Find out what you need to know when selecting a Variable Frequency Drive (VFD). It's more than just specifying the Horse Power!
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It’s tempting to select a variable frequency drive based on horse power alone. After all, if you select a drive that has the same or more horsepower than your motor you are using, then you should be good to go – right? Well … not necessarily. There are six other things you need to consider when specifying a drive: To make 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 at least that full load current. That is a great starting point, but you also need to make sure the drive can handle any overload conditions you may expect during startup or during periodic or intermittent extra loading. Looks like this one is rated for 150% for constant torque applications and 120% overload for Variable torque applications. So make sure you know what your system overload conditions are. If you expect to exceed these overload specs, then up size the drive until you find one that can handle. You will also need to know if your application is for Variable Torque like fans and pumps or constant torque like conveyors and machine control. If you are not sure, assume constant torque. Knowing the application type is important because all the spec tables are segregated by application type. As the altitude increases, the air becomes less dense, which reduces its ability to cool. Since there isn’t as much cooling available you may need to oversize the drive to compensate for the decrease in cooling. GS4 AC drives are designed to operate at 100% capacity at altitudes up to 1000 meters. By the time you get to 3000m, the drive can only be expected to generate about 80% of it’s rated current. Note that this curve assumes these temperatures for these temperatures for the different types of standards. At 1000m GS4 drives can be operated up to 50 or 40 degrees C. But at 3000 meters the max air temperature can only be 40 or 30 degrees C. For example, Denver is at 1600 meters. So GS4 drives can only be expected to output about 95% of the normal current. Which brings us to temperature. AC Drives generate a lot of heat so if the drive is in an enclosure there is the possibility that you will exceed the max temperature ratings of the drive if the enclosure doesn’t have additional cooling or ventilation. This table in the user manual tells us Drives up to 30 or 40 hp can operate up to 40 or 50 deg C depending on if the ventilation cover is removed or not. Of course, removing that cover does change your IP Rating. We have similar max temperatures with the larger drives. They can operate up to 40 or 50 deg C depending on whether they have a conduit box installed or not. They don’t have ventilation covers and that conduit box restricts the airflow. Can you go above those limits? Sure, but you have to derate the drive like this which is 2% for every 1 degree C above the max. with an absolute maximum operating temperature of 60 degrees C. So make sure you have measured or calculated what the temperature in your cabinet or room will be when the drive is running to be sure you don’t exceed these limits or if you do exceed the normal limits, that you derate the drive accordingly. A variable frequency drive takes the three phase sinusoidal input, rectifies it and then chops it so 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 - so the motor feels like it is getting this, when it is really just getting a bunch of square pulses. The rate that the pulses are sent is what we call the carrier frequency. The higher the carrier frequency the more accurately we can reproduce the effective sin wave into the motor. If we have a low carrier frequency which gives us only a few pulses to work with per cycle, then we’ll get a really crude approximation of a sin wave. So you would think we always want a really high carrier frequency – right? Well, no. In fact, you usually want the lowest carrier frequency you can handle. Parameter 2.10 is for setting the carrier frequency. And down here at the bottom there is a great little chart. It tells us if you want to minimize motor noise and vibration, then you want a higher carrier frequency because you get a smoother sin wave. But most of the time you want to minimize EMI, you want to minimize heat and minimize power consumption, which means you want a lower carrier frequency. What carrier frequency should you use? Most of the time the default is fine. But if you find need to reduce the noise or reduce the power consumption or reduce the heat dissipation … being able to modify the carrier frequency is a great option to have. So you need to select a drive that is rated for the motor you want to use, can handle any temporary overload conditions you expect, is designed for the type of application you have, that will still support your motors needs at the altitude, and can output the current your motor needs while operating in the temperature of your environment. And you can tune the carrier frequency if you need to lower the power consumption and operating temperature of the drive. Click here to learn more about GS4 Variable frequency drives, click here to learn more about AutomationDirect’s free award winning support options, and here to subscribe to our video channel so you are notified when we post new videos.