Lesson 11 Flashcards#
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1. What is an alternator?
A device that converts mechanical energy into electrical energy through electromagnetic induction. These are typically used in automotive and other combustion-engine systems.
2. What is an electrical bus?
A conductor that provides power to devices connected in parallel between the bus and a common ground.
3. What voltage do all devices connected to a bus receive?
All devices connected to a bus receive the same voltage.
4. What is the definition of ground?
A reference point in an electrical circuit from which voltages are measured, typically defined as 0 V.
5. What are the voltage properties of an electrical bus?
Because devices are connected in parallel, the voltage drops across all devices are the same.
6. What are the current properties of an electrical bus?
The total current entering the bus is equal to the total current leaving the bus, in accordance with KCL.
7. What is a common ground and why is it used?
A common ground is a shared node (often the metal frame of a vehicle) that provides the return current path for all connected devices. It defines 0 V, the reference from which all bus voltages are measured. Without it, there would be no complete circuit for current to flow back to the source.
8. Why do vehicles such as cars and airplanes use the metal frame as a common ground?
Using the metal frame as a common ground simplifies wiring. Each device only needs a single wire run from the appropriate bus to receive power; the current return path is provided automatically by the frame, eliminating the need for a second wire all the way back to the source.
9. What is a block diagram in the context of power distribution?
A block diagram is a simplified representation of a power system where generators, transformers, buses, converters, and loads are shown as labeled blocks with connecting lines. It captures the essential structure of the system without drawing every wire, making large or complex systems easier to understand and design.
10. How do you apply KCL at a bus to find the source current?
Treat the bus as a single node. The current supplied by the source (entering the node) must equal the sum of all branch currents drawn by the individual loads (leaving the node):
I_source = I_load1 + I_load2 + … + I_loadN
11. How should a circuit breaker be sized for a branch?
A good rule of thumb is to rate the circuit breaker 10–15% above the maximum expected current draw for that branch. This provides headroom for small transient currents while still protecting the circuit. In practice, you then select the nearest available breaker rating above that calculated value.
12. What happens when multiple loads share the same bus?
Each load receives the same voltage (the bus voltage). The total current the source must supply is the sum of all individual load currents. Adding another load to the bus increases the total current demand on the source but does not change the voltage seen by the other loads.
13. Why might a vehicle power system have multiple voltage levels?
Different devices require different operating voltages. Rather than designing each device to accept only one voltage, a transformer is used to step the main bus voltage up or down to create an additional bus at the required voltage level. This allows a single generator to power a diverse set of equipment efficiently.
14. How do you calculate the turns ratio needed between two buses at different voltages?
The turns ratio a equals the primary (input) bus voltage divided by the secondary (output) bus voltage:
a = V1 / V2
Both voltages should be expressed in the same form (both RMS or both peak) before dividing.
15. How do you work backward from a load to find the required source current in a multi-stage system?
Start at the load: calculate the power consumed by all loads on the final bus. Then move backward through each stage — for each converter or transformer, use its efficiency (P_in = P_out / η) and the transformer current relationship (I1 = I2 / a) to find the current entering that stage. Continue until you reach the source bus and sum all branch currents there using KCL.