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Transformers
GFEs - MOD 3 - Transformers - ILT '13
19
Other
Professional
07/01/2011

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Cards

Term

Step–up transformer

Definition

 A transformer that increases the voltage from primary to secondary… or from the lesson plan:

If secondary windings are more than primary, flux will cut more coils in the secondary and induce higher voltage;

Term

Step–down transformer

Definition

 A transformer that decreases the voltage from primary to secondary… or from the lesson plan:

If there are fewer secondary windings than primary windings, less voltage will be induced than is applied.

Term

Primary winding

Definition
 Coil producing original magnetic field
Term
Secondary winding
Definition
Coil in which voltage is induced
Term

Turns ratio

Definition
 Ratio of the primary turns to the secondary turns(primary is the first number)
Term
Calculate the secondary voltage from given information
Definition

Vp/Np= Vs/Ns

 

Vp is primary voltage.

Np is number of primary winding turns.

Vs is induced secondary voltage.

Ns is number of secondary winding turns.

Term

Describe the operation of a transformer during no load conditions.

Definition
When AC is applied to a primary winding, a small current called exciting current flows in the primary.  Coil current is limited by the counter-EMF (CEMF), which is equal to applied voltage at no load.  Unloaded transformer has minimum primary current and rated secondary voltage available
Term
Explain why a transformer should never be connected to a DC source
Definition
If the primary of a transformer is connected to DC voltage, the absence of counter-EMF will allow the primary winding to burn up due to the low resistance and high current
Term

Describe the relationship between current and the number of turns in a loaded transformer.

Definition

Ip*Np=Is*Ns

Ip = Primary current

Np = # of Primary turns

Is = Secondary current

Ns = # of Secondary turns

Term
Describe the relationship between voltage and current in a loaded transformer.
Definition

Vp*Ip = Vs*Is 

Vp = Primary voltage

Ip = Primary current

Vs = Secondary voltage

Is = Secondary current

Term
Explain why transformers make it practical to transmit AC power over long distances
Definition

They allow alternating electrical energy to be transferred between circuits, raising or lowering the voltage as necessary.  High voltage and low current transmission lines have greatly reduced (I2R) losses.  Less current is required at a high voltage than a low voltage to transmit a given amount of power.

Term
Calculate the rating of a transformer from given information
Definition

Transformer capacity is rated in apparent power(VA).  Output rating is based on maximum current without exceeding design temperature.  Transformers and AC machines in general are rated in volt-amperes to preclude using power factor.

Preal=Papparent*power factor

Real power is denoted in Watts

Apparent power is denoted in VA

Term

Describe the five losses that occur in a transformer

Definition

1)  I2R losses in the form of heat from current flow in the windings.

2) Magnetic flux leakage is any flux that does not link the primary and secondary and is wasted.

3) Third loss is eddy currents, limited by laminating the core.

4) Fourth loss is hysteresis loss, high quality soft iron or silicon iron minimizes resistance of the core to realigning of domains.

5) Fifth loss is saturation loss, by ensuring capacity of the transformer is greater than maximum input power; the core will not become saturated.

Term

Single Phase transformer Connections, Describe methods to supply a residential customer with 120 VAC and 240 VAC from a single distribution transformer.

Definition

Local transformers that supply single-phase, relatively low voltage power, to residential and small industrial loads are distribution transformers.  Usually are supplied from one leg of a three phase line.  Secondary has a center-tap and two end connections.

Term

Describe the advantages and disadvantages of a three–phase transformer compared to a bank of three single–phase transformers.

Definition

1) ADV: Three-phase transformers occupy less space than three single-phase transformers of the same rating.

2) ADV: Three-phase transformers are slightly more efficient and have a lower initial cost.

3) DISADV: Three single-phase transformers, If one transformer fails, the other two phases will still be supplied.

Term

OA

Transformer Cooling

Definition
Transformers in an oil bath with no forced circulation of oil or air.  Cooling is reliant on natural circulation of oil and air
Term

OA w/radiators

 transformer Cooling

Definition

Same as above, but with radiators that take advantage of natural air movement to cool the oil.

Term

OA/FA

 Transformer Cooling

Definition
Transformer in an oil bath with forced air(fans) blowing air across a radiator to cool the oil(similar to your car)
Term

FOA

Transformer Cooling

Definition

Transformers in an oil bath with forced air and forced oil(oil pumps) circulation for the most available cooling(similar to the main transformer)

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