Eddy Currents, AC Current and Transformers Flashcards
What are eddy currents?
Currents produced in a conductor by magnetic fields
(Lenz’ law in conductors)
(these are the anti currents that are produced in transformers)
Why does the magnet take longest to fall through the full copper pipe?
- Magnet in freefall
- Plastic not a conductor so no eddy currents (still in freefall)
- Copper pipe incomplete so can’t create eddy currents (still in freefall)
- Eddy current reduce acceleration
How are eddy currents created in this copper pipe?
- Flux linkage decreasing above -> current creates attracting field upwards
- Flux linkage increasing below -> current creates repelling field upwards
How does eddy current braking work?
- Part of disk leaving field -> current creates attraction to electromagnet
- Part of disk entering field -> current creates repulsion to electromagnet
- The attraction and repulsion causes the velocity of the disk to decrease
When can eddy current braking not be used?
To hold a car stationary on a slope
(no change in flux linkage)
How does the oscilloscope trace look for an AC current?
(When the time base is switched on)
How does the oscilloscope trace look for an AC current?
(When the time base is switched off)
How does the oscilloscope trace look for an DC current?
(When the time base is switched on)
How does the oscilloscope trace look for an DC current?
(When the time base is switched off)
For AC supply what is Vrms and how is it calculated?
The equivalent DC voltage that would supply the same average power
For AC supply what is Irms and how is it calculated?
The equivalent DC current that would supply the same average power
Label V0 and Vp→p on this AC oscilloscope trace
What is wrong with this calculation?
When using electricity formulas must use rms values for voltage (and current)
How does a step up transformer work?
- AC current flows through primary coil and create a magnetic field (right hand grip rule)
- Magnetic field flows through secondary coil
- Changing flux linkage in secondary coil larger
- Greater emf induced from faradays law (so bigger voltage)
How does a step down transformer work?
- AC current flows through primary and creates a magnetic field (right hand grip rule)
- Magnetic field flows through secondary coil
- Changing flux linkage in secondary coil smaller
- Smaller emf induced from faradays law (so smaller voltage)
Why does a transformer only work with AC supply?
If supply is DC
Flux linkage in secondary coil doesn’t change as it doesn’t flip
So emf isn’t induced (Faraday’s law)
How do you calculate the voltage (rms) in the secondary coil?
This equation always works (no matter what efficiency) Always use (rms) voltages
How do you calculate the efficiency of a transformer?
(The voltages and currents must be rms values)
What are the 4 main causes of thermal loss in a transformer?
- Large Eddy currents in magnet (P=I2R)
- Large currents in primary and secondary coils (large currents are inefficient)
- Hysteresis losses (magnet’s resistance to change in flux linkage)
- Flux losses (not all flux passing through secondary coil)
How are the main causes of thermal losses in a transformer reduced?
- Large Eddy currents in magnet
- Large currents in primary and secondary coils
- Hysteresis losses
- Flux losses
- Laminate the core (make it into sheets so eddy currents don’t flow properly)
- Use wire with low resistance
- Use soft iron core for magnet (stops the core from becoming magnetised)
- Wind primary coil over secondary coil
Why are step up transformers used to transport electricity over long distances?
Smaller currents = smaller energy losses to thermal
(P=I2R)
