Flashcards in True Or False - DC Machines Review Deck (60):
The direction of rotation for a DC series motor may be reversed by interchanging only the field leads
True. The direction of current flow through the armature does not change, just the field polarity
In a cumulative compound motor, reversing the series field connections will change the direction of rotation
False. But it will change the motor to differential compound
The best method of reversing rotation for a shunt motor with interpoles is to interchange the leads at the brushes
False, this would change the polarity of the field and the armature as well, thus the same rotation
It is best to reverse the shunt field leads of a cumulative compound motor to reverse rotation.
False, this would convert the machine to a differential compound motor
If the supply lines to a series DC motor are reversed, the armature will rotate in the opposite direction.
False, both the field and armature current directions are reversed resulting in no change in rotation
A two pole motor has its field windings series connected, and then paralleled with the armature. This indicates a series motor.
False, parallel with the armature means a shunt motor
An under compounded generator has better voltage regulation than a straight shunt generator (of equal rating).
True, only differential compound has poorer voltage regulation than the shunt generator
A series DC motor may be connected as a shunt motor, and a shunt motor may be connected as a series motor, if necessary.
False, due to the physical differences of the shunt and series windings
As a general rule, to change rotation for a compound motor, it is best to reverse current flow through the armature and reverse both field windings.
False, results in same rotation
The starting torque of a cumulative compound motor, per amp of armature current, is considered higher than that for a shunt motor.
True. Series has good starting torque, shunt is poor, and compound is between them
Differential connected motors have lower starting torque than cumulative connected compound motors.
True, the series field opposes the shunt
The series motor has better speed regulation than a cumulative compound motor.
False, no load speed is very high
The load current of a self excited shunt generator is simply reversed by reversing the direction of rotation.
False, there would be no buildup of voltage without reversing the field connections
The polarity of a separately excited shunt generator can be reversed, simply by changing the direction of rotation, or interchanging the field leads
A shunt motor, started with resistance inserted in its field, will develop higher starting torque than without resistance.
False. High field resistance - low field current - low field flux - low torque
Differential compounded motors are used more extensively than cumulatively compound motors.
If the armature circuit voltage of a motor with a constant load is reduced, then the speed will increase.
False, results in lower armature current, lower torque and lower rpm
Less current through the field of a shunt motor will cause a speed increase.
True. Lower flux, lower CEMF, higher armature current, higher torque and rpm
If an over compounded motor is differentially connected, load changes may create unstable speed conditions.
True, poorer speed regulation closer to series motors. Reversing under high load could cause it to over speed.
The starting torque of a differential motor is much higher than the starting torque for a cumulative type motor.
False, fields oppose
If the series field of a cumulative compound motor is bypassed (shorted out), the speed of the motor will increase.
True. Less flux, less CEMF, higher torque
A cumulative motor is to be tested for proper connections. Rotation will be the same if run as a series motor (shunt field lead opened), or as a shunt motor (series field shorted out).
True, would reverse direction of differentially compounded
A differential motor started under heavy load may start up in a direction opposite to that desired.
True, the series flux may overpower the shunt flux due to high inrush current
A shunt generator may be either self or separately excited
The torque of a motor calculated by HP = TN/5252 is the full load torque.
If the speed of a shunt motor is increased, by a field current change, then the armature current will also increase momentarily.
True, a decrease in field current decreases the flux and CEMF, this would increase the armature current, torque and rpm
Very small HP rated (fractional) series motors will normally not reach destructive speeds if unloaded.
True, the residual flux is sufficient to maintain some CEMF
With suitable compounding, a generator can deliver a higher voltage at full load than at no load.
True, over compounding
The torque of a series motor always increases as the square of the armature current.
False, only up to the point of saturation
A generator is flat compounded when the series field current is equal to the shunt field current
False, when V(no load) = V(full load)
A shunt motor may have its field separately excited
True, good for speed control as it does not affect the armature voltage
In every DC generator, there is a fundamental motor action that occurs.
A DC motor can be used as a generator.
In a short shunt generator connection, the shunt field is directly across the terminal lines.
A DC series motor is considered to have a definite no load speed.
During starting, all DC motors have high CEMF
False, no CEMF at start
A DC generator can be operated as a motor
A DC shunt motor develops a higher starting torque, per amp of armature current, than a series motor.
False, series is best
A shunt generator supplies a constant output current under a varying load.
False, the terminal voltage drops due to armature reaction, IR drops and further field weakening
For constant speed drives, the best motor to select is a shunt motor
True, low speed regulation, called a constant speed motor
A cumulative compound motor has a definite no load speed.
A shunt motor, without load, develops a CEMF almost equal to the applied voltage.
True, limits the armature current
If any DC motor is started across the line, the inrush current, for all practical purposes, is determined by the armature circuit resistance.
True, this is why reduced voltage starting is used
If a series motor draws 50% of its rated current, then the torque produced is very nearly 25% of rated
True, torque is proportional to Iᴀ² (up to saturation)
Armature reaction is not as significant for DC motors as it is for DC generators
False, equally significant
In high torque applications, such as cranes and hoists, a shunt motor is much better suited than a series motor.
False, the opposite is true
Field strengthening in a shunt motor will cause its speed to decrease
True, increased flux increases the CEMF, decreases Iᴀ, decreases torque and rpm
If the field of a series motor under load is opened, a high speed may occur due to the flux reduction
False, open armature stops the motor
If the field of a shunt motor burns open, the motor will always stop.
False, decreases flux and CEMF, increases Iᴀ, torque and rpm
If connections remain unchanged, a cumulative compound motor used as a generator will be differentially compounded.
True, armature becomes a source causing Iᴀ to reverse
Compensating windings are connected in series with the armature and embedded in the main pole face. They are connected so that the current flow through them is in a direction opposite to the armature conductors immediately adjacent to them.
True, used to reduce armature reaction
A differential compound generator is best suited for a constant voltage output under varying loads
False, best used for DC arc welders
When the speed of a shunt motor drops, it is probably a sign that more resistance has been inserted in the field circuit.
False, more resistance would mean less flux, less CEMF, more armature current, more torque and rpm
The fundamental motor equation is that "Terminal voltage equals generated voltage plus the IR drop in the armature".
True. Vᴛ = CEMF + IᴀRᴀ
A reduction of CEMF, in a DC motor, permits more current to flow through the armature.
The DC motor that does not have a definite no load speed is the differential compound motor
False, series motor
Under load, the electrical neutral plane of a DC motor shifts in the direction of armature rotation.
False, motor shift is opposite to rotation
A compound DC motor has the following connections; L1 to F2 to A2; L2 to F1 to S2; and A1 to S1. It is connected cumulative compound for CW rotation
If the diverter rheostat on a cumulative compound DC generator is increased, then the terminal output will decrease.
False, the diverter rheostat is in parallel with the series winding. An increase in resistance would increase the current through the series winding, increase the flux and increase the generated voltage