Why do we need variable speed drives (VSD)?

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Reasons for using  variable speed drives

There are many and diverse reasons for using variable speed drives. Some applications, such as paper making machines, cannot run without them while others, such as centrifugal pumps, can benefit from energy savings.

In general, variable speed drives are used to:

  1. Latch the speed of a drive to the process requirements
  2. Latch the torque of a drive to the process requirements
  3. Save energy and improve efficiency

The needs for speed and torque control are usually fairly obvious.

Modern electrical VSDs can be used to accurately maintain the speed of a driven machine to within ±0.1%, independent of load, compared to the speed regulation possible with a conventional fixed speed squirrel cage induction motor, where the speed can vary by as much as 3% from no load to full load.

The benefits of energy savings are not always fully appreciated by many users. These savings are particularly apparent with centrifugal pumps and fans, where load torque increases as the square of the speed and power consumption as the cube of the speed.

Substantial cost savings can be achieved in some applications.

Variable speed drive (VSD) - Application examples
Variable speed drive (VSD) - Application examples

An everyday example, which illustrates the benefits of variable speed control, is the motorcar. lt has become such an integral part of our lives that we seldom think about the technology that it represents or that it is simply a variable speed platform. lt is used here to illustrate how variable speed drives are used to improve the speed, torque and energy performance of a machine. It is intuitively obvious that the speed of a motorcar must continuously be controlled by the driver (the operator) to match the trafiic conditions on the road (the process).

In a city, it is necessary to obey speed limits, avoid collisions and to start, accelerate, decelerate and stop when required.

On the open road, the main objective is to get to a destination safely in the shortest time without exceeding the speed limit.

The two main controls that are used to control the speed are the accelerator, which controls the driving torque, and the brake, which adjusts the load torque.

A motorcar could not be safely operated in city traffic or on the open road without these two controls. The driver must continuously adjust the fuel input to the engine (the drive) to maintain a constant speed in spite of the changes in the load, such as an uphill, downhill or strong wind conditions. On other occasions he may have to use the brake to adjust the load and slow the vehicle down to standstill.

Another important issue for most drivers is the cost of fuel or the cost of energy consumption. The speed is controlled via the accelerator that controls the fuel input to the engine.

By adjusting the accelerator position, the energy consumption is kept to a minimum and is matched to the speed and load conditions. Imagine the high fuel consumption of a vehicle using a fixed accelerator setting and controlling the speed by means of the brake position.

Fundamental principles

The following is a review of some of the fundamental principles associated with variable speed drive applications.

Forward direction

Forward direction refers to motion in one particular direction, which is chosen by the user or designer as being the forward direction. The Forward direction is designated as being positive (+ve). For example, the forward direction for a motorcar is intuitively obvious from the design of the vehicle.

Conveyor belts and pumps also usually have a clearly identifiable forward direction.

Reverse direction

Reverse direction refers to motion in the opposite direction. The Reverse direction is designated as being negative (—ve).

For example, the reverse direction for a motor car is occasionally used for special situations such as parking or un-parking the vehicle.


Motion is the result of applying one or more forces to an object. Motion takes place in the direction in which the resultant force is applied. So force is a combination of both magnitude and direction. A Force can be +ve or —ve depending on the direction in which it is applied.

A Force is said to be +ve if it is applied in the forward direction and —ve if it is applied in the reverse direction. In SI units, force is measured in Newtons.

Linear velocity (v) or speed (n)

Linear velocity is the measure of the linear distance that a moving object covers in a unit of time.

lt is the result of a linear force being applied to the object. In SI units, this is usually measured in meters per second (m/sec). Kilometers per hour (km/hr) is also a common unit of measurement. For motion in the forward direction, velocity is designated Positive (+ve). For motion in the reverse direction, velocity is designated Negative (—ve).

Angular velocity (to) or rotational speed (n)

Although a force is directional and results in linear motion, many industrial applications are based on rotary motion. The rotational force associated with rotating equipment is known as torque. Angular velocity is the result of the application of torque and is the angular rotation that a moving object covers in a unit of time.

ln SI units, this is usually measured in radians per second (radlsec) or revolutions per second (revlsec).

When working with rotating machines, these units are usually too small for practical use, so it is common to measure rotational speed in revolutions per minute (revlmin).


Torque is the product of the tangential force F, at the circumference of the wheel, and the radius r to the center of the wheel. In SI units, torque is measured in Newton-meters (Nm). A torque can be +ve or —ve depending on the direction in which it is applied. A torque is said to be +ve if it is applied in the forward direction of rotation and —ve if it is applied in the reverse direction of rotation.

Using the motorcar as an example, Figure 1 illustrates the relationship between direction, force, torque, linear speed and rotational speed. The petrol engine develops rotational torque and transfers this via the transmission and axles to the driving wheels, which convert torque (T) into a tangential force (H).

Torque (Nm)  = Tangential Force (N) × Radius (m)
Figure 1 - The relationship between torque, force and radius
Figure 1 - The relationship between torque, force and radius

No horizontal motion would take place unless a resultant force is exerted horizontally along the surface of the road to propel the vehicle in the forward direction. The higher the magnitude of this force, the faster the car accelerates.

In this example, the motion is designated as being forward, so torque, speed, acceleration are all +ve.

Reference: Practical Variable Speed Drives and Power Electronics – Malcolm Barnes

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About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. D.arasu
    Jul 20, 2016

    Danfoss drives side have lached start ,but we r need same lached start reverse
    How to will do,which parameter will change ,have that designs of parameters and terminal connection pls will share to me…..Sir ……

    Thank you
    Arasu. …..

  2. Rick A.
    Jun 02, 2014

    good description

  3. Joseph Ainoo
    Mar 07, 2014

    Is there anyway I can be with you to work together.

  4. Joseph Ainoo
    Mar 07, 2014

    It is very good to get understanding of some electrical tips or details of terms used in electrical engineering technologt department. Please am seeking for employment as an Electrical Technician of 5 years experience both industrial and domestic wiring.

  5. Helena
    Nov 27, 2011

    Thank you very much! I used to work with Danfoss variable speed drives, and had very good expirience with them.

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