Processing, June 2020

Figure 1 Modern VFDs like this model from AutomationDirect have many settings and operating modes each of which must be properly selected for best process or equipment operation JUNE 2020 www processingmagazine com 13 Modern VFDs have many operating modes and hundreds of con guration parameters However for each application the user must determine the appropriate operating mode this is usually not recommended for a few reasons The first is that a typical fan cooled motor is designed to cool itself properly at full speed but will cool less at lower speeds Running such a motor at low speed will not generate enough air ow for the necessary cooling leading to overheating and subsequent reduced lifespan or motor failure Inverter duty motors counter this problem with heavy duty insulation and thermal properties that allow convection cooling to protect against overheating issues A standard motor may have a NEMA insulation class rating of F while an inverter duty motor might be rated at H which increases the allowable operating temperature by 45 F is adds a substantial margin to the operating range Electrically the rotor in an inverter duty motor is specially designed to maximize efficiency for any control mode and also improve a VFDs capability to monitor induced current As we will see this latter point is important for SV control Classic VFD control V Hz For typical motors full speed motor operation is achieved at rated line voltage and frequency For industrial motors in North America this is quite commonly 480Vac three phase at 60Hz Depending on how many poles the motor has the 100 rated motor speed rotations per minute RPMs at full voltage and frequency could be 1800 which is very typical or some other value e following discussion assumes a motor rated with the preceding values A classic V Hz control scheme would output both the voltage and frequency in a linear fashion generating 100 motor speed at 480V and 60Hz Operating the motor at 50 speed would call for 240V and 30Hz One limitation of this approach is the open loop nature of this control mode is occurs because while the VFD is outputting appropriate power values it really has no feedback about how the motor is actually performing or rotating so there is no con rmation that the desired speed is being produced A second limitation is that it may only be able to control a motor down to around 2 or 3 of the speed setpoint which may not be suf cient for some applications On the beneficial side many V Hz VFDs let the user select from other standard or user de ned custom non linear output curves For instance this can tailor the output for high inertia equipment where a greater starting torque is needed such as for centrifugal fans and pumps making them perform better and last longer Another useful feature of the V Hz mode is that one VFD can drive multiple motors at the same time which can be bene cial for equipment like warehouse conveyors where many motors need to operate at the same speed While using V Hz mode doesnt provide the most accurate speed control many common pump fan and conveyor applications dont require much precision and dont experience signi cantly varying loads Figure 2 If a running load does vary the motor speed will slow down below the commanded frequency a condition called slip A VFD operating with V Hz cant detect a motor slowdown Many VFDs include a tuning mode where the motor characteristics can be evaluated automatically by the VFD allowing compensation to be applied For V Hz operation tuning is not required but it is recommended for best results In many cases a new VFD can simply be con gured with the horsepower and voltage rating of the target motor and immediately put into service Virtually closing the loop with sensorless vector VFDs operating in SV mode are not really sensorless they just dont have an external sensor to obtain feedback from a motor If an external speed sensor is used the most common type is an encoder installed with additional wiring from the motor to the VFD External sensors may be required for the most accurate control of some types of equipment However a VFD operating in SV mode monitors the voltage and current of the motor via the already connected power leads and then mathematically determines the motor speed with good accuracy is is a simpler and less costly solution than installing and connecting an encoder While not as positive as a dedicated sensor SV provides su cient feedback in most applications to enable pseudo closed loop operation Because the VFD now has information regarding the commanded and actual speed it can vary the output voltage and current to generate the necessary torque so the motor achieves the desired speed is form of closed loop operation is constantly in e ect allowing the motor control system to quickly adapt to any varying load conditions As compared to the V Hz mode the SV mode delivers higher starting torque tighter speed control under varying loads the ability for low speed operation down to 1 for maximum rated speed and the possibility of generating up to 200 of rated torque for a brief time For a VFD to operate a motor in SV mode the drive should be tuned for the motor This is a quick and simple function usually performed when the equipment is rst commissioned A limitation as compared to the V Hz mode is that only one motor can be connected to a VFD running in SV mode due to the way the motor windings are monitored The SV mode can be used for an application of any type and it is generally superior to V Hz mode Applications with varying loads and speed sensitivity include printing lines textile manufacturing and CNC machinery operation The better or best VFD mode depends on application Modern VFDs have many operating modes and hundreds of con guration parameters e most capable units even include on board logic controllers and PID loops However for each application the user must determine the appropriate operating mode SV is a more recent development and will provide the best performance for most applications where there is one motor associated with each drive For good control where the best accuracy isnt needed V Hz is a quick and easy choice Kevin Kakascik is a technical marketing engineer at AutomationDirect Over his 20 year career he has held controls engineering positions for machine OEMs entertainment industry systems integrators and material handling systems integrators where he estimated designed commissioned and started up systems Kevin has worked at AutomationDirect since 2013 in technical and marketing roles He holds a bachelors degree in Computer Science and Engineering Technology and an associates degree in Electrical Engineering Technology AutomationDirect www automationdirect com Figure 2 Opposite AutomationDirect VFDs include V Hz control mode standard which is good for many applications such as centrifugal pumps fans and material conveyors Images courtesy of AutomationDirect