VFD (VARIABLE FREQUENCY DRIVE)

Component No. 1:Six pulse rectifier: Converts the 3 ph AC voltage into DC voltage

Component No. 2: Capacitor bank : Energy storage which stabilizes the intermediate ckt DC voltage

Component No. 3: Six pulse IGMT inverter: Converts the DC V to ACV & vice versa. Motor                                                  operation is controlled by switching the IGMTS.

Component No. 4: Motor control: The motor control is based on Direct torque control (DTC) method. Two phase currents & DC link voltage are measured and used for the control, the third phase current is measured for earth fault protection.

Ø  How VFD is used as a Energy saving?

Because of it reduces the motor speed as per demand. When the speed is reduced he power consumption will also reduces. Eg. Power = (Speed)³.

Ø  What happens when DC supply is given to AC motor?

No rotating magnetic field will be generated & hence the motor will not rotate.. No back emf will be generated. Reactance will not com as frequency is zero. Rotor impedance will remain isolated from equivalent ckt of stator. Only resistance of stator ckt (which is very low) will be there. Hence stator current will be very high so motor will burn.

Ø  What will happen if AC power supply is given to the rotor of an alternator?

Excitation AC current is constant. The rotor will be rotated by a motor or turbine. It will cause emf induced in the rotor. If we give AC supply to the rotor, the flux will be variable & cause emf induced in the stator & it will act as a motor.

Ø  What is the effect of PF on the size of alternator?

If PF is less, KVA rating on the generator will be more. Because KW= KVA/PF. If PF reduces, KVA rating goes up for the same KW rating. So generator size will be more than the normal. For low PF, current will also high and increase in losses at all stages.

Ø  What is the major difference between PT & CVT?

PT is inductive step down transformer used for measurement of voltage & protection. Where a CVT is capacitive voltage transformer consisting a stock of series connecting capacitor. The voltage across the capacitor is used to measure the voltage. It also serves the purpose of power line carrier communication.

Ø  Why ACB is not used in place VCB at high transmission system?

VCB means vacuum at higher potential. There is a change of ionization of air molecules but if it is vacuum. Without a vacuum there will not be any change of ionization of air molecules. Vacuum has a high arc quenching property compared to air. Its dielectric strength is equal to 8 times of air.  VCB works on zero natural current principal.

Ø  Why layers of charcoal & salt are used in Earth pits?

The layers of charcoal & salt helps to maintain low resistance for earth fault current because of ionic behavior of salt & charcoal. They will maintain moisture around the earth pit.

Ø  How % impedance affect on the size of the power transformer?

% impedance is used to determine the max. fault current. The Transformer is capable of delivering to a down stream short ckt current.

Fault current = FLC x 100/% Imp. Fault current is also named as MVA.

Fault MVA = 1.73 x Fault current x V

Ø  What is our max demand/

3500KVA

Ø  Total connected Load?

4250 KW

Ø  What will happen if MD exceeds & what will you do?

I f MD exceeds, we have to pay penalty. To avoid this we will cut our unwanted load,

Ø  In how much time MD is changed in meter?

Max. in 6 minutes

Ø  In substation, what are the merits for human safety?

1.      Remote control panel

2.      Insulation mat in front of panel

3.      Insulated tools & meters

4.      Electrical safety shoe, HT gloves, face screen etc

5.      Fire extinguishers.

Ø  When we are using 433V supply, why 33Kv supply is taken from MSEB?

By increasing voltage, we have minimized the current due to following advantages:

1.      Due to minimized current, cable size & area is decreased.

2.      Range of switching capacity is also decreased.

3.      Saving money.

Ø  Types of CT?

1.      Metering CT, 2. Protection CT.

Ø  Difference between Metering CT & Protection CT?

Metering CT is connected on Load current and protection CT is connected on Fault Current. Fault current is always more. So metering CT will not work on Protection CT but protection CT will work on Metering CT but accuracy will not be there. Saturation current of Metering CT is low & that of Protection CT is high. Saturation current is depend upon the material of core.

Ø  How Earthing is checked?

By earth tester or by test lamp, keeping between phase & earth.

Ø  How much the earth resistance?

It should be between  0.5 ohm to 1 ohm

Ø  What will do if earth resistance is 3 ohm? And what will you do?

Solution of salt is to be added in earth pit. Still it is not decreasing we will provide parallel earthing.

Ø  Type of earthing.

SN	PLATE EARTHING	PIPE EARTHING
1	Size of plate: 60mm x 60mm x 3mm	Pipe size: dia 25mm, 2.5 M length,
2	Length of pit – 3 mtrs	Length of pit – 3 mtrs
3	Plate will vertical	No of holes with 7cm gap in the pipe
4	Layer of salt & charcoal	Layer of salt & charcoal
5	Not less than 8 SWG copper wire	Not less than 8 SWG copper wire

Ø  Why charcoal is used?

It is a soft conductor so its conductivity increases

Ø  Why salt is used?

It maintenances wetness

Ø  Why grey color is used for electrical panel & motors?

Cooling efficiency in grey color is more compared to other colors.

Ø  What are the insulation materials?

Mica,  Wood, Bakelite, Asbestos, Rubber, PVC, glass.