SSC JE 2014 Electrical question paper with solution(Evening-shift)

An induction motor is a single excited machine. The crawling word it self-suggest crawl means moving at low speed.
This characteristic is the result of improper functioning of the motor which means either motor is running at a very slow speed or it is not taking the load.

The resultant speed is nearly 1/7th of its synchronous speed. This action is due to the harmonics fluxes produced in the gap of the stator winding of odd harmonics like 3rd, 5th, 7th, etc.

Synchronous speed Ns is

Ns = 120f/P

Ns = 120 × 50/4 = 1500 rpm

Slip speed = Ns – Nr

= 1500 – 1440

= 60 rpm

Why does low voltage winding of the transformer placed near to core?

The LV winding of the Transformer is placed near the core in order to reduce the cost of insulation and the size of the Transformer. The insulation is directly proportional to the voltage so, If the HV winding of the Transformer is placed near the core, the insulation would have to be thicker which leads to higher cost.

Placing the HV winding after the LV winding makes the much lesser thickness of insulation for the HV winding.

In star connection

VL = √3VPh

IL = Iph

In delta connection

VL = Vph

IL = √3Iph

VL = line voltage

Vph = phase voltage

IL = line current

Iph = phase current

Line-to-line voltage V_L  = √3 × Vp

V_L = √3K

In an Auto-transfer the power is transferred both inductively and conductively

For inductive power transfer in auto-transformer is

Total power input × (V1 -V2)/V1

And power transfer conductively is given as

total power input × V2/V1

For medium and large capacity transformers circular coils are used because they are mechanically stronger.It is more economical to use circular shape coils around the stepped core.

For the same area of the iron core, required by magnetic flux the diameter of the circumscribing circle (d) get reduced with an increase in the number of steps.

The reduction of diameter “d” will reduce the mean length turn of the winding around the core. Thus lesser copper (mean turn wise) will be used and the cost will reduce.

The core area also can be better utilized for cooling purposes and mechanically the core structure will be stable with stackings intact.

Hopkinson’s test is also referred to as the regenerative or Back-to-Back test.  It is a full load test and required two identical machines are connected parallel, whereas the first machine gets excited from the source so it acts as a motor and is mechanically coupled with the next machine and makes it run as a generator.

The electrical output power from the generator (second machine) is fed to the motor(First machine) and vice-versa. Hopkinson test is performed to measure the efficiency of the DC machine.

Terminal voltage of DC shunt generator is

V = E – IaRa

Where E is generated EMFI

Ia is armature current

Ra Armature resistance

V = 230 – 0.1 x 200

V = 230 – 20

V = 210 v

• The commutator is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
• The commutator converts the alternating current generated in the armature into the unidirectional current

Fleming left-hand rule is applicable to DC motor because direction of rotation of motor or direction of force experienced i.e motoring action can be determined by Fleming’s left hand rule.