MCQs on Electrical Drives

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01․ To save energy during braking-----------------braking is used?
all of the above

In regenerative braking energy is given back to the source at the time of braking.

02․ The concept of V/f control of inverters driving induction motors resuls in
constant torque operation
speed reversal
reduced magnetic loss
hormonic elimination

So, if we maintain the flux is a constant value, then torque also will be a constant value.

03․ Which of the following are electrical braking methods ?
all of the above

Braking is to stop the motor intentionally. The basic braking adapted is mechanical, which include noise, temperature rise, maintenance and repair. To reduce these effects electrical braking is used.These are all the different types of electrical braking methods. Electric braking is effective but alone it cannot stop the motor effectively. Therefore electrical braking is accompanied with mechanical braking which becomes effective braking system. In regenerative braking the motor feeds back the supply. In Plugging the motor first tends to slow and rotate in reverse direction, then the mechanical brake is applied.

04․ ———– was the first city in India to adopt electric traction.

Electric traction was introduced on Indian Railways in the year 1925 on 1.5 KV DC and the first electric train ran between Bombays Victoria Terminus and Kurla along the Harbour Line on February 3rd 1925. The total distance of 9.5 miles and that was flagged off by the Governor of Bombay “Sir Leslie Orme Wilson”.

05․ Which of the following state capital is not on broad gauge track?

Previously, in Jaipur the broad gauge track is not there. Now the Indian railway department has converted the meter gauge track into broad gauge track.

06․ Which braking is not possible in series motor?
Regenerative braking.
Dynamic braking.
Counter current braking.
Rheostat braking.

In case of regenerative braking back emf is greater than the supply voltage. In series motor back emf cannot exceed the supply voltage. So regenerative braking is not possible.

07․ Polarity of supply voltage is reversed in which type of braking?
Regenerative braking.
Dynamic braking.
None of these.

The polarity of supply voltage is reversed while motor is running, the type of braking is called counter current braking / plugging. In this case direction of rotation is reversed and direction of motor torque is also reversed.

08․ In industries which electrical braking is preferred?
Regenerative braking.
Dynamic braking.
None of the above.

Usually braking means converting kinetic energy to electrical energy and then to heat energy but in case of regenerative braking the kinetic energy converts to electrical energy and then feeding back to supply but it is not possible to feeding back to supply because of power electronic devises so that electrical energy will be charge into capacitors. Hence it is advantage to industries.

09․ Full form of VVVF control
variable voltage VAr frequency.
VAr variable voltage frequency.
variable VAr voltage frequency.
variable voltage variable frequency.

VVVF stands for variable voltage variable frequency control. The effect of VVVF speed control is to retain the shape of the torque-speed curve, but shift it along the speed axis. For VVVF control, because the shape of the torque-speed curve is the same at all frequencies, it follows that the torque of an induction motor is the same whenever the slip speed (rpm) is the same. With VVVF control, the speed range possible is from about 10 % to 150 % of rated speed

10․ What type electric drive is used in cranes?
Both A and C.

In multimotor drive system there are several drives, each of which serves to actuate one of the working parts of the driven mechanism.