Motors test Flashcards
Counter-electromotive force (CEMF) or back-EM are terms describing voltage that is induced into the armature of a DC motor as it begins rotating. T or F
True
A device that converts electrical energy into mechanical energy is called a(n)______
Electric motor
A series motor may be operated with no load connected. T or F
False
The compound motor connection that should generally be avoided is the _________.
Cumulative
Differential
Series
Shunt
Differential
The speed regulation of a DC motor is proportional to the ________
Armature Resistance
Motor with the best speed characteristics?
Series
Motor with the best Torque characteristics?
Shunt
A compound motor has ____ and ______ field windings.
Shunt and Series
In a DC motor, terminals Ay and Az are connected to the shunt fields. T OR F
False
Series field windings are connected in series with the armature. They are wound with ____ turns of fairly _____ wire.
Few, Large
Shunt field windings are connected in parallel with the armature. They are wound with _____ turns of fairly ____ wire.
Many, small
The __________
ride against the commutator
segments and are used to connect the armature to the external circuit of the DC machine.
Brushes
Brushes are made from a material that is softer than the ________
bars of the
commutator.
Copper
How does the speed vary as the mechanical load applied to the series motor increases, i.e., as the motor torque increases.
The speed decreases nonlinearly when the motor torque increases.
The DC motor brush leads are generally labeled ____ and ____.
A1 and A2.
When a motor has only one set of field windings mounted on a DC motor that is connected in parallel with the armature, the motor is called a _____ motor.
Shunt.
Is the cumulative compound motor a good compromise of the characteristics of the series and shunt connected motors?
The characteristics of a cumulative compound motor is a good compromise of the series and shunt motor characteristics.
Can a series connected motor be started with no mechanical load connected to it?
A series motor must never be started with no mechanical load because the speed increases to a very high value, which can damage the motor (motor runaway).
What happens to the speed of a shunt connected motor as the load applied to it increases?
The motor speed varies little as the mechanical load varies
Select types of overload protection devices available for motor starters.
Magnetic
Thermal
Electronic
All of the above
All of the above
When an overload relay is added to a magnetic contactor, the combined device is now called a magnetic motor starter, provided that the contactor is HP rated. T or F
True
The primary reasons for utilizing automated control circuits for motors is to reduce costs by automating processes and to improve safety and productivity. T or F
True
What is the most common application for using one of the N.O. auxiliary contacts as part of a three-wire control circuit?
Holding/maintaining contact
Upon applying power to a magnetic motor starter, the coil produces a humming sound.
The pole contact surfaces are clean (no carbon, dirt or debris) and the plunger appears to be moving freely.
What is the most likely cause of the noise and what corrective action should be taken in order to repair it?
Under voltage, correct voltage source problem.
Upon applying power to a magnetic motor starter, the coil produces a humming sound.
The pole contact surfaces are clean (no carbon, dirt or debris) and the plunger appears to be moving freely.
What is the most likely cause of the noise and what corrective action should be taken in order to repair it?
Under voltage, correct voltage source problem.
What problems are likely occurred with a DC motor if the brushes are NOT properly set up to the correct neutral plane position (Zero Point)?
Arcing and sparking at brushes
Damage to commutator
Power loss
Efficiency reduction
All of these
All of these
To reverse the direction of a shunt motor, the wire leads for both the armature and shunt field should be swapped. T or F
False
What conditions can cause the contact’s surface to be excessively worn or pitted?
Loose contact pressure
Debris/dirt on the contact surfaces Weak or damaged springs excessive current
All of the above
All of the above
What typically happens to the speed, CEMF and current of a shunt motor as load is added to the motor?
The speed drops, the CEMF reduces causing current to increase through the armature and strengthens its magnetic field which increases
torque to meet the added load requirements.
What advantages does Dynamic Braking have over mechanical braking methods?
Dynamic Braking does not require additional equipment
Provides very smooth deceleration without any wear and tear
Less maintenance is required due to no mechanical brake parts very little extra electrical equipment is required
All of these
All of these
How does changing the resistance value in a Dynamic Braking circuit affect the length of time to stop the motor completely?
The counter torque produced during dynamic braking changes accordingly with changes in current affected by resistance.
Which best describes the advantages of using a drum switch to reverse the direction of a DC motor?
Easy to use, heavy duty, the handle returns to OFF (center) position when let go
Maintains the selected direction and turns the motor off when changing directions, has heavy duty contacts
Easy to operate, small and less expensive
All of the above
All of the above
What two purposes does the MCR serve in a control circuit?
The MCR is used to provide low voltage protection and safely shut down an entire section or all of a machine.
Reduced voltage starting methods are used instead of full voltage starting for a motor control circuit is to reduce the high inrush current and/or reduce starting torque to reduce the mechanical stress on equipment being driven by the motor. T or F
True
The advantage of reversing direction of a DC Shunt Motor through the armature instead of the field winding, is that smaller, less expensive devices can be used due to the lower current of the armature winding. T or F
False
A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of 1650 RPM.
The motor is connected to a 250 VDC source and measures a current draw of 175 Amps.
What is the Power output (watts) of the motor?
45532.51
34541.88
56254.25
43750.4
Power output = Output HP X 746
Power output = 46.303 × 746
Power output = 34541.88
A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of
1650 RPM.
The motor is connected to a 250 VDC source and measures a current draw of 175 Amps.
What is the Power input (watts) of the motor?
79500
68250
55650
43750
250 v x 175 amps = 43750 watts
A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of 1650 RPM.
The motor is connected to a 250 VDC source and measures a current draw of 175 Amps.
What is the efficiency of the motor?
57%
53%
79%
85%
Efficiency = Power output ÷ Power input X 100
Efficiency = 34541.88 watts ÷
43750 X 100
Efficiency = 78.9 % or 79%
A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of 1650 RPM. The motor is connected to a 250 VDC source and measures a current draw of 175 Amps.
What is the HP of the motor?
61.036
21.258
7127
46.303
Output Horsepower = T(Nm)x Speed (RPM) ÷ 7127
Output Horsepower = 200(Nm)x 1650 (RPM) ÷ 7127
Output Horsepower = 46.303
46.303 HP
