EMC ( Transformers) Ch 1

Question 1

What type of electrical energy is normally used to supply industry?

Answer 1

Three-phase AC power is commonly used because it’s more efficient for powering large motors and industrial equipment.

Question 2

What are the two important reasons for using 120 V in machine control systems?

Answer 2

1. Safety – 120 V is considered safer than higher voltages for control circuits.
   
   2. Standardization – Many control devices (like relays, contactors, timers) are designed for 120 V operation.

Question 3

There is 480 V AC available in a given plant. To obtain 120 V, what turns ratio is required?

Answer 3

Turns ratio = Primary voltage / Secondary voltage = 480 / 120 = 4:1

Question 4

If voltage drops to 456 V, what is the resulting secondary voltage (4:1 turns ratio)?

Answer 4

Secondary voltage = 456 V / 4 = 114 V

Question 5

Draw the symbol for a dual-primary, single-secondary control transformer with all lead designations.

Answer 5

• H1, H2, H3, H4 – Primary leads (H1-H3 and H2-H4 are paired for series or parallel)
• X1, X2 – Secondary leads

Question 6

What parts of a transformer contribute to temperature rise?

Answer 6

• Copper winding
• Iron core
(The enclosure does not generate heat but may retain it.)

Question 7

What may happen to control system components under low voltage (poor transformer regulation)?

Answer 7

•Relays or contactors may fail to operate or drop out unexpectedly

•Undervoltage can damage sensitive electronics or cause malfunctions

Question 8

Draw a complete circuit: dual-primary control transformer on 480V 3-phase to 120V control system.

Answer 8

• Primary side: H1/H2 (or H1-H4 & H2-H3 in series for 480V)
• Connection across two phases of 3-phase 480V
• Secondary side: X1 to control line, X2 to neutral/common

Question 9

Transformer reducing voltage from primary to secondary is called

Answer 9

Step-down transformer

Question 10

Draw a block diagram showing standard components of a UPS.

Answer 10

• AC Input (from utility)
• Rectifier/Charger
• Battery Bank
• Inverter
• AC Output (to load)
• Bypass Circuit

Question 11

What happens when power is lost in a UPS system?

Answer 11

The battery and inverter immediately take over, providing uninterrupted power to the load.

Question 12

What factors cause an imperfect utility power supply?

Answer 12

Voltage fluctuations
• Frequency variations
• Power surges or sags
• Harmonic distortion
• Blackouts or brownouts

Question 13

Transformer regulation: No-load = 120V, full-load = 110V. What is the percent regulation?

Answer 13

120-110 10
———— x100 —— x 100 =9.09
110 110

Question 14

What happens if a 60 Hz transformer is used on a 50 Hz supply?

Answer 14

Increased core saturation, causing:
• Overheating
• Humming
• Reduced efficiency
• Possible damage

Question 15

What utility supply issues make a UPS advisable?

Answer 15

• Frequent blackouts
• Voltage dips/surges
• Poor power quality
• High harmonic distortion
• Sensitive equipment needing clean power