Medium Voltage Variable Frequency Drives Design Requirements

This article is for design requirement of Medium Voltage Variable Frequency Drives (AFD) for use with 4,160 or 13,800 V, 60 Hz induction motors.  This document is mainly based on lEC Standards ANSI/IEEE. Main keywords for this article are What is Medium Voltage Variable Frequency Drives Design Requirements, VFD Performance, VFD Basic Design, VFD Driver and Motor Protection, VFD Diagnostics.

VFD References

International Electrotechnical Commission (IEC)
60044 (series) Instrument transformers
60076-6 Power Transformers – Part 6: Reactors
60529 Degrees of protection provided by enclosures (IP Code)
61000-4-5 Electromagnetic compatibility (EMC) Part 4-5: Testing and measurement techniques – Surge immunity test
61800-2 Adjustable speed electrical power drive systems Part 2: General requirements – Rating specifications for low voltage adjustable frequency a.c. power drive systems
61800-3 Adjustable speed electrical power drive systems Part 3: EMC requirements and specific test methods
61800-5-1 Adjustable speed electrical power drive systems Part 5-1: Safety requirements – Electrical, thermal and energy

Medium Voltage Variable Frequency Drives Design Requirements

What is Medium Voltage Variable Frequency Drives


  • Drive shall be designed and constructed in accordance with IEC 61800. Drive shall convert fixed voltage, three-phase, 60 Hz power to an adjustable voltage and frequency three-phase, ac power for stepless motor speed control over the speed range as shown in SES IEC E10-X02. Drive shall be designed to accept power from one of the medium voltage systems available in the plant as shown in data sheet.
  • The motor voltage rating shall be optimized considering overall impact on the power system and economic evaluation.
  • This specification prescribes the general requirements for all drives. See Following data sheets, and one line diagram for specific requirements.

    What is Medium Voltage Variable Frequency Drives Design Requirements, VFD Performance

  • Drive shall include an input/isolation transformer or input reactor, an input converter (rectifier), a direct current link, output converter (inverter), protection equipment, control/monitoring panel, harmonic filters and other accessories.
  • Drive shall be capable of operating the squirrel cage induction motor described in data sheet. Drive shall be designed to continuously operate the motor up to the nameplate ratings shown in data sheet. Additionally, drive shall have the overload capability specified in following paragraph.
  • It is preferred that the drive outputs will not produce pulsating torques in the mechanical drive train. If there is a torque pulsation caused by the drive output, drive vendor shall perform a torsional analysis for the mechanical drive train to ensure there are no vibration problems throughout the speed range.
  • The design of the motor shall be coordinated with the design of the drive (see following parapragh) to account for common mode voltages, and shall be suitable for essentially a square wave current waveform. The motor winding insulation system shall be designed for line to neutral voltages equal to or greater than 2 times rated system voltages. The motor turn to turn insulation shall be suitable for high dV/dt stresses.
  • When it is desired to retrofit a drive to an existing motor, a suitable drive output filter shall be furnished. The output filter shall provide a sinusoidal waveform with a maximum of 5 percent total harmonic distortion.

VFD Performance

  • Drive shall be capable of operating on a steady state basis with input voltage variations of ±10 percent voltage and ±5 percent frequency. An output voltage regulator shall be provided to automatically maintain correct V/Hz ratio within 1 percent of setpoint, despite incoming voltage variations.
  • Drive shall provide near harmonic free operation to the connected power source. For individual or simultaneous operation of the drives, the level of harmonic current and voltage distortion shall not exceed the limits defined in IEC 61800-3. If harmonic filters are required to meet these requirements, the drive vendor shall provide the filter, and be responsible for the design and manufacturing of the filter. Compliance shall be verifiable by field measurements of harmonic distortion differences at point of common coupling (drive input) with and without drive operating.
  • Drive shall have a minimum efficiency of 95 percent when measured at rated output and torque. A loss penalty will be imposed for failure to meet the efficiency quoted in vendor’s proposal, at the rate shown for loss evaluation in above data sheet.
  • Drive output current shall approximate actual sinewave current. The output voltage and frequency shall vary proportionally, thus maintaining a constant V/Hz value up to a nominal frequency. Above that nominal frequency, output voltage shall remain constant, thus causing the drive to operate in the constant kVA range.
  • Drive input power factor shall not be less than 0.9 lagging for the total speed/load range.
  • Drive shall be capable of supplying 115 percent of rated output (power and torque) continuously with input conditions specified. Drive shall have a short time overload rating of 150 percent for 1 minute.
  • Output voltage shall be separated by 120 degrees plus or minus 1 percent, and phase to phase voltages shall not be out of balance by more than 2 percent.
  • Drive shall not be damaged by an open circuit or short circuit condition at the output terminals. Drive shall limit output current to 230 percent of rated current for all faults on drive output.
  • Drive shall be designed for speed regulation of ±0.5 percent over the speed range without feedback.
  • Drive shall be designed to meet electromagnetic compatibility (EMC) requirements in accordance with IEC 61800-3.
  • Drive shall be capable of riding through power system disturbances caused by faults on the power grid lasting up to 30 cycles. Drives shall also be capable of riding through voltage dips up to 20 percent caused by starting of other motors on the bus. Neither of the above events shall negatively impact operation of drive or connected load.
  • Audible noise generated by the controller shall not exceed a sound pressure level of 75 dBA measured 1.5 m from any surface of the enclosure.
  • If specified on data-sheet, Drive shall be provided with a maintenance bypass switch to allow motor to be operated at normal speed, when the drive is being maintained.
  • Drive shall be designed and manufactured in a way that if any one component fails, drive shall still operate the motor.

VFD Basic Design

  • The design shall be field service proven with a minimum of 3 years actual field service with similarly rated equipment. Vendor shall include a user list and complete reference contact information with proposal.
  • Drive shall be a current source inverter type using two 6 pulse thyristor controlled rectifier bridges in series to provide an 18 pulse input converter.
  • Two dc link inductors shall be provided to filter the dc current ripple and provide isolation between the rectifier and inverter bridges. The reactors shall be dry type air-core, convection cooled, with copper windings and shall be located in one of the drive cubicles. The reactors shall conform to IEC 600766.
  • An output bridge shall be provided to convert the dc current to a variable voltage and frequency ac current.
  • Input to the power converters shall be protected by surge arresters from voltage surges and harmonic resonance on the incoming line. The surge arresters shall be metal oxide varistor type.
  • Drive shall be controlled by a microprocessor based system, which integrates the operator interface, control and regulation function. A separate isolated microprocessor based system shall be employed for the protection, diagnostics and data logging functions. Low voltage control circuits shall be protected from surges in accordance with IEC 61000-4-5.
  • Drive shall be designed using modular functional sub-systems. The subsystem limits shall be selected to group components by function and common operational characteristics. The sub-systems shall facilitate maintenance and help ensure safe working conditions for the mechanic.
  • The power converters shall have the components mounted in modular groups, with each module containing a positive and negative leg of series connected thyristors. All associated suppression and gate isolation components shall be mounted on the respective modules.
  • Printed circuit boards shall be high quality industrial grade with keyed, gold plated edge connectors. Circuit boards shall conform to IEC 61800-5-1 section
  • The boards and components shall be high temperature rated and be suitable for operation in module enclosures with only internal natural draft air circulation.
  • Instrument transformers required for control and indication shall be located in the associated switchgear or drive cabinets as indicated on the one line diagram. Instrument transformers shall comply with IEC 60044.

VFD Enclosure

  • Drive components shall be factory mounted and wired in a grouping of dead front, grounded, free standing enclosures as specified in data sheet above. The enclosures shall be rigidly constructed with structural shapes and side panels a minimum of 3 mm thick. The cubicles shall be bolted together to form a continuous switchgear type assembly.
  • The enclosure base shall be designed to allow rolling equipment into position. The base shall have provisions for jacking, fork lift handling and hoisting.
  • Enclosures shall have full height hinged doors (minimum 3 mm steel) with provisions for locking all doors, and interlocking of doors to high voltage compartments containing switchgear.
  • Drive enclosure shall conform to IEC 60529 (min IP21), and shall be suitable for installation in an indoor, unclassified area.
  • All enclosure external openings exceeding 6 mm in width shall be provided with stainless steel screens to prevent the entrance of, for example, snakes and rodents. The maximum screen mesh opening width shall be 6 mm.
  • The enclosure shall be designed for front access only, and side by side and back to wall mounting.
  • All power buses shall be tin or silver plated copper, conservatively sized for the maximum current, and fully insulated for rated voltage. Materials and instructions for termination of incoming power and motor conductors shall be included by vendor.
  • A copper ground bus running the entire length of the drive assembly shall be provided. The minimum size shall be 6.4 x 50.8 mm. Each cabinet shall be bonded to the ground bus. Connectors shall be provided for connecting ground conductors of the supply circuit and motor feeder, and for connections to the plant ground grid.
  • Grounded metal barriers shall separate the various drive modules, power bus, control components and wiring.
  • All field control connections shall be made on terminal blocks external to the drive components inside the enclosure.
  • Connection points for inputs and outputs of different voltage levels shall be segregated from each other. If necessary, this may be accomplished through the use of terminal barriers and covers.
  • Where control wiring is run through a metal sheet or barrier, bushings, grommets, or other mechanical protection shall be provided.
  • All internal wiring shall be terminated with no more than two (2) conductors per terminal block point.
  • Terminal blocks and each terminal shall be permanently marked with numbers  shown on wiring diagrams. Sleeve type wire markers shall be provided on each end of each wire showing the wire number assigned on wiring and schematic diagrams.
  • All devices and meters for normal operator interface shall be mounted on the exterior of the front door(s).
  • Each enclosure section shall be provided with space heaters. The space heaters shall be rated for 240 Vac, 1 phase, 60 Hz and shall operate on 120 Vac, 1 phase, 60 Hz supplied from external source. Rating shall be specified in above data sheet. The space heater shall be thermostat controlled with manual bypass switch. Each space heater circuit shall have its own power contactor and ammeter installed. Each circuit must be capable of being isolated independently.
  • When required, an additional normally closed heavy duty auxiliary contact shall be provided on the motor starter contactor, to allow motor space heater operation when the motor is stopped. Motor space heaters can be fed from motor starter CPT if equipped. CPT shall be appropriately sized to accommodate the additional heater load. Motor space heater may be fed from a remote power panel. A warning label with white engraved letters on a red background shall be provided on the front of all motor starter cubicles that have an external heater power feed. The warning label shall be written in English:


  • Each enclosure section shall be provided with internal illumination lamp  operated by door switch, a convenience plug socket and switch.
  • Removable type eye bolts / lifting lugs shall be provided on all enclosures.

VFD Cooling

  • Drives shall utilize direct forced air cooling or closed loop liquid cooling for  power components. Direct forced air cooling shall be preferred for drives rated 373 kW and smaller.
  • Direct air cooled drives shall be provided with 2-100 percent capacity redundant air blowers. It shall be possible to operate the drive continuously at rated conditions with one cooling blower out of service. A fan failure alarm shall be provided. Air inlets shall be equipped with cleanable stainless steel air filters. Blowers shall be sized and cabinet shall be arranged to allow exhaust air to be ducted outside the building.
  • When a liquid cooling system is required, vendor shall supply a complete system with all required components, controls, motor starters, plumbing and interconnecting wiring installed.
  • Motors for pumps and cooling fans shall be in accordance with Induction Motors 150 kW and below.

VFD Operational Features

  • Drive shall have the following door mounted manual controls and indicating lights:
    a. Stop-permissive Run Selector Switch
    b. Status indication for Run, Stop, Trip, Cooling Failure
    c. Auto – Manual Switch (key operated)
    d. Speed adjustment
  • Drive functions and parameter settings shall be software programmable. Drive shall be suitable for direct interface with industry standard communications systems and programmable logic controller (PLC) interface if specified in data sheet above. An RS485 port and an RS232/422 serial port shall be provided for direct field access and remote communication.
  • The digital keypad shall allow operator to enter exact numerical settings. A plain English user menu shall be provided in software as a guide to parameter setting (rather than codes). Drive parameters shall be factory set electrically erasable programmable read-only memory (EEPROM), and resettable in the field through the keypad. Password security shall be available to protect drive parameters from unauthorized personnel.
  • A digital display shall provide diagnostic messages and identify parameters in English language without the use of codes. The display shall be capable of displaying:
    a. Output kW
    b. Motor speed
    c. Output frequency
    d. Output volts for each phase
    e. Output amps for each phase
    f. Manual or auto mode
  • Digital display shall provide the following alarms:
    a. Motor stalled
    b. Loss of speed command
    c. Drive over temperature
    d. Low cooling fluid level (liquid cooled system)
    e. High cooling fluid temperature (liquid cooled system)
    f. Output ground fault
    g. One series thyristor failure (with indication of actual device)
    h. Loss of any cooling water pump (liquid cooled system)
    i. Loss of any cooling fan (air cooled system)
    j. Incoming phase loss
    k. Gate drive circuit failure
    l. Inverter over voltage
    m. Inverter under voltage
    n. Output over frequency
    o. Output over current
    p. Output over voltage
    q. Input over current
    r. Input over voltage
    s. Loss of control voltage
    t. Gate power supply under voltage
  • Drive shall start, stop, accelerate, and decelerate under adjustable controlled speed ramps. After ramp-down, ‘Stop’ shall remove power to the motor, not simply provide a zero speed setting.
  • Programmable automatic restart after any trip resulting from over current, over voltage, or under voltage shall be provided. For safety, drive shall shut down and require manual reset and restart if the automatic reset/restart function is not successful after one attempt.
  • Speed droop shall be provided to reduce drive speed on transient overloads. Drive shall return to the set speed after the transient overload is removed.
  • Individually adjustable settings for start, stop, entry, and slope shall be provided. Minimum and maximum speed set points shall be provided.
  • Analog speed reference input (process control follower) shall be provided. A proportional and integral set point process controller with menu driven selection and programming via door-mounted keypad shall be provided.
  • Drive shall be able to restart into a connected coasting motor (forward or reverse direction) without resultant damage to the controller, or causing nuisance tripping.
  • Drive shall automatically reset after any under voltage trip without the need for local/manual reset.
  • The acceleration rate shall be independently adjustable.
  • The deceleration rate shall be independently adjustable.
  • Motor current limit shall be adjustable from 20 to 135 percent of rated current.
  • Motor overload trip shall be adjustable from 20 to 135 percent of rated current.
  • Frequency shall be adjustable from 0.2 to 400 Hz or as given in data sheet.
  • Drive shall be pre-programmed for the ramp-up and ramp-down times shown  in above data sheet, and shall be re-programmable in the field to allow for changes.
  • Drive shall be capable of starting and accelerating the motor and its load to a pre-selected speed.

VFD Driver and Motor Protection

  • Drive shall have necessary components to protect itself and the motor against  motor overload, motor or drive internal faults, and incoming ac line disturbances. The following shall be included:
    a. Short circuit protection for single phase or 3 phase short circuit on drive output terminals
    b. Static instantaneous over current and over voltage trip with inverse over current protection
    c. Over speed (over frequency) protection
    d. Line under voltage protection
    e. Power unit over temperature protection
    f. DC bus discharge circuit for protection of operator and service personnel
    g. Electronic motor replica overload protection with motor resistance temperature detector (RTD) input
    h. Motor current unbalance and differential protection
    i. High motor temperature alarm/shut down
    j. Loss of control signal input
  • Drive shall have a ground fault detection scheme on the output.

VFD Diagnostics and Data Logging

  • A microprocessor based diagnostic system, which includes self-check, shall be furnished to monitor the system and display faults and operating conditions. Microprocessor systems shall be products of drive manufacturer. This system shall be independent of the operator interface, control and protection microprocessor system.
  • This system shall monitor the internal conditions of control and power system components through a method of self checking during run up and normal operation. The diagnostic system shall have an independent secure power  system to ensure operation during power upsets, and to ensure no data is lost during power interruptions.
  • The system shall store drive data in three logs: Operational and Set-up Parameter Log, Fault log and Historical Data Log.
  • The Operational and Set-up Log shall contain all drive set-up, tuning and configuration parameters.
  • The data log records shall be accessible via an RS232/RS422 or RS 485 serial link, and line by line on the keypad display.

VFD Input and Output Interfaces

Form C contacts shall be wired to terminal blocks for all provided remote inputs and  outputs.

  • Each output shall be an isolated form ‘C’ contact rated 10 A at 600 Vac. The  following minimum AFD status indications shall be provided:
    a. Alarm annunciator – summary
    b. Trip annunciator – summary
    c. Local control
    d. Remote control
    e. Output contactor open
    f. Output contactor closed
    g. Bypass contactor closed
    h. Spare
  • The following 4-20 mA dc output signals shall be provided:
    a. Output voltage
    b. Output current
    c. Output frequency
  • Input contacts shall be rated 10 A at 600 Vac. The following minimum inputs to the AFD shall be provided:
    a. Input signal (4-20 mA – frequency command)
    b. Motor start
    c. Motor stop
    d. Input breaker open
    e. User trip

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