Motor controls 27-30
Refer to the circuit shown in Figure 27-5. When the START button is pressed, the motor does not start. After a 4-second time delay, control relay CR de-energizes. Which of the following could cause this problem?
1M starter coil is open.
1. A dual voltage 240/480-volt motor is to be used for part winding starting. Which voltage must be used and why?
240V, the motor must always be operated on the low voltage setting of the motor.
3. The nameplate of a part winding motor indicates a full load current rating of 72 amperes. What current rating should be used when sizing the overload heaters?
36 *(72/2)
Maximum starting torque for a split phase motor is developed when the run-winding current and start-winding current are how many degrees out of phase with each other?
90* degrees out of phase with each other
How is the braking force of drum type brakes controlled?
A spring is used to adjust the amount of pressure the brakes you exerts against the drum to control the amount of breaking that takes place when the motor is stopped
What is a watchdog timer?
A timer that disconnects power to the motor if the second winding is not energized in a predetermined amount of time.
A direct current shunt motor is to be operated in the counterclockwise direction. Should the A1 or A2 lead be connected to the F1 lead?
A1 should be connected to the F1 lead
What is the advantage of a capacitor start induction run motor over a resistance start induction run motor?
An increase of starting torque
Why are mechanical brakes often used on cranes?
Because mechanical brakes have the advantage of holding a suspended load
How can a compound motor be operated at above normal speed or over speed?
By applying full voltage to the armature and reduced voltage to the shunt field.
How can a compound motor be operated at below normal speed or under speed?
By applying full voltage to the shunt field and reduced voltage to the armature.
How is the direction of rotation of a split phase motor changed?
By changing the run or start windings in relation to each other. The most common method is to reverse the start winding because this doesn't interfere with the run winding connection of the dual voltage motor.
How is the dynamic braking force of an alternating current motor controlled?
By connecting DC current to the stator winding
How is dynamic braking for direct current motors accomplished?
By permitting power to remain connected to the shunt field when the motor is stopped, and reconnected the Armature to a High wattage resistor
What is the most common method of reversing the direction of rotation of a direct current motor?
By reversing the current flow through the armature
What type of split phase motor does not have to disconnect the start winding when the motor reaches approximately 75 percent of its rated speed?
Capacitor start induction run
What device is generally used to disconnect the start windings of a split phase motor for open type motors that are not hermetically sealed?
Centrifugal switch
What type of motor changes speed by changing the number of stator poles?
Consequent pole motors
What type of armature winding is generally used for machines designed to operate on low voltage and high current?
Lapwound
Name three methods of braking a motor.
Mechanical braking, dynamic braking, and plugging
What prevents a multispeed fan motor from being damaged when it is operated at a low speed?
Multi speed fan motors are wound with high impedance windings to prevent damage to the stator of the fan.
How is a plugging stop accomplished?
Plugging is accomplished with three phase motors by disconnecting the motor from the powerline and momentarily reversing the direction of rotation
What device is generally used to accurately stop a motor when a plugging stop is used?
Plugging switch or zero speed switch
Explain the differences between series and shunt field windings in construction and how they are connected in the motor
Series field windings are constucted with few turns of large wire and are connected in series. Shunt field windings contain many turns of small wire and are connected in parallel.
What are the three basic types of direct current motors?
Series, shunt, and compound
What type of direct current motor is generally referred to as a constant speed motor?
Shunt motor
Name three factors that determine the amount of induced voltage.
Strength of the magnetic field, length of the conductor, and speed of the cutting action
Refer to the circuit shown in Figure 27-5. When the START button is pressed, the motor does not start. Which of the following could not cause this problem.
TR1 timer coil is open
The amount of counter torque developed by a direct current generator is proportional to what?
The Armature current
What is the advantage of dynamic brakes over mechanical brakes?
The advantage of dynamic braking is that there are no mechanical brake shoes to wear out
What is the principle of operation of a direct current motor?
The attraction and repulsion of magnetism
When using dynamic braking for a direct current motor, how is the braking time controlled?
The breaking time can be controlled by adjusting the resistance value
What is the disadvantage of dynamic brakes when compared to mechanical brakes?
The disadvantage is that dynamic brakes cannot hold a suspended load
What are the three major types of split phase motors?
The resistance start induction run, capacitor start induction run, and capacitor start capacitor run
What is the characteristic of a motor that is flat compounded?
The series and shunt fields are balanced, the series field compensates for any motor losses and causes the motor to operate at almost constant speed from load to no load.
Explain the difference between cumulative and differential compounding.
When both fields are connected in such a way that current flows in the same direction and aid each other in the production of magnetism it is called cumulative compounding. If the series and shunt fields are connected to where the current flows in opposite directions, they oppose each other in the production of magnetism which is called differential compounding.
Refer to the circuit shown in Figure 30-10. When the START button is pushed and the motor starts in the forward direction, the plugging switch closes. What prevents the reversing contactor from energizing when the plugging switch contact closes?
When the F contactor is energized all F contacts change position, this makes the N.C. F contact in series with the R contactor to open. Ultimately stopping the flow of current to the R contactor when the plugging switch closes.
Are the stator windings of a motor designed for part winding starting connected in parallel or series?
they're designed to be connected in parallel with each other
