Electrical machines miscellaneous
- A synchronous generator is feeding a zero power factor (lagging) load at rated current.
The armature reaction is
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The distributor of current and fluxes with phase angle φ = + π 2 
For lagging load is the current will be shifted in space by an angle π 2
from maximum emf which coincide with centre of pole. The field created by the armature reaction mmf will be in opposite to main field flux and will therefore by wholly dimagnetizing effect.Correct Option: B
The distributor of current and fluxes with phase angle φ = + π 2 For lagging load is the current will be shifted in space by an angle π 2
from maximum emf which coincide with centre of pole. The field created by the armature reaction mmf will be in opposite to main field flux and will therefore by wholly dimagnetizing effect.
- A 220 V DC machine supplies 20 A at 200 V as a generator. The armature resistance is 0.2 ohm. If the machine is now operated as a motor at same terminal voltage and current but with the flux increased by 10%, then ratio of motor speed to generator speed is
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For Generator, E = V + Ia Ra
= 200 + 20 × 0.2 = 204 V
For Motor, V = E – Ia Ra
E = 200 – 20 × 0.2 = 196 V∴ E1 = N1 × φ1 E2 N2 φ2 ⇒ 204 = N1 × 1 196 N2 11 and N2 = 196 = 0.87 N1 204 × 1.1 Correct Option: A
For Generator, E = V + Ia Ra
= 200 + 20 × 0.2 = 204 V
For Motor, V = E – Ia Ra
E = 200 – 20 × 0.2 = 196 V∴ E1 = N1 × φ1 E2 N2 φ2 ⇒ 204 = N1 × 1 196 N2 11 and N2 = 196 = 0.87 N1 204 × 1.1
- A 400 V, 50 Hz, three phase balanced source supplies power to a star connected load whose rating is 12 √3 k VA, 0.8 pf (lag). The rating (in kVAR) of the delta connected (capacitive) reactive power bank necessary to bring the pf to unity is
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Given VA readings = 12 √3 kVA.
cos φ = 0.8 pf (lag)
To bring power factor to unity cos φ of delta connected network = 0.8 pf (leading)
sin φ = 0.6
kVAR (Reactive Power) = 12 √3 × 0.6 = 12.47 kVARCorrect Option: D
Given VA readings = 12 √3 kVA.
cos φ = 0.8 pf (lag)
To bring power factor to unity cos φ of delta connected network = 0.8 pf (leading)
sin φ = 0.6
kVAR (Reactive Power) = 12 √3 × 0.6 = 12.47 kVAR
- In a DC machine, which of the following statements is true ?
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By the use of interpoles the armature cross flux to neutralized and in addition some flux is pr oduced there. This addition flux induces rotational emf in the commuted coil in such a direction as to oppose the reactance voltage thus we get zero current in commutated coil and a sparkless commutation.
The effect of armature reaction under pole shoe can be neut r alized by using compensat ing winding. It neutralizes the armature flux.Correct Option: B
By the use of interpoles the armature cross flux to neutralized and in addition some flux is pr oduced there. This addition flux induces rotational emf in the commuted coil in such a direction as to oppose the reactance voltage thus we get zero current in commutated coil and a sparkless commutation.
The effect of armature reaction under pole shoe can be neut r alized by using compensat ing winding. It neutralizes the armature flux.
- For a single phase capacitor start induction motor which of the following statements is valid ?
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For changing the direction of rotation, we have to change main winding terminal and direction magnetic flux will be reversed.
Correct Option: B
For changing the direction of rotation, we have to change main winding terminal and direction magnetic flux will be reversed.
