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Advantages of Inverter Drives

What are the advantages of using an Inverter Drives?

1) Speed Control

Inverter Drives, or Variable Frequency Drive's (VFD's) are able to control the speed of motors according to an input command into the drive. This could be a fixed set speed entered through the drive's keypad, a command communicated through an industrial field bus network such as Profibus DP or a reference signal input directly into the drive's input terminals. Changing the speed ensures the process is optimised both in terms of performance and energy consumption. For example if a motor was connected to a fan and the flow of air needed to be reduced, without an Inverter Drive attached to the motor the only way to reduce the flow of air is to introduce a mechanical flow regulator. By doing so the air flow reduces but the speed of the motor doesn't reduce and the power required does not significantly change. See the diagram below;

Using a mechanical damper to control airflow

If an Inverter Drive is now connected to the motor, the speed of the motor can be reduced to regulate the air flow and the power requirement is also reduced. See the diagram below;

Reduced power requirements using an inverter drive


2) Slow start and slow stop (soft start)

The intelligence of the drive means that different acceleration (and deceleration) profiles can be programmed. Selecting a less aggressive start / stop profile can greatly reduce the stress on the mechanical components of the drive line ensuring a more reliable and long life system. It also may be very important to the success of the process to which the motor is attached. For example, assume the motor was attached to a conveyor belt with bottles standing on it, if the conveyor belt accelerated quickly all the bottles might fall over!

No motor speed control.....

Starting without an inverter can be a very bumpy start   

With motor speed control provided by an inverter drive......

 A soft start with an inverter


3) No speed reducer mechanics required

Fixed speed motors often require speed reduction mechanics to achieve the correct operating speed. These can take the form of gearboxes or belts and pulleys. This creates more cost and more maintenance requirements. The variable speed nature of the inverter drive can eliminate the need for these mechanical interfaces.

No need for mechanical reducers such as pulleys or gearboxes.

4) Reduction in total power requirements

Given that the inverter drive has the ability to operate at an optimum power consumption it follows that the overall power requirements of a factory will be reduced.

Overall power requirements are reduced

5) Motor rotational direction can be changed easily

The direction of rotation of the motor can only be changed by re-wiring the supply to the motor when the motor is wired directly to the supply. However, by using an Inverter Drive the direction of rotation is simply set by using the digital keypad.

Its simple to change direction


Diagrams reproduced by kind permission of Yaskawa Electric.

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How does an inverter work?

An inverter when used in the context of motor speed control can also be known as a variable frequency drive (VFD). It essentially generates a varying frequency three phase AC voltage to effect a change in the speed of a motor. It achieves this by converting the incoming power supply into a DC voltage and then generating a three phase AC voltage from this DC supply. The development of electronics since the manufacture of the first semiconductors has seen the speed and processing power increase enormously which has made it possible to, not only digitally synthesise the required AC frequency for any given speed of the motor but to also analyse the motor current and rotor position.

Why is it called an inverter?

The term inverter only relates to the final part of the VFD's electronic architecture, the part that converts DC voltage to AC. There is no clear technical reason for the use of the term 'inverter' as it is generally believed to refer to the inversion of the early mechanical process of converting AC voltage to DC, sometimes referred to as an 'inverting converter'.

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