Practical Variable Speed Drives and Power Electronics
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.
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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. It 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 (ω) 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