For machine control and other applications, using frequency converters to control motor speed can save energy, but it's not as simple as just adding a frequency converter. What other factors need to be considered to improve the efficiency of a motor control system?
In addition to the many features of modern variable frequency drives (VFDs), the ability to operate AC induction motors at speeds above 60Hz is also a significant advantage. By reducing motor speed to meet application requirements, VFDs decrease the power needed to keep the load moving. Most importantly, VFDs do not operate at full load, thus saving energy.
Most people are already familiar with the above information, so there's no need to debate whether or not to use a frequency converter. Let's assume the frequency converter purchase is complete, but is that all there is to saving money using one? Like most things in life, it's much more complex. How can we fully utilize frequency converter devices to achieve energy savings?
01. Correctly install the frequency converter
First, it's necessary to briefly discuss the necessity of proper inverter installation. If an inverter explodes or blows due to improper installation, no one can save money by reducing energy costs. Many inverter manufacturers are happy to offer extended warranties for products installed and wired by personnel who have passed their review or factory training. As a factory trainer, I strongly recommend employing installers who have been sent to the factory for internships and learned all the details of proper inverter installation. Training should include a variety of scenarios to avoid the risks associated with incorrect operation.
Proper installation can reduce problems and extend the life of the frequency converter. A good installation technician will notice issues that may cause premature inverter failure, such as high ambient temperature, long motor lead length, and power factor correction capacitors. If premature failure can be avoided, some frequency converters can be used for a very long time, thus significantly improving the return on investment.
02. Selection Considerations for Motor-Inverter Systems
The frequency converter itself is an indispensable key component for energy saving, but it is only one part of the entire system. The other two most important parts are the motor that drives the load and the cable connecting the frequency converter and the motor.
First, ensure the motor can handle the pulse-width modulation (PWM) voltage sent by the inverter. Some older motors, besides being inefficient, have poor insulation systems that may not be able to handle the voltage generated by the inverter, exacerbated by excessively long motor leads. Generally, for any application with motor leads exceeding 100 meters, an output reactor should be considered between the motor and the inverter to ensure that the first turn of the stator (where the insulation is thinnest) does not fail. As with many things, a trade-off needs to be considered. Any kind of output filtering (such as a load reactor) may introduce losses. However, sometimes a slight decrease in efficiency is worthwhile because it can extend the motor's lifespan.
Another factor to consider is the motor size. The application requires a motor capable of continuously transmitting the torque needed to power the load. A moderately sized motor is preferable to an oversized one. An undersized motor can suffer from a poor power factor, which can significantly reduce the efficiency of the entire system. An oversized motor may offer reassurance because the heat loss of all components is well below its threshold, so the system won't fail quickly; however, a properly sized motor will be more efficient. A properly sized motor can also extend the lifespan if selected and installed correctly, so the key trade-off is choosing the most suitable motor.
Precautions for motor cables typically relate to cable length and specifications. Consult the inverter manufacturer's installation documentation to find the appropriate cable specifications. As mentioned earlier, excessively long leads can lead to premature motor failure unless precautions are taken. Large impedance variations can cause reflections of the PWM pulses from the inverter (which have correspondingly high dv/dt values). This isn't an all-or-nothing situation. The greater the impedance difference, the more pulses are reflected. Sometimes, the only solution is to suppress the steep dv/dt of the pulses through filtering. However, filtering itself introduces energy loss and additional installation costs.
03. Automatic tuning of the frequency converter
Generally, the closer a motor's rated speed is to its synchronous speed, the higher its efficiency. To optimize motor operation, the frequency converter requires an accurate value for the motor's rated slip. When connected to the motor, the frequency converter performs excellent autotuning, resulting in a very good motor slip value.
To obtain optimal torque-per-ampere performance from a new frequency converter, proper autotuning is required. An effective autotuning procedure for the frequency converter will help the drive establish a good motor model within the converter. From a factory perspective, the frequency converter is likely set up for a typical motor that matches its specifications. This means that the motor's rated current value in the frequency converter will be based on the voltage and power values specified in NPPA 70:NEC. In addition, the motor model also needs expected losses in the windings and magnetic field. Values such as losses are rarely specified on the motor nameplate. These loss values are named "line resistance (Ω)" and "leakage inductance (%)".
▲The automatic tuning operation of the new frequency converter is very simple and can control the motor more effectively.
After inputting some typical nameplate values into the inverter, it is best to test the inverter on the motor. The end result will likely show a reduction in output current when running at the same speed and load as the pre-tuned motor. It is strongly recommended to run the auto-tuning function on a motor without any load. Adding a load will cause deviations in current and slip measurements. Since resistance changes with temperature, the internal temperature of the auto-tuned motor should also ideally be close to its normal operating temperature.
04. How to handle load fluctuations in the system?
Another way to help a frequency converter achieve optimal efficiency from its application is to train it to handle load fluctuations. During low-load operation, reducing the output voltage to its optimal point helps minimize power consumption and maximize efficiency. Energy-saving modes in frequency converters can help maximize efficiency, especially in V/Hz applications such as fans and pumps. It can be helpful to think of energy saving as the reciprocal of torque increase. One approach is to increase the voltage from normal levels to increase output torque; another is to decrease the voltage to minimize power consumption, thus using electrical energy more efficiently.
▲Shutting down the frequency converter under low-demand conditions helps save energy.
Sometimes, if the load is too low, the frequency converter may need to stop operating. This is known as "hibernation." It's called hibernation because although the inverter output is off, it remains operational and monitored to determine when it can be woken up again to drive the load when conditions are met. This type of operation is common in pumping applications, where the pump's operation can vary depending on demand or time of day. The frequency converter needs to be programmed to determine under which conditions (e.g., operating at minimum speed or when output pressure is too low) it will enter hibernation or wake up.
The last possibility for energy saving relates to what's commonly known as "standby power consumption." This refers to the power consumption of devices even when they're not turned on. Think of a flat-screen TV. Even when the device isn't "on," it still consumes some residual power. Perhaps its power consumption is small and not worth worrying about, but consider how many other devices consume power when they're off. Then, multiply that by all the houses and businesses in the world, and the worry accumulates.
The same principle applies to frequency converters. Even when not running, a frequency converter consumes some current through the rectifier and bus capacitors. Some manufacturers have developed a feature that uses a small DC power supply to briefly power the frequency converter's "brain" while simultaneously opening the input circuit breaker. Because the brain still has power, it can close the input circuit breaker whenever a new operating command is received. Perhaps the power consumed by a single frequency converter during non-operation periods doesn't justify adding extra equipment, but what if you have 50 or 100 frequency converters? Then the power waste can be quite significant.
Variable frequency drives (VFDs) can match motor speed to load demand, resulting in significant energy savings. However, other factors, such as motor specifications, cable selection, auto-tuning, and energy-saving features, should also be considered to maximize energy efficiency.
