This is the first article in a series on motor starting.   Motor starting and it's associated problems is well know to many who have worked on large industrial processes,  but  are less know by others  with less experience or those who are brand new to the topic.  Hopefully this series will provide an introduction to motor starting,  a refresher for  those who have spent time away and conceivably the odd interesting observation  to the experts.

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Electric motors are one of the most common pieces of electrical equipment in service.   For over a hundred years we have been using motors and during that time there has been relatively little change in how they function.   Of all motor types, the induction motor is by far the most widely used in industrial and building applications.  Within this article, I'll primarily be concentrating on the application of motor starting in connection with induction motors. 

Induction motors convert electrical power into rotating power and rely on the interaction of magnetic fields to achieve this.  Motor starting is the period of time from when the electrical supply is connected to the motor until the motor accelerates to full speed.  During this time, the build up of magnetic fields and back emf introduce transient conditions on the electrical system.  These transient events can affect the electrical supply system and other equipment connected to it.  Limiting the transient effects and ensuring that motors accelerate the mechanical load correctly are the key reasons consideration is given to motor starting.  

This figure illustrates what happens during starting.   During the starting period the magnetic fields within the motor and back emf are building, the mechanical load is accelerating and a current significantly larger than the motors normal full load running current is drawn.  This startup current can by as  high as five to eight times the full load current.  Duration of the starting period is dependant on the combination of the motor and mechanical load and can be anything from fraction of a second to thirty seconds or longer.  During the startup period, the high levels of current required  can have detrimental affects on the electrical supply system and other connected equipment.

During design of any electrical system, consideration is given to the ensuring that the steady state running period conditions are adequately catered for.  Cables are sized for this and voltage drops across the electrical system calculated on this basis.  During starting however, cables are going to be carrying more current and the system voltage drops will become much larger.  This is particularly apparent if large motors are started and/or many motors are started at the same time.  Voltage drops within an electrical system may affect other equipment; even to the extent of causing failures.  In addition, if the voltage drop to the motor itself is too great, the motor may be unable to develop sufficient torque to accelerate the mechanical load.

Overcoming these problems has been a concern of engineers since motors first started to achieve widespread adoption.  Over the years, many methods and techniques have been developed to address these issues ;  each with it's own advantages and limitations.  Recent decades have seen massive strides in the utilisation of electronics in providing electrical power to motors;  which not only enable a great deal of control over the starting current, but can allow the motor to operation with very specific acceleration characteristics.   Subsequent posts wwill look at some of the more commonly used methods of motor starting, which are:

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

For the follow on articles I will  be endeavoring to complete  and post these on a weekly basis (biweekly if I find I am short on time).   In the next article, I'll start the discussion on each of the starting methods with a consideration of Direct On Line starting.  As we will see this is the easiest method and has high starting torque, but has the disadvantage of drawing large starting currents during the starting period. 

Edited 12 August 2011 - Reduced Voltage During Staring

Many of the staring methods to be discussed in subsequent articles rely on a reduction in voltage supplied to the motor during staring.  It can be useful to understand in a very general way how this will affect the motor.

If the voltage is reduced then the current drawn will be reduced proportionally.  However, the torque will be reduced according to the square of the reduction.

For example, if the voltage is reduce by a factor of 2 then the current will be reduced by a factor of 2.  However the starting torque will be reduced by the square of this, i.e. by a factor of 4. 

Motor Starting - Direct On Line >