Motor power factor is a measure of how efficiently an electric motor uses the current it draws from the power supply. Expressed as a number between 0 and 1 (or 0% and 100%), it represents the ratio of real working power to the total apparent power flowing through the circuit. A motor with a power factor of 0.85 converts 85% of its apparent power into useful work, while the remaining portion circulates as reactive power that the motor needs but doesn’t convert into motion.
How Power Factor Works
In any AC (alternating current) circuit, voltage and current rise and fall in waves. In a perfect scenario, these two waves peak at exactly the same moment, and all the power delivered does useful work. That ideal state is called unity power factor, or 1.0. In reality, induction motors cause the current wave to lag behind the voltage wave. The angle between these two waves is called the phase angle, and the cosine of that angle equals the power factor.
When the phase angle is zero, the cosine is 1.0 and power factor is perfect. When the angle grows to 90 degrees, the cosine drops to zero, meaning no real work gets done at all. Most industrial motors operate somewhere in between, typically with a lagging power factor because the current trails behind the voltage.
Real, Reactive, and Apparent Power
To understand power factor, it helps to picture a right triangle. The horizontal side represents real power (measured in kilowatts), which is the energy that actually turns the motor shaft. The vertical side represents reactive power (measured in kVAR), which is the energy that creates the magnetic field inside the motor but doesn’t produce mechanical work. The hypotenuse is apparent power (measured in kVA), which is the total power the utility has to deliver through its lines.
Power factor is simply the ratio of the horizontal side to the hypotenuse: real power divided by apparent power. A low power factor means the reactive side of the triangle is large relative to the real power side, so the utility has to push far more total current than what’s actually doing useful work. That extra current heats up wires, loads transformers, and wastes capacity across the entire electrical system.
Why Motors Have Low Power Factor
Every induction motor needs a magnetic field to operate, and creating that field requires a special component of current called magnetizing current. This current runs roughly 90 degrees ahead of the voltage, which means it’s almost entirely reactive. Under no-load conditions, when the motor is spinning but not driving anything, the magnetizing current dominates and the power factor can drop very low.
As the motor takes on more mechanical load, the real (working) portion of the current increases while the magnetizing current stays roughly the same. This shifts the balance, and the power factor improves. That’s why a motor running at full load typically has a much better power factor than one running at partial load. A motor sized too large for its task will spend most of its time lightly loaded, dragging down its power factor unnecessarily.
The design of the motor itself also matters. The internal magnetic properties that determine how much magnetizing current is needed vary from one motor to the next. Motors with a larger reactive component in their magnetizing current will have a lower power factor, while designs that minimize this reactive draw achieve better numbers.
Power Factor vs. Efficiency
These two concepts are related but measure different things. Motor efficiency is the ratio of mechanical power coming out of the shaft to the electrical power going in. It accounts for losses inside the motor itself: heat from the windings, friction in the bearings, and magnetic losses in the core. Power factor, on the other hand, describes how much of the current drawn from the grid is doing useful work versus sustaining the magnetic field.
A motor can be highly efficient at converting electrical energy to mechanical energy while still having a poor power factor. The inefficiency from low power factor shows up not inside the motor but in the electrical system feeding it: higher currents flowing through cables, transformers, and switchgear, all generating unnecessary heat and consuming capacity. Improving efficiency reduces losses inside the motor. Improving power factor reduces losses in the distribution system delivering power to it.
Why Utilities Penalize Low Power Factor
When your power factor is low, the utility has to deliver more total current to supply the same amount of real, usable power. That extra current occupies capacity in transformers, transmission lines, and generators that could serve other customers. To recover those costs, most utilities impose penalties on industrial customers whose power factor falls below a set threshold.
The most common threshold is 0.85 (85%), though some utilities set it at 0.90 or even 0.95. Penalties are typically calculated based on excess reactive power. The utility determines how much reactive power you would draw at the threshold power factor, then charges you for every kVAR above that limit. For a facility with dozens of motors, those charges can add up to thousands of dollars per month.
Correcting Power Factor With Capacitors
The most common fix is installing power factor correction capacitors. Capacitors generate reactive current that is the opposite of what inductive motors consume. By supplying reactive power locally, capacitors reduce the amount of reactive current your facility draws from the utility. In the power triangle, adding capacitors shrinks the reactive (vertical) side, which shortens the apparent power (hypotenuse) and brings the power factor closer to 1.0.
Capacitors can be installed at three general points. Placing them directly at the motor is the most efficient option because it cancels the reactive current right where it’s produced, reducing losses in every cable and breaker between the motor and the utility meter. Alternatively, capacitor banks can be placed at a motor control center or at the main service entrance to correct the power factor for a group of loads at once. Centralized banks are easier to manage but don’t reduce internal wiring losses as effectively as capacitors placed at individual motors.
How Variable Frequency Drives Affect Power Factor
Variable frequency drives (VFDs) control motor speed by converting incoming AC power to DC and then back to AC at whatever frequency the motor needs. The rectifier stage at the input of a typical VFD naturally produces a displacement power factor of about 0.95, regardless of what the motor is doing on the output side. That means motors fed through VFDs generally don’t need separate capacitor correction.
However, VFDs only help the motors they’re connected to. In many industrial facilities, only about 40% of motor loads use drives, leaving a large share of direct-connected motors still pulling significant reactive current. For those remaining loads, capacitor correction or other strategies are still necessary to keep the overall plant power factor above the utility’s threshold.
One important caution: power factor correction capacitors should not be installed on the output side of a VFD. The drive’s switching electronics can interact with the capacitors in damaging ways. When a facility adds VFDs and capacitors together, the capacitors are placed upstream, on the line side of the drive, or at the distribution panel level.
Practical Signs of Low Power Factor
You won’t feel low power factor the way you’d notice a tripped breaker or an overheating motor. The most visible sign is usually on your utility bill, either as a direct power factor penalty or as a higher demand charge driven by elevated kVA readings. Some utilities list the measured power factor on the bill; others just apply the surcharge without highlighting the underlying cause.
Electrically, low power factor means higher current flowing through your wiring for the same amount of real work. This can show up as transformers running hotter than expected, voltage drops across long cable runs, or breakers and conductors operating closer to their rated limits than they should be. If your facility has a power meter that displays kVA and kW separately, dividing kW by kVA gives you a real-time snapshot of your power factor.