In Dc Machine The Number Of Slots Per Pole Usually Lies

1. In Dc Machine The Number Of Slots Per Pole Usually Lies Within
2. In Dc Machine The Number Of Slots Per Pole Usually Lies Across
3. In Dc Machine The Number Of Slots Per Pole Usually Lies Given

Choice of specific electric and magnetic loadings in DC machine

Effect of higher value of q

« Back CONTENTS Next » 1. The number of parallel path in a simplex lap winding is equal to (a) number of poles (b) number of pole pairs (c) two (d) two times number of pole 2. The number of parallel path in a simplex wave windings are equal to (a) two. N = number of turns per coil. Whereas in a 4 pole machine, 180 electrical degrees is equal to 90 mechanical degrees. Most DC machines are designed in such a. There are various types of DC Motors. They are Shunt motor, Series motor and Compound motor. Also See: Construction of DC Generator. The EMF equation of DC Motor is given by the equation shown below: Where, E b is the induced emf of the motor; N is the speed; P is the number of poles; ϕ is the flux per pole. Z is the total number of conductors.

Note: Since armature current I_a and number of parallel paths A are constants and armature diameter D must be as less as possible or D must be a fixed minimum value, the number of armature conductors increases as q = I_a Z / A π D increases.

(a) As q increases, number of conductors increases, resistance increases, I²R loss increases and therefore the temperature of the machine increases. Temperature is a limiting factor of any equipment or machine.

(b) As q increases, number of conductors increases, conductors/slot increases, quantity of insulation in the slot increases, heat dissipation reduces, temperature increases, losses increases and efficiency of the machine reduces.

(c) As q increases, number of conductors increases, armature ampere-turns per pole AT_a / pole = (I_a Z / 2 A P) increases, flux produced by the armature increases, and therefore the effect of armature reaction increases. In order to overcome the effect of armature reaction, field mmf has to be increased. This calls for additional copper and increases the cost and size of the machine.

(d) As q increases, number of conductors and turns increases, reactance voltage proportional to (turns)² increases. This leads to sparking commutation.

Effect of higher value of B_av

(a) As B_av increases, core loss increases, efficiency reduces.

(b) As B_av increases, degree of saturation increases, mmf required for the magnetic circuit increases. This calls for additional copper and increases the cost of the machine.

It is clear that there is no advantage gained by selecting higher values of q and B_av. If the values selected are less, then D²L will be large or the size of the machine will unnecessarily be high. Hence optimum value of q and B_av must be selected.

- In general q lies between 15000 and 50000 ampere-conductors/m.

- Lesser values are used in low capacity, low speed and high voltage machines.

- In general B_av lies between 0.45 and 0.75 T.

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1. The number of parallel path in a simplex lap winding is equal to
(a)number of poles
(b)number of pole pairs
(c)two
(d)two times number of pole

2. The number of parallel path in a simplex wave windings are equal to
(a)two
(b)number of poles
(c)number of poles pairs
(d)no of coils

3. The number of parallel path in a 8-pole duplex wave winding is equal to
(a)2
(b)4
(c)8
(d)16

4. The number of parallel path in a 8-pole duplex lap winding is equal to
(a)4
(b)8
(c)16
(d)32

5. In DC machine, the fractional pitch winding is used to
(a)produce vibration
(b)reduce sparking
(c)avoid hunting
(d)avoid magnetic locking

6. In a DC machine, the sparking at the brushes is due to
(a)low contact resistance
(b)reactance voltage
(c)armature reaction
(d)over excitation

7. When the field excitation of a DC generator increases, its generated emf
(a)decreases
(b)increases
(c)remain same
(d)first increases then constant

In Dc Machine The Number Of Slots Per Pole Usually Lies Within

8. The field coil of a DC machine are usually made of
(a)copper
(b)aluminium
(c)carbon
(d)mica

9. Then insulating material used between commutator segment is generally
(a)paper
(b)mica
(c)plastic
(d)fibre

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10. The function of dummy coils in DC machines is to
(a)reduce armature reaction
(b)provide alternate flux
(c)reduce losses
(d)provide mechanical balancing

11. Permeability is reciprocal of
(a)susceptibility
(b)reluctivity
(c)permittivity
(d)conductivity

12. The purpose of using dummy coil in a DC machine is to
(a)reduce losses
(b)reduce armature reaction
(c)provide mechanical balance to armature
(d)improve power factor

13. The rotor part of a DC machine carries
(a)armature winding
(b)field winding
(c)either field or armature winding
(d)none of the above

14. If a field winding designed for series connection is connected in parallel, the machine will
(a)draw very high current
(b)stand still
(c)run normally
(d)run at slow speed

15. In the commutative compound machine, the mmf of the series field winding will ________ to the mmf of the shunt field winding.
(a)add
(b)minus
(c)same
(d)half

16. Which is the basic dominant factor in the production of the magnetic flux in a compound machine?
(a)Shunt field mmf
(b)Series field mmf
(c)Both shunt and series field mmf
(d)None of the above

17. The principle of dynamically induced emf is used in
(a)generator
(b)battery
(c)transformer
(d)all of the above

18. A dummy coil may required in a DC machine, if the machine is
(a)lap wound
(b)wave wound
(c)either lap or wave wound
(d)none of the above

19. In a DC Machine, the frog leg wound armature are designed for the use with
(a)high current high voltage
(b)high current low voltage
(c)moderate current and moderate voltage
(d)low current and low voltage

20. Which of the following is main factor to determine the size and rating of and electric machine?
(a)Current
(b)Temperature
(c)Speed
(d)Voltage

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In Dc Machine The Number Of Slots Per Pole Usually Lies Across

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In Dc Machine The Number Of Slots Per Pole Usually Lies Given

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