How to select the right AC Drive
Part 1 of 4
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hello and welcome to three steps to selecting the right ac drive this is part one introduction automationdirect offers four different models of ac drives the gs1 mini drive which is available in three configurations the gs2 micro drive also available in three configurations the durapulse sensorless vector drive offered in two configurations and the hitachi sj300 flux vector drive there are several benefits of using an ac drive to power an induction motor versus using across the line or soft start starting methods these include adjustable speed and direction control through keypad entry analog input signals or communications which helps eliminate mechanical parts controlled starting current acceleration and stopping which reduces starting currents and prolongs the life of the motor and mechanical systems realized energy savings and reduced power line disturbances versus using across the line starters elimination of some mechanical transmission components since ac drives provide speed and torque control allowing for simple mechanical transmission designs to be used and a variety of control methods including four to 20 milliamp loops plus or minus 10 volt signals encoder following and commands via serial communication deciding to use an ac drive and properly applying one is greatly dependent on the type of system to be controlled let's review some of the most important characteristics of a system that should be considered to successfully apply an ac drive knowing the type of load is important some load types require higher starting torques while some loads require faster speeds and some fall in the middle most all applications fall within the following four load types variable torque loads which require less torque at speeds below rated base speed typically as the speed decreases the torque decreases with the square of the speed and the horsepower decreases by the cube of the speed some example applications are centrifugal and vacuum pumps fans and blowers and agitators constant torque loads which require constant torque at speeds up to rated base speed no matter how fast the load is being moved the amount of torque does not change some example applications are conveyors machine tools cranes and elevators constant horsepower loads which run above the rated speed of the motor where the amount of torque available decreases with respect to the speed thereby keeping horsepower fairly constant some example applications are winders and machine tools impact loads which are intermittent in the amount of torque is not a function of the speed the drive system typically drives a flywheel which in turn delivers the energy to the load some example applications are punch presses crushers and shears correct sizing of the drive is critical to successfully control the system many resources are available in books and on the website to assist in determining the size of the system generally this involves determining the system inertia as seen by the motor this information is needed coupled with the torque and horsepower requirements speed required for the load and the torque of the motor duty cycle and application type also play an important role in system sizing when replacing an existing ac drive system you can generally use the same size drive how the system operates is important as well the speed range of the application how quickly the system needs to respond to the command and the duty cycle all need to be considered type of motor use should be correct for the application if a drive is being added to an existing application check that as a minimum the motor is inverter duty rated to avoid doing internal damage that will shorten the life of the motor applications that require constant torque at near zero speed should use a vector duty motor in a new application consider all available motor options enclosure types include drip proof totally enclosed severe duty and explosion proof many motor manufacturers have a variety of specialized enclosures to fit the application cooling methods typical motor cooling methods are fans blowers water or convection cooled fan cooling employs a fan that is attached to the motor shaft it is more effective when the speed of the motor is at or near the base speed blower cooled motors allow for constant cooling which is independent of the motor speed water cooled incorporates recirculating water to remove the heat from the motor convection cooled means that a fan blower or water method is not used the heat is dissipated through a specially designed case that incorporates fins which aids in cooling brake and encoder options brakes can be added to applications that require that the load doesn't move when the power to the motor is removed elevator type applications generally use brakes if tight speed regulation is required an encoder is used to provide feedback information bearing options different bearing types are available for most all motor manufacturers for general use applications ball bearings are used whereas if a pulley system is installed directly on the motor shaft then roller bearing should be used in general understanding as much as possible about the application will help you avoid some common problems when applying an ac drive undersizing the drive not specifying a needed accessory or using an incorrect motor type are a few of the common mistakes generally made when applying an ac drive now that we know some facts let's start with step one selecting the right model