# Why does reactive power not output useful work in a motor?

They say that negative power from an inductive circuit means that power is flowing from the inductor back to the source, so no work is done. For a motor, even though the power or current is flowing back to the source, because it is a closed circuit, the current also flows through the coil of the motor, generating the magnetic field it needs to operate, so in the end, isn't negative power also generating useful work?

• The part that generates useful work is not reactive. May 4, 2022 at 16:01
• Reactive power is stored and "replayed" into the supply. So obviously you can't get useful work out of it : if you did, it wouldn't be reactive, it wouldn't flow back to the supply.
– user16324
May 4, 2022 at 19:04

isn't negative power also generating useful work?

Reactive power is not negative power. Positive (real) power would be defined as either power flowing from source to load (or load to source) while negative (real) power would be defined as the opposite direction. So positive and negative is just mathematical convention for the direction that real power is flowing.

Reactive power is just real power that flows circulates around the circuit but is never dissipated so never performs work. It flows from source to load but then flows back from load to source at some future point in time.

In a three phase system it could be circulating between the phases rather than between source and load. Either way it's not dissipated so doesn't perform work.

It never performs work but since it is still flowing and thus still takes up ampacity in the wires.

the current also flows through the coil of the motor, generating the magnetic field it needs to operate.

Ahhh, but a magnetic field doesn't inherently perform work just like a permanent magnet doesn't inherently perform work. Supporting a magnetic field alone is not work anymore than an asteroid hurling through space is performing work by virtue of the kinetic energy in its motion.

A motor is a bit more complex than an inductor. With an inductor, an AC current converts input electrical power into magnetism, then magnetic energy is converted back into electrical energy returned to the circuit. There is energy going forth and back, but no net energy in one cycle (assuming there is no resistance).

Now with a motor, you also have the magnetic interaction with the rotor/ stator, plus the back emf that translates the mechanical reaction of the load being driven by the motor into the electrical source. The difference between a pure inductor and a motor (or in general an RL load) is that the only part of the input energy is returned to the circuit (in one cycle).

The energy not returned is either lost in resistances or "absorbed" by the back emf thus converted into mechanical energy. The energy that is returned accounts for the reactive power (P = E/t being t the cycle period)

Some clarification is needed to correct misunderstandings.

• Generator power is negative, and loads use positive power.

• Reactive current in AC is cyclic (+/-) but is out of phase by 90 degrees to voltage.

• Because of the law of conservation of energy, the sum of all inputs and outputs is zero. Thus in math, we express real power supplied as negative and used power as positive.

• Reactive [VA]power(VA) or VAR's for AC circuits, Inductive current cycles in both polarities by lagging the current by 90 degrees after voltage.

• Real resistance from losses or work done draws current in-phase and this of real to total power ratio is called Power Factor. pf<=0.25 might be expected with no real load and p.f.>= 0.8 at full load .

Reactive energy serves many purposes to store energy in cycles and and magnetically couple and attract the moving rotor to the moving magnetic field. Think of this Reactance or Inductance like a flywheel that takes far more power to get up to speed quickly than rated power which is far more power than no-load at max RPM.

Inductance and magnetic material is like a flywheel which stores magnetic energy as inertia to smoothen the torque per cycle. , inductance per pole and rotation stores electrical energy and demands excess current to startup. The inductance also magnetically creates the rotating magnetic field to attract the rotor with a rotating force. As it moves with full speed and intertia it also generates voltage called back ElectroMotive Force (back EMF or BEMF). This is like a unicycle propels the peddles if you coast and is proportional to speed. Thus the only current with no load is only needed to push and pull the peddles to sustain the speed from losses.