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AuCom provides comprehensive motor control solutions for both Low Voltage and Medium Voltage applications, including Soft Starters, Drives, and Motor Control Centres.
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GET YOUR MOTOR RUNNING

The complete guide to motor starting and control

Five technical papers in one reference, covering what determines starting performance, how the principal starting and control methods differ, and how to select the right approach for the application as a whole.

GET THE HANDBOOK

Or read the five technical papers individually

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A practical guide on motor theory

Choosing how to start and control a motor means looking beyond the starter itself.

The electrical supply determines how much starting current and voltage disturbance can be tolerated. The motor determines how much torque is available, while the driven machine determines what is required throughout acceleration. The process then determines whether the motor simply needs to reach operating speed or whether speed and torque must continue to be controlled during operation.

Get Your Motor Running works through these relationships step by step, helping you understand the application before deciding which starting or control method best fits it.

Inside the handbook

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_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 on its own 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 on its own 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 on its own 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 on its own 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 on its own

A look inside

Worked examples, ratings tables and equations throughout, in metric units and to IEC convention. 

  • GYMR Preview 1
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The right solution is not always the most sophisticated one

Direct-on-line starting may be entirely appropriate where the supply can tolerate the starting current and the machine can withstand full-voltage acceleration. A soft starter may be the better fit where controlled starting is required but the motor runs at fixed speed. Where speed or torque must be adjusted during normal operation, a variable frequency drive provides the control the process requires.

The electrical supply determines how much starting current and voltage disturbance can be tolerated. The motor determines how much torque is available. The driven machine determines what is required throughout acceleration. The process then determines whether the motor simply needs to reach operating speed, or whether speed and torque must continue to be controlled during normal running.

The handbook works through those relationships so that the selection is made around the requirements of the complete system.

 

Get the complete handbook

Access the handbook opens straight away. We will also email you the link so you can come back to it when you need it.

Common questions

What is the difference between a soft starter and a variable frequency drive? A soft starter controls the voltage applied to the motor during starting and stopping. Once the start is complete, the motor runs at supply frequency. A variable frequency drive controls both voltage and frequency and stays in control during normal operation, so motor speed can be varied continuously. A soft starter suits fixed-speed applications that need controlled acceleration. A VFD is required where speed or torque must change as part of the process.
Does reducing starting current always reduce starting torque? Yes. Under reduced-voltage starting, initial motor torque varies approximately with the square of motor current at the locked-rotor condition. Halving the starting current leaves roughly a quarter of the locked-rotor torque. This is why a starting method has to be assessed against the load torque requirement, not against the current limit alone.
Why does star-delta starting reduce current and torque to about one third? In star, the voltage across each motor winding is reduced to 1/√3 of the line voltage. Starting line current and starting torque both fall to approximately one third of their full-voltage delta values. The method gives only two fixed conditions, so the torque available in star has to be sufficient to accelerate the load before transition.
What does an AC-53 utilisation code tell me? It defines the duty a soft starter rating applies to: starting current as a multiple of full-load current, start time, and either the on-load duty cycle and starts per hour (AC-53a, non-bypassed) or the off time between starts (AC-53b, bypassed). A starter rated for one combination may have a different usable rating under another, which is why an ampere rating alone does not establish whether a starter suits an application.
Will a bigger motor solve a starting problem? Not on its own. Where starting current is restricted, the torque available depends on the motor's locked-rotor current, locked-rotor torque and speed/torque characteristic rather than rated power. Motors with similar rated power and full-load performance can have substantially different starting characteristics, so selecting a motor with more suitable starting characteristics is often more effective than selecting a higher-rated one.What is the difference between a soft starter and a variable frequency drive?