Why do we need variable speed drives (VSD)?

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.

Substantial cost savings can be achieved in some applications.

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).

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

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.

Force

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

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).

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