## Inductive circuits

## Inductive circuits

(OP)

I’ve been trying to understand inductive loads and voltage drop. I though if voltage drops on an inductive circuit the current will also decrease. Is this correct? I understand if on phase of a 3 phase motor circuit one phase is high resistance this causes an imbalance causing the other two phases to draw excessive current due to the back EMF becoming smaller. Any clarification will be greatly appreciated.

Thanks again guys

Thanks again guys

## RE: Inductive circuits

## RE: Inductive circuits

## RE: Inductive circuits

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

## RE: Inductive circuits

[tangent warning] Constant power loads provide destabilizing feedback because low voltage causes higher currents. Over the last decade there has been a huge shift from incandescent (i.e. constant impedance) lighting over to LED lighting that operates as constant power. This shift potentially leaves the grid more vulnerable to voltage collapse.

Although induction motors are typically modeled as constant power with respect to voltage, typical induction motors do provide damping with respect to frequency changes. Motor loads typically respond to declines in system frequency by slightly reducing power consumption. This feedback helps to achieve a stable balance between load and generation. When motors are put behind VFDs, the load become constant power with respect to both voltage and frequency. As more loads are put onto VFDs, controlling the frequency of the grid becomes harder.

[/tangent]

## RE: Inductive circuits

I’ll see your silver lining and raise you two black clouds. - Protection Operations

## RE: Inductive circuits

## RE: Inductive circuits

Dropping 115 volts across a motor contactor would lead to a very large amount of heat to dissipate within the contactor. Typically a "high resistant" in a contactor/joint/spice just means high in comparison to the expected value. For example, a joint might have 0.001 ohm of resistance instead of the expected 0.0001 ohm. This would increase the heat generated by the joint by 10x, but would have negligible impact on the rest of the circuit.

On the original question about a single phase with low voltage, the explanation of reduced back EMF is misleading. An alterative way to analyze imbalanced voltages is by splitting the voltages into positive and negative sequence components. Unbalanced voltages produce a negative sequence a torque that rotates opposite to the direction of the motor. The motor then draws an additional amount of positive sequence current to offset the negative sequence torque generated by the unbalanced voltage.

As a thought experiment, picture two identical motors attached to the same shaft. Apply normal voltage to the first motor. For the second motor, swap two of the phases so it tries to drive the shaft the opposite direct. For the second motor, apply 2% of rated voltage. The second motor will attempt to slow down the shaft, and this setup would lead to significant increase in total current feeding the two motors.

## RE: Inductive circuits