Separation can improve delivery time and allow exact replacement.
by Miles Mahaffy
June 17, 2019

Motor independence is a premise that the variable frequency drive (VFD) selection should not be tied to a specific motor and, conversely, the motor selection should not be limited by a VFD. By decoupling the selection of a motor and VFD, users can select the appropriate motor for their application based on performance, energy usage and availability. Motor independence is important as users explore the use of motors beyond the traditional three-phase induction variety.

Driven by Motor Efficiency

The standard three-phase induction motor (also known as three-phase asynchronous) has long been the motor of choice for commercial and industrial applications. The design of these motors continues to evolve as government regulations increase the demand for higher efficiency levels. Electric motors are a prime target for improvements in efficiency to reduce the global use of electricity, and motor manufacturers are responding. Today’s induction motors feature lower current requirements and offer improved efficiency.

The desire for improved efficiency is leading to the use of other three-phase motor technologies to gain greater electrical capability. Permanent magnet (PM) and synchronous reluctance (SynRM) motors are two possible options. PM motors can be used in both constant and variable torque applications, making them suitable for a wide range of applications. They can also be used in applications requiring high torque and low speed and potentially eliminate the need for a speed reduction gearbox.

PM motors use permanent magnets imbedded in the rotor of the motor to generate the rotor magnetic field. Using magnets instead of inducing current in the rotor (such as in an induction motor) can improve the efficiency of the motor. The initial costs of these motors are typically higher, but lifetime operational costs may offset this.

SynRM motors use induced magnetic reluctance in the rotor to develop torque. Because there is no current flow in the rotor, the losses are reduced compared to an induction motor. These motors are more suitable for variable torque applications, such as fans and centrifugal pumps, due to lower starting torque. The cost of SynRM motors is lower than PM motors since they lack the permanent magnets.

The efficiency regulation requirements are dictated locally. In the United States, federal regulations 10 CFR 431.25 through 431.26 set the standard for efficiencies of motors produced with the latest revision for standard National Electrical Manufacturers Association (NEMA) motors updated in 2016. In the European Union (EU), the minimum efficiency levels are specified by Regulation (EC) No. 640/2209, with EU Regulation No. 4/2014. With each revision of the standards, the efficiency levels become stricter, potentially requiring the use of new motor technologies.

Motor Selection

An equipment manufacturer or end user will need to begin selection of the motor based on the application. Power and voltage are the two easiest variables to consider. Other more complex variables make the decision more challenging: starting torque, load type (variable or constant torque), peak torque required (breakdown torque), environment, etc.

The type of motor selected depends on the application applied. Standard induction motors are versatile and can be used on several applications. If higher efficiencies are desired, a PM or SynRM may be beneficial. If a PM motor is selected, should it be a surface PM motor or an internal PM motor? Both types have different characteristics and challenges when controlling them with a VFD. SynRM motors have their own distinct control requirements.

In addition to the above factors, motor selection must consider availability.

Users are faced with several issues:

  • local suppliers who can provide quick delivery in the event of failure
  • the availability of an exact replacement or an equivalent motor
  • the cost of keeping a spare motor if an exact replacement is required

VFD for Control

The selection of the VFD should be based on current, voltage, application and environment, and not its ability to control the motor. This is motor independence.

Control electronics have improved so that multiple-control algorithms can exist in one VFD to control most three-phase AC motor technologies. Typically, a parameter selection is all that is needed to indicate motor type. An automated identification process that can detect variations of motors could be used.

The advantage to equipment manufacturers and end users is having to support only one VFD for multiple applications and motors (plus perhaps a second VFD for small motors for cost purposes).

An installed motor could be changed to a different motor type if, in the future, it becomes a requirement or if the motor fails and needs to be replaced. However, if a motor is tied to a VFD type, that flexibility goes away.

The idea of matching a specific VFD to a specific motor has the advantage of implied responsibility of pairing. Conversely, selecting a VFD that controls all motor types provides flexibility.

Another advantage of motor independence is evident when there is no motor VFD match for the application. For example, the VFD will be in an environment that requires a higher enclosure class, such as Type 4, but the VFD paired with a specific motor is only available in Type 1 or Type 12 enclosures.

Another situation may arise if the motor must be replaced but the exact substitute is not available, requiring the replacement of both the motor and VFD, which can add costs.