Reactive Power Consumption

J

Thread Starter

JRH

I am wondering about the interface of power factors with mechanical components. I understand the electrical side of power factors, i.e. what the electrical panel outputs. I know that it is advantageous to have a low power factor because more of the power is used (i.e. more of what is actually payed for is translated into actual energy).

But what sort of limits exist when the power gets to the mechanical components? If you have an AC induction motor, is there a particular minimum reactive power needed to run the motor? And if you do not supply this, will the motor shift the phase angle between the voltage and current to compensate?

For example, if you had a motor which you originally supplied a pf of 0.78 or something when you installed it, and you decided to install pf correction capacitors to change the pf to 0.99, do you actually get 0.99 of what you pay for (other than the capacitor inefficiency introduced)? or does the motor drain some of the real power to create necessary reactive power needed to run the motor?
 
JRH... I suggest you start with the white paper:

www.Imphotonics.com/pwrfact.htm

It is a very good reference that explains the connection (please excuse the pun) between induction motors and power-factor. It also exposes several myths related to the application of power-factor correction for PFC for motors.

Regards, Phil Corso
 
B

Bruce Durdle

It is not advantageous to have a low power factor - ideally, the pf should be close to 1.

Once you get through the motor, power factor ceases to have any meaning. Mechanical power on the shaft is all active power, and the active power consumed by the motor will be proportional to this - so if you have say an unloaded pump the active power will be low, while if you load the pump the active power drawn by the motor will increase in proportion.

The reactive power drawn by the motor is related to the energy needed to maintain the motor's internal magnetic field. The current drawn by an inductive winding lags the applied voltage by 90 degrees. The magnitude of current is dependent on the supply voltage and will not change with load. So if the motor is unloaded the total effect is a low active power draw with a relatively high reactive power draw, while if the motor is loaded the active power is increased while the reactive power is unchanged. Since power factor is the ratio of active power to the "total" or "apparent" power (square root of (active power squared + reactive power squared)) the power factor for the unloaded motor is low while for the loaded motor it is high.

The motor circuit will draw whatever it takes from the supply - you cannot simply decide to feed a load at 0.7 pf. Power factor correction operates because capacitors draw a current that is 90 degrees ahead of the applied voltage, effectively cancelling some of the inductive current. The capacitors can also be thought of as "supplying" reactive power for the inductive motor to "consume". If you don't provide adequate reactive power, ultimately the effects will ripple through back to the alternators supplying the grid and have an effect there - but that's another whole new issue.

Cheers,
Bruce.
 
D

David Ford, PE

Bruce answered your question well and I would like to add a couple things. First, the power company may penalize you for a poor power factor. Second, if you have a poor PF you will not get full use of your conductors, requiring you to run larger wires to counteract heating.

Fix this and increase the useful amps of you plant system. Finally, be careful about trying to correct PF too close to 1; a leading PF, say on weekends, can be disastrous.
 
A
Bruce,

thank you for this answer but I want to ask you to complete your explanation when the reactive power absorbed by the inductive loads gets back to the generators what actually happens? in other words, if the reactive power is not dissipated and just moves between source and loads so, why do we always be told that the generator supplies reactive power all the time?

thanks in advance
Ahmed
 
Hi Ahmed,

There are two ways of looking at a power system. In the usual approach, the values of interest are RMS power, voltage or current and we are not concerned with the time variations. Effectively, the numbers we are concerned with are averages taken over a number of cycles. In the second approach, we need to take account of the time variations of the various quantities involved in some detail.

The average value of reactive power over a cycle is 0, and in any calculations using RMS or average values it will not appear. However, it has a real physical effect over a single cycle - you cannot have a varying voltage across a capacitor without having an associated power flow, with the same applying to current in an inductor. For alternating capacitor voltage or inductor current, there is an associated alternating reactive power. Over a cycle, reactive power flows into a capacitor or inductor when the voltage or current is increasing, and out when these are decreasing.

To be able to include this effect in calculations, it is easier to pretend that "reactive power" has a significant value in the long term as well as over a single cycle. While the direction of "real" power is given by the movement of energy from a source to a load, this cannot be applied in the reactive case. The "direction" of positive reactive power flow is simply a convention and has no physical significance. However, just as with current or real power, there must be a balance in the circuit at all times.

I hope this helps.

Bruce.
 
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