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[new guy]

An ac motor has a running amperage of 10 amps and the measured voltage is 120volts ac. If the voltage drops to 110volts, what amperage would you expect to measure on a meter. HIGHER than 10 amps or LOWER than 10amps? 9.2 amps? , 10.8 amps? or??

[SETEC_Astronomy]

My guess as a hobbyist is lower amps. A lower voltage can't push as much current through a given resistance than a higher voltage. I really have no clue.

[Craig]

OHM'S LAW

In 1827 a German scientist named Georg Simon Ohm discovered a relationship between the voltage that existed across a simple electric circuit and the current through that circuit. He determined that the magnitude of a current is, in general, proportional to the magnitude of the emf that produces it. When he doubled the potential difference he found that the current was doubled; when he tripled the potential difference the current was tripled, and so on. Ohm discovered that as the voltage increased, the current increased in direct proportion to the applied voltage, maintaining a constant ratio of voltage to current. Ohm's law is stated as follows:

The current produced in a given conductor is directly proportional to the difference of potential between its end points.

Therefore, if the voltage decreases, the current (amps) will also decrease.

[ecerfoglio]

An ac motor has a running amperage of 10 amps and the measured voltage is 120volts ac. If the voltage drops to 110volts

An AC (induction) motor's current depends on the speed it is rotating.

When the input voltage is decreased, il lowers its speed (** only slightly **) and increases its current (and power factor = Cos Fi) so it can continue to provide the same power.

[dacflyer]

it will be lower...

[Externet]

Hi.
If the voltage supply drops from 120V to 110V, the motor current will diminish.

If the voltage drops from 120V to 110 V because the motor encountered a higher effort, friction, or workload, the current will increase.

[Chris Smith]

Current is not “pushedâ€

[jollyrgr]

Ohm's law is good for linear loads, such as a resistor, when changing voltages. This is not a linear case. A motor's amperage will vary with physical load. You put a larger load on the motor it will draw more amps to maintain the same RPM. Now follow me on this. An AC motor's RPM will usually be determined by the line frequency. (A Universal motor is an AC/DC motor so don't include that in this discussion.) So if the RPM is the same and the amount of physical load on the shaft is the same the voltage drop will cause the motor current to go up to maintain the same RPMs.

Think of it this way. You can have a car engine spinning at the same RPM going the same speed. But you have to press harder on the gas pedal to go up a hill compared to level surfaces.

[rshayes]

The behavior of a motor depends to a great extent on what load it is driving. Some loads are non-linear, such as air blowers. Often AC motors have non-linear responses and may have negative resistance regions. A synchronous motor might well draw mmore current at a lower voltage in order to maintain the load at synchronous speed.

In most cases, I would expect the speed and current drain of the motor to decrease with a decrease in motor voltage. There are probably many exceptions, and I don't think I would claim that this is a hard and fast rule.

A motor with tachometer feedback would probably draw more current with decreasing input voltage in order to maintain the output power of the motor constant.

[ecerfoglio]

A synchronous motor might well draw mmore current at a lower voltage in order to maintain the load at synchronous speed

A motor with tachometer feedback would probably draw more current with decreasing input voltage in order to maintain the output power of the motor constant.

As I posted yesterday, an "AC motor" probably means an induction or asynchronous one.

As their name implies, asynchronous motors don´t keep a constant speed but it varies only a little:

ie a 4 pole 60 Hz motor may have a (full load, full voltage) speed of 1730 RPM (1800 RPM minus about 4 %),
With an 8% reduction in voltage (110 V vs 120 V) it will reduce its speed to something like 1700 RPM (1800 RPM minus about 5.5 %), which gives a reduction in speed of only 1.7%.

The load it is driving may be non linear, but it will take a power that varies from proportional to the speed (linear) to proportional to it's cube (a blower), so the power drops from 1.7% (linear) to 5.1% (P = k n^3).

If the voltage drops more than the power the motor will draw more current or use a higer power factor (=cos Fi) (or both)