Motor Starting - Introduction 

August 11th, 2011

By

Steven McFadyen

on

Electric motor driving a pump

Motor starting and its associated problems are well-known to many people who have worked on large industrial processes.  This post is a quick introduction to motor starting.

Motors have been in use for over 100 years, and during that time there has been relatively little change in how they function. The induction motor is by far the most widely used motor in industrial and building applications. As such, this book concentrates primarily on the application of motor starting in connection with induction motors.

Induction motors rely on the interaction of magnetic fields to convert electrical power into rotating power. The build-up of magnetic fields and back electro-motive force, or back emf, during the motor starting time introduces transient conditions into the electrical system. These transient events can affect the electrical supply system and other equipment connected to it. The key reasons consideration is given to motor starting are: to limit the transient effects; and to ensure that the motor accelerates the mechanical load correctly

Motor starting time, starting current, and starting transients

Motor starting time is the period from when the electrical supply is connected to the motor to when the motor accelerates to full speed. The length of the starting period is dependent on the combination of the motor and mechanical load, and it can be anything from a fraction of a second to 30 seconds or longer.

High levels of current are required during the start-up period, and they can have detrimental effects on the electrical supply system and other equipment connected to it. The duration of starting transients depends on the load characteristics and how long it takes the motor to run up to speed.

The figure below illustrates what happens during motor starting. During the starting period a current significantly larger than the motor’s normal full load running current is drawn, the magnetic fields within the motor and back emf increase, and the mechanical load accelerates. The start-up current can be as high as five to eight times the full load current.

Motor current during starting and running

Electrical systems are designed to cater for the steady state running period conditions. The cables are sized to accommodate the steady state running conditions, and voltage drops across the electrical system are calculated based on the steady state conditions.

During the motor starting period the cables will carry more current than during the steady state running period. System voltage drops will also be much larger during the starting period than during the steady state running period – this becomes particularly apparent when large motors are started, and / or if many motors are started at the same time.

If the voltage drop to the motor itself is too great during the starting period, the motor may be unable to develop sufficient torque to accelerate the mechanical load. In addition, voltage drops within an electrical system may affect other equipment, even to the extent of causing failures.

As the use of motors became widespread, overcoming motor starting problems became a concern for engineers. Over the years, many methods and techniques – each with its own advantages and limitations – have been developed to address the issues around motor starting.

The more commonly used methods of motor starting are:

• Direct On Line
• Star Delta
• Auto-Transformer
• Primary Resistance
• Rotor Resistance
• Electronic Soft Start

DOL and star-delta are by far the most commonly employed methods of motor starting. However, there have recently been massive strides made in the utilisation of electronics in regulating electrical power to motors, and electronic starting is fast catching up with DOL and star-delta. These advances can be utilised to allow the motor to operate with very specific acceleration characteristics.

This introduction to motor starting is an extract from my short introductory book on the subject.  If you want to delve into motor starting and understand how to the various types of starter work, please check out book. 

Table of Contents
Introduction to Motor Starting
Direct On Line Starting
Star-delta Starting
Auto-transformer Starting
Primary Resistance Starting
Rotor Resistance Starting
Electronic Soft Start
Variable Frequency Drives
Summary of Motor Starting Methods
How To Calculate Motor Starting Time
Useful Motor Technical Information
Typical Motor Starting Design Information
List of Symbols & Glossary   

The book is available in paperback eBook format at all Amazon stores. 

Motor Starters are electrical devices that are used to start and stop a motor safely. They are similar to relays and switch powers ON/OFF – although, unlike a relay, it also provides a low voltage and overcurrent protection.

Motor Starters serve the functions of safely starting/stopping a motor, reversing the motor’s direction, or protecting the motor from low voltage and overcurrent. A motor starter comprises two main components that complement each other to control and protect the motor. These are,

– Electrical Contractor: This contractor switches the power supply of a motor ON/OFF by making/breaking the contact terminals.

– Overload Protection Circuit: This circuit protects the motor from potential harm due to overload conditions. A huge current through the rotor may damage the winding as well as other appliances connected to the supply. It senses the current and breaks the power supply.

There are different types of motor starters, such as,

– Wye-delta open transmission is a somewhat standard electromagnetic starter system designed to safely reduce the voltage in the operation of major commercial equipment. The system is suitable for and often implanted in the operation of air compressors and pumps.

– Soft-start solid-state, often used in most major commercial equipment – a soft start motor starter is a reduced voltage starter that performs its function via the use of fluid, magnetic forces, or steel shot to lower startup current and control torque. Soft-start motor starters are frequently used in generators, conveyor systems, and other general-purpose functions. 

– Wye-delta starter OEM, a system that features sub-panel mounting, 120-volt coils, and a starter wye-delta timer for control systems that initially refrain from incorporating timer functions.

Importance of Motor Starters

Motor starters are one of the major inventions for motor control applications. As the name may imply, a starter is an electrical device that controls the electrical power needed to start a motor. These electrical devices are also used to stop, reverse, and protect electric motors.

A motor starter is also very important for starting an induction motor. This is due to its low rotor impedance, which depends on the slip of the induction motor, which is the relative speed between the rotor and stator. Induction motors have a high inrush current that is 5 to 8 times the normal rated full load current. Such volumes of current can easily damage or burn the windings of the motor rendering the machines useless. It can also cause a huge dip in the supply voltage, which leads to damaging other appliances connected to the same particular line.

In order to protect the motor from such huge volumes of currents, motor starters are used to limit the initial current for shorter durations at startup, and once the motor attains a certain speed, the normal power supply to the motor is resumed.

How do Motor Starters Work?

Motor starters usually consist of two devices, a contractor that completes the circuit to the motor and an overload relay that monitors the current the motor draws. This overload shielding device is set to a pre-determined maximum load that the motor can safely handle. When a condition occurs that causes the motor to surpass the maximum load, the device opens the motor starter control circuit, and then the motor is turned off.

Conclusion

When an electric motor is started, a large amount of current is required to turn the motor. This current is supplied by the motor starter. The motor starter consists of a contactor, which is an electrically controlled switch, and a set of coils. When the contactor is energized, the coils create a magnetic field that closes the contactor, allowing current to flow to the motor.