VSD advantages, disadvantages, selection criteria …

1 APPLICATION energize - May 2005 - Page 44 This document seeks to help users to understand and simplify the process of selection , installation and the use of a VSD. Users should see it as a long-term relationship and investment when acquiring a motors are the workhorses of industry and commerce. Each day large numbers of industrial electric motors are sold throughout the world. These motors are reliable and inexpensive. However they are fixed speed devices, and most applications require a variable speed output from the motor. Frequently this is obtained by coupling some mechanical device (clutch, gear, belt and pulley, etc.) to the motor. These methods are inefficient in terms of mechanical wear and energy advantagesVSDs have many advantages and manufacturers of VSDs claims different advantages for their units.

2 This paper will cover a couple of basic functions which we use with most ControlA fundamental function of a VSD is to adjust the speed of an electric motor. The basic command frequency for VSDs is normally from 0 Hz to 50 Hz, but mostly with the capability to be adjusted up to 400 Hz. If the base frequency of a motor is 50 Hz then the final speed will be 8 times the base frequency of the motor with the command frequency set at 400 Hz. Due to their design, it is not practically normal for standard induction motors to operate at these high frequencies. In practice a command frequency set point of between 25 Hz and 75 Hz is acceptable without compromising performance or introducing any mechanical damage to the motor.

3 At low frequency set points, care must be taken that there is enough cooling for the motor produced by the mechanical fan. At high frequency set points mechanical failure may occur due to the mechanical design of the motor bearings normally rated at the design speeds of 2, 4, or 6 poles. At high frequency command speeds, care should be taken as torque loss may be experienced. Torque controlBasic torque control is possible in an open loop system; however, the actual system response required must be considered. In an open loop system the VSD monitors the motor current and adjusts the voltage to perform torque control, depending on the installation . If the current of the motor does not vary sufficiently, very inaccurate results will be controlWith the aid of an optional interface card most VSDs have the ability to be used as a low cost position controller.

4 Items to be taken into consideration are the dynamic response of the motor and control system. As a rule of thumb an open loop system with a standard squirrel cage induction motor is approximately 400 radians/second. In a closed loop system with a standard squirrel cage induction motor, feedback is approximately 600 radians/second. A full servo system is approximately 1000 radians/second. 1 radian/second = rpm or 2 radians (rad.) in 360 or 1 radian = .Smooth controllable starting and stoppingA Simple adjustment of the time required to accelerate the motor from rest to full speed (Starting), normally 50 Hz, and from full speed to rest (Stopping), ensures a smooth controllable start and stop sequence.

5 This reduces mechanical wear on the machine. Various types of starting and stopping curves are available by setting the correct parameters in the VSD as illustrated in figure 1a to savingWe know that a Direct On Line (DOL) starter will supply full voltage to the motor at the supply frequency with the current uncontrollable. The motor will use as much current as the load VSD advantages, disadvantages, selection criteria and installation tipsThe purpose of this document is to assist and guide users when they acquire a variable speed drive (VSD) and to help them through the many precarious pitfalls to successfully select, install and operate their R G van der Merwe, Motiontronix and C Hoogendoorn, Circuit Breaker IndustriesFig.

6 1: Acceleration and deceleration energize - May 2005 - Page 45requires - normally between 600 to 700% of the full load current of the the days of Soft Starters and VSDs our alternative to control the starting current was with Star-Delta starters, which reduced the starting current to approximately 200%. Our next best device today to limit the starting current is to fit a Soft Starter. With Soft Starters we use the Phase Angle principle to control the voltage and therefore reduce the starting current while at the same time producing a smooth controllable start. The limitation is that these units are very basic and have limited adjustable time settings, normally from 0 to 60 current limiting features on VSDs ensure that when you accelerate a motor from rest, you will not exceed more than 100% of the full load current of the motor.

7 Replacing DOL starters with VSDs will reduce the Current Demand when starting motors. VSDs will deliver maximum torque at the motor shaft while limiting the current to the full load current setting of the motor in the is the responsibility of every individual to use forms of energy effectively and is already well known in the Heating and Ventilation industry that volumes, flows and pressures of centrifugal fans, pumps and compressors can be controlled by mechanical means to match the demands of the system. Many of them do not consider the immense amount of energy ( watts, money) that can be saved by using modern reliable electronic the efficiency of the system can be improved the power demand drops proportionally with the increased efficiency.

8 Almost all fans, pumps and compressors are over designed (just in case) and seldom do they work at their maximum designed efficiency an inverter to a fan, pump or compressor motor varies the motor speed which then varies the characteristics of the fan, pump or compressor to operate at a different efficiency, producing energy Affinity Laws say that flow is proportional to speed and kilowatts vary as the cube of the flexibility to set up and configure a VSD for various applications, Constant torque, Variable torque, Hoisting and many others, allow users to customise units to suit their needs. See illustrations in Figs. 2 and friendlyMost VSD are supplied with basic LCD or LED display keypads, with which the user can adjust parameters such as acceleration time, deceleration time, full load current, etc.

9 This allows the user to customise the inverter for his application. Most VSDs have advanced units that could copy parameters from one unit to another. Apart from this basic function, most units available today are supplied with serial communication ports to interface with personal computers that allow users to analyse the behavior of their to interface with other intelligent control systemsOur demanding society forces managements to know what is happening in their plant and processes. Information from VSDs is not only for the engineer s benefit, but also allows managements to see if they could increase their production safely without overloading the process or plant. This is normally done via the serial interface.

10 It is also possible to integrate units into a complex network e c h a n i c a l w e a r a n d tearIt is to the advantage of the users that where mechanical wear is part of the process, users could speed up or slow down their application to deliver the necessary production. In a case study it was found that due to excessive wear, the life of a pump was approximately six months. A VSD was fitted to regulate the flow as the flow reduced due to impeller wear. This enabled the user to increase the speed of the pump to produce the required flow, increasing the life of the pump from six months to twelve months. The other benefit was that when the pump was still new the speed could be reduced according to the necessary flow required, saving energy and decreasing the wear on the disadvantagesVSDs Due do not have many disadvantages with respect to industrial standards and regulations.