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BLD-ELEC05-06 · Lesson 9 · 26 clock hours · demo

Variable-Frequency Drive (VFD) — Principles and Parameter Setting

Objective. Students program a variable-frequency drive with basic parameters—motor nameplate data, acceleration/deceleration ramps, minimum and maximum frequency—and run a motor through a defined speed profile.

What a Variable-Frequency Drive Does

A variable-frequency drive (VFD) controls the speed of an AC induction motor by changing the frequency and voltage of the power it delivers. Because the synchronous speed of an AC motor is directly proportional to supply frequency, reducing frequency reduces speed; increasing frequency increases speed. This lets a single motor run a pump, fan, or conveyor at precisely the speed the process requires, saving energy and reducing mechanical wear compared with fixed-speed operation.

Inside the VFD: Three Sections

Rectifier section. Incoming three-phase AC (typically 208–240 V or 480 V) passes through a bridge rectifier made of diodes or SCRs. The rectifier converts AC to pulsating DC.

DC bus. Large filter capacitors (and sometimes inductors) smooth the pulsating DC into a steady DC bus voltage. The bus voltage is roughly 1.35 times the line-to-line AC input voltage. This bus is the energy reservoir the inverter draws from.

Inverter section. Six insulated-gate bipolar transistors (IGBTs) switch on and off thousands of times per second under control of the drive's microprocessor. This switching pattern is called pulse-width modulation (PWM). By varying the width and timing of the pulses, the inverter synthesizes an AC output waveform whose frequency and voltage can be set anywhere within the drive's range. The result at the motor terminals looks like variable-frequency, variable-voltage AC.

Control board. A microprocessor stores parameters, reads the keypad or remote inputs, runs protective algorithms, and drives the IGBT gate signals. This is where you enter motor data and configure operating behavior.

Motor Nameplate Parameters

Before running a motor from a VFD, you must program the drive with data from the motor's nameplate. Critical entries include:

Entering incorrect nameplate data is one of the most common causes of nuisance trips and motor damage. Always read the actual nameplate; never guess.

Acceleration and Deceleration Ramps

The accel ramp (ramp-up time) defines how many seconds the drive takes to accelerate from 0 Hz to maximum frequency. A 10-second accel ramp means the output frequency rises steadily over 10 seconds. Longer ramps reduce inrush current and mechanical shock to couplings and driven equipment.

The decel ramp (ramp-down time) defines how long the drive takes to coast the output from maximum frequency down to 0 Hz. A 15-second decel ramp is longer than the accel ramp in this exercise — common in applications where a heavy load (flywheel, large fan) must be slowed gently to avoid DC-bus overvoltage trips caused by regenerative energy.

If a load decelerates too fast and pumps energy back into the DC bus faster than the drive can handle, the drive will fault on overvoltage. Extending the decel ramp or adding a dynamic braking resistor solves this.

Minimum and Maximum Frequency

Maximum frequency is the highest output frequency the drive will produce, typically set to 60 Hz for standard 60 Hz motors (though some applications run motors above 60 Hz when the motor and driven equipment are rated for it).

Minimum frequency keeps the motor turning slowly enough for its cooling fan to work and prevents the load from stalling. A common minimum is 6–10 Hz, though the exact value depends on the application and whether the motor has separate forced cooling.

Synchronous Speed and Slip

Synchronous speed (in RPM) = (120 × frequency) ÷ number of poles.

A four-pole motor at 60 Hz has a synchronous speed of (120 × 60) ÷ 4 = 1,800 RPM. Actual shaft speed is slightly less due to slip — typically 2–4% for standard induction motors. At 30 Hz the synchronous speed of the same motor is 900 RPM; at 45 Hz it is 1,350 RPM. Measuring shaft RPM with a tachometer and comparing to the calculated synchronous speed confirms the drive is producing the commanded frequency and the motor is running normally.

Safety Before You Touch the Drive

VFDs store lethal DC voltage on the bus capacitors even after AC input power is removed. Always wait the manufacturer's specified discharge time (often 5–10 minutes) after de-energizing before opening the enclosure — the warning label on the drive states the required wait time. Follow your facility's lockout/tagout (LOTO) procedure: de-energize, lock out the disconnect, verify absence of voltage at the input terminals with a meter rated for the voltage class.

Wear appropriate PPE: safety glasses, insulated gloves rated for the voltage, and hearing protection if the motor and drive are run at high load. Never defeat a drive's fault or ground-fault protection during training exercises. Keep hands clear of the motor shaft and any belt or coupling while the motor is running.

Key terms10

RectifierThe section of a VFD that converts incoming AC voltage to DC using diodes or SCRs.
DC busThe filtered DC link inside a VFD, formed by capacitors (and sometimes inductors), that stores energy between the rectifier and inverter sections.
InverterThe output section of a VFD that uses IGBTs switched by PWM to synthesize variable-frequency, variable-voltage AC from the DC bus.
Pulse-Width Modulation (PWM)A switching technique in which the on-time (width) of voltage pulses is varied at high speed to produce an averaged output waveform of a desired frequency and voltage.
Accel rampThe programmed time, in seconds, for a VFD to increase output frequency from 0 Hz to maximum frequency; longer ramps reduce inrush current and mechanical shock.
Decel rampThe programmed time, in seconds, for a VFD to decrease output frequency from maximum back to 0 Hz; must be long enough to prevent DC-bus overvoltage from regenerative energy.
Synchronous speedThe theoretical no-load rotational speed of an AC motor's magnetic field, calculated as (120 × frequency) ÷ number of poles, expressed in RPM.
SlipThe difference between synchronous speed and actual rotor speed in an induction motor, expressed as a percentage; necessary for torque production.
Motor nameplate parametersThe rated voltage, current, frequency, speed, and power data printed on a motor's nameplate that must be entered into a VFD so the drive can protect and control the motor correctly.
Dynamic braking resistorAn external resistor connected to the VFD's DC bus that dissipates regenerative energy as heat during rapid deceleration, preventing overvoltage faults.

Worked examples4

1. Calculate the synchronous speed of a four-pole AC induction motor when a VFD commands an output frequency of 30 Hz.
  1. Identify the formula: Synchronous speed (RPM) = (120 × frequency) ÷ number of poles.
  2. Substitute values: (120 × 30) ÷ 4.
  3. Multiply numerator: 120 × 30 = 3,600.
  4. Divide by number of poles: 3,600 ÷ 4 = 900 RPM.
Answer: 900 RPM synchronous speed at 30 Hz.
2. Calculate the synchronous speed of the same four-pole motor when the VFD is set to 45 Hz.
  1. Use the same formula: Synchronous speed (RPM) = (120 × frequency) ÷ number of poles.
  2. Substitute values: (120 × 45) ÷ 4.
  3. Multiply numerator: 120 × 45 = 5,400.
  4. Divide by number of poles: 5,400 ÷ 4 = 1,350 RPM.
Answer: 1,350 RPM synchronous speed at 45 Hz.
3. The tachometer reads 862 RPM when the VFD is commanding 30 Hz on the four-pole motor. Is this within the 5% tolerance of the calculated synchronous speed of 900 RPM?
  1. Calculate 5% of the synchronous speed: 900 × 0.05 = 45 RPM.
  2. Determine the acceptable range: 900 − 45 = 855 RPM (lower bound); 900 + 45 = 945 RPM (upper bound).
  3. Compare measured value: 862 RPM is between 855 RPM and 945 RPM.
  4. Conclusion: 862 RPM is within the 5% tolerance band.
Answer: Yes, 862 RPM is within tolerance (855–945 RPM). Note that actual shaft speed is slightly below synchronous speed due to slip, so readings just under synchronous speed are expected and normal.
4. At 60 Hz, the synchronous speed of the four-pole motor is 1,800 RPM. The tachometer reads 1,746 RPM. Calculate the percentage slip.
  1. Identify the slip formula: % slip = ((synchronous speed − actual speed) ÷ synchronous speed) × 100.
  2. Subtract: 1,800 − 1,746 = 54 RPM.
  3. Divide: 54 ÷ 1,800 = 0.03.
  4. Multiply by 100: 0.03 × 100 = 3%.
Answer: 3% slip — a normal value for a standard NEMA design B induction motor at or near full load.

The math in this lesson2

Learners compute synchronous speed as 120f/P at 30, 45 and 60 Hz and percent slip from tachometer readings.
7.RP.3A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor-led teardown of a sample VFD: identifies rectifier, DC bus, inverter section, and control board; explains how pulse-width modulation creates variable-frequency output. · Students work in pairs to enter motor nameplate parameters into a training VFD using the keypad, set accel ramp to 10 seconds and decel ramp to 15 seconds, and verify parameter entry against a printed parameter checklist. · Speed profile test: students command the motor to run at 30 Hz, 45 Hz, and 60 Hz using the keypad; measure and record shaft RPM with a tachometer and output frequency on the drive display, then compare to calculated synchronous speed.
Materials: Training VFD (1–2 HP, suitable for 208–240 V three-phase training motor); three-phase training motor; parameter checklist handout; tachometer; clamp ammeter; manufacturer VFD programming manual; NCCER Electrical Level 3 motor controls module
Check: Parameter checklist signed off by instructor; speed profile test: measured RPM within 5% of calculated value at each frequency; all three speeds verified — pass requires all three within tolerance.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Motor Controls — VFD Theory
1. Which section of a VFD converts the incoming AC supply voltage into DC?
Motor Controls — Speed Calculations
2. A six-pole induction motor is operated from a VFD set to 45 Hz. What is the synchronous speed?
Motor Controls — VFD Parameter Setting
3. Why is the decel ramp in this exercise set longer (15 s) than the accel ramp (10 s)?
Safety — LOTO and Electrical Hazards
4. Before opening a VFD enclosure for inspection after de-energizing the AC input, a technician must:
Motor Controls — VFD Commissioning
5. Which motor nameplate value primarily sets the overload current protection threshold inside the VFD?
Motor Controls — VFD Theory
6. What does pulse-width modulation (PWM) allow the VFD inverter section to do?
Motor Controls — Application and Protection
7. A student programs the VFD with a minimum frequency of 5 Hz on a totally enclosed fan-cooled (TEFC) motor and runs it continuously at that speed. What is the primary risk?
Motor Controls — Measurement and Verification
8. During the speed profile test, the tachometer reads 1,323 RPM when the VFD displays 45 Hz on a four-pole motor (calculated synchronous speed 1,350 RPM). The pass criterion is within 5%. Does this reading pass?
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