Special Control Circuits Flashcards Preview

Electrical Controls and Switching Theory Period 4 SAIT-Alberta > Special Control Circuits > Flashcards

Flashcards in Special Control Circuits Deck (15)
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What are the three basic types of timing devices?

Spring wound interval timers;

Mechanical and electronic time switches;

Timing relays.


What are the two basic timed contact functions of a timing relay?

Time delay on energization;

Time delay on de-energization.


When locating a TDOE contact in a schematic, which way does the "arrow" point?

Up - NEMA;

Left - IEC.


When locating a TDOD contact in a schematic, which way does the "arrow" point?

Down - NEMA;

Right - IEC.


What does "dip" switch stand for?

Dual in-line package.


Which timed relay function requires an extra start terminal on the relay?



How is a mechanical friction motor brake electrically operated?

It is released by a solenoid which is energized with the motor coil.


How is dynamic braking achieved in a DC motor?

The power source to the armature is replaced by a resistor {so current can flow}. The shunt field voltage is maintained, which generates a lorrentz force in the still rotating rotor in the direction opposite rotation.

{Final braking is usually achieved with a friction brake.}


How is dynamic braking achieved in an AC motor?

The ac source to the stator is replaced by a DC source, which creates a static magnetic field. As the rotor continues to rotate through the field, current is induced into the rotor, which creates a magnetic field around the rotor bars, which tend to "lock on" to the stationary field.


What must be considered when applying DC injection braking to an AC motor?

The DC voltage will not see any inductive reactance in the coil.


What is plugging?

When the stator connection of a three phase motor is reversed in order to stop the motor.


When is plugging most often used?

In applications which require immediate stopping of the motor.


Why should plugging not be done frequently?

The rotor bars will overheat and may melt.


What is anti-plugging?

When a rotary switch is placed in the control circuit in order to prevent a motor from being reversed until it has come to a complete stop.


What is the terminal that is usually grounded on a 120V control transformer?