science

The synchronous speed Ns of a four-pole

machine connected to a 50 Hz supply will be:

• 200 rpm

• 750 rpm

• 1500 rpm

• 3000 rpm

four-pole induction motor has a synchronous speed of 1500 and a rotor speed of

1425 rpm, calculate the percentage slip

• 2%

• 5%

• 52.5%

• 75%

A laminated cylinder of silicon steel with copper

or aluminium bars slotted into holes around the

circumference and short-circuited at each end of

the cylinder, is one description of:

• a cage rotor

• an electromagnet

• a linear motor

• an induction motor

All electric motors with a rating above 0.37kW

must be supplied with:

• protection by MCB

• protection by HBC fuses

• a motor starter

• remote stop/start switches

A star delta starter:

• increases the initial starting torque of the motor

• reduces the initial starting current of the

  motor

• gives direct connection of the main voltage to

  the motor during starting

• requires only three connecting conductors

  between the motor and starter

An auto-transformer starter:

• increases the initial starting torque of the motor

• increases the initial starting current of the

  motor

• gives direct connection of the mains voltage

  to the motor during starting

• requires only three connecting conductors

  between the motor and starter.

A series d.c. motor has the characteristic of

• constant speed about 5% below synchronous

  speed

• start winding 90° out of phase with the run

  winding

• low starting torque but almost constant speed

• high starting torque and a speed which varies

  with load.

A shunt motor has the characteristic of:

• constant speed about 5% below synchronous

  speed

• start winding 90° out of phase with the run

  winding

• low starting torque but almost constant speed

• high starting torque and a speed which varies

  with load

A three-phase induction motor has the characteristic

of:

• constant speed about 5% below synchronous

  speed

• start winding 90° out of phase with the run

  winding

• high starting torque but almost constant

  speed

• high starting torque and a speed which varies

  with load.

A single-phase induction motor has:

• variable speed control operated by a centrifugal

  switch

• a start winding approximately 90° out of phase

  with the run winding

• high starting torque and almost constant speed

• high starting torque and a speed which varies

  with load.