• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer

Motion Control Tips

Automation • Motion Control • Power Transmission

  • News
    • Industry News
    • Editor Blogs
    • Video
  • Controls
    • HMIs
    • PC-Based Controllers
    • PLCs + PACs
    • Stand-Alone Controllers
    • Software
  • Drives
    • Servo Drives
    • Stepper Drives
  • Encoders
    • Absolute Encoders
    • Incremental Encoders
    • Rotary Encoders
  • Mechanical
    • Bearings
    • Brakes + Clutches
    • Belt + chain
    • Couplings
    • Gears + Gearing
    • Lubrication
    • Shock + Vibration Mitigation
    • Springs + Rings + Seals
  • Linear
    • Actuators
    • Linear Motors
    • Linear Encoders
  • Motors
    • AC Motors
    • DC Motors
    • Brushless Motors
    • Gearmotors
    • Piezo Motors
    • Servo Motors
    • Stepper Motors
  • Systems
    • Conveyors + linear transport systems
    • Gantries + Stages
    • Rotary Tables
    • Grippers + End Effectors
    • Robotics
  • Networks
    • Connections + Sliprings
    • Fieldbuses
    • I/O
    • Sensors + Vision
  • FAQs
    • Motion Casebook
    • Motion Selection Guides
  • Suppliers
You are here: Home / FAQs + basics / FAQ: What are medium voltage AC drives, and where are they used?

FAQ: What are medium voltage AC drives, and where are they used?

March 18, 2017 By Danielle Collins Leave a Comment

Recall that the basic equation for electric power is P = VI, or, power = voltage multiplied by current. This means that for a given power level, voltage and current are inversely proportional. In other words, the higher the supply voltage, the lower the current draw will be. For AC motors that are used in high-power applications, operating at a low voltage causes the motor to draw very high current, resulting in higher energy usage, lower efficiency, and higher cost. As you can see from the power equation, current draw can be reduced by increasing the voltage supplied by the drive.


The full equation for three-phase power is:

P = V * I * PF * 1.732

Where:

P = power (watts)

V = voltage (volts)

I = current (amps)

PF = power factor (supplied by the manufacturer)

The power factor is the ratio of working power to apparent power—essentially, how effectively the electricity is being used.

The constant 1.732 is the square root of 3, and is used for 3-phase power to account for the fact that all three phases do not produce the same amount of power at the same time.


AC drives are generally classified as low voltage, medium voltage, and high voltage, although the classifications for each type vary by manufacturer and even by standard. The ANSI C84.1 standard defines low voltage as 240 to 600 VAC, while medium voltage is typically 2300 or 4160 VAC (3300 or 6600 VAC in Europe and most of the rest of the world), but it can be as high as 69,000 VAC. High voltage, according to ANSI C84.1, is 115,000 to 230,000 VAC.

Based on the three-phase power equation above, here’s an example of the difference in current draw between a low voltage and a medium voltage drive:

If a 1000 hp (746 kW) motor operates at 480 V, the current draw will be:

746,000 W = 480 V * I * 0.87 * 1.732

I = 1031 A

The same 1000 hp (746 kW) motor, operating at 4160 V, will have a current draw of:

746,000 W = 4160 V * I * 0.87

I = 206 A

Medium voltage drives can be either current source inverter or voltage source inverter types, although VSI types are more popular due to high reliability and low harmonic distortion. AC drives in this configuration are referred to as medium voltage multi-level voltage source PWM drives.

medium voltage ac drives
Schematic of a medium voltage multi-level VSI-PWM drive.
Image credit: Robert Hanna

Recall that inverters convert DC power back to AC power at the required frequency and voltage. The term “multi-level” refers to the output of the inverter, and PWM is pulse-width-modulation – which switches the DC voltages in order to create the AC power. In the multi-level design, the inverter uses multiple DC voltages (rather than just two voltages, as with a typical 2-level inverter) to synthesize the AC waveform, making it nearly sinusoidal in nature. This allows the PWM switching frequency to be reduced and reduces the dV/dt, which reduces harmonics. Most medium voltage AC drives also include a multi-phase transformer on the front end, which works in conjunction with the multi-level inverter to reduce harmonics. And for applications with highly dynamic loads, an active front end may also be used. The active front end reduces harmonics on the line (input) side by monitoring the harmonic level and actively filtering the waveform to provide dampening.

medium voltage ac drives
Voltage output from a two-level inverter (left) and a multi-level inverter (right).
Image credit: Rijil Ramchand

The term dV/dt refers to the change in voltage over time. In PWM applications, it describes the rapid voltage rise at each pulse of the PWM waveform.


Medium voltage AC drives generally benefit applications that require motors from 500 to 20,000 hp, where the difference in current draw between low voltage and medium voltage becomes substantial. Motors in this range are often used to operate large compressors pumps, and fans in power stations, petrochemical plants, water/wastewater treatment facilities, and mines.

You may also like:

  • pulse rectifier
    FAQ: What is a pulse rectifier and what kinds are…
  • inverters
    FAQ: What are current source inverters and voltage source inverters?
  • reflected waves
    What are VFD reflected waves and why are they harmful?
  • rectifier
    What is a rectifier?
  • Yaskawa-A1000-drive-family
    How to handle imbalances from light loads on VFDs

Filed Under: AC Motors, Drives + Supplies, FAQs + basics, Featured

Reader Interactions

Leave a Reply

You must be logged in to post a comment.

Primary Sidebar

POWER TRANSMISSION REFERENCE GUIDE

DESIGN GUIDE LIBRARY

“motion
Subscribe Today

RSS Featured White Papers

  • Specifying electric rodless actuators: Ten tips for maximizing actuator life and system performance
  • The truth about actuator life: Screw drive survival
  • Top Ten Tips: How to specify electric rod-style actuators for optimal performance, reliability and efficiency

Footer

Motion Control Tips

DESIGN WORLD NETWORK

Design World Online
The Robot Report
Coupling Tips
Linear Motion Tips
Bearing Tips
Fastener Engineering.

MOTION CONTROL TIPS

Subscribe to our newsletter
Advertise with us
Contact us
About us
Follow us on TwitterAdd us on FacebookAdd us on LinkedInAdd us on YouTubeAdd us on Instagram

Copyright © 2022 · WTWH Media LLC and its licensors. All rights reserved.
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media.

Privacy Policy | RSS