How to select variable frequency drives to improve motor control and efficiency

To select variable frequency drives, it is necessary to consider some tips that will guarantee the investment over time. Advances in high-speed electric motor technology, along with improvements in the cost and performance of VFD (variable frequency drive) systems, make the direct coupling of an electric motor without gears to a pump worth considering.

Generally, a variable frequency drive takes an AC power supply and converts it to DC power. The speed control part of the drive then uses the DC voltage to create DC pulses at a variable frequency to drive the output motor to speeds other than 3,600 rpm or 1,800 rpm, or other speeds, depending on the number of poles the motor was designed to run at. A 60 or 50 Hz AC supply voltage.

How big should the variable frequency drive be? The size of the VFD should be chosen based on the motor's maximum current at maximum demand and not based on motor horsepower. Constant starting, stopping, and dynamic loads affect the electronics within the VFD much more than they affect the local power bus and a full voltage motor starter. Therefore, maximum demand current should be used. 

It can be tempting to size a variable frequency drive (VFD) based solely on power. However, there are other factors to consider before selecting the best VFD for your factory. That's why, in this article, we'll explain how to select VFDs to improve motor control and efficiency, as well as their service life and other parameters.

How to Select Variable Frequency Drives to Improve Motor Control and Efficiency in Seven Steps

1. Understanding the benefits and features The greatest benefit is the reduction in energy consumption when using devices such as blowers, fans and centrifugal pumps at lower speeds. Other advantages of using VFDs include:

• Adjustable engine speed and torque.
• Reduced engine acceleration on startup.
• Controlled arrest and reversal.

Within the features, the 'c' is described.Variable frequency drive capacity To vary the motor speed allows for the optimisation of the work required by a machine or process because only the necessary speed is provided. This contrasts with operating the motor at full speed and accelerating the output, which is inefficient, reduces motor lifespan, and increases maintenance of the motor and regulation equipment.

When selecting variable speed drives, it should be taken into account that controlled acceleration reduces starting current and extends motor lifespan. This is especially true in applications that require frequent starting and stopping, such as Variable frequency drives for the food industry that help improve plant safety.

2.  Selecting the VFD based on load size: The sizing of VFDs often requires more than simply matching the rating to the motor. The operating profile of the load it controls must be considered. Constant or variable loads, frequent starts and stops, or continuous operation should also be taken into account when selecting equipment.

It must be sized according to maximum torque demand rather than just horsepower. Under certain conditions, the motor may demand more power and/or torque, and oversizing may be necessary when dynamic loads or impact load create temporary overload conditions. 

3. Determine the braking options. A VFD may also need a bit of help to slow down a load. While it can stop moderate inertial loads, high inertial loads can cause an overvoltage condition in the drive. For rapid deceleration of heavy loads, an external dynamic braking resistor should be considered.

The braking resistor allows the variable frequency drive to produce additional braking torque by reducing the voltage generated by a decelerating motor. Without a braking resistor, typical VFDs provide approximately 20 percent braking torque. 

4. VFD Interface VFDs are controlled via wired, discrete, and analogue inputs/outputs (I/O); or by digital communications. Discrete inputs to the VFD, usually outputs from an PLC, are used to start and stop the drive, though manual push buttons and selector switches can also be used.

Discrete outputs are a factor of great importance that should be considered when selecting variable frequency drives because they include fault present, frequency reached, non-zero speed and local/remote indication. Some higher-end units also include frequency outputs for speed reference. The analogue output of the drive, when available, also has the same signal levels. 

5. Understanding digital communication options Digital communication protocols allow commands and information to be communicated between a PLC and the VFD via a single cable, as opposed to many cables. Protocols range from simple serial interfaces to more advanced Ethernet and fieldbus communication options, such as EtherNet/IP.

These communication interfaces allow the VFD to be controlled by a master device, such as a PLC or other advanced controller. And can eliminate the need for wired analogue and discrete I/O, allowing for the monitoring of speed, current, fault and other parameters of the drive. 

6. Specify operating parameters. From a control perspective, the VFD should not be routinely stopped by opening a contactor on the input voltage supply, as this reduces its lifespan. This should only be done for emergency stop purposes. 

Disk I/O or communications must control start-stop in all other instances. These and many other installation and operating procedures are described in the manual and must be followed carefully, and the VFD supplier should be contacted with any questions.

7. Handling noise and harmonics VFDs generate electrical noise and harmonics that can cause damage to motors, equipment, transformers, and power wiring. Fortunately, filters and line or load reactors can minimise many problems. 

Most installation instructions for selecting variable frequency drives recommend the use of passive harmonic filters, such as AC line reactors and chokes. These devices reduce harmonics and protect VFDs from transient overvoltages on the line side of the drive.

Using manufacturers' websites and educational information can make anyone an expert when selecting frequency inverters, provided they recognise the factors to consider when incorporating variable frequency drives for industrial processes. For more information, we invite you to subscribe to Technology for Industry Newsletter.