Question

Question (1)

Explain with relevant diagrams, resistance commutation and how it helps in improving commutation.

 

Question (4)

A 240 V, 4 – pole shunt motor running at 1000 r.p.m. gives 15 h.p. with an armature current of 50 A and field current of 1 A. The armature winding is wave connected and has 540 conductors. The armature resistance is 0.1 Ω and the drop at each brush is 1 V. Calculate (i) the useful torque (ii) the total torque (iii) useful flux per pole and (iv) iron and friction losses.

 

Question (5)

A 1100 V, 50 Hz, delta-connected induction motor has a star-connected slip-ring rotor with a phase transformation ratio of 3.8. The rotor resistance and standstill leakage reactance are 0.012 Ω and 0.25 Ω per phase respectively. Neglecting stator impedance and magnetizing current, determine (i) the rotor current at start with slip-rings shorted (ii) the rotor power factor at start with slip-rings shorted (iii) the rotor current at 4 % slip with slip-rings shorted (iv) the rotor power factor at 4 % slip with slip rings shorted (v) the external resistance per phase required to obtain a starting current of 100 A in the stator supply lines.

Answer 1

Question 1:

Methods of Improving Commutation:
There are two practical ways of improving commutation i.e. of making current reversal in the short-circuited coil as sparkless as possible. These methods are known as (i) resistance commutation and (ii) e.m.f. commutation (which is done with the help of either brush lead or interpoles, usually the later).

Resistance Commutation

та lut BGh kb IL = Ia + = 51A V=240v. 3000rpm Та - 50 A and If Ta + If V=Eb + IQ Bat Bruch voltage drop. each bruch es er Go Brush voltage drop = no. of brushes x v.Drop - 2x12v. Eb - V-raha-2 2.30-50X0.1-2 +223 votos. Fb = GOXA Z- фхыр of flux. per pol. per pole in coebere. no. of Conductors (Speed nom = no. of polees. t =p for Lap connected coinding no. of parallel paths A=2 for coave connected winding tobxGOXA ZNP = 12-8888 mcob. P A 223 XGOX2 (Llis 540X 1000 X 4 223 X JO 69 11150 X60 tx 1000 (1) Tatal torque (TT) = ? Poe EbxIQ = 11:15060. Pd 2NTI (pocoer developed at armature) 60 Tr 60 sxx 106.47465 N-m 1 hp = 736 watte. Go Shoeft poccer = 15 hp = 15x736 = 11.040 kco, PGh= 10040 coatt., Tsh-Shaft torque en N-m (L. Tsh 105.42423 43 N-m (Troos) due to friction and from LOGG - Ty or Ta Toh 1.05 0422628 8- m, ITNXTs b (lv): Proos (Iron & friction loos) - 110.watts. asetewedbafit torque also called as cuse free torque 1104OXO 2000 LOGt Torque GO trurit pecoer +106cels.

This method of improving commutation consists of replacing low-resistance Cu brushes by comparatively high resistance carbon brushes.From Fig., it is seen that when current I from coil’ C reaches the commutator segment b, it has two parallel paths open to it. The first part is straight from bar ‘b’ to the brush and the other parallel path is via the short-circuited coil B to bar ‘a’ and then to the brush. If the Cu brushes (which have low contact resistance) are
used, then there is no inducement for the current to follow the second longer path, it would preferably follow the first path. But when carbon brushes having high resistance are used, then current I coming from C will prefer to pass through the second path because
(i) the resistance r1 of the first path will increase due to the diminishing area of contact of bar ‘b’ with the brush and because (ii) resistance r2 of second path will decrease due to rapidly increasing contact area of bar ‘a’ with the brush.

Hence, carbon brushes have, usually, replaced Cu brushes. However, it should be clearly understood that the main cause of sparking commutation is the self-induced e.m.f. (i.e. reactance voltage), so brushes alone do not give a sparkless commutation; though they do help in obtaining it.

The additional advantages of carbon brushes are that

(i) they are to some degree self-lubricatingand polish the commutator and

(ii) should sparking occur, they would damage the commutator less than when Cu brushes are used.

But some of their minor disadvantages are :

(i) Due to their high contact resistance (which is beneficial to sparkless commutation) a loss of approximately 2 volt is caused. Hence, they are not much suitable for small machines where this voltage forms an appreciable percentage loss.

(ii) Owing to this large loss, the commutator has to be made some what larger than with Cu brushes in order to dissipate heat efficiently without greater rise of temperature. (iii) because of their lower current density (about 7-8 A/cm2 as compared to 25-30 A/cm2 for Cu brushes) they need larger brush holders.

Thank You

Answer 2

Question 4

та lut BGh kb IL = Ia + = 51A V=240v. 3000rpm Та - 50 A and If Ta + If V=Eb + IQ Bat Bruch voltage drop. each bruch es er Go Brush voltage drop = no. of brushes x v.Drop - 2x12v. Eb - V-raha-2 2.30-50X0.1-2 +223 votos. Fb = GOXA Z- фхыр of flux. per pol. per pole in coebere. no. of Conductors (Speed nom = no. of polees. t =p for Lap connected coinding no. of parallel paths A=2 for coave connected winding tobxGOXA ZNP = 12-8888 mcob. P A 223 XGOX2 (Llis 540X 1000 X 4 223 X JO 69 11150 X60 tx 1000 (1) Tatal torque (TT) = ? Poe EbxIQ = 11:15060. Pd 2NTI (pocoer developed at armature) 60 Tr 60 sxx 106.47465 N-m 1 hp = 736 watte. Go Shoeft poccer = 15 hp = 15x736 = 11.040 kco, PGh= 10040 coatt., Tsh-Shaft torque en N-m (L. Tsh 105.42423 43 N-m (Troos) due to friction and from LOGG - Ty or Ta Toh 1.05 0422628 8- m, ITNXTs b (lv): Proos (Iron & friction loos) - 110.watts. asetewedbafit torque also called as cuse free torque 1104OXO 2000 LOGt Torque GO trurit pecoer +106cels.

Answer 3

Question 5: