Question

23. What limits the amount of current through the armature of a DC motor when it is running? 24. What is CEMF and what cause it? 2 25. What are the three factors that determine the amount of CEMF produced in a motor? 26. What are the 3 basic types of DC motors? 3 27. Which type of DC motor has the highest starting torque? 28. Which type of DC motor has the best speed regulation? 29. What type of DC motor should never be operated without a load attached? 30. How do you reverse the direction of a DC motor? 31. How can a DC Shunt motor be made to operate below base speed? 32. How can a DC Shunt motor be made to operate above base speed? ? 33. Who is credited with the first use of the term horse-power?

any help is greatly appreciated !

thanks in advance !

Answer 1

. When a DC motor is runing, back emf limits the amount rmature current - Current a de motor nakes it rotate in magnetic field, but because of Lena's low (the effect opposes the Cause) a back emt or counter emt is generated which opposes this current. When load increases, back emt increases limiting the current in the armature. 29) Counter - electromotive force (CEMF) also known as back emt is generator output of a dc motor at a given speed. When the current in coil interacts the magnetic field, the coil starts rotating but due to current in the coil, a counter emt is generated by Lenz's loul Which opposes the current Change in the Coil, which is CEME. CEMF = Abblied Voltage - Armature drop

in dc motor. - By haraday's clous, it is known that a coil rotating in magnetic field induces voltase. This is the factor due to whes back emt is induced in dc motors. The armature Coil rotating in magnetic field is the cause of CEMF. Due to CEME, a motor rotates. 25 CEMf is determined by following factors in motor- → Speed of the motor (N) → field flun for encitation of motor (0). → Ratio of total number of armature concluctors (z) to no- of parallel paths in armature winding (A). je (24) CEMF is given by 8-No. of pokes (E=826 N- Speed in 8.b.5.

(26) Types of DC motors Self- permanent Magnet DC Motor encited At Separately encited Encitation js giren from a separate - Circuit One these types of DC Motors, Perma- nent magnets are used to create the magnetic field. field winding does not require a Seported Source. field is feed from the arsnature itself. It is of two types. (Series & shunt) field winding requires a Seborde soure to supply Series Shunt Shunt & G

DC Series motors have highest starting torque, which is why they are best for traction and drives which require high starting torque. for any motor, 1 Torque alblun). (Armature a current) ),е еф. Та but for series motor, . Ia $ a Ia - as field is in series as frera Et field with oronature Corries arronature - Armature 1. Current. M2 So, (A x I?) Series motor Now, at the time of starting because of absence of CEMF, a high (ia) flows through motor, which gives high Starting torque (Ixla ²) ce T

8) Amotor which has nearly, a Constant speed or the difference between no load and full load is Very less is said to have good speed regulation, which is true for DC Shunt motors. 1. So, shunt motors have best speed regulations. 20 DC series motor should never be operated without load attached. At no load, very small current flows through the armature, which results in very small flun. But (@at ) flun is inversly proportional to the speed making the speed dangerously Torque high, which may speed destroy the motor • Load

o The direction of DC motor can be reverse by either changing the field terminals or armature terminals but not both. & for below bose speed in DC 32) Shunt motor, armature voltge Contral is used. When armature voltage is reduced, back emb reduces making the speed slower. But armeture voltage cannot be increased above rated voltage, so a speed below bose speed is obtained for speed above bose speed, field flun Control is used. The field when reduced speed of the motor increases (oxt) for constant load torque (33) James Watt - (termed horsepower to measure power of steam engines).