Variable Frequency Drives (VFD) are commonplace in the design of mechanical systems. Where “VFD” is the most common terminology used, they are also called adjustable frequency drives, variable speed drives, and AC drives. Their primary use is to adjust the speed and/or torque in an electro-mechanical system by varying input frequency and voltage.
So when should VFD be employed as part of a mechanical design? That’s a topic for a future post! However, the intent of this post is to share some of the common electrical design considerations used in VFD selection.
What’s in a Name?
The term “VFD” commonly refers to two different pieces of equipment, depending on the application. Technically speaking, a VFD consists of a Variable Frequency Controller and Operator Interface. It has connections for standard input power (60 or 50 Hz Sine Wave), variable frequency output power (square wave), and external controls (sensors, building automation controls, building management systems). For the sake of this post, we’ll call this a “Stand Alone VFD.” Another common configuration is a “Stand Alone VFD” coupled with an overcurrent protective device (OCPD), such as a circuit breaker or fuses. We’ll call this a “Combination VFD.”
Although the industry uses the same name for both configurations, there are two slightly different systems. Similarly, we use the term “Motor Starter” for the contactors and coils used to connect the motor to the power sources, and “Combination Motor Starter” to describe a “Motor Starter” coupled with a fused disconnect.
How are VFDs Sized?
The sizing of the equipment is straightforward. VFDs are chosen by voltage, phase, and horsepower (HP). Next, it is important to use the FLA (full load amps) of the motor to finalize the VFD size. Once this is selected, a few other considerations need to be factored in.
An oversized VFD can be used for a smaller HP motor. The advantage of this application is that if the motor requirements increase, the VFD may be large enough to still handle the increased load. The disadvantages for this are that the VFD will be more expensive, and the feeder to the VFD needs to be sized to match the VFD, not the motor, again increasing the initial cost.
Constant Torque or Variable Torque
Centrifugal equipment, such as fans or pumps, most often utilizes variable torque drives. This allows the best access for fine-tuning control for energy savings and proper sequence operation. Constant torque VFDs are used for applications requiring a constant level of torque at all speeds. Examples of this include conveyors, positive displacement pumps, punch presses, and extruders.
VFDs are often used to control variable motorized operations as part of building mechanical process or HVAC systems. These operations may include start/stop, change speed, constant speed, limits, ramping, forward/reverse, or energy conservation. The commands to perform these operations are communicated to the VFD through some form of operator interface – the simplest of these being local manual control such as an external switch, push button, or keypad. However, other more sophisticated methods include automatic control from an external process control signal. These methods may include remote control signals generated as digital or analog inputs, digital/relay, or analog outputs from some control device associated with the equipment tasked to provide variable control to the motor. Examples of these signals are a voltage range (0 – 10 Vdc), current range (0 – 20 mA) from a temperature sensor or pressure transducer, or a set of dry contacts on a control relay. Other control methods include multiple motors on a single drive, lead/follow, closed loop/PID, and cascade control card (fixed and variable stages).
A building automation system (BAS) may be used to enable/disable the VFD control of a motorized process through a host of information including schedules, energy optimization, and electrical demand limiting. A direct digital controller (DDC) associated with the process equipment manages the necessary information obtained from various control devices wired into it (points) and enables/disables the VFD, based upon the BAS programming. The DDC may also send a reference signal command that may correspond to a motor speed required to maintain a control set point. A serial communications protocol may be used to allow a DDC controller to communicate directly with a VFD over a local area network (LAN) or Bus (RS-485 connection). Much more sophisticated functions and information management are made possible through this form of communication. Some other communication methods include Ethernet protocols such as BACNet and LonWorks, which form more of a Global Network of multiple control systems hosted by a PC-based graphic user interface (GUI).
Either circuit breakers or fuses can be used as the overcurrent protection for a VFD. Fuses are less likely to have nuisance outages than circuit breakers, and are a better option for circuits that have a high-fault current. However, fuses can drop out one leg of a three-phase circuit and cause damage to a motor if not properly equipped with phase-loss detection. Circuit breakers can easily be reset if tripped and do not require spares to be stored, which can be costly.
Providing the proper enclosure for the surrounding environment is essential. There are standard configurations known as NEMA (National Electrical Manufacturers Association) configurations that dictate what environment for which an enclosure should be rated. NEMA 1 is for standard locations. Most indoor applications will be in a NEMA 1 enclosure. NEMA 3R is for wet locations. VFDs installed outside, or in wet locations inside, will be in a NEMA 3R enclosure.
Line and Load Reactors
Line Reactors are used to help protect the VFD from harmful disturbances (surges, spikes, transients, etc.) produced on the distribution system and reduce the harmonics produced by the VFD from bouncing back onto the distribution system. They are placed in series with the VFD on the line side of the VFD.
Load Reactors are used on the load side of the VFD to protect the motor and wiring where the distance between the VFD and motor is very long. High voltage is created by noise spikes and high frequency waves. The insulation of the cables is not rated for the high voltage spikes and over time will deteriorate. A good rule of thumb is for distances more than 100 feet, a Load Reactor should be installed.
VFDs expose the load-side cables to high voltage, intense heat due to corona discharge, and electromagnetic interference. Standard PVC-insulated single wire conductors are not designed to withstand these effects and can actually create more problems. The cross-linked polyethylene insulation on VFD cables withstands the intense heat created by the corona discharge. Shielding within the cable prevents the EMI from escaping and being picked up on other equipment in the area. Cable geometry and grounding reduce the high voltage wave effect created by the variable frequency drive. VFD cables should be used on the load-side of the VFD, from the VFD all the way to the motor.
In the end, mechanical systems do not work without control and power. VFDs offer a great way to dial in the operation of your mechanical system, but if they do not function properly with the control and power requirements, they are more of a boat anchor. These electrical considerations will allow a VFD to make your system work as the mechanical engineer intended.
Have questions about VFDs? Contact us to discuss your questions and challenges.
- Craig G. Malesic, LC, PMP, EIT – Email or 717.434.1558
- Brian E. Pitzer, PE – Email or 717.434.1550
- Rich M. Lindemon, PE, LEED AP – Email or 717.434.1563