ELECTRICAL EASY

Variable Frequency Transformer or simply VFT is quite a new technology to connect two asynchronous Grids. Asynchronous Grids mean two power systems operating at two different frequencies. The world’s first VFT was installed and commissioned in the year of 2004 at Hydro- Quebec’s Langlois substation, where it is used to exchange up to 100 MW of power between the asynchronous power grids of Quebec (Canada) and New York (USA).  First thing to note here that though the name suggests it to be Transformer but construction wise it is like a Slip Ring Induction Motor. The basic concept behind the VFT is a rotary transformer with three phase windings on both rotor and stator. A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the VFT. VFT is a controlled bi-directional device allowing flow of power in both directions i.e. from one Grid to another and vice ...

We have already discussed about staring methods of Single Phase Induction Motor. If you miss that please read here, 1)      Revolving Field Theory of Single Phase Induction Motors 2)      Starting Methods of Single Phase Induction Motors The schematic diagram of Capacitor Split Phase Motor is shown in figure below. Like in resistor split phase motor , there are two windings, Main and Auxiliary winding but the basic difference between the two method is that in Capacitor Split Phase Motors a capacitor of suitable value is connected in series with the auxiliary winding.  Capacitor is connected in series with the auxiliary winding to obtain the desired time phase displacement between the auxiliary winding current Ia and main winding current Im. A centrifugal Switch is also provided the cut out auxiliary winding when the speed of Single Phase Induction Motor reaches 70 to 80% of synchronous speed. You may like to...

As discussed in earlier post  Revolving Field Theory of Single Phase Induction Motors , a single phase induction motor with main stator winding alone has no inherent starting torque as the main stator winding only produces stationary pulsating air gap flux wave. For the development of starting torque, rotating field at the starting must be produced. There are various methods of starting a single induction motor, which can be classified as below, 1)     Split phase starting 2)     Shaded pole starting 3)     Repulsion motor starting 4)     Reluctance starting Generally a single phase induction motor is known by the method employed for its starting. Basically the selection of a particular single phase induction motor and choice of its starting method depends upon the following factors: 1)     Torque speed characteristics of load from starting to normal operating speed 2)  ...

Revolving Field Theory is used for the analysis of behavior of Single Phase Induction Motor. Before going into the details of Revolving Field Theory, first we should know why do we need to study this theory? Well, a Single Phase Induction Motor with its main winding alone can be represented as shown in figure below. When a Single Phase Induction Motor is supplied with a voltage source to its stator, an alternating flux is developed by the stator in the air gap which is stationary but pulsating in nature. This flux links with the Rotor winding and produces a current in the rotor bars. For the assumed direction of stator flux Øs as shown in figure above, the current distribution along the periphery of the Squirrel cage Rotor bars will be as shown in figure. This distribution of current in the rotor produces a flux Ør. Notice that the direction of rotor produced flux Ør is in opposition with the stator produced flux Øs which is in accordance with the Lenz’s law. As...

This is the simplest mode of starting of Induction Motor where the stator is directly connected to the mains supply as shown in figure below. The motor starts with its own characteristics. When it is switched on, the motor behaves like a transformer with its secondary, formed by the very low resistance rotor cage, in short circuit. There is a high induced current in the rotor which results in a current peak in the mains supply: Current on starting = 5 to 8 times of rated current. You may like to read, Difference between Induction Motor and Transformer Concept of Transformer Action Such a large current of short duration do not harm rugged squirrel cage induction motor , but the high current may cause objectionable voltage drop in the power supply line feeding to the Induction Motor. This large voltage drop causes undesirable dip in the supply line voltage which will affect the operation of other equipment connected to the same source of supply. If the su...

As per the equivalent circuit representation, a three phase Induction Motor is generalized transformer but there are some differences between the Induction Motor and a Transformer, they are summarized below. The basic and most important difference is that even though the equivalent circuit of Induction Motor and Transformer is same rotor of motor rotates where as secondary of transformer do not rotate. Another difference is that Transformer is an alternating flux machine while Induction Motor is rotating flux machine. Rotating flux is only possible when three phase voltage or poly phase which is 120° apart in time is applied to a three phase winding or poly phase winding 120° apart in space then a three phase rotating magnetic flux is produced whose magnitude is constant but direction keeps changing. In transformer the flux produced is time alternating and not rotating because the three phase windings are not 120° apart in space rather they are wound on the limbs of Transformer. There...