For example if I connect a 90v motor to a 120v battery source doesn’t that overheat and blow my motor?
Why is it though when it’s 120v from the wall then the motor is still fine?
In: Engineering
When given the correct **voltage**, a properly functioning electrical device will determine, by itself, how much **current** to draw.
A device on a 15A circuit will determine on its own how many amps to draw (up to 15). A 90V motor hooked up to 120V isn’t the correct voltage, so all bets are off. Therefore, there’s no contradiction.
If it really is a 90v motor designed to be hooked up to 120v current, it’ll have a transformer to step the voltage down to 90v, so that won’t be a problem. You might have a problem because a battery outputs *direct* current instead of *alternating,* though.
In the general case of your title question, current is pulled, not pushed. The 15A rating isn’t what *is* delivered, but what *can be* delivered without the circuit overloading.
Having a 15A circuit doesn’t mean everything on the circuit gets 15A. It means that circuit will allow up to 15A to be drawn before the breaker pops.
The devices connected to the circuit will only draw what they need. This is because of ohms law. V=IR. The V is constant so the current (I) is determined by the R, which is the device load.
The amount of current drawn is limited by the appliance, not the circuit. Imagine being at a buffet where there are 15 cakes available for you to eat. You will not burst because you can only eat 1 or 2 cakes while there are so many available. You eat just the cakes you want to and leave the rest. If you eat more then 15 cakes though something is going to fail and the pastry chef will blow a fuse.
Some good comments here already, but since you mentioned tools, here’s an analogy that might click:
“Why does your air compressor (110psi 6CFM) not overload tools that only use 1.8 CFM, 2.5 CFM, 5.2 CFM (etc. anything less than 6)?
It’s basically the same idea. Voltage is like pressure, Amps are like flow rate. The tool will use as much “flow rate” as its design requires. The combination of flow rate and pressure together describes how much “power” is required to run the tool; that’s Watts!
You can also have multiple tools hooked up at the same time, but if their combined CFM is greater than the compressor can supply, the pressure is going to drop – voltage actually does the same thing, but the compressor in that case is usually pretty big (e.g. a power plant). So your Amps aren’t limited by the source, they’re limited by the breaker. Limit is the key word there. The 15A isn’t what’s coming through the wire all the time, it’s how much is *allowed* to come through the wire.
A 15 Amp circuit doesn’t mean that it always has 15 amps flowing through it, it means it’s designed to safely withstand up to 15 amps.
The device using the electricity is what determines how many amps are flowing, if for some reason the combine load of all the devices on a circuit would exceed 15 amps the breaker would trip to protect the wiring in the building.
When you turn on your kitchen sink, why doesn’t your house immediately flood, as it would if you opened a fire hydrant?
This is the “hydraulic analogy”. Voltage is water pressure, current is the flow rate.
My background is fluids, helps me to think of electricity as a water analog. Maybe it will help you too. Voltage is like water pressure. Put 120psi (volts) in something designed for 90psi and it will come apart. Voltage is an electrical potential, pressure is a hydraulic potential.
Amperage is like water flow. If I want 5gpm water flow, I need a certain pressure to get that much water to flow through the pipe. Flow may be limited by an upstream valve (circuit breaker) that will restrict flow (current).
A motor will only draw the amps it needs to spin at its design speed. So a 15A saw motor may only use 2 A when it’s free spinning and doing no work. When you start a cut, the blade slows down and the motor pulls more amps to try to stay at the design speed. So a 120V device will always use 120V, it will have a varying amount of current flow.
Easy simplistic way to think of it. Voltage is pushed, current is pulled.
120v to your 90v WILL overheat it.
15a available at 90v to your motor WON’T overheat it.
Electric current is pulled by the tool rather pushed but the electrical panel. A 15A circuit means it can deliver up to 15A when a tool is plugged in but when there is nothing plugged in the circuit is at 0A.
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