How to successfully recognize power quality problems

Measuring Motor Displacement PF

Start measuring motor displacement power factor by selecting which of the two methods to use based on the transformer configuration supplying the 3-phase induction motor. Either method will give the same results. Method #1 is for the grounded-Y source.

It is simple and can be applied in most situations, since virtually all of the low voltage motors in commercial and light industrial facilities are fed from a grounded-Y source.

Method #2 is for floating sources sometimes found in heavy industrial facilities.

Method #1: Grounded-Y source

To check if the source is grounded-Y, measure voltage of each phase to ground. If the readings are equal, you can use this measurement method.

Set-up:

Clamp the current probe on any phase (with arrow on clamp pointing towards the motor). Then, attach the red voltage probe to the same phase and the black probe to ground (not to another phase).

• Active Power: Read kW and multiply by 3: kW_MOTOR = 3 × kW
• Displacement PF: Read DPF. (Not necessary to measure kVA)

Method #2: Three-wire Source

With floating-Y, floating-delta or grounded-delta, the voltage will be different for at least one of the phase-to-ground readings. (For the floating source, the phase-to-ground voltage is unpredictable, since it depends on phase to ground capacitance).

Method #2 is known as the Two-Wattmeter method.

Set-up:

First connect the black voltage probe to any phase. Then connect the red voltage probe and the clamp (arrow towards the load) together on a second phase. Record kW₁.

Then move red probe and clamp to the third phase (do not move the black probe). Record kW₂. Record kVA (kVA of either phase will be more or less equal if the current unbalance is within limits).

• Active Power: Kw_MOTOR = kW₁ + kW₂ (If either kW reading is negative, as might happen on a very lightly loaded motor, it would be subtracted instead of added)
• Apparent Power: kVA_MOTOR = kVA × 1.73.
• DPF = kW_MOTOR / kVA_MOTOR

Example (Two-Wattmeter):

Measurements:

• kW₁ = + 1.52
• kW₂ = + 1.74
• kVA = 2.41

Calculations:

• Kw_MOTOR = kW₁ + kW₂ = (+ 1.52) + (+1.74) = 3.26 kW
• kVA_MOTOR = kVA × 1.73. = (2.41) (1.73) = 4.17 kVA
• DPF = Kw_MOTOR / kVA_MOTOR = 3.26 / 4.17 = 0.78