Beyond the purchase: How to get the most from your variable frequency drive

VFD performance insights

• Using installation technicians that have been vetted or factory trained can substantially increase the lifespan of a VFD.
• When installing a VFD, paying proper attention to the motor that drives the load and the cable connecting the VFD and the motor are integral to ensuring optimal performance.
• Getting the best torque per amp performance out of a VFD also requires that end-users perform a proper auto-tune.

Besides the many other features on a modern variable frequency drive (VFD), the simple ability to run an AC induction motor at speeds other than 60 Hz is a very wonderful thing.

By slowing the motor down to meet the demand of the application, a VFD reduces the power required to keep the load moving.  Power usage over time equals energy, so the bottom line is that a VFD saves energy by not running at full load.

Most people are already familiar with all the information in the previous paragraph, so maybe using a VFD isn’t up for debate. Assume instead that the VFD purchase has been made already. But is that all there is to properly using a VFD to save money?  Like most things in life, it’s slightly more complex than that. How can a VFD be optimized for energy savings?

Proper VFD installation

A brief discussion of a proper VFD installation is needed, first. No one saves money through energy cost reduction if a VFD blows up or melts down after improper installation. Many VFD manufacturers are happy to offer extended warranties on products if they are installed and wired by people that they have vetted or factory trained. (As a factory trainer myself, I emphatically endorse using installers that have been sent to the factory to practice and learn all the ins and outs of proper VFD installation.  Training includes many different scenarios to provide the experience needed to avoid pitfalls.)

Proper installation can avoid problems and short VFD lifespans.  A good installation technician will notice environmental issues that can lead to premature VFD failures like high ambient temperatures, long motor lead lengths and power factor correction capacitors. If an early demise can be avoided, some VFDs can last a very long time, making the return on investment (ROI) very high indeed.

Figure 1: Proper torquing of power wiring done by a certified installer is one of the steps that help the drive’s lifespan and increase VFD return on investment. Courtesy: Yaskawa America

Motor sizing for the motor-VFD system

By itself, the VFD can be integral to energy savings but the drive is only one part of the whole installation. The two other parts that matter the most are the motor driving the load and the cable connecting the VFD and the motor.

The motor is probably the more important of the two items. First, make sure that the motor is up to the job of handling the pulse-width-modulation (PWM) voltage that the VFD will be sending it. Some older motors, besides being inefficient, were manufactured with inferior insulation systems that may not be able to handle the voltage created by a VFD and exacerbated by a long motor lead. Generally, any application that has motor leads over 100m should consider an output reactor between the motor and VFD to help protect that first turn in the stator (where the insulation is thinnest) from failing. Like many things, there is a tradeoff to be considered. Any kind of output filtering, like that load reactor, will come with its own losses. However, the slight decrease in efficiency is worth the extended life of the motor.

Another motor consideration is the size. Applications need a motor capable of continuously delivering the torque that the load requires. A properly sized motor is preferable to an oversized motor. An under-loaded motor can suffer from a poor power factor which greatly decreases the efficiency of the whole system.  Address this tradeoff. An oversized motor might provide peace of mind that the system will not fail any time soon since heat losses in all of the components are well below their thresholds, but the properly sized motor will be more efficient. If correctly selected and installed, the properly sized motor also will live a long life, so the tradeoff is heavily weighted towards the properly sized motor.

For new installations, the motor selected will most likely be very efficient due to government regulations regarding motor efficiencies. Generally, the closer the rated speed of the motor is to the synchronous speed, the more efficient the motor is. To optimally run a motor, a VFD needs an accurate value for the motor’s rated slip. A good auto-tune done by the VFD while attached to just the motor can yield a very good value for motor slip. See more on auto-tuning later and how it applies to VFD control methods.

Figure 2: An output reactor is sometimes necessary for a VFD installation. Courtesy: Yaskawa America

Motor-VFD cable considerations

Motor cable considerations generally relate to the cable length and gauge. The proper cable gauge should be found by consulting the VFD manufacturer’s installation documents. As mentioned earlier, long lead lengths can actually lead to premature motor failure unless precautions are observed. Large changes in impedance can cause a point of reflection for the PWM pulses (with their corresponding high dv/dt values) coming from the VFD. It is not an all or nothing thing. The greater the difference in impedance, the more of the pulse is reflected. Sometimes the only fix is to dampen the steep dv/dt of the pulses through filtering. Again, the filtering components come with their own set of energy losses and additional installation costs.

Figure 3: A dv/dt filter will sometimes be necessary in VFD applications with very long motor lead lengths. Courtesy: Yaskawa America.

VFD auto-tuning

To get the best torque per amp performance out of a new VFD, a proper auto-tune is needed. An effective auto-tuning routine for a VFD will help the drive build a good motor model in the VFD. From the factory, the drive is most likely set up for a typical motor that matches the VFD size. That means that the rated motor amps value in the VFD will be based on NPPA 70: National Electrical Code (NEC) values for a motor of that voltage and horsepower.  On top of that, the motor model will require values for expected losses in windings and magnetic fields. Values like losses are rarely specified on the motor nameplate. These loss values are named things like “Line to Line Resistance (Ω)” and “Leakage Inductance (%).” It is probably best left to the VFD to run its own tests on the motor after some of the typical nameplate values are input to the drive. The end result will most definitely show a reduction in output current while running the same speed and load as the pre-tuned motor. It is highly recommended that the auto-tuning function be run on a motor without any load attached. An attached load will skew the current and slip measurements. It is also preferable that the auto-tuned motor’s internal temperature is close to its normal running temperature due to resistance changing with temperature.

Figure 4: An auto-tune operation for a new drive installation is simple and will lead to more efficient control of the motor. Courtesy: Yaskawa America

How to deal with light loads in a motor-VFD system

Another way to help a VFD get the best efficiency out of the application is to train it on how to deal with situations where the load fluctuates. During operation at low loads, decreasing the output voltage to an optimal point can help minimize power consumption and maximize efficiency. An energy savings mode in a VFD can help maximize the efficiency, especially for V/Hz applications seen in fans and pumps. It can be helpful to think of energy savings as the inverse of torque boost. One increases the voltage from the normal levels to increase the output torque, and the other decreases the voltage to minimize power usage, resulting in a more efficient use of energy.

Sometimes it may be necessary for the VFD stop running if the load becomes too low. This is referred to as sleep. It is called sleep because despite the drive output turning off, the VFD remains in a run state and monitors to see if the conditions of the applications warrant waking up to start driving the load again. This type of operation is common on pumping applications where the demand on the pump can change based on usage or time of day. The VFD will need to be programmed with what states (such as running at a minimum speed or outputting too low of a pressure) will need to occur to cause the drive to go to sleep or wake back up.

Figure 5: Having the VFD stop during times of low demand can help with energy savings. Courtesy: Yaskawa America

A final energy savings possibility relates to what is colloquially called “vampire power.” It refers to the power that devices draw even when they are not turned on. Consider an LCD flat-panel TV. Even when it’s not “on,” a certain residual power is drawn. Maybe it is not enough power by itself to warrant concern, but consider how many devices in are also drawing power while they are off. Then, multiply that by all the houses and businesses in the world, and concern accumulates. The same principle applies to VFDs. Even when the VFD is not running, it is drawing some current in through its rectifier and bus capacitors.  Some manufacturers have created a function that uses a small DC power supply to keep the VFD “brain” alive while opening up an input breaker when it has not been running for a long enough time. Because the brain still has power, it can close the input breaker whenever a new run command is received. Maybe a single drive’s non-running power draw does not warrant the extra equipment, but what if you have 50 or 100 VFDs? The power waste could be significant.

A VFD by itself can be a big win for energy savings due to its ability to match motor speed to load demands. However, there are other factors such as motor sizing, cable selection, auto-tuning and energy saving functions that should also be considered to maximize those energy savings.

Paul Avery, senior product training engineer, Yaskawa America. Edited by David Miller, Content Manager, Control Engineering, CFE Media and Technology, dmiller@cfemedia.com. 

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