chapter 12 class9 science

A D.C. series motor is that which

• has its field winding consisting of thick wire and less turns
• has a poor torque
• can be started easily without load
• has almost constant speed

A three point starter is considered suitable for

• shunt motors
• shunt as well as compound motors
• shunt, compound and series motors
• all D.C. motors

As -the load is increased the speed of D.C. shunt motor will

• reduce slightly
• increase slightly
• increase proportionately
• remains unchanged

As there is no back emf at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit

• a resistance is connected in series with armature
• a resistance is connected parallel to the armature
• armature is temporarily open circuited
• a high value resistor is connected across the field winding

By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?

• Frame
• Shaft
• Commutator
• Stator

D.C. generators are installed near the load centres to reduce

• iron losses
• line losses
• sparking
• corona losses

D.C. generators are normally designed for maximum efficiency around

• full-load
• rated r.p.m.
• rated voltage
• all of the above

D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration We will select the

• series motor
• shunt motor
• compound motor
• any of the above

D.C. series motors are used

• where load is constant
• where load changes frequently
• where constant operating speed is needed
• in none of the above situations.

D.C. shunt motors are used for driving

• trains
• cranes
• hoists
• machine tools