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From time to time someone will say

From time to time someone will say

From time to time someone will say

From time to time someone will say something like this about car components like 12 V DC air compressors and industrial devices driven by 120 VAC induction motors

"Use right size cables, if there is voltage drop in the cables the compressor will take the same power and compensate loss of voltage by taking more amps."

Is there any truth to such a statement ?

RE: From time to time someone will say

I don't know if it's proper to say that a compressor compensates for reduced voltage by drawing more current, which seems a little illogical given that the voltage is lower, but a compressor is definitely not a fixed impedance.

As demonstrated recently by a family member who connected an ice cream freezer to a truck battery via an inverter.
With 8 gage battery cables, the compressor refused to start.
With 4 gage battery cables, the compressor started right up and ran happily.

Mike Halloran
Pembroke Pines, FL, USA

RE: From time to time someone will say

I believe understanding the "how this happens" is in the concept of applied forward emf and back emf generated by an electric motor.

Current draw is determined by the degree that generated back emf is less than applied forward emf and, internal resistance... any external condition that causes generated back emf to drop e.g. applied load, applied voltage influence, will cause current draw to increase. The "motor effective circuit" diagram in the reference below illustrates this relationship visually.


Perhaps the simplest explanation is that a motor (as an energy transformation device) takes electrical power input in the form of voltage and current. If the applied voltage drops, and the motor is expected to drive the same load (rotational speed and torque), current must increase.. conservation of energy.

RE: From time to time someone will say

Motors are not resistors and do not follow Ohm's law. Battery cables are simple resistors. DC motor are simpler to understand than AC. At zero load DC motors will generate a back emf (voltage) close to the supply voltage, so there is little net voltage to drive current through the resistance of the motor, and the current draw is low. Adding load will slow the motor so it generates less back emf and it will draw more current to try to maintain speed.

The speed of an AC induction motor is controlled by the applied frequency. If you reduce the voltage it will not slow down but will draw more current to increase torque in order to try to maintain speed.


RE: From time to time someone will say

In some respects, the OP is correct. Considering that the load is essentially a constant, then the drive power of the compressor must try to be constant as well. So long as the compressor isn't at its limits, the back EMF is such that the power supply will see a load consistent with the power draw. When the supply voltage is lower, the compressor operate closer to its maximum limit, so the back EMF drops, allowing the lowered power supply to provide the same power, which means more current.

I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: From time to time someone will say

I doubt you'd find it applies very well to DC motors since you typically control DC motor speed by varying the DC voltage. In most cases, you'll find the DC motor just runs slower and the load receives less mechanical power.

Within a certain operating window, AC motors will run very close to a fixed speed (speed varies by 2-3%). So, you also get a fixed load speed, meaning the load is taking a fixed mechanical power from the motor shaft. The motor input power has to balance with the mechanical power so if you drop the voltage a bit then the current will go up a bit. But, drop the voltage too much and it all goes to crap because the motor just stalls and the load quits using any shaft power.

RE: From time to time someone will say

You can analogize using a water hose to represent the wire. A small hose will not take the same "current" that a larger one will. That smallness represents the "resistance" of the wire. So if the water powered motor you have needs a 2 inch inside diameter hose to supply the correct amount(current or amps) and pressure (voltage) then you restrict that hose to a 1/2 inch inside diameter hose to supply your motor, it is obvious you will not get the same power from that motor.

RE: From time to time someone will say

As a thought experiment the claim is that if you put a resistor in series with the load the current will increase. In a DC machine that seems unlikely. With an AC circuit I suppose it could happen.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: From time to time someone will say

In a AC induction motor, the motor is a spinning at the speed of the frequency(synchronous speed) with a maximum of 2% slip from the main frequency(50/60 HZ).When a voltage drop occurs the torque available at the shaft will be reduced and at the same time the rotor will try to main it slip with the main frequency and draw more current in try to maintain the 2% slip.

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