How to select/choose a VFD (Variable Frequency Drive)

 

 

 variable frequency drive

              It’s tempting to pick a variable frequency drive supported horsepower alone. After all, if you decide on a drive that has the same or more horsepower than your motor you're using, then you ought to be good to go – right? Well … not necessarily. There are six other belongings you got to consider when specifying a drive: to form sure the drive can handle the motors current demands, just read the motors full load current requirement right off the name plate and find a drive that is rated for a minimum of that full load current.

  vfd rating calculation        

     That's an excellent start line , but you also need to form sure the drive can handle any overload conditions you'll expect during startup or during periodic or intermittent extra loading. seems like this one is rated for 150% for constant torque applications and 120% overload for variable torque applications.

          So, confirm you recognize, what your system overload conditions are. If you expect to exceed these overload specs, then up size the drive until you discover one which will handle. you'll also got to know if your application is for variable Torque like fans and pumps or constant torque like conveyors and machine control. If you're unsure , assume constant torque. Knowing the appliance type is important because all the spec tables are segregated by application type. because the altitude increases, the air becomes less dense, which reduces its ability to chill . Since there isn’t the maximum amount cooling available you may have to oversize the drive to catch up on the decrease in cooling.

  vfd overload            

    GS4 AC drives are designed to work at 100Êpacity at altitudes up to 1000 meters. By the time you get to 3000m, the drive can only be expected to get about 80% of its rated current. Note that this curve assumes these temperatures for these temperatures for the various sorts of standards. At 1000m GS4 drives are often operated up to50- or 40-degrees C. But at 3000 meters the max air temperature can only be 40- or 30-degreesC.

          For instance , Denver is at 1600 meters. In GS4 drives can only be expected to output about 95% of their rated current. Which brings us to temperature. AC Drives generate tons of warmth so if the drive is in an enclosure there's the likelihood that you simply will exceed the max temperature ratings of the drive if the enclosure doesn’t have external cooling or ventilation. This table within the user manual tells us Drives up to 30 or 40 hp can operate up to 40 or 50 deg C counting on if the ventilation cover is removed or not. Of course, removing that cover does change  IP Rating. we've similar max temperatures with the larger drives. they will operate up to 40 or 50 deg C depending on whether or not they have a conduit box installed or not. They don’t have ventilation covers and that conduit box restricts the airflow. are you able to go above those limits?

         Sure, but you've got to derate the drive like this which is 2% for each 1 degree C above the max. with an absolute maximum operating temperature of 60 degrees C. So confirm you've got measured or calculated what the temperature in your cabinet or room are going to be when the drive is running to make certain you are doing not exceed these limits or if you do exceed the traditional limits, that you simply derate the drive accordingly.

vfd selection criteria for motors        

       A variable frequency drive takes the three-phases in usoidal input, rectifies it then chops it therefore the voltage sent to the motor looks like this on each phase. The VFD very accurately controls the width of these pulses – we call that pulse width modulation - therefore the motor seems like it is getting this, when it's really just getting a bunch of square pulses. the speed that the pulses are sent is what we call the carrier frequency. the upper the carrier frequency the more accurately we will reproduce the effective wave into the motor. If we've a coffee carrier frequency which gives us only a couple of pulses to figure with per cycle, then we’ll get a very crude approximation of a wave. So, you'd think we always need a really high carrier frequency – right?

           Well, no. In fact, you always want rock bottom carrier frequency you'll handle. Parameter 2.10 is for setting the carrier frequency. And down here at rock bottom there's a great little chart. It tells us if you would like to attenuate motor noise and vibration, then you would like a better carrier frequency because you get a smoother sine wave. But most of the time you would like to minimize EMI, you would like to attenuate heat and minimize power consumption, which suggests you would like a lower carrier frequency.

   variable frequency drive for 3 phase motor      

            What carrier frequency do you have to use? Most of the time the default is ok . But if you discover got to reduce the noise or reduce the facility consumption or reduce the warmth dissipation … having the ability to modify the carrier frequency may be a great choice to have. So, you would like to pick a drive that's rated for the motor you would like to use, can handle any temporary overload conditions you expect, is designed for the sort of application you've got , which will still support your motors needs at the altitude, and may output the present your motor needs,while operating in the temperature of your environment. And you'll tune the carrier frequency. If you got to lower the facility consumption and operating temperature of the drive.