CHAPTER 28 POWER POINT Flashcards
a force that acts at a distance and is caused by a magnetic field.
MAGNETISM
a device that attracts iron and steel because of the molecular alignment of its material•May be permanent or temporary
Magnet
the invisible lines of force that make up the magnetic field
Magnetic flux
Magnetic Flux (force) lines travel in same direction
Unlike magnetic poles
________ poles attract
Unlike magnetic poles
______poles repel
Like magnetic poles
Magnetic Flux (force) Lines travel in opposite directions.
Like Magnetic poles
produced any time electricity passes through a conductor.
Magnetic field
the magnetic field produced when electricity passes through a conductor. Is a temporary magnetic force, because the magnetic field is present only as long as current flows.
Electromagnetism
On standard ____ (cycle) power frequencies, the current passes the zero point ____ times per second.
60 Hz
120
a device that converts electrical energy into a linear, mechanical force.
Solenoid
A solenoid uses __________.
Electromagnetic force
Provide a liner, mechanical force when electricity is applied to the coil.
Solenoid
A solenoid is utilized in applications that use short strokes at ______.
low force
In a solenoid, the ______ is removed from the coil by a spring.
iron core
In a solenoid a magnetic field produced around coil when current flow through _____.
Windings
In a solenoid a magnetic force draws ____ into a coil.
Iron core
In a solenoid where is the drum mounted?
on the motor shaft
A solenoid is used to open what when it is energized?
brake
What is used to close the brake when Solenoid is de-energized?
spring
Solenoids develop linear force in applications that use _______ at low force.
short strokes
Solenoids common electrical applications are to:
•Control the flow of water or gas in: Washing machines, Dishwashers, Dryers, Control door locks, Stamp information, Dispense coins
an electric device that uses electromagnetism to change voltage from one level to another or to isolate one voltage from another.
Transformer
Primarily used to step up or step down voltage.
Transformers
A transformer consist of two or more coils of insulated wire wound on a __________.
laminated steel core
coil to which the voltage is connected on the input side.
Primary coil
coil in which the voltage is induced on the output side
Secondary coil:
when the magnetic flux lines from one expanding and contracting magnetic field cut the windings of a second coil, a voltage is induced in the second coil.
The principle of electromagnetic mutual induction
are materials that can be magnetized easily.
Ferromagnetic materials
are materials that cannot be magnetized easily.
Paramagnetic materials
are materials that cannot be magnetized under normal conditions.
Diamagnetic materials
In transformers to calculate the relationship between the number of turns and the voltage:
Ep/Es = Np/Ns
In a transformer ______ is connected to primary.
Supply voltage
In a transformer _____ connected to secondary.
loads
In a step-up transformer there are more turns of wire on the ______.
Secondary coil
In a step-up transformer Induced voltage is ____ than applied voltage.
Higher
In a step-down transformer ____ turns if wire on secondary coil.
fewer
In a step-down transformer induced voltage ___ than applied voltage.
lower
Transformers are designed to transform power at one voltage level to power at _____________.
another voltage level
When dealing with transformers used voltage and current rating, don’t use______.
wattage ratings
Rate transformer for its _____ because it is the _______ of the transformer to which the loads are connected.
output capabilities
To calculate the efficiency of a transformer use this formula __________.
Eff = Ps/Pp x 100
Eff =
efficiency in %
Ps =
power of secondary circuit in W.
Pp =
Power if primary circuit in W.
The basic types of transformers include:
- Appliance/equipment
- Control
- Bell/chime
- Instrument (current)
- Distribution
- Isolation
- Neon Sign
- Power transformers
All transformers have a power rating_______.
VA or kVA
Indicates the amount of power the transformer can safely deliver. It is not an absolute value.
Power rating
100 VA rated transformer is not destroyed if required to deliver 110 VA for a ______.
short time
Transformers are used to deliver power to a set _______.
number of loads
A transformer is overloaded when it is required to deliver __________________.
more power than its rating
•Methods to dissipate heat in a transformer include:
- Self-air cooled
- Forced air cooled
- Liquid-immersed/self-air cooled
- Liquid-immersed/forced-air cooled
Transformers that dissipate heat through the air surrounding the transformer.
Self-air cooled
Transformers that use a fan to move air over the transformer
Forced air cooled
Transformers that use refined or synthetic oil to help cool the transformer windings
Liquid-immersed/self-air cooled
Transformers that use refined or synthetic oil and fans to cool the transformer
Liquid-immersed/forced-air cooled
is used anytime power is delivered to a residential, commercial, industrial, construction, or other site.
A transformer
The _____ of the transformer is based on the amount of expected power required
size
Should be stored in parallel with each other so their unlinked poles touch. ________
Bar magnets
Should be stored with a soft iron bar placed across the pole ends.
Horseshoe magnets
To size a 1 Φ transformer, apply the following procedure:
- Determine the total voltage required by the loads if more than one load is connected.
- Determine the amperage rating or kVA capacity required by the load(s). Add all loads that are (or may be) ON concurrently.
- Check load(s) frequency on the nameplate.
- Check the supply voltage to the primary side of the transformer.
To calculate kVA capacity of a 1 Φ transformer when voltage and current are known, apply the following formula:
KVAcap=E x 1/1000
kVACAP =
transformer capacity in kVA
E=
voltage in V
I =
current in A
1000 =
constant
To size a 3 Φ transformer, apply the following procedure:
- Determine the total voltage required by the loads if more than one load is connected.
- Determine the amperage rating or kVA capacity required by the load(s). Add all loads that are (or may be) ON concurrently.
- Check the frequency of the load(s) on the nameplate. The frequency of the supply voltage and the electrical load(s) must be the same.
- Determine the type of 3 Φ voltage available.
- Check the supply voltage to the primary side of the transformer.
To calculate the kVA capacity of a 3 Φ transformer when voltage and current are known, apply the following formula:
kVACAP = E x 1.732 x 𝑰/𝟏𝟎𝟎𝟎
kVACAP =
transformer capacity in kVA
E=
voltage in V
1.732 =
constant for 3 Φ power
1000 =
constant
Some transformers are provided with a _________ to provide a uniform secondary voltage when the primary voltage varies due to line voltage drop.
tap charger
No _________ and no power is used if no load is connected to the transformer.
current flows
To calculate transformer current draw when kVA capacity and voltage are known, apply the following formula:
I = kVACAP x 𝟏𝟎𝟎𝟎/𝑬 Where kVACAP = transformer capcity in kVA E= voltage in V 1.732 = constant for 3 Φ power I = current in A
To calculate current draw of a 3 Φ transformer when kVA capacity and voltage are known, apply the following formula:
I = kVACAP x 𝟏𝟎𝟎𝟎/(𝑬 𝐱 𝟏.𝟕𝟑𝟐) Where: kVACAP = transformer capcity in kVA E= voltage in V 1.732 = constant for 3 Φ power I = current in A 1.732 = √3
Temperature rise in a transformers is the temperature of the _________ above the existing ambient temperature.
windings
To calculate the derated kVA capacity of a transformer operating at a higher than normal ambient temperature condition, apply the following formula:
kVA = rated kVA x maximum load
Where
kVA = derated transformer capacity in kVA
Rated kVA = manufacturer transformer rating in kVA
Maximum load = maximum transformer load in %
Are used to derate transformers in high ambient temperatures.
Transformer derating charts
The average temperature of the air that cools a transformer over a 24-hour period.
Transformer Special Ambient Temperature Compensation
Serve to dwellings is normally 1 Φ, 120/240 V.
Electric in residential
In ____ used for general-purpose receptacles and general lighting.
120 V
_______ used for heating, cooling, cooking, et c.
240 V
Three 1 Φ transformers may be connected to develop ________.
3 Φ voltage
May be connected in a wye or delta configuration.
Three-Phase Residential Transformer Connections
The voltage output and type available for the loads is determined by whether the transformer is connected in a _________.
wye or delta configuration
Three-Phase, Delta-to-Delta Connections
Three transformers may be connected in a delta-to-delta connection. Used to supply 3 Φ voltage on the secondary.
Used to supply both 1 Φ and 3 Φ voltage.
The ends of each transformer are connected together.
Three-Phase, Wye-to-Wye Connections
This type of connected depends on:
The incoming supply voltage
The requirements of the loads
The practice of the local power company
Delta-to-Wye/Wye-to-Delta Connections
Loads connected to a transformer should be connected so the transformer is as electrically __________________.
balanced as possible
occurs when each coil of transformer carries the same amount of current for the various loads.
Electrical balance
Connecting points that are provided along the transformer coil.
Available on some transformers to correct for excessive voltage conditions.
Located on the primary side of the transformer.
Transformer taps
Standard transformer tap connections are provided for ___and __ of rated primary voltage.
2%
5%
Additional power is required when the capacity of a transformer is insufficient for the power requirements of the load(s).
Single-Phase Transformer Parallel Connections
Single-phase transformers may be connected in parallel as long as:
- Primary and secondary voltage ratings are same
- Frequencies are same
- Tap settings are same
- Impedance of either transformer is within ±7% (93% to 107%) of the other.
Three-Phase Transformer Parallel Connections. May be connected in parallel as long as certain conditions are met:
- Primary and secondary voltage ratings are same
- Frequencies are same
- Tap settings are same
- Impedance of either transformer is within ±7% (93% to 107%) of the other.
- Angular displacement of transformer banks is same
It is dangerous and illegal to improperly dispose of transformer oil containing ________________________. Check local and state codes for proper transformer-oil handling procedures.
poly-chlorinated biphenyls (PCBs)
