Input power factor and reactive power consumption
The focus in all energy efficiency initiatives is to minimize the losses, improve the system efficiency and reduce overall power consumption. With “power” people usually refer to active power and this is basically correct. Main purpose of VFD is to reduce active power. However, VFD also helps to reduce the consumed reactive power. Did you know that? This is sometimes forgotten in the considerations.
Reactive power, unlike active power, cannot be transformed into useful work. Therefore the name reactive… Nevertheless, transmission of reactive power is associated with losses same as active power. By reducing the reactive power the losses in the transmission or distribution grid are reduced. And there is one more benefit: Reduced reactive power consumption generally increases the stability of the grid. But this is not a direct benefit for the consumer, at least as long as there is no blackout 😉
Most utilities charge the consumers for both active and reactive power. Therefore, reducing the reactive power consumption generates additional savings and helps to further reduce the payback time of a VFD.
So how does the VFD help to reduce the reactive power consumption? We will demonstrate it on several examples of induction motor drives.
What is the power factor of an induction motor?
Induction machines are the work horses of the industry and most motor drives include this type of machines. Their power factor is inherently inductive, i.e. less than 1.0. The motor therefore consumes both active and reactive power. In case of direct on-line motor the power is supplied directly from feeding grid. Induction motor rated for 5 MW output with nominal power factor 0.9 and 97% efficiency consumes at full load 5.155 MW active power, 5.727 MVA apparent power and 2.5 MVAr reactive power. Although power factor 0.9 sounds quite high, the reactive power consumption is definitely not negligible.
So let’s assume the most common situation for a benchmark:
– Induction machine operated as direct on-line
– Induction machine fed from voltage source inverter (VSI) with diode rectifier (DFE)
What is a power factor of a diode rectifier?
Diode is the most common semiconductor. As soon as positive polarity voltage is applied across the diode, the device starts to conduct current. In fact, there is a small on state voltage (forward bias in the figure below right), but it is almost negligible. The principle of a diode is described by a V-A (Volt-Ampere) characteristic:
The fundamental power factor is defined by the angle φ between voltage and current (PF = cos φ). When voltage and current are exactly in phase, the angle is zero and power factor is unity (cos 0° = 1). Since the diode has only negligible forward bias it is clear that the power factor is inherently very close to unity. Practically there is a commutation of current from one phase to another one. The commutation is not instant, but the rate of change of current depends on the reactance in the commutation path. This reactance is defined by the transformer(short circuit) impedance and grid impedance. For typical transformer design with voltage impedance 6 to 10% and reasonably strong grid the power factor of diode rectifier at full load is approx. 0.95. This is a value that almost all VFD manufacturers state in their catalogues for diode front end drives. In fact, the power factor at partial load increases since the current is lower and with the same rate of change (di/dt) the commutation takes less time.
Power factor versus load
Here comes the main difference:
– Power factor of induction machine generally drops with decreasing load *
– Power factor of diode rectifier increases as the load decreases.
* There might be a slight increase when reducing the load from 100% down to 75%, but below 75% the power factor is dropping and below 50% the drop becomes sharp.
Considering that VFD driven applications do not run all the time at full load, the improvement in power factor and reactive power consumption is significant.
The figure below illustrates the comparison of input power factor of 4 MW, 6 pole motor (992 rpm) operated
a) direct on-line
b) supplied from VFD (voltage source inverter with diode rectifier)
Benchmarks
Let’s compare the data of direct on-line and VFD supplied motors for several cases:
Case 1: 4 MW slipring 6-pole motor versus VFD supplied motor with same rating
Case 2: 7 MW squirrel cage 4-pole motor versus VFD supplied motor with same rating
Case 3: 12 MW squirrel cage 4-pole motor versus VFD supplied motor with same rating
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Case 1 (4MW_6-pole)
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Case 2 (7MW_4-pole)
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Case 3 (12MW_4-pole)
The above examples considered induction motor drives as the most common type of drives in the industry. With synchonous machines the picture would look different as synchronous machines typically operate with power factor 1.0 or even with capacitive power factor (overexcited) to compensate other loads. However, synchronous machines are typically only used in high power range or in special process performance applications. VFD can be equipped with active front end and reach the same behavior (power factor 1.0 or capacitive power factor in VAR compensation mode).
Summary
Three model cases illustrate how much reactive power can be saved when the induction motor is supplied from VFD instead of operating direct on-line. The savings are in the range 30-50% at rated load and even larger (in percentage) at partial load. The reduction of reactive power is additional advantage of VFD besides the energy savings. It might reduce the electricity bill from the utility, reduces voltage drop in the system, contributes to more stable grid and might reduce the sizing of incoming cables and input step-down transformer.
