Using Variable Speed AC Drives to improve Power Factor


power factor diagram


Using Variable Speed AC Drives to improve Power Factor.


The use of Variable speed AC Drives (VSD) to control motor speed has the advantage of clearly improving the power factor and thus reducing losses in the supply cables and transformers. It also avoids the cost of investment in power-factor-correction equipment.


Why Do Electric Motors Cause Low Power Factor?

The use of AC induction motors is essential for industry and utilities. AC induction motors consume more than 50 per cent of the energy used in industry. As compared to other type of loads, motor loads have relatively poor power factor. Poor power factor causes higher line currents, which causes additional heat in line cables and transformers. ‘The power factor is especially low in cases when the motors are oversized and are running with a light load.


To produce the required rotating torque and speed, the induction motor takes both active current and reactive current from the power supply. The rotating torque of the motor is created as an interaction between the active current component and the magnetic field. The field is produced by the reactive current component. Light load takes less active current but the magnetic field, as well as the reactive current, stays constant. This means that the power factor decreases with decreasing load


When to improve Power Factor

Power plant generators are designed for PF 0.8 to 0.9. Therefore, if the actual demand-side power factor is lower than the designed level, either the generator current increases above the rated current or the active power output has to be limited. For that reason, the power companies put limits on reactive power consumed by the customers. The limits usually are set for large industrial or public customers only.


Customers have to pay a power factor penalty if power factor falls below a certain limit. The limits can vary widely from 0.8 to 0.97. Connected electric motors are the main reason for reduced power factor. The rated power factor of a standard motor depends on its rated power and this is typically 0.85 but can be much lower if the motor is lightly loaded.


How to correct the Power Factor?

There are many different methods to improve the power factor or compensate for the reactive power:

– At the power plant, the excessive reactive power can be compensated by increasing the excitation of synchronous generators; or, by using separate rotating synchronous compensators.

– At transmission or transformer stations, the reactive power can be compensated by power-factor-correction capacitors. The capacitors can be installed to improve the power factor for a single load or an entire power system.

– At the plant level, the power factor correction can be accomplished by using power-factor-correction capacitors, or by using variable speed AC drives. When AC drives ate used, power-factor-correction capacitors should not be used, because it is usually unnecessary, and because drive harmonics could damage power factor capacitors.



– The power factor topic is interesting and important for a number of parties within the power generation and consumption marketplace:

– Industrial, commercial and domestic customers have a desire to get the most cost effective electrical installation to serve their machinery. Low power factor can mean extra losses and penalty payments to the utility for excessive reactive power.

– Power production and transmission companies want to sell as much active power as possible to their customers. Low power factor can reduce the generating and transmission capacity.

– Manufacturers of power-factor-correction equipment are willing to sell capacitor banks and automation equipment to help improve the power factor.

– Consultants have an interest to help the power companies, consumers and other interest groups with audits and plans for better energy economy and achieving higher power factors.

– Drives and motors manufacturers can also help to improve the power factor with variable speed drives. VSDs help solve the power factor problem while improving process control, saving electrical energy, and reducing machinery wear.


More in using Variable Speed AC Drives to improve Power Factor

The full ABB article explains why the input current to the Variable speed AC Drive can be lower than the output current, and also gives some guidelines when compared with fixed speed applications or other speed control methods like DC drives. Go to ABB for the complete article


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