# Voltage drop due to large amount of current in Parallel circuit

There is this one particular thing I don't understand. I was going through the text book exercise and found this question.

What happens if the machine being driven by the motor jams, so that the rotor suddenly stops turning?

The book stated that

There would be no Back EMF induced on the coil by the magnets of motor and large amount of the current would pass through the coil which can melt the coil.

I have no problem understanding this. After this, there was written that

When the motor is first turned on, there's a momentary surge of current until the motor picks up speed. This surge causes greater-than-usual voltage drop in the power lines supplying the current. Similar effects are responsible for the momentary dimming of lights that can occur in a house when an air conditioner or dishwasher motor starts.

I don't understand how voltage drop happens. Isn't the wiring of houses done such that every equipment is parallel to the power line? If it is done in parallel, shouldn't the voltage to every equipment be same? What I have understood is, at the beginning there is no back emf as the coil is not moving causing large amount of current to flow, which will cause less-than-usual current to flow from other equipment such as light bulb which causes its dimming. I think I am missing something important here. Any hints or help would be kindly appreciated!

There is this one particular thing I don't understand.

If there's only one, you're way ahead of the rest of us.

I don't understand how voltage drop happens. Isn't the wiring of houses done such that every equipment is parallel to the power line?

Yes, but the power line itself has some resistance. Here's a simple model:

When one of the loads draws a very large current, it causes a drop across the feed line resistance, represented by R1 in my model.

If it is done in parallel, shouldn't the voltage to every equipment be same?

Yes, that's why a drop caused by one load (say R1) can also affect the voltage seen by another load (say your overhead lights).

What I have understood is, at the beginning there is no back emf as the coil is not moving causing large amount of current to flow, which will cause less-than-usual current to flow from other equipment such as light bulb which causes its dimming.

Yes, you're missing the step where the high current from the motor causes voltage drop across the feed line, which lowers the voltage available to the resistive loads. The lower voltage is what causes them to draw less-than-usual current.

Even if the power at the delivery transformer can be considered nearly ideal, there will still be non-zero resistance on the feeder from there to the house. So while the motor may be on a dedicated circuit, it can still cause a voltage drop at the panel to all other circuits.

Yes all the circuits are in parallel and will have basically the same voltage, but that voltage can fluctuate depending on the total current.

• Can you please explain further?. I still don't get why does the voltage drops Commented Dec 25, 2022 at 15:03
• @goodplayer1, presumably you have heard of Ohm's law. If a large current is drawn through the supply wires, there is a drop between the source end and the load due to the resistance of the wires. Commented Dec 25, 2022 at 16:30