# Special transformers

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 Author: johnbowens ID: 125396 Filename: Special transformers Updated: 2011-12-31 03:21:47 Tags: Special transformers NJATC IBEW JATC electrical inside special Folders: Description: ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers. Show Answers:

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1. Control transformers are often referred to as what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 183
Power supplies
2. Control transformers are used to provide what sort of voltage power to control systems and devices? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 183
Low-voltage power.
3. What transformers are used to measure high voltage present on power lines? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 183
• Potential transformers.
4. The most common control transformers have how many primary coils and secondary? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers PP 183
There are two primary coils and one secondary coil on this type transformer.
5. When the control transformer has the two primary coils wired in parallel what happens? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 183
The voltage is stepped down from 240 V to 24 V. Primary: parallel is 240V and series is 480V.
6. Control transformer sizes typically range in what VA range? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 184
50 VA 23 KVA is the size range for these transformers.
7. The size of the control transformer can be determined by what method? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 185
By adding the power requirements of the components in the control circuit you can sizing the transformers.
8. When does the voltage output of the control transformer decrease? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 185
With an undersized control transformer the total load connected to a control transformer exceeds the rating of the transformer, this happens.
9. An undersized control transformer can make it difficult to troubleshoot the circuit. Give examples of how. ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 185

What are the two power ratings for a relay, contractor, or starter? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 185
For example, a relay, contactor, or starter has two power ratings: one for holding in the armature and one for pulling in the armature, therefore a starter, relay, or contacted cannot complete the circuit, but can be done so manually.

A lower rating which is the hold-in VA rating, and a higher rating which is the pull-in VA rating.
10. What NEC section list requirements for transformers used motor control circuits? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 187
430.72C
11. NEC section 430.72C says that control transformers which occur in the primary winding of less than _____ shall be protected by overcurrent devices 500% or less of the rated primary current. ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 187
Two Amps
12. What is common industry practice in regards to protection of the control transformer? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 187
It is common industry practice protect this type of transformer on both the primary and secondary side.
13. A grounded system in regards to a control transformer is what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 188
A control system with a grounded secondary in the control transformer, making it easy to test voltage to ground on the output side.
14. A floating system is what? And what cannot be done when troubleshooting this type of control transformer? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 188
• A control system without a grounded secondary in the control transformer, which means a voltmeter cannot be used to troubleshoot the circuit by measuring voltage to ground, but relative to the transformer instead.
15. A circuit that has more than one point connected to earth ground, with a voltage potential difference between the two ground points is what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 189

What type of transformer can prevent ground loops?
A ground loop.

• An isolation transformer.
16. What safety precautions are taken for heater controls? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 190
Heater controls are usually designed to be normally open and only operate when needed.
17. What type of transformer is a transformer used to supply low-voltage to power circuits, such as for doorbells, annunciators, and similar systems. ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 190
Bell transformers.
18. Bell transformers operate like a standard step down control transformer. They usually have an insulated panel with four screws for connections. Describe the configuration of the screws for voltage connections. ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 190
• One of the screws is the common and the other three are taps that allow different voltages to be taken off the coil. These taps are usually 6V, 12V, 16V, and sometimes 24V.
19. What is a precision two winding transformer used to step down high-voltage to allow safe voltage measurement? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 190
A potential transformer.
20. Although potential transformers are usually fairly small, they're used to measure very high voltages but are typically rated at what VA? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 191
These transformers are typically rated at 500 VA or less.
21. A transformer correction factor is what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 192
A number provided by the manufacturer that is used as a multiplier to correct for inaccuracies. The correction factor correct for effects of magnetizing current or internal phase shift created by the internal inductance of the transformer.
22. Current transformers are unique for what reason? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 193
Because these usually have only one winding they are unique. The primary is connected in series with the line load.
23. When the primary on a current transformer has a large current rating what may be done with the primary winding? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 193
The primary winding may consist of a straight conductor passing through the center of the magnetic circuit. This straight conductor represents a one turn winding.
24. The secondary over current transformer must never be allowed to do what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 194

Why?

Therefore, a current transformer should always have what done to the secondary when not connected to an external load?
This must never be allowed to be open when the primary circuit is energized.

When the circuit is operational, the load on the secondary keeps the magnetizing currents low, keeping turn to turn potential low.

The secondary should be shorted when not connected to an external load.
25. Three types of current transformers in general use are what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 195
Window, bar, and wound.
26. A transformer that consists of a secondary winding wrapped around a core and a primary sent through the opening in the core. ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 195
Window current transformer.
27. In a window transformer what is passed through the window and acts as the primary? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 195
A power line Is passed through the window and acts as a primary.
28. A special type of window current transformer with a solid bar placed permanently through the window is what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 195
A Bar current transformer.
29. A Bar current transformer is found in installations where the potential is what KV or less? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 195
These transformers are found in installations 25KV or less.
30. A transformer with separate primary and secondary windings wrapped around a laminated core. ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 195
• What is the current transformer.
31. This type of current transformer is found in the high-voltage side of substations and has a primary conductor the carries the current and a low turn secondary for the output current. ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 195
Wound current transformer.
32. The secondary of a current transformer is always rated at what amperage? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 197
At 5 amps on the secondary regardless of the current rating of the primary.
33. Common applications of current transformers include what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 197
Power metering, motor current monitoring, and variable speed drive monitoring are common applications for this type of transformer.
34. In power metering a current transformer provides what in comparison to a potential transformer? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 197
The current transformer provides a way to measure the current, and a potential transformer provides way to measure the voltage in this type of metering.
35. What has current transformers on the lines to the starter to monitor the motor currents? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 197
Motor starters have these type of transformers on the lines.
36. What is used to transfer the load in the event of a power failure? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 193
• Voltage relays.
37. What is part of instantaneous electronic trip IET circuit that takes the drive off-line if a sudden rise in current occurs that exceeds the rating of the drive? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 197
Variable speed drive current transformers that monitor the incoming current is part of this.
38. A variable transformer is a transformer used to make fine adjustments to the output voltage. A typical variable transformer can be adjusted from what percentage of the input voltage? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 198
This transformer can be adjusted from 0% to about 170% of the input voltage.
39. What type of transformer is usually wired is an autotransformer with no electrical isolation between the source and the output? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 198
• Variable transformers are usually wired this way.
40. A typical variable transformer consists of what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 19
This consists of a copper winding on a toroidal core.
41. What type of power loss & inefficiencies does a variable transformer usually have? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 198
These transformers can have extremely low power losses with efficiencies as high as about 98%.
42. An application of variable transformers is what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 200

Other applications of variable transformers include?
Voltage regulation is an application of this type of transformer.

Temperature control, control lighting, and laboratory equipment testing for over-voltage and under-voltage conditions, are other applications of this type of transformer.
43. What is used to adjust location of the output tap from the variable transformer? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 200
A stepper motor is used for this.
44. What can be used with buck boost transformers to create a voltage regulation system? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 200
Variable transformers can be used with this.
45. An application of variable transformers is what ? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 200
As a voltage regulator is an application of this type of transformer.
46. The two most common control transformer secondary voltages are what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 201
24V and 120V are the two most common secondary voltages with this type of transformer.
47. The secondary output of potential transformers is? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 201
120 V is the secondary output of this transformer.
48. A current transformer is used to do what? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 201
This transformer is used to step down a high current to make it easier to measure.
49. What transformers are used together to measure the power delivered to the customer from the utility? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 201
Current and Potential transformers.
50. A variable transformer uses what that rotates across the winding to provide any desired output voltage within the design range? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 201
A wiper or brush is used to provide any desired output voltage within the design range.
51. A current transformer is used to step down the line current to make it easier to measure, thus creating what type of voltage transformer? ATP & NJATC, Transformers principles & applications, 2006; Chapter 10: special transformers, PP 193
A modified step-up voltage transformer. Since the power on the primary and secondary of a transformer is the same.

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