Controls and Switching Circuits Flashcards Preview

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

Flashcards in Controls and Switching Circuits Deck (16)
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1

How is the direction of a three phase induction motor reversed?

Interchanging any two leads to the motor.

2

 Why is only one overload relay needed in a reversing magnetic motor starter containing 2 contactors?

Because only one motor is involved.

3

What is the function of an interlock?

When the action of one device is intended to affect the action of another device.

4

Why is a mechanical interlock required in a reversing magnetic motor starter?

So that both sets of contacts cannot close at once, preventing a short circuit.

5

Why is an electrical interlock required on a reversing magnetic motor starter?

To prevent burnout of the contactor coil in the event that the start button is held on.

{This occurs because the current in the coil which is not allowed to fully pull in is 4 to 10 times it's regular value.}

6

What are the typical terminal numbers for an IEC holding contact?

13 and 14.

7

What are the typical terminal numbers for an IEC overload contact?

95/96 NC;

97/98 NO.

8

How is rotation of a motor changed without first pressing the stop button?

With a set of push button interlocks, which simultaneously deenergize one coil and energize the other.

9

May all motors be reversed at speed?

 

No, some high inertial loads may cause permanent electrical and mechanical damage to the circuit and equipment if the motor is reversed prior to stopping.

10

What is motor jogging?

When a motor is repeatedly energized in short bursts at rated voltage.

11

What is motor inching?

When less than rated voltage is applied to a motor to slowly creep it forward or back.

12

What is positive jogging?

A connection method which disables the motor from turning on if the jog button is released quickly.

13

When Jogging, when is it necessary to use a larger starter than is normally required?

When a motor is jogged more than five times per minute.

14

What is the starter classification of an IEC AC-4 motor starter?

Useful for plugging and jogging/inching.

15

When numbers are on the right side of a schematic diagram, what do they represent?

The rungs of contacts controlled by the relay located in the rung that they are next to.

16

What is another name for three wire control?

Low voltage protection.

{Containing holding contacts}