Unit 31 DC Motors

Counter-electromotive force (CEMF) and back-EMF are names for the voltage induced into the armature of a DC motor.

True

Permanent magnet motors have _____ efficiency than conventional field-wound motors.

higher

shunt field motors are connected in ______ with the windings

parallel

What provide(s) the magnetic field necessary for the operation of a DC machine?

pole pieces

purpose of a field loss relay

prevent a compound motor from becoming a series field motor.

The ServoDisc motor is also often called a _____ motor.

printed-circuit

When a motor is connected differential compound, the _____ determines the direction of rotation of the motor at no load or light load.

shunt field

Pulse Width Modulation (PWM)

speed of the disc serve motor can be varied by changing amount of voltage supplied to the armature

servomotor

•small, lightweight armature that contains very little inertia •2-6 poles •run at high speed but can be stopped/reversed quickly

A square wave voltage varies between 0 V and 12 V. The positive pulses are 50 μs wide and there is a 50 μs gap between them. What is the DC average of the waveform?

6 V

1 hp=

746 watts

A square wave voltage varies between 0 V and 12 V. The positive pulses are 75 μs wide and there is a 25 μs gap between them. What is the DC average of the waveform?

9 V

A device that converts electrical energy into mechanical energy is called a(n) _____.

motor

The servomotor featuring a fiberglass and copper rotor is the _____ motor.

ServoDisc

The compound motor connection that should generally be avoided is the _____.

differential

When the field winding of a DC motor is connected in parallel with the armature, the motor is called a _____ motor.

shunt

cumulative compound

windings aid each other in magnetism

differential compound

windings oppose each other in magnetism

permanent magnet motor

•contain wound armature and brushes like a conventional dc motor. •pole pieces are permanent magnets •more efficient than conventional than dc motors

brushless dc motor

rotor/armature has permanent magnets. No wound armature, commutator, or brushes

speed relation

amount by which the speed decreases as load is added

Counter Electomotive Force

induced voltage is opposite of the polarity of the voltage applied

commutator

performs function of a rotary switch

The speed of the disc servomotor can be varied by changing the amount of voltage supplied to the armature. The voltage is generally varied using _____.

pulse-width

series field motors are connected in ____ with the windings

series

A compound motor has _____ field windings.

series and shunt

The speed regulation of a DC motor is proportional to the _____.

armature resistance

In a compound DC motor control circuit, the field loss relay will disconnect the _____ if power is lost to the _____.

armature; shunt field

The motor that does not contain a wound armature, commutator, or brushes is the _____ motor.

brushless DC

When a DC machine is used as a generator, the _______________ performs the function of a mechanical rectifier to change the AC produced in the armature into DC before it exits the machine through the brushes.

commutator

The pushing or turning force created by the magnetic field of the pole pieces and the magnetic field of the loop or armature is called _______________.

torque

field loss relay

protective device connected in series with the shunt field

Small permanent magnet motors that have lightweight rotors are often used as _____.

servomotors

The DC motor, known as a "constant speed motor," is the _____ motor.

shunt