How DC Motors Affect To Power Factor?

If d.c motor is fed by rectifiers,there is a possibility of harmonics in the a.c side which can change the pf

Power factor: It is the cosine of angle between voltage and current in an A.C. circuit.

It is very clear from the above definition that, power factor is only related to AC systems. There are no sine/cosine waves in DC, so power factor is a non-issue.

However, If the DC motor is fed through AC power, the AC power would need to be converted. In that conversion process, you will have to use rectifiers (and probably transformers). The conversion process will have poor power factor itself. Than, in this case power factor of systems may be affected.

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Why would the rectification process have low PF? When the DC load is drawing its current at the rated voltage, the V and I are "in phase" so to say. This current is coming from the AC side anyway, so it has to be in phase. So i can't really see why the PF can be anything other than 1.

KVS! I too agree with you! Rectifiers generally operate at very close to UPF. This is claimed as an advantage by many VFD manufacturers too. But, it is only the Displacement PF that would be close to Unity. The Distortion Pf would be very poor. Hence, the Total Pf which is a multiple of Displacement Pf and the Distortion PF would fall drastically. But, thank God! Our electricity boards still charge us only for the displacement PF and not for the Total PF.

Never having to deal with this, i simply did not know ! Thanks and GA!

Superb links, thank you. As i said to EE65, i just did not know, so i learnt something here. Pity you didn't register, and also voted yourself OT !! WHY for heaven's sake ? This post has to be most ON TOPIC. i will try with my one GA !

Non-linear loads, such as rectifiers, distort the current drawn from the system. Because current in these systems is interrupted by a switching action, the current contains frequency components that are multiples of the power system frequency. Distortion power factor is a measure of how much the harmonic distortion of a load current decreases the average power transferred to the load.

In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor. The devices for correction of the power factor may be at a central substation, spread out over a distribution system, or built into power-consuming equipment.

Power factor correction in non-linear loads :

Passive PFC

The simplest way to control the harmonic current is to use a filter: it is possible to design a filter that passes current only at line frequency (e.g. 50 or 60 Hz). This filter reduces the harmonic current, which means that the non-linear device now looks like a linear load. At this point the power factor can be brought to near unity, using capacitors or inductors as required. This filter requires large-value high-current inductors, however, which are bulky and expensive.

Active PFC

An "active power factor corrector" (active PFC) is a power electronic system that controls the amount of power drawn by a load in order to obtain a power factor as close as possible to unity. In most applications, the active PFC controls the input current of the load so that the current waveform is proportional to the mains voltage waveform (a sine wave). The purpose of making the power factor as close to unity (1) as possible is to make the load circuitry that is power factor corrected appear purely resistive (apparent power equal to real power). In this case, the voltage and current are in phase and the reactive power consumption is zero. This enables the most efficient delivery of electrical power from the power company to the consumer.

Thank you.

i still am a little confused. Rectifier is not the load here, the DC motor is. A 3Φ bridge will be almost ripple free, and the inductance of the motor will perhaps smooth it out further. Will it still have all this distortion pf etc?

have a look at this

Especially towards the end, but, may be go through. It deals basically with SCR but that is a diode bridge with alpha=0.