Introduction to Motor Starting

Many people who have worked on large industrial processes are familiar with motor starting and its associated problems. This is a brief overview of motor starting.

Motors have been in use for more than a century, and their operation has remained relatively unchanged. The induction motor is by far the most common motor used in industrial and construction applications. As a result, this book focuses primarily on the application of motor starting in the context of induction motors.

Induction motors use magnetic field interaction to convert electrical power into rotating power. During the motor starting process, the accumulation of magnetic fields and back electro-motive force, or back emf, introduces transient conditions into the electrical system. These transient events can have an impact on the electrical supply system and other connected equipment. The primary reasons for considering motor starting are to limit transient effects and to ensure that the motor correctly accelerates the mechanical load.

Starting transients, starting time, and starting current of a motor

The motor starting time is the time it takes from connecting the electrical supply to the motor to reaching full speed. The length of the starting period is determined by the motor and mechanical load and can range from a fraction of a second to 30 seconds or longer.

High current levels are required during the startup period, and they can be harmful to the electrical supply system and other equipment connected to it. The duration of starting transients is determined by the load characteristics and the time it takes the motor to reach full speed.

The diagram below depicts what happens when a motor starts. During the starting period, a current much greater than the motor’s normal full load running current is drawn, the magnetic fields and back emf increase, and the mechanical load accelerates. The start-up current can be five to eight times that of the full load current.

Starting and running motor current

Starting and running motor current Electrical systems are built to handle steady-state operating conditions. The cables are sized to accommodate steady-state running conditions, and voltage drops across the electrical system are calculated using those conditions.

The cables will carry more current during the motor starting period than during the steady state running period. System voltage drops will also be much greater during the starting period than during the steady state running period; this is especially noticeable when large motors are started and/or when many motors are started at the same time.

If the voltage drop to the motor is too great during the startup period, the motor may be unable to generate enough torque to accelerate the mechanical load. Furthermore, voltage drops within an electrical system can affect other equipment, even causing failures.

Engineers became concerned about motor starting problems as the use of motors became more widespread. Many methods and techniques have been developed over the years to address the issues surrounding motor starting, each with its own set of advantages and limitations.

The following are the most commonly used motor starting methods:

Online Direct

Delta Star


Initial Resistance

Resistance of the Rotor

Soft Start with Electronics

DOL and star-delta are by far the most commonly used motor starting methods. However, significant progress has recently been made in the use of electronics in regulating electrical power to motors, and electronic starting is quickly catching up with DOL and star-delta. These developments can be used to enable the motor to operate with highly specific acceleration characteristics.

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