Motor drives (VFDs)

Why it matters

Most industrial motors are three-phase induction motors. Connected straight to the supply, they run at one speed. A variable frequency drive (VFD) lets a pump, fan or conveyor run at exactly the speed the process needs, with soft starts and big energy savings. VFDs are everywhere, and they have their own faults, settings and hazards.

Motor speed and frequency

An induction motor's magnetic field rotates at the synchronous speed:

synchronous rpm = 120 × frequency (Hz) / number of poles

A 4-pole motor on 60 Hz: 120 × 60 / 4 = 1,800 rpm. The rotor runs a little slower because of slip, so the nameplate might say 1,750 rpm. On 50 Hz supplies the same motor would turn at 1,500 rpm synchronous.

Change the frequency, and you change the speed. At 30 Hz the same motor runs at about 900 rpm.

Inside a VFD

A typical drive has three stages:

1. Rectifier: diodes convert the incoming three-phase AC into DC.
2. DC bus: large capacitors smooth the DC. On a 480 V drive the bus sits around 650 V DC.
3. Inverter: fast transistors (IGBTs) switch the DC bus on and off thousands of times a second. By varying the width of the pulses (pulse width modulation, PWM), the drive builds an output that acts like a sine wave at whatever frequency and voltage it chooses.

The drive keeps the volts-per-hertz ratio roughly constant so the motor's magnetic flux, and therefore its torque capability, stays steady. A 460 V, 60 Hz motor gets about 230 V at 30 Hz. More advanced drives use sensorless vector or closed-loop vector control for better torque at low speed.

Why plants use VFDs

• Energy: for centrifugal pumps and fans, power falls with the cube of speed (the affinity laws). Running a fan at 80 percent speed uses roughly 0.8³ ≈ 51 percent of full-speed power, compared with throttling a damper.
• Soft starting: the drive ramps up, avoiding the large inrush current and mechanical shock of across-the-line starting.
• Process control: speed follows a setpoint from a PLC or a PID loop, for example holding pipe pressure constant.
• Ramps and braking: controlled acceleration and deceleration, and braking resistors for high-inertia loads.

Control and common parameters

A VFD gets start/stop and speed reference from its keypad, hard-wired terminals (digital inputs and a 0 to 10 V or 4 to 20 mA analog input) or a network (EtherNet/IP, PROFINET, Modbus). Common parameters:

• Motor nameplate data: voltage, current, frequency, rpm, power.
• Minimum and maximum frequency.
• Acceleration and deceleration times.
• Current limit and motor overload settings.
• Stop mode: ramp to stop or coast.

Worked example. A conveyor should run between 20 and 60 Hz, with speed set by a 4 to 20 mA signal. At 12 mA the reference is halfway, so the drive runs at 20 + 0.5 × (60 − 20) = 40 Hz. On a 4-pole motor, synchronous speed is 120 × 40 / 4 = 1,200 rpm.

Faults you will see

• Overcurrent: acceleration too fast, a jammed load, or a short in the motor cable.
• Overvoltage on the DC bus: deceleration too fast with a high-inertia load (the motor acts as a generator). Lengthen decel time or add a braking resistor.
• Undervoltage: supply dips.
• Overtemperature: blocked cooling fans or dirty heat sinks.
• Ground fault: damaged motor insulation or cable.

Drives keep a fault log: read it before resetting. Repeated resets hide a real problem.

Side effects

The fast PWM switching causes electrical noise, voltage spikes on long motor cables and shaft currents that can damage bearings (the fluting you saw in the bearings lesson). Fixes include proper shielded cable, grounding, output reactors and shaft grounding rings. Motors on VFDs should be inverter-duty rated. Self-cooled motors at low speed also cool poorly, because their fans slow down too.

Common mistakes

• Setting a very short deceleration time and getting overvoltage trips.
• Running a standard motor slowly at full torque for long periods, so it overheats.
• Putting a contactor between the drive and motor and opening it while running.
• Resetting faults without reading the log.

Safety

The DC bus capacitors stay charged after the drive is switched off. Drives carry a label stating how long to wait (often several minutes) before opening the cover, and the bus voltage must be verified as zero with a properly rated meter before touching anything. A drive can also restart a motor automatically after a fault reset or power return. Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

Recap

• Synchronous rpm = 120 × Hz / poles; a VFD varies frequency to vary speed.
• Rectifier, DC bus, IGBT inverter with PWM; volts per hertz stays roughly constant.
• Fans and pumps save energy with the cube of speed.
• Read the fault log; respect stored energy in the DC bus.

Try it

Open the Do tab and take the quiz on how VFDs control speed.