Lesson 5.13 — Variable Frequency Drive (VFD) Fundamentals

§01

Overview

A VFD (Variable Frequency Drive) — also called an Adjustable Frequency Drive (AFD), Inverter, or AC Drive — controls the speed of an AC induction motor by varying both the frequency and voltage of the power delivered to it.

VFDs are found on nearly every new piece of industrial equipment and are increasingly common in retrofits. Understanding them is essential for any industrial technician working on process systems, HVAC, conveyors, pumping systems, or compressors.

What a VFD Does — At a Glance

Speed control: Run a motor anywhere from 0 Hz to well above nameplate speed
Energy savings: On variable-torque loads (fans, pumps), power savings can exceed 50%
Smooth starting: Eliminate inrush current spikes and mechanical shock on start
Process control: Maintain precise flow, pressure, or speed setpoints
Motor protection: Built-in overload, overcurrent, overvoltage, and ground fault protection

Industry Context

The U.S. Department of Energy estimates that motors account for approximately 70% of industrial electrical energy consumption. VFDs on variable-torque loads (pumps and fans) represent one of the largest single opportunities for energy savings in industrial facilities. A pump running at 80% speed uses only 51% of the power it would use at full speed — purely from the physics of fluid dynamics.

§02

Learning Objectives

Upon completing this lesson, you will be able to:

L5-13-01
Describe the three power stages of a VFD: converter (rectifier), DC bus, and inverter
L5-13-02
Explain V/Hz (volts per hertz) control and why the ratio must remain constant across the operating speed range
L5-13-03
Identify common VFD parameters: acceleration time, deceleration time, minimum/maximum frequency, motor FLA, and carrier frequency
L5-13-04
Calculate motor output speed from VFD output frequency using the nameplate RPM ratio method
L5-13-05
Describe common VFD fault codes and their causes: OC, OV, UV, OL, GF, OH, PF, SL
L5-13-06
Identify VFD input/output wiring connections — main power, control terminals, and communication
L5-13-07
Explain common VFD hazards: DC bus stored voltage, harmonics on output, and reflected wave voltage on long cable runs
L5-13-08
Perform a basic VFD startup commissioning checklist for a new installation

§03

Prerequisites

Complete the following lessons before beginning this one:

5.3 — AC Fundamentals 5.7 — Three-Phase Power 5.11 — Power Supplies & Rectifiers 5.12 — Motors & Motor Starters

Key Knowledge Required

You should be comfortable with AC voltage and frequency concepts, how a 3-phase induction motor develops torque, and the difference between AC and DC circuits. If those concepts are shaky, review the prerequisite lessons first — VFDs combine all of them simultaneously.

§04

Why VFDs Exist — The Energy Case

The Old Way: Fixed Speed + Throttling

Traditional pump and fan systems ran at full speed constantly. To control flow, operators partially closed a valve or damper. The motor kept working at nearly full power — all that extra energy was wasted as pressure drop and heat across the valve. It is the mechanical equivalent of driving with one foot on the accelerator and one on the brake at all times.

The VFD Way: Match Speed to Demand

A VFD slows the motor when less flow is needed. Because of the Affinity Laws for centrifugal fans and pumps, even small reductions in speed produce very large reductions in power consumption.

The Affinity Laws for Centrifugal Loads

Flow ∝ Speed | Pressure ∝ Speed² | Power ∝ Speed³ Affinity Laws — valid for centrifugal fans and pumps

Flow is proportional to speed — halve the speed, halve the flow
Head / Pressure is proportional to speed squared — halve the speed, get only one-quarter the pressure
Power is proportional to speed cubed — halve the speed, use only 1/8 of the power

Worked Example

A pump motor running at 100% speed uses 100 kW. If a VFD reduces speed to 60%:
Power = 0.60³ × 100 kW = 0.216 × 100 = 21.6 kW
That is a 78.4% energy reduction — while still delivering 60% of rated flow. Compare that to throttling a valve at full speed, which still consumes nearly 100 kW.

Interactive — Affinity Laws Simulator

Drag the slider to see how flow, pressure, and power change with motor speed according to the affinity laws.

75%

Motor Speed (% of rated)

25%100% (Full Speed)

Flow

75%

∝ Speed¹

Pressure / Head

56%

∝ Speed²

Power Consumed

42%

∝ Speed³

Note: Affinity laws apply only to centrifugal loads (fans, pumps, blowers). Constant-torque loads (conveyors, positive-displacement pumps, compressors) have different power vs. speed relationships.

§05

VFD Internal Power Stages

Every standard VFD contains three fundamental power stages. Understanding each stage is essential for safe work and systematic troubleshooting.

VFD Power Stage Block Diagram — 3ϕ AC Input to Motor Output

■ Stage 1: Converter (Rectifier) — AC to DC ■ Stage 2: DC Bus — Filter and Store ■ Stage 3: Inverter (IGBTs) — DC to Variable AC

1

Converter — Rectifier Section

Takes 3-phase AC input (typically 480 VAC, 60 Hz) and converts it to pulsating DC using a full-wave diode bridge (6 diodes in a 3-phase bridge configuration). Peak DC output voltage is approximately 1.35 × V_L-L = 648 VDC peak for 480 V input. Filter capacitors on the DC bus smooth this ripple to approximately 675 VDC steady DC. Diodes are passive components, so this stage generates relatively little heat.

2

DC Bus — Intermediate Storage

Maintains a stable intermediate DC voltage. Contains large electrolytic capacitors — the source of dangerous stored charge after power-off. DC bus voltage varies with input voltage. The drive continuously monitors bus voltage:

Bus drops below threshold → UV (Undervoltage) fault
Bus spikes above threshold from regenerative braking → OV (Overvoltage) fault

Some drives include a DC bus choke (series inductor) to reduce harmonic distortion fed back into the AC input supply.

3

Inverter — Output Section (IGBTs)

Six IGBT (Insulated Gate Bipolar Transistor) power transistors arranged in a 3-phase bridge. These high-speed switches turn on and off at the carrier frequency (typically 4–16 kHz) using Pulse Width Modulation (PWM). By varying the on/off duty cycle, the inverter synthesizes a variable-frequency, variable-voltage AC waveform that the motor interprets as approximately sinusoidal.

Higher carrier frequency → smoother motor operation, less audible noise, more drive heat
Lower carrier frequency → more audible motor hum (at carrier harmonics), less drive heat

⚠ Safety — DC Bus Discharge Time

After removing input power, the DC bus capacitors can retain dangerous voltage for several minutes. Always wait a minimum of 5 minutes and then measure DC bus voltage at the designated test points (usually labeled DC+ and DC−) with a CAT III rated meter before touching internal components. The display going dark means the low-voltage control supply has discharged — the main bus may still be at 600+ VDC.

§06

V/Hz Control — Volts per Hertz

An AC induction motor's magnetic flux (and therefore its torque-producing capability) depends on the ratio of voltage to frequency — V/Hz. If this ratio changes outside the design range, problems occur:

Frequency drops, voltage stays high: V/Hz ratio increases → magnetic flux saturates → excessive magnetizing current → motor overheats, may trip thermal protection or destroy winding insulation
Frequency drops, voltage drops too much: V/Hz ratio decreases → insufficient flux → reduced torque → motor may stall or slip excessively under load

The VFD must maintain a constant V/Hz ratio throughout the operating speed range — both voltage and frequency are adjusted together in proportion.

V/Hz = V_rated ÷ f_rated = constant Output voltage must scale proportionally with output frequency throughout the speed range

V/Hz Example — 480 V / 60 Hz Motor

Nameplate: 480 V at 60 Hz → V/Hz ratio = 480 ÷ 60 = 8.0 V/Hz

VFD Output Frequency	Required Output Voltage	Calculation	Motor Speed (%)
15 Hz	120 V	15 × 8.0	25%
30 Hz	240 V	30 × 8.0	50%
45 Hz	360 V	45 × 8.0	75%
60 Hz	480 V	60 × 8.0 (full voltage)	100%

Above 60 Hz — Field Weakening Region

The VFD cannot output more than 480 V (limited by DC bus voltage and motor rating). Above 60 Hz, voltage is held constant at 480 V while frequency continues to increase. The V/Hz ratio therefore decreases, magnetic flux decreases, and available torque decreases. The motor enters constant-power mode — higher speed but proportionally lower torque. Most VFDs allow up to 120 Hz output, but torque at 120 Hz is only 50% of rated torque. This is acceptable for some fan and blower applications but not for constant-torque loads.

Technician Tip — Low-Speed Voltage Boost

Many VFD manufacturers offer a "V/Hz boost" or "IR compensation" feature that slightly raises the output voltage at very low frequencies (below ~10 Hz). This compensates for stator winding resistance voltage drop at low speeds, improving low-speed starting torque. If a loaded motor stalls at low speed despite correct parameter settings, check whether a voltage boost can be enabled. Do not confuse this intentional feature with a misconfigured V/Hz curve.

§07

Key VFD Parameters

Every VFD manufacturer uses different parameter numbering and naming conventions, but the fundamental parameters below are present on every drive. Knowing what each parameter controls allows you to program any drive with its manual in hand — you are looking for the function, not the number.

Parameter	Typical Range / Values	What It Does
Motor FLA	Set to motor nameplate FLA (amps)	Sets the overload (OL) protection threshold. The drive uses this to calculate I²t thermal accumulation. Critical — must match motor nameplate exactly for the connected winding configuration.
Motor Voltage	208 / 230 / 460 / 480 / 575 V	Sets the rated motor voltage. Must match motor nameplate for connected winding. Determines the V/Hz ratio baseline used by the drive.
Motor Poles	2 / 4 / 6 / 8	Number of motor poles from nameplate. Required for accurate RPM display on the keypad and for some advanced control modes.
Maximum Frequency	50–120 Hz	Upper speed limit. Normally set to motor nameplate Hz (60 Hz). Can be increased for overspeed applications — consult motor manufacturer for overspeed rating before exceeding nameplate frequency.
Minimum Frequency	0–30 Hz	Lower speed limit. Prevents motor from running too slowly (loss of shaft-mounted cooling fan effectiveness, stall risk). Typically 10–15 Hz for pump/fan applications. Set to 0 for hoists or positioning applications.
Acceleration Time	0.1–6,000 sec	Time for the drive to ramp from 0 Hz to maximum frequency after a Start command. Too short → OC fault (current spike). Too long → slow process response. Fans/pumps: 30–60 sec typical.
Deceleration Time	0.1–6,000 sec	Time to ramp from maximum frequency to 0 Hz on a Stop command. Too short on high-inertia loads → OV fault (DC bus overvoltage from regeneration). Add a dynamic braking resistor for fast decel of high-inertia loads.
Carrier Frequency	2–16 kHz	IGBT switching frequency. Higher → quieter motor operation, smoother torque ripple, but more drive heat generation (may require derating current). Lower → audible motor hum, less drive heat.
Reference Source	Keypad / 0–10 V / 4–20 mA / Network	Where the speed setpoint (frequency command) comes from. Keypad = local manual control. Analog input = remote signal from PLC or process transmitter. Network = Modbus RTU, EtherNet/IP, etc.
Control Source	Keypad / Terminal Strip / Network	Where Run and Stop commands originate. Must match the physical control method designed into the panel. Mismatched control source is a common commissioning error that prevents the drive from starting via remote control.

⚠ Critical — Motor FLA Setting

The Motor FLA parameter is the single most important commissioning step. Set it too high and the drive will not protect the motor from overload — the motor will overheat and fail. Set it too low and the drive will nuisance-trip on normal load. Always set it to the nameplate FLA for the actual operating voltage. Many motors have dual-voltage ratings (e.g., 230/460 V) — verify which winding configuration is connected (Y vs. delta) before setting FLA.

§08

VFD Speed Calculation

The exact motor output speed depends on synchronous speed minus slip. For practical troubleshooting and commissioning work, use the simplified ratio method with nameplate data:

N_output ≈ N_nameplate × (f_VFD / f_rated) Simplified VFD Speed Formula — nameplate ratio method (accurate to within slip, typically <3%)

N_sync = 120 × f / Poles N_rated = N_sync × (1 − slip) Exact calculation — slip typically 2–5% at full load for standard induction motors

Example Calculations

Motor: 4-pole, nameplate 1,760 RPM at 60 Hz (slip ≈ 2.2% at full load)

VFD Set Frequency	Calculated Speed (Ratio Method)	% of Rated Speed
60 Hz (full)	1,760 × (60/60) = 1,760 RPM	100%
45 Hz	1,760 × (45/60) = 1,320 RPM	75%
30 Hz	1,760 × (30/60) = 880 RPM	50%
15 Hz	1,760 × (15/60) = 440 RPM	25%
90 Hz (overspeed)	1,760 × (90/60) = 2,640 RPM	150% (field weakening)

Interactive — VFD Speed Calculator

Enter motor nameplate data and VFD output frequency to calculate motor speed and approximate centrifugal load process values.

Nameplate RPM

Rated Frequency

VFD Set Frequency (Hz)

§09

Common VFD Fault Codes

VFD fault codes are standardized enough across brands that you can diagnose most drives using the same systematic approach. Codes vary slightly by manufacturer — always confirm with the drive's manual — but the underlying causes and first-checks below apply universally.

Code	Fault Name	Common Causes	First Check / Fix
OC	Overcurrent	Mechanical jam or locked rotor; short in motor cable or windings; acceleration time too short; motor undersized for actual load	Extend accel time (most common fix). Megger test motor cable and winding insulation. Check for mechanical jam in driven equipment. Verify motor HP vs. load.
OV	Overvoltage (DC bus too high)	Decel time too short for load inertia; high-inertia load regenerating energy into DC bus; input line voltage too high	Extend decel time. For high-inertia loads (large fan, flywheel), add a dynamic braking resistor. Check input supply voltage.
UV	Undervoltage (DC bus too low)	Input power interrupted or voltage sag; blown input fuse; loose connection on input terminals; upstream breaker tripped	Measure all 3 input phase voltages L-L. Check input fuses (all three) and tighten input terminal connections. Check upstream overcurrent device.
OL	Overload (thermal)	Motor mechanically overloaded; motor too small for application; Motor FLA parameter set too low; ambient temperature too high	Measure output current with TRUE RMS clamp meter vs. motor nameplate FLA. Investigate mechanical load. Verify Motor FLA parameter is set correctly.
GF	Ground Fault	Damaged motor cable insulation (conductor to shield/ground); motor winding insulation failure; moisture in motor or junction box	Disconnect motor at drive output terminals (U/V/W). Megger test cable and motor separately. Identify and repair the insulation fault before reconnecting.
OH	Overheat (heatsink)	Drive cooling fan failed; ventilation inlet blocked; high ambient temperature; carrier frequency set too high causing extra losses; drive undersized	Verify drive cooling fan is running. Clear ventilation. Reduce carrier frequency setting by 2–4 kHz. Confirm ambient temperature is within drive specification.
PF	Phase Loss / Phase Fault	One or more input phases absent; blown input fuse on one phase; loose terminal connection; single-phasing on upstream source	Measure all three input phase voltages L-L and L-N. Check all input fuses. Inspect and tighten input terminal screws and upstream connections.
SL	Speed Loss / Stall	Motor could not reach commanded speed within timeout; belt slipping or broken; excessive mechanical load; ramp time too short for high-inertia load	Check driven equipment for jams or mechanical binding. Inspect belt/coupling. Increase acceleration time. Verify load requirements vs. motor and drive sizing.

Diagnostic Tip — Fault History Log

Most VFDs store the last 5–10 faults in a fault history log with timestamp and output current/voltage recorded at the moment of fault. Always check the fault history before clearing a fault — patterns tell you everything. OC always at startup? Accel time. OV always at 30 seconds into decel? Fast ramp on high-inertia load. Access the fault history through the drive keypad parameter menu (typically labeled "Fault Log" or "Event Log").

Fault Diagnostic Sequence

When you arrive at a tripped VFD, use this sequence every time:

Record the fault code before clearing — and check the fault history for pattern
Note the output current at fault — was it at FLA? At 200%? This tells you severity
Understand the cause before clearing — clearing without understanding usually means it trips again immediately
Clear and restart only when the cause is identified and corrected or is understood to be intermittent
Monitor while running — watch output current, frequency, and DC bus voltage during the startup sequence

§10

VFD Wiring Overview

VFD wiring has two distinct sections: main power wiring (high voltage, heavy gauge) and control wiring (low voltage signal conductors). These must always be kept separated — never run them in the same conduit.

Main Power Terminals

R / L1 480 V 3-phase AC input, Phase A

S / L2 480 V 3-phase AC input, Phase B

T / L3 480 V 3-phase AC input, Phase C

U / T1 3-phase AC output to motor, Phase A

V / T2 3-phase AC output to motor, Phase B

W / T3 3-phase AC output to motor, Phase C

DC+ / DC− DC bus test points; braking resistor terminals

PE / GND Earth / safety ground connection

Control Terminals (Typical)

COM Control common (0 V signal reference)

FWD Forward run — dry contact closed to COM starts drive

REV Reverse run — dry contact to COM for reverse direction

AI1 Analog speed reference input (0–10 V or 4–20 mA)

AO Analog output — speed or current feedback (4–20 mA)

RO1A/B/C Relay output — fault alarm or run status (SPDT contact)

+24V Drive-supplied 24 VDC for digital input power (some drives)

Critical Wiring Rules

1

Separate control wiring from power wiring in all conduits. Never install control wiring (AI, FWD, REV, AO signals) in the same conduit as power wiring (input R/S/T or output U/V/W). The VFD output generates severe electromagnetic interference that will corrupt analog speed signals and cause erratic speed commands, unstable control, or nuisance trips. Maintain at least 6 inches of physical separation or use shielded twisted-pair control cable routed perpendicular to power conduits where crossings are unavoidable.

2

Size input overcurrent protection per NEC 430.52 for VFD input current. Install input fuses or a circuit breaker sized for VFD input current (which is higher than motor FLA due to harmonic content). Follow the drive manufacturer's recommendation — typically 150–200% of the drive's input current rating. Do not size based on motor FLA alone.

3

Use shielded cable for output runs over 50 feet. For output cable runs over 50 feet, use shielded cable (dedicated VFD cable preferred; THHN in metallic conduit is acceptable as an alternative). Connect the shield to ground at the drive end only. Grounding both ends creates a ground loop that can cause interference and nuisance trips.

4

Never install power factor correction capacitors on the VFD output side. PFC capacitors are designed for 60 Hz sinusoidal sources. The VFD output is PWM at 4–16 kHz, which will cause the capacitors to resonate at the carrier frequency, draw destructively high currents, and destroy themselves — and potentially damage the drive's IGBT output stage in the process.

5

Output contactors must only switch when the drive output is at zero Hz. Installing a bypass contactor between the VFD output and motor is sometimes done, but switching an output contactor under load produces an arc and voltage spike that can damage the drive's IGBTs. Interlock the bypass contactor electrically and in the control program so it can only open or close when drive output frequency is confirmed at zero.

⚠ Bypass Mode Caution

Many VFD panels include a bypass contactor that allows the motor to run across-the-line (DOL) when the VFD is faulted. Always confirm the system operating mode before working. A bypass-mode motor appears identical from the load side, but it starts with full inrush current, always runs at full speed, and is protected only by the motor overload relay — not the VFD's electronic protection.

§11

Reflected Wave Voltage — Long Cable Runs

This concept becomes critically important on installations where the VFD and motor are separated by long cable runs — a common situation in industrial plants where drives are in a central MCC and motors are distributed throughout the facility.

The Problem

The VFD output produces rapid voltage pulses (PWM switching, typically with rise times of 0.1–1 microsecond). On long cables, these pulses travel from the drive to the motor and encounter an impedance mismatch at the motor terminals (motor windings present high impedance at MHz frequencies compared to the cable's characteristic surge impedance). The voltage pulse reflects back toward the drive, and the incident pulse and reflected pulse add together at the motor terminals.

V_{reflected peak} ≈ 2 × V_{DC bus} ≈ 2 × 675 V ≈ 1,350 V Worst-case peak voltage at motor terminals — 480 V VFD on a long cable run

Why It Matters

Standard motors have 600 V insulation class — repeated 1,350 V spikes accelerate insulation degradation, causing premature motor failure (often after 2–5 years vs. 20+ year normal life)
The problem worsens with faster IGBT switching (higher dV/dt) and longer cable length
Short runs (under 50 ft at 480 V) rarely cause issues with modern drives
The reflected wave also radiates EMI that can disrupt sensors and control systems nearby

Solutions (in order of cost and effectiveness)

Inverter-duty motor (NEMA MG1 Part 31 rated): Motor insulation rated for 1,600 V peak. This should be standard practice on all new VFD installations regardless of cable length.
Output line reactor (series inductor): Slows the rise time of voltage pulses (reduces dV/dt), reducing peak voltage at the motor. Cost-effective. Install between VFD output terminals and the motor cable. Required when cable run exceeds 100 ft at 480 V with a standard motor.
dV/dt filter: More aggressive RC or LC filter that more significantly reduces both the voltage peak and rise time. Used on very long runs or particularly sensitive equipment.
Sine wave filter: Converts the PWM output to a true sinusoidal waveform. Completely eliminates reflected wave issues and allows use of standard (non-inverter-duty) motors. Most expensive option but provides the best motor protection and lowest EMI.

Field Rule of Thumb for 480 V Systems

Under 50 ft: no action required with modern drives and inverter-duty motor.
50–100 ft: inverter-duty motor minimum; consider output reactor.
Over 100 ft: output line reactor required; inverter-duty motor required.
Over 200 ft or very fast switching: dV/dt filter or sine wave filter recommended.

§12

Basic VFD Commissioning Checklist

Use this checklist when commissioning a new VFD installation or replacing a failed drive. Complete all 13 items before declaring the installation ready for service. Click or tap each item to mark it complete.

Checklist Progress 0 / 13 complete

After Commissioning

Document all completed parameter settings in the panel drawing set or equipment file. Take a photograph of the keypad parameter listing if the drive has a printable summary screen. Write the key parameters (FLA, accel, decel, max Hz, min Hz, reference source, control source) on a label inside the panel door. Future technicians — including you six months from now — will thank you for this.

§13

Knowledge Check — Assessment

Answer all five questions. Immediate feedback is provided after each submission. If you miss a question, review the indicated section before proceeding.

Q1 — A 480 V VFD has its DC bus charged. The SAFE waiting time after disconnecting input power before opening the drive enclosure is:

A. 30 seconds — the large capacitors discharge quickly once power is removed B. Wait until the keypad display goes completely dark, then it is safe to open the enclosure C. At least 5 minutes minimum, then verify DC bus voltage with a CAT III meter at the designated test points before touching any internal components D. 1 minute per 100 V of bus voltage (approximately 7 minutes for a 675 VDC bus)

Q2 — A pump motor running at 100% speed uses 50 kW. Using the affinity laws, approximately how much power does it consume at 50% speed?

A. 25 kW — power is directly proportional to speed B. 12.5 kW — power is proportional to speed squared C. 6.25 kW — power is proportional to speed cubed D. 1 kW — power decreases exponentially at low speeds

Q3 — A VFD is controlling a 4-pole motor with a nameplate of 60 Hz, 1,760 RPM. The VFD is set to output 45 Hz. What is the approximate motor output speed?

A. 1,760 RPM — an induction motor always runs at nameplate speed regardless of VFD frequency B. 1,320 RPM C. 2,200 RPM — reducing frequency increases motor speed D. 880 RPM

Q4 — A VFD trips with an OV (Overvoltage) fault every time it decelerates from full speed to a stop. The most likely cause is:

A. The input supply voltage is consistently too high, overcharging the DC bus when the drive is stopped B. The motor is too large for the drive and draws excessive current during the deceleration ramp C. The deceleration time is too short — the high-inertia load is regenerating energy back into the DC bus faster than it can be dissipated, spiking bus voltage above the OV threshold D. The Motor FLA parameter is set too low, causing the drive to misidentify the deceleration current as an overload condition

Q5 — Why should power factor correction (PFC) capacitors NEVER be installed on the output side (between VFD and motor) of a variable frequency drive?

A. They will unnecessarily reduce power factor below 1.0, increasing energy consumption B. They will generate interference on the 4–20 mA analog control signal connected to the drive's AI1 terminal C. The VFD output is PWM at 4–16 kHz — capacitors present very low impedance at these frequencies, creating destructive resonance that draws enormous current and destroys the capacitors and potentially the drive's output IGBT stage D. Capacitors are passive components that are only electrically functional on the input (supply) side of electrical equipment

§14

Summary & Key Takeaways

01

Three Stages: Converter → DC Bus → Inverter

Every VFD converts 3-phase AC to DC (converter/rectifier stage), stores and smooths it on a capacitive DC bus (~675 VDC for 480 V input), then uses IGBTs switching at carrier frequency via PWM to generate variable-frequency AC output. The DC bus retains lethal voltage after power-off and must be verified with a meter before touching internal components.

02

V/Hz Ratio Must Remain Constant

The VFD adjusts both voltage and frequency together to maintain constant V/Hz ratio (8.0 V/Hz for a 480 V/60 Hz motor). Above rated frequency, voltage is capped at nameplate — motor enters field weakening with reduced torque at higher speed.

03

Affinity Laws: Power Scales as Speed Cubed

For centrifugal fans and pumps: Power ∝ Speed³. Running at 50% speed uses only 12.5% of full-speed power. Running at 80% speed uses only 51.2% of full-speed power. This cube relationship is the primary energy savings mechanism and economic justification for VFD retrofits.

04

Safety: DC Bus and TRUE RMS Measurement

Wait 5+ minutes after disconnect, then verify DC bus with a CAT III meter before touching anything inside the drive enclosure. Always use a TRUE RMS clamp meter on VFD output leads — standard averaging meters give incorrect readings on PWM waveforms and will not show true current.

05

Fault Codes Tell the Story

OC = current spike (jam, short, or fast accel). OV = DC bus too high (fast decel or regeneration). UV = input power problem. OL = motor overloaded or FLA set wrong. GF = insulation fault. OH = drive cooling problem. Always check fault history before clearing.

Next Lesson

5.14 — Control Circuits: Ladder logic, relay logic, and PLC basics as applied to motor control circuits. You will learn how the Start, Stop, Forward, and Reverse signals that command a VFD are generated by the control circuit elements covered in that lesson — completing the picture from power distribution through process control.