3.12: DC Motor-Generator Theory Flashcards
- The voltage induced in a conductor moving in a magnetic field depends on the
strength of the magnetic field and:
(a) the length of the conductor and the relative speed of movement
(b) the resistance of the conductor and the speed of movement
(c) the area of the conductor and the direction of movement into or out from the
magnetic field.
(a) the length of the conductor and the relative speed of movement
- In a shunt -wound DC motor, back emf is :
(a) Caused by the contact resistance between the brushes and the commutator
surfaces
(b) Generated by the armature rotating in the magnetic field,
(c) Generated by eddy currents in the field pole pieces.
(b) Generated by the armature rotating in the magnetic field,
When the armature in a motor rotates in a magnetic field, voltage is induced in its windings. This voltage is called the back or counter emf and is opposite in direction to the voltage applied to the motor from the external source.
The current flowing through the armature, therefore, decreases as the counter emf increases.
- Which DC electrical motor is most suitable for high starting torque
requirements?
(a) The shunt motor.
(b) The series motor.
(c) The compound motor.
(b) The series motor.
A series motor should never be started without some mechanical resistace or the motor may operate at such a high speed that the armature falls apart
- In a DC Series Motor the back emf is:
(a) greater than the terminal voltage
(b) equal to the terminal voltage
(c) less than the terminal voltage.
(c) less than the terminal voltage.
Initially when the motor is turning slowly the back emf is small and therefore the current through the field windings and armature is high, generating strong magnetic fields which produce high starting torque. As the motor speeds up, the back emf increases, which causes torque to fall and speed to increase
- Reversal of rotation of a shunt-wound DC motor can be achieved by:
(a) Reversing the field connections,
(b) Reversing the armature connections,
(c) Reversing either the field or armature connections
(c) Reversing either the field or armature connections
- A primary characteristic of a DC shunt - wound motor is that its:
(a) Speed is approximately constant over a wide range of loads,
(b) Torque output is approximately constant over a wide range of speeds,
(c) Armature current is very low when the motor has stalled.
(a) Speed is approximately constant over a wide range of loads,
The starting torque is considerably less than that developed by a series motor of equal size but when all load is removed, it assumes a speed slightly higher than the loaded speed. For this reason, considered a constant speed motor
- The principal advantage of a series field wound motor is the:
(a) RPM is very stable, even with load changes
(b) starting torque is higher than on all other electric motors
(c) motor does not need brushes.
(b) starting torque is higher than on all other electric motors
- To reverse the direction of a DC Compound Wound Motor, it is necessary to
reverse the connections to the:
(a) series field
(b) shunt field
(c) the armature
(c) the armature
Note: the compound wound DC motor incorporates best parts of both series wound and shunt wound dc motors
- What happens to a series wound DC Motor if the external DC supply polarity is
reversed?
(a) The motor will not change the direction of rotation
(b) The motor will change the direction of rotation
(c) the motor will stop rotating
(a) The motor will not change the direction of rotation
- In a DC Shunt Wound Generator, the generated EMF is:
(a) greater than the terminal voltage
(b) the same as the terminal voltage
(c) less than the terminal voltage
(a) greater than the terminal voltage
- Armature reaction in a DC generator can be corrected by:
(a) Increasing the number of windings in the armature,
(b) The use of interpoles or compensating windings,
(c) Through adjustment of the brush resistance.
(b) The use of interpoles or compensating windings,
- The function of a commutator fitted to a generator is :
(a) to convey the current to and from the windings
(b) to convert the a.c. generated into a d.c. output
(c) to prevent sparking when the load changes
(b) to convert the a.c. generated into a d.c. output
- Which of the following DC Motors delivers the greatest starting torque:
(a) Shunt wound
(b) Series wound
(c) Compound wound
(b) Series wound
- Which of the following DC Motors delivers the most consistent speed:
(a) Shunt wound
(b) Series wound
(c) Compound wound
(a) Shunt wound
DC Series Motors - High starting torque, speed drops with increasing load
Shunt wound - low starting torque but constant speed
- Thermal runaway is a serious consideration when using:
(a) DC Shunt Motors
(b) DC Series motors
(c) Compound Motors.
(b) DC Series motors
Wave winding =?
Copper cable on brushes is known as?
What are the 3 types of DC generators (wound)?
What happens resistance and voltage in a series wound DC gen?
What happens resistance and voltage in a shunt wond DC gen?
What is best winding for starting?
What is best winding for constant speed?
If given Peak value, how would you find RMS?
If given RMS value, how would you find Peak?
If given peak value, how would you find Peak to Peak?
Voltage induced in a conductor depends on the …. (3)
In a Shunt wound DC motor, how is back EMF generated?
What are x2 characteristics about a series wound motor?
How would you reverse the direction of a shunt wound motor?
What should be noted regarding a shunt wound motor in terms of speed?
How would you reverse the direction of a Compound-wound motor?
What happens in a Series wound motor if power supply polarity is swapped?
What should be noted about EMF and terminal voltage in a shunt-wound generator?
Armature reaction in a DC generator can be corrected by use of _________ or _____________
Thermal runaway is associated with what type of motor?
Identify each type
Draw a schematic of each of the following:
(a) Shunt motor
(b) Series motor
(c) Compound motor
