Line C Electronic Proportioning

Question 1

Where are the potentiometers housed on a steam boiler with a single point positioning system utilizing a bridge circuit?

Answer 1

Modulation motor and modulating Pressuretrol

Question 2

Referring to the figure below, what is the fixed resistance between points B & W of the bridge circuit?

Answer 2

135 Ω

Question 3

Referring to the figure below, once released to modulate, what would cause the modulation motor to rotate open?

Answer 3

A decrease in resistance between R & B and an increase of resistance between R & W of the modulating control.

Question 4

Referring to the figure below, what will happen to the firing rate if the wiper (R) in the modulating control moved towards (W)?

Answer 4

The burner firing rate will decrease

Question 5

Referring to the figure below, what would indicate that the bridge circuit is balanced?

Answer 5

The motor stops rotating.

Question 6

What is the purpose of the N.C. limit switches in the open winding and closed winding circuits of the modulation motor?

Answer 6

To prevent over rotation of the motor

Question 7

What device is NOT part of the 4–20 ma control loop?

Answer 7

Sensor