When it comes to pinpointing precision, minimizing utility costs, and managing electrical energy, variable frequency drives, are one of the most successful strategies available today, through a varied voltage and frequency of energy supplied to that motor. In other words, VFD technology is essential in situations which do not require an electric motor to run at full and constant speed. A downside is the electrical stresses that are imposed on a motor winding that is not designed for VFD use, resulting in a reduced life span. Variable frequency drive rated motors have winding insulation and cooling systems designed to be compatible with the VFD non-sinusoidal waveform, when the motor is applied properly within a defined installation environment.
Variable frequency drives give engineering and maintenance managers more control over machines such as HVAC systems, for when the motor load changes, the application’s motor speed needs to change to not waste energy, so they (manually or automatically) instruct the VFD to match the new load requirement. As advancements in the technology used for VFD motors continue to emerge, many companies are taking a closer look into the potential benefits of variable frequency drive applications, and when they should be using them in their organization.
Where VFD Motors Excel
There are two types of VFD rated motors: “suitable for Inverter use” and “Inverter Duty”.
“Suitable for Inverter use” are motors constructed with VFD (or inverter) rated magnet wire. They do not have special cooling systems nor additional insulation treatments. These motors are normally rated for a speed range of 4:1 (15Hz to 60Hz) on variable torque loads and 2:1 (30Hz to 60Hz) on constant torque loads. These are used mainly on HVAC and other variable torque applications where ODP (open drip proof) motors are appropriate as they are lower cost. TEFC (totally enclosed fan cooled) motors are also available.
“Inverter Duty” motors will have additional insulation treatments, a lower temperature rise, may have an external cooling system, a special nameplate, and are typically TEFC. They may have a variable torque speed range of 20:1 (3Hz to 60Hz) and a constant torque speed range of 10:1 (6Hz to 60Hz), though these speed ranges vary among motor manufacturers. “Inverter Duty” motors are more expensive and are generally used in constant torque industrial processes.
A good way to define the relationship between variable frequency drives and motors, is to imagine the amount of pressure you put on the accelerator in your car. Just as situations when driving prompt different speeds, in many industries, different circumstances need an adjustable flow of energy.
One of the most obvious applications, ideal for VFD motors, is the HVAC system. HVAC solutions do not require a constant supply of energy at the same level or speed to provide comfort in a residential or commercial building. Primarily, VFD applications are only for three-phase induction motors, but they have advantages to offer in any circumstance which requires the variable control of speed and energy in a system. In a HVAC system, VFD motors can manage the flow rate throughout the building, allowing for the maintenance of a particular temperature in certain rooms, or the delivery of different temperatures at particular times throughout the day.
Although a number of HVAC systems used within commercial buildings are using alternative methods for the distribution of conditioned air – such as variable volume fans – these systems aren’t as reliable or efficient as VFD systems.
The Energy Saving Applications
Obviously, the ability to adjust and control the amount of speed and power an application uses has one major benefit — saving energy. The driving equipment of electric motors, such as fans and pumps, generally operate at a constant speed. However, if you have applications that do not use full speed at all times, you can reduce energy costs by reducing the amount of work the motor has to do at certain times, using a variable frequency drive.
A VFD allows you to match the speed of the equipment driven by the motor, to the load requirement. Although the amount of energy saved will depend on the cube of the motor’s speed, electrical motors account for over 65% of the power consumed in the industry today. Simply switching to a VFD system can be enough to reduce the amount of energy consumed by any company by up to 70%.
Today, variable frequency drives are one of the most efficient and successful energy management tools ever applied to HVAC systems and other electric motor applications. Not only are modern VFD motors reliable, and easy to use, but their affordable nature ensures that managers quickly reap the rewards of their investments through reduced utility bills.
Precision and Durability
Although most industry professionals consider the primary benefits of VFD systems to be the energy savings they can provide, the truth is that these VFD and motor solutions can also be useful in various circumstances. When properly applied, variable frequency drives can be the most effective motor controllers in the industry — offering flexibility of control combined with efficiency and precision.
For example, operating motors at the most efficient speed for a specific application can lead to fewer mistakes, therefore ensuring that production levels increase, and waste levels reduce. What’s more, variable frequency drives can help equipment last longer and suffer from less downtime as a result of reduced frequency of maintenance. After all, fixed-speed loads subject motors to a high-starting torque, as well as surges in the current that can be significantly larger than the full-load current. Because the VFD can control the motor’s voltage and frequency according to optimum standards, it can reduce machinery downtime from issues such as over-voltage, under-voltage, thermal overloads and more.
Modern variable speed drives can also run motors in specialized patterns in order to further reduce electrical and mechanical stress, and advances in the technology available mean that the applications of VFD motors are constantly evolving.
Should You Use a VFD Motor?
Any situation that requires the modification of speed and energy in the operation of a motor, to produce greater efficiency and precision, while reducing maintenance and utility costs can, and should make use of a variable frequency drive with the proper VFD rated motor.