Controls and Switching Circuits

Question 1

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

Answer 1

Interchanging any two leads to the motor.

Question 2

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

Answer 2

Because only one motor is involved.

Question 3

What is the function of an interlock?

Answer 3

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

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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.}

Question 6

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

Answer 6

13 and 14.

Question 7

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

Answer 7

95/96 NC;

97/98 NO.

Question 8

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

Answer 8

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

Question 9

May all motors be reversed at speed?

Answer 9

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.

Question 10

What is motor jogging?

Answer 10

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

Question 11

What is motor inching?

Answer 11

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

Question 12

What is positive jogging?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

Useful for plugging and jogging/inching.

Question 15

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

Answer 15

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

Question 16

What is another name for three wire control?

Answer 16

Low voltage protection.

{Containing holding contacts}