Term
| What does a basic transformer consist of (3)? |
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Definition
- Primary winding
- Secondary winding
- Common steel core
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Term
| Describe theory of transformer operation |
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Definition
- Alternating current causes a magnetic field to expand and collapse on the primary winding.
- The expanding and collapsing magnetic field provides the relative motion necessary to induce a current in the secondary winding.
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Term
| How do transformers transfer electrical energy? |
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Definition
| Electromagnetic induction |
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Term
| What is the relationship between the primary and secondary winding voltages? |
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Definition
Eprimary/Nprimary = Esecondary/Nsecondary
or
Esecondary = Nsecondary x (Eprimary/Nprimary)
...where E = votage, and N = number of turns |
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Term
Describe the relationship for the following properties of a transformer with an open/unloaded secondary:
- Primary current to applied voltage,
- CEMF to applied voltage
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Definition
- Primary current LAGS applied voltage by 90o
- CEMF is 180o out of phase with the applied voltage[image]
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Term
| Definition of excitation current (2) |
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Definition
- Current flow through the primary with an open secondary,
- The amount of current required to produce the magnetic field and supply core losses
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Term
What percentage of average power is attributed to excitation current for power and distribution transformers? |
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Definition
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Term
| What do dots on transformer terminals indicate? |
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Definition
Which primary and secondary terminals are positive and negative at the same instant |
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Term
| What happens to a loaded transformer current/voltage phase angle as current increases? |
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Definition
As the load on the secondary increases, the phase angle between primary voltage and current decreases towards zero degrees.
[image] |
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Term
| In an ideal transformer, what is the relationship between power in the primary and secondary? |
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Definition
Primary power is equal to secondary power
Ep Ip = Es Is |
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Term
| No-Load Transformer losses (3) |
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Definition
- Core losses
- Hysterises (changing magnetic fields)
- Eddycurrent (circulating current)
- I2R losses (heat due to wire resistance)
- Dielectric losses (limits of insulation)
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Term
| Loaded Transformer Losses (3) |
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Definition
- Losses due to load
- I2R losses in primary and secondary
- Stray losses due to dlux in windings, clamps, etc.
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Term
| Describe transformer noise losses |
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Definition
| Due to changes in flux density, the core steel laminations change length |
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Term
| How are transformers rated? |
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Definition
- Kilovolt-ampers (product of full load current and voltage)
- Temperature (power losses)
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Term
| Voltage and KVA rating for POWER TRANSFORMERS |
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Definition
- 2400 to 500,000V
- over 500KVA
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Term
| Voltage and KVA rating for DISTRIBUTION TRANSFORMERS |
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Definition
- 120/240V for residential,
- 480, 600, 2400V for industrial
- Up to 500 KVA
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Term
| Describe a potential transformer |
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Definition
Transform (step-down) tansmission or distribution voltage for use with low voltage measuring instruments
[image] |
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Term
| Describe a current transformer |
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Definition
Steps down current for instrumentation & metering – provides a low current connection for high current systems. Disadvantage - HIGH VOLTAGE
[image]
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Term
| What special precaution applies to current transformers? |
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Definition
| Always short before detaching load, attach load before removing short. |
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Term
| What is unique about an auto transformer? |
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Definition
| It consists of only one winding |
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Term
| How are the primary and secondary divided in an auto transformer? |
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Definition
A common connection (tap) divides the single winding into the primary and secondary.
[image] |
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Term
| Advantages of auto transformer |
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Definition
- Lower cost,
- Less weight,
- Good efficiency
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Term
| Disadvantage of auto transformer |
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Definition
| Not safe for use in distribution systems since they provide a conducting path between the primary and secondary |
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Term
| Describe the three methods of transformer cooling used at LGS |
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Definition
- Self cooled - Sufficient surface area for air cooling
- Forced air cooled - Fans or blowers used
- Oil-cooled - Insulating oil serves as cooling medium, natural or forced circulation
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Term
| Describe the 4 standard transformer connections |
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Definition
- Wye-Wye (High voltage / reflected harmonics)
- Delta-Delta (Not normally used - grounding difficult)
- Wye-Delta (Natural step-down)
- Delta-Wye (Naural step-up)
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Term
What type of connection is this?
[image] |
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Definition
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Term
What type of connection is this?
[image] |
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Definition
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Term
What type of connection is this?
[image] |
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Definition
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Term
What type of connection is this?
[image] |
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Definition
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Term
| Function and operation of transformer tap changers |
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Definition
- Permit changing the turns ratio to control secondary voltage
[image] |
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