Skip to content
Close
SEARCH
Amconex Logo       Benshaw Logo
AuCom provides comprehensive motor control solutions for both Low Voltage and Medium Voltage applications, including Soft Starters, Drives, and Motor Control Centres.
Product Category
aucom_emx4_products_header
AuCom specialises in delivering tailored motor control solutions across a wide range of industries, from HVAC to Oil & Gas, to Mining & Quarrying.
Industries
aucom_mining_industries_header
In addition to our product range, AuCom provides specialised services to support the smooth and efficient operation of your systems.
Services
aucom_field-support_services_header
Explore our comprehensive resources, from product guides to technical insights, designed to support and empower your operations.
Resources
aucom_brochure_resources_header
Whether you need to contact our team, find support, or connect with our global network, AuCom is here to help anytime, anywhere.
Contact
aucom_hotline_contact_header
Our Company
Careers
News & Articles
aucom_el-hammam-pumping-stations_articles_header
Irrigation pivot crop GettyImages-160019835_super_CMYK 3

Reduced-voltage starting of three-phase induction motors

Get Your Motor Running White Paper Series

Download Get Your Motor Running Part 1: Reduced-Voltage Starting

Part 1 of five. Understand why motor rating alone is not enough to predict starting performance when current is limited.

Specifying a larger motor does not necessarily give you more starting torque. When starting current is restricted, two motors with the same power rating can behave very differently depending on their locked-rotor current, locked-rotor torque, and speed/torque characteristics.

Part 1 shows you how to assess that relationship before selecting the motor or starting method. Using real motor data, it compares nine 110 kW motors and shows how the torque available at the same current limit can range from just 24% to 65% of full-load torque.

This paper answers:

  • Why can two motors with the same rating have very different starting performance?
  • How does rotor design affect locked-rotor current and starting torque?
  • How much torque will a motor produce at a given starting-current limit?
  • How much starting current is required to produce a specified torque?
  • What does the motor's speed/torque curve tell you about whether the load will accelerate successfully?

The Get Your Motor Running Series

Five technical papers take you from the fundamentals of motor starting through to the selection and application of soft starters and variable frequency drives.

aucom_gymr_handbook The Complete Series: A Practical Guide to Motor Starting and Control The complete guide to motor starting and control. All five Get Your Motor Running papers in one reference, from what determines starting performance through to selecting and applying soft starters and variable frequency drives. Worked examples and ratings guidance throughout. Read the Handbook aucom_gymr_p1 Reduced-voltage starting of three-phase induction motors What happens when starting current is restricted, and why motor design decides the outcome. Nine four-pole 110 kW motors with similar rated power, speed and efficiency are compared at the same current limit: at 300% FLC the available initial torque ranges from 24% to 65% of full-load torque. Covers locked-rotor current, locked-rotor torque and the speed/torque curve. Read Part 1 aucom_gymr_p2 Starting torque requirements of machines and motor loads What the driven machine requires from the motor during starting. Breakaway torque, work torque, acceleration torque and inertia, with worked examples for a punch press, a centrifugal fan, a compressor and a flywheel, plus the equations for full-load torque, acceleration torque and reflected load inertia. Read Part 2 aucom_gymr_p3 Motor starting solutions The principal methods compared: direct-on-line, primary resistance, auto-transformer, star-delta, soft starter and variable frequency drive. Covers open-transition and closed-transition switching, why the transition point matters more than the initial current reduction, and a side-by-side comparison of all six methods. Read Part 3 aucom_gymr_p4 Solid-state soft starters Controlled starting for fixed-speed applications. Open-loop and closed-loop control methods, from timed voltage ramp through to acceleration control. Also covers SCR thermal duty, AC-53 utilisation codes and ratings tables, motor protection, semiconductor fuse coordination, and power factor correction capacitor switching. Read Part 4 aucom_gymr_p5 Variable frequency drives How a VFD controls speed and torque across the operating range. V/F, sensorless vector and closed-loop vector control, the constant-torque and constant-power regions, load characteristics and energy use, drive and motor selection, harmonics and mitigation, braking methods, and synchronous transfer. Read Part 5