Just added a page to the tools, which will allow you to calculate the synchronous speed, slip and rated torque for an induction motor. Not a particularly complicated calculation, but anything the makes life easier is welcome.

Induction Motor Calculator 

IEC 61439 'Low-voltage switchgear and controlgear assemblies', specifies standard arrangements of switchboard (call forms of internal separation). The are labelled as Form 1, Form 2, Form 3 and Form 4. Forms 2, 3 and 4 are further broken down into Form2a, 2b, 3a, 3b, 4a and 4b.  Each Form relates to the internal separation of the busbars, functional units and terminals, each being defined as:

This is the eighth and final post in my series on Motor Starting. What I’ll do here is summarise what we have been through and pull everything together.

Electric motors are one of the most common items of electrical equipment in service. Despite there usefulness of electric motors there are issues in starting motors and various methods are used to overcome these. Of particular concern it the growth of magnetic fields and back emf during starting leading to large currents and torque during this period. These currents and torque can have negative effects on both the electrical system and mechanical load.

Breathing a sigh of relief, we are now onto the final motor starting system to be considered in the series.

Before talking about electronic soft starting, is is worth mentioning that the development of power electronics has and continues to revolutionise that way motors are used and the applications to which they can a applied. You could write books (and some people have) on the application of power electronics to the running and control of motors. This is outside the scope of what we are looking at and I will concentrate only on the application to starting (and then in only a very general way).

Rotor resistance starting is the second starting method to discuss involving the addition of resistors into the external circuit. In this case the resistors are connected into the rotor circuit. This is also the only method I’ll be discussing where you need to get access to the rotor windings; consequently a standard squirrel cage motor cannot be used and a slip-ring motor is required.

So far we have looked at direct on line starting and reduced voltage methods of starting, (star-delta and auto-transformer). In this post we start on starting methods which rely on the use of resistors to limit the current. Primary resistance starting is concerned with the introduction of resistance into the stator windings. Rotor resistance, which I will discuss in the next post is concerned with the introduction of resistance in the motor rotor. Resistors inserted into the primary of the motor increase the overall impedance of the circuit and reduce the starting current.

Like star-delta starting, the use of an auto-transformer is a reduced voltage starting method. Unlike star-delta which forces a fixed reduction in voltage, the use of an auto-transformer enables almost any range of voltage to be considered during the starting period. Given tapping's on the transformer, the voltage can be varied through the stating period.  The ability to vary the level of voltage applied during stating not only allows the starting current to be constrained within limits, but also allows for closer matching of the starting torque to the mechanical requirements. The major downside (at least compared to star-delta) is the increased cost due to the auto-transformer.

Having looked at DOL starting in the last article, we will now consider Star Delta Starting (or Wye/Delta in the US). This is the first reduced voltage starting method we will look at. Voltage reduction during star-delta starting is achieved by physically reconfiguring the motor windings. During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3. This also reduces the torque by a factor of three. After a period of time the winding are reconfigured as delta and the motor runs normally.