Disadvantages of Low Power Factor

In our previous Post we have already studied about “what is power factor?” Now, in this post we will study about the “disadvantages of low power factor” and we will also talk about the “types of power factor”. So, let’s start.

The low power factor is highly undesirable as it causes an increase in current, resulting in additional losses of active power in all the elements of power system from power station generator down to the utilisation devices.

In order to ensure most favourable conditions for a supply system from engineering and economical standpoint, it is important to have power factor as close to unity as possible.

That’s why a lot of emphasis is given on improving power factor specially in industries.

The power factor plays an important role in AC circuit since power consumed depends upon power factor.

As we know that,

It is clear from above that for fixed power and voltage, the load current is inversely proportional to the power factor. Lower the power factor, higher is the load current and vice-versa.

A low power factor results in following disadvantages:

1. Greater Conductor Size

If we have poor power factor, it means that we are drawing additional current for the reactive component of the load. This additional power means we are drawing additional current which will increase the current flow through electricity network.

As we know that cables are rated to handle a certain amount of current flowing through them. So, if lot of large consumers with low power factor, are connected to the network then the cables could overload.

Also, cross-sectional area of cable is directly proportional to the amount of current it carries. So, to carry a large amount of current, suppliers have to increase the size of conductor which will increase the cost.

The suppliers could also struggle to meet the demand and capacity agreements. Moreover, no new customer will be able to connect to the utility until they either replace the cable with larger conductor size or install additional cables.

To understand this, let’s take an example.

Example: Suppose there is a single-phase AC motor having an input of 10kW on full load and the terminal voltage being 250V. Find out the input full load current at

(i) Unity power factor

(ii) 0.8 power factor

Solution: There is a single-phase AC motor

Input Power; P = 10kW

Terminal voltage = 250V

(i) At unity power factor i.e. pf = 1

Active power is given by


The input full load current will be;

IL = 40 Amps

(ii) At 0.8 power factor; pf = 0.8

From general definition of power factor, we know that

Now, the input full load current at 0.8 pf will be

IL = 50 Amps.

Conclusion: If we compare case(i) and case(ii) then we will find out that at low power factor of 0.8, the current through the supply cables and motor conductors increased upto 50A. Therefore, the size of the conductor must also be increased.

2. Large Copper (I2R) Losses

The large current at low power factor causes more I2R losses in all the elements of supply system. This results in poor efficiency.

3. Large kVA Ratings of Equipment

The electrical machineries [eg. Alternator, Transformer, Switchgear, etc.] are always rated in kVA.

Note: The electrical machineries are rated in kVA because the power factor of the load is not known when the machinery is manufactured in the factory.

Now, as we know that

Hence, it is clear that kVA rating of the equipment is inversely proportional to power factor. The smaller the power factor, the larger is the kVA rating. Therefore, at low power factor, the kVA rating of the equipment has to be made large which makes the equipment bulky and expensive.

4. Poor Voltage Regulation

If large consumers have poor power factor, it means that they are increasing the current flow through the electricity network which introduce greater voltage drops across alternators, transformers, transmission lines and distributors which means that we will receive less voltage at our homes and offices.

This reduces the supplier’s distribution capacity and degrades the performance of utilization devices.

In order to keep the receiving end voltage within permissible limit, voltage regulators are required which increases the cost of operation.

5. Reduced Handling Capacity of System

The low lagging power factor reduces the handling capacity of all the elements of the system because the reactive component of current (ISinɸ) prevents the full utilization of installed capacity.

6. Power Factor Penalty

If large customers have poor power factor then they will increase the current flow through the electricity network which will result in voltage drop across all the elements of power system and increased copper losses. This reduces the distribution capacity of the suppliers.

Therefor, to compensate for their losses, customers having low power factor are charged by the suppliers which is known as Power Factor Penalty.

Due to power factor penalty, electricity bills of such customers increase. Power factor penalty is more prominent for industrial customers. That’s why power factor is given so much importance in industries.

Suppliers also gives incentives on total bill amount if a customer is able to maintain power factor at/above a particular limit.

Example: In Maharashtra, India

Utilities gives 10% penalty on total bill amount if  

Suppose, if total bill amount = Rs.10,000

Then, bill with penalty = Rs.11,000

Utilities gives 7% incentives on total bill amount for

Suppose, if total bill amount = Rs.10,000

Then, incentive given = Rs.700

Decreased bill amount = Rs,9300

Why we should improve power factor?

Due to following reasons, we should improve power factor:

  • For better utilization of electrical machineries.
  • For better utilization of conductors.
  • To reduce size of the conductors.
  • To reduce copper (I2R) losses.
  • For better voltage regulation.
  • To improve handling capacity if the system.
  • To avoid penalties and to get incentives from utilities.

Types of Power Factor

Basically, there are two types of power factor which depends on the nature (linear and non-linear) of the load:

  • Displacement Power Factor
  • Distortion Power Factor

Displacement Power Factor

In case of linear loads, the degradation of power factor is just because of the reactive component (inductor or capacitor) of the load.

The reactive element introduces phase difference between voltage and current or we can say that it introduces a phase displacement between voltage and current due to which power factor degrades. This type of power factor is known as Displacement Power Factor.

As we know that, in case of purely resistive load, voltage and current remains in same phase which results in unity power factor.

Power Factor = Cosɸ = Cos0o = 1

But in presence of reactive element, a phase displacement is introduced between voltage and current due to which power factor goes down (becomes less than 1).

Power factor due to displacement between voltage and current is known as Displacement Power Factor.

Phase difference between voltage and current = ɸ

Power Factor = Cosɸ; 0 < Cosɸ < 1

Note: If the load is inductive in nature then displacement power factor can be improved by using capacitor banks.

Distortion Power Factor

In actual system, apart from linear loads, we also have non-linear loads. For example, inside variable frequency drives and servo drives, rectifier bridge is present which is non-linear load.

These non-linear elements introduce harmonics in the current waveform which means that apart from the fundamental frequency of current, harmonics are also present in the current waveform.

Because of this harmonic distortion of current, power factor gets degraded. So, in case of non-linear loads, power factor is known as Distortion Power Factor.

In case of non-linear loads, we can not improve power factor just by applying capacitors because harmonics is also present in the current. Therefore, we have to remove harmonics in the current which can be achieved by applying Active/Reactive Harmonics Filter.

Now, in actual system where both linear and non-linear loads are present, the total power factor of the system is given by relation:

Watch “what are the disadvantages of Low power factor ?”

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