# Voltage v/s current

#### nagumuthu

Joined Mar 7, 2007
1
Hi Guys,

I'm asking a basic question.

While 3 phase motor is running, if voltage is dropping on one of the phases due to some reasons, will the load take more current..

I wish to know why it takes more current..

I'm not too strong on basics and advice any websites to understand the basics of voltage and current relationship on 3 phase supply.

Thanks a lot.

#### thingmaker3

Joined May 16, 2005
5,084
Current will be proportional to voltage, but will be lagging a bit behind in time.

Reading through Chapter 10, Volume II of All About Circuits may help answer many of your questions on 3-phase systems.

Also, the earlier chapters of Volme II will help you understand the phase relationship between voltage and current and voltage in inductive circuits.

#### subtech

Joined Nov 21, 2006
123
First lets assume the motor is connected to a load of some sort be it an air compressor or whatever. It is a fact that to turn the compressor or overcome its resistance to turn, it takes watts or true power. If the motor is loaded to its nameplate rating (lets say 10 h.p.) then the current drawn from the source will be very close to the amount stated on the nameplate of the motor which is the full load amperes rating or "FLA". If the source voltage to the motor decreases by 5%, then the current must increase to supply the same amount of watts, or true power to keep compressor running (E*I). Up to a point, this works ok.
But, if the supply voltage drops too low and the resultant current magnitude goes to high, the motor will overheat (I2R losses in the conductors/windings) and damage or failure will result.
It is also very important on the three phase motors to keep the voltages balanced as well as within the allowed tolerance for voltage magnitude.