methods, from simplest to most capable:

1. Hardwired digital inputs: Run forward, Run reverse, fault reset. Two-wire control uses a maintained contact; three-wire uses separate Start and Stop pulses.
2. Analog speed reference: 0-10 V or 4-20 mA from a PLC analog output. 4-20 mA is better over long cables and lets the drive detect a broken wire.
3. Modbus RTU over RS-485: commands, speed and status over two wires, at modest update rates; see RS-485 wiring explained for termination and biasing.
4. Industrial Ethernet: PROFINET, EtherNet/IP and others. The PLC sends a control word and speed setpoint and reads back status, speed, current and fault codes. Siemens drives use PROFIdrive telegrams; PowerFlex drives in Studio 5000 use add-on profiles.

Whatever the method, wire the drive's fault and ready status back to the PLC so the program knows the difference between "stopped" and "tripped".

Where the energy savings come from

For centrifugal fans and pumps, the affinity laws say that flow is proportional to speed, pressure to speed squared, and power to speed cubed. In the ideal case, running at 80% speed needs about half the power (0.8 x 0.8 x 0.8 = 0.51). Real systems save less, because of static head, motor and drive losses, but the effect is large, which is why drives on fans and pumps are common. The motor and VFD tools in our free engineering calculators let you check the numbers for your own motor; they are on the resources page. A conveyor or positive-displacement pump is a constant-torque load, and slowing it saves far less.

Installation matters

Many drive problems are installation problems:

• Shielded motor cable, with the shield bonded all round at both ends through EMC glands, and a good earth.
• Harmonics: the rectifier draws non-sinusoidal current from the supply. Line reactors or DC chokes reduce it, and [IEEE 519](https://standards.ieee.org/ieee/519/10677/) (current edition 2022) is the standard often used to set limits.
• Long motor cables can cause voltage spikes at the motor; the drive manual gives maximum lengths and when an output filter is needed.
• Braking: loads that drive the motor, such as fast deceleration of a large fan or a lowering hoist, push energy back into the DC bus. A braking resistor or regenerative unit handles it.

Safety: STO is not isolation

Most modern drives have Safe Torque Off (STO), a safety function defined in IEC 61800-5-2 that stops the drive producing torque in the motor. STO is used in machine safety circuits, but it does not isolate the supply. Before working on a drive or motor, isolate and lock off, then wait the discharge time printed on the drive: the DC bus capacitors stay charged after the supply is switched off.

Troubleshooting common faults

Read the fault history before resetting anything. The last few fault codes, with the speed and current at the time, often tell you whether the problem is the drive, the motor, the load or the settings.

Free VFD training on EDWartens

• Variable Frequency Drives: Wiring, Programming, PLC and Network Control (beginner): choosing a drive, wiring, keypad setup, 2-wire and 3-wire control, speed references, PLC control, Modbus RTU and fieldbus on an ATV320, constant-pressure pumping, safety and troubleshooting.
• Allen-Bradley PowerFlex Drives (beginner): PowerFlex 525 parameters, wiring and EtherNet/IP control from Logix.
• Siemens SINAMICS Drives (intermediate): G120 and S120 with STARTER and Startdrive, BICO, STO and SS1.
• Industrial Motor Control (beginner): contactors, overloads and starters, the circuits drives often replace.

The drives and motion courses page and the process control path list related courses. Learning is free in full: credited video lessons from independent YouTube creators, written notes, a practice task per module and a 15-question final assessment (60% to pass, three attempts, then a 24-hour wait). Create a free account so progress is saved.

The optional EDWartens Certificate of Completion is a small one-off fee, US$8.99 for a beginner course. It can be checked at edwartens.com/verification. It is not a Schneider, Rockwell or Siemens certification and is not accredited.

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Variable Frequency Drives: Wiring, Programming, PLC and Network Control

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