How Does Back EMF Cause a Voltage Drop in Household Circuits?

In summary: There's something else responsible for that, and that something is the power supply or source that's driving the motor. In summary, when a vacuum cleaner is plugged into a household circuit, the inrush current causes a momentary voltage drop in the circuit. This is due to the resistance in the wiring and other factors. As for the design of a motor, the input voltage will always be limited by the power supply or source, even as the back emf increases with the motor's speed.
  • #1
Jimmy87
686
17
Hi pf, I have been reading about back EMF and just had a few questions. I found this article on wiki (http://en.wikipedia.org/wiki/Counter-electromotive_force) and the last paragraph talks about the effects of back emf when you plug a vacuum cleaner into your wall. It says that you can notice an incandescent bulb dim which I tried and it does indeed do this. I understand the concept of back emf and that when you first switch on the vacuum it draws a large current as the back emf is zero at this point. How does this cause a momentary voltage drop in your household? The only thing I can think of is that this same effect happens in a car due to the internal resistance of the car battery but there is no battery in a household circuit so what is responsible for this voltage drop? Is there a source of internal resistance at the voltage input to a house?

The other question I had it what is it about the design of a motor that keeps the input voltage above the back emf. If you consider a motor which starts to spin then the back emf grows and grows as the motor spins faster and faster. So shouldn't it keep doing this until it reaches a point where the back emf equals the input emf which would mean there would be no net emf to keep driving the motor?

Thanks for any help offered!
 
Physics news on Phys.org
  • #2
Jimmy87 said:
How does this cause a momentary voltage drop in your household?
The wiring in your household has resistance. If your bulb and vacuum cleaner are on the same circuit, then the bulb will see a momentary voltage drop due to the large inrush current flowing to the vacuum. There's more to it than just resistance, though.

Jimmy87 said:
The other question I had it what is it about the design of a motor that keeps the input voltage above the back emf.
There's always a practical limit to what voltage you can apply to the motor, so, as you say, there will always be some speed at which you can't drive current into it due to its back-EMF.

The motor doesn't have a say in what voltage you apply to it, however.
 

1. What is Back EMF and how is it generated?

Back EMF, or back electromotive force, is a phenomenon that occurs in electrical circuits when the magnetic field created by the current in a coil of wire changes. This change in the magnetic field induces a voltage in the same coil, opposing the original current flow. It is generated when a current-carrying conductor experiences a change in magnetic field, such as when a motor or generator is in operation.

2. How is Back EMF used in motors and generators?

In motors and generators, Back EMF is used to regulate the flow of current and control the speed and torque of the device. In a motor, the Back EMF opposes the applied voltage, limiting the amount of current that can flow and preventing the motor from burning out. In a generator, the Back EMF is used to produce a counter voltage, allowing for the conversion of mechanical energy into electrical energy.

3. What are some common applications of Back EMF?

Back EMF is commonly used in various types of motors, including AC and DC motors, as well as generators. It is also utilized in household appliances, such as refrigerators and washing machines, to control the speed and efficiency of the motor. Additionally, Back EMF is used in power electronics for energy conversion.

4. How is Back EMF measured and calculated?

Back EMF can be measured using a voltmeter connected to the motor or generator. The magnitude of the Back EMF can be calculated using Faraday's law, which states that the induced voltage is equal to the rate of change of magnetic flux through the coil. This can be represented by the equation E = -N(dΦ/dt), where E is the induced voltage, N is the number of turns in the coil, and dΦ/dt is the rate of change of the magnetic flux.

5. What are the benefits and drawbacks of Back EMF in electrical circuits?

The main benefit of Back EMF is its ability to regulate the flow of current and control the speed and torque of motors and generators. This helps to prevent damage to the devices and improve efficiency. However, Back EMF can also cause voltage spikes and interference in circuits, which can be problematic for sensitive electronic components. Therefore, proper measures must be taken to mitigate these effects.

Similar threads

Is this a valid way to measure the Back EMF of a DC motor?
    • Mechanical Engineering
Replies
14
Views
4K
How does back EMF contribute to the operation of an autotransformer?
    • Electrical Engineering
2
Replies
64
Views
5K
Back EMF: Explaining the Counter Voltage Induced by Change in Current
    • Classical Physics
Replies
4
Views
3K
Internal Resistance of a Car Battery
    • Introductory Physics Homework Help
Replies
8
Views
12K
Why Does Back EMF Decrease as Current Derivative Decreases in an LR Circuit?
    • Electromagnetism
Replies
10
Views
3K
Why does Back EMF produce so much voltage?
    • Electromagnetism
Replies
6
Views
3K
Internal resistance of the battery and energy transformed
    • Introductory Physics Homework Help
Replies
14
Views
2K
Understanding Back EMF: How It Affects Motor Performance and Power Consumption"
    • Electrical Engineering
Replies
11
Views
4K
Unraveling the Mystery of EMF Internal Resistance
    • Introductory Physics Homework Help
Replies
2
Views
1K
Solving Resistor Circuit Homework: DC Motor, Rf, Rr, E, If, Ir, Pth
    • Introductory Physics Homework Help
Replies
13
Views
8K

Hot Threads

Recent Insights

Back
Top