Variable frequency drive configuration, high-efficiency operation, and permanent magnet motors

VFD configuration for permanent magnet alternating current (PMAC) motors and other considerations can lead to optimal system performance, providing the desired energy savings in motor applications.

11/20/2013


Figure 1 shows an example of motor efficiency in 3 hp fan application, with the NovaTorque PremiumPlus+R PMAC motor demonstrating a 5%-12% efficiency improvement over an induction motor. Courtesy: NovaTorquePermanent magnet alternating current (PMAC) motors are seeing increasing demand in variable-speed, high duty-cycle motion control applications due to their higher efficiencies and energy saving potential across different speed and torque ranges. Figure 1 shows an example of motor efficiency in a 3 hp fan application, with a high-efficiency PMAC motor demonstrating a 5%-12% efficiency improvement over an induction motor. A variable frequency drive (VFD) is needed to control both PMAC motors and induction motors in variable speed applications. VFD configuration for PMAC motors and other considerations can lead to optimal system performance, which can mean the difference between success and failure in achieving the desired energy savings in motor applications.

The term “PMAC” is used to designate permanent magnet motors that have sinusoidal back-EMF (back electromotive force) and can be efficiently driven by three-phase sine wave output VFDs. “Brushless dc” or “permanent magnet dc” motors are those with trapezoidal back-EMF driven by simple trapezoidal output drives. PMAC motors sometimes also are referred to as “brushless PM” or “ECPM” (electronically commutated permanent magnet).

Sensorless control

In the last few years, many of the major VFD manufacturers have introduced “sensorless” PMAC motor control capability to their low-cost drive models. Previously, use of permanent magnet motors was restricted to servo systems or specialized applications employing closed-loop feedback control. PM motors were effectively excluded from fan, pump, and other workhorse applications because of the cost and installation complexity of the associated closed-loop control systems. Now, with the addition of sensorless PMAC control algorithms to VFDs, the opportunity exists to reap the energy-saving benefits of permanent magnet motors in a wide range of variable-speed, high duty-cycle applications.

VFD configuration complexities

Configuration of VFDs for sensorless PMAC motor control is more complex than that for induction motors for two reasons:

1. Control algorithms for induction motors have been developed and refined over a couple of decades whereas sensorless PMAC control is still relatively new.

2. There is more variability among different vendors’ PMAC motors than there is for induction motors.

VFD configuration for induction motors has reached the maturity of a routine operation: Nameplate motor characteristics are entered into the VFD, an auto-tune procedure is typically run, and the induction motor is then ready for use. With PMAC motors, more motor data may be required, including information not provided on the motor nameplate. In addition, PMAC motor performance may vary considerably with different VFDs, and is dependent on the suitability of the specific sensorless PMAC control as well as proper entry of the appropriate parameter configuration into the VFD.

When selecting a VFD for PMAC motor operation, both the VFD and motor manufacturer should be consulted for technical advice. The PMAC motor manufacturer will likely have a list of recommended or “qualified” drives that have been verified to deliver the efficiency and robust performance that the PMAC motor has been designed to deliver. They may also have developed, optimized, and tested VFD configurations, made available as a “packaged solution” with both a PMAC motor and pre-programmed VFD.

PMAC motor characteristics

In addition to standard motor nameplate characteristics shared with induction motors (rated power, rated speed, rated frequency, full load amps, and nominal voltage), the PMAC motor characteristics of winding inductance, winding resistance, and motor back-EMF must be properly configured in the VFD. These values are critical for successful motor operation.

It should be noted that the sophistication of VFD auto-configuration for PMAC motors is steadily improving. Several manufacturers now include auto-tuning procedures that remove some requirements for manual configuration of VFD parameters. However, in situations where the PMAC motor and VFD are not provided as a packaged solution, it is necessary to know the characteristics of the PMAC motor, and understand the specific control algorithms offered by the VFD. This information is required to determine if the motor-drive combination is suitable for the performance goals of the motion control application.

Motor winding inductance

PMAC motor designs fall into two primary categories: surface mount magnet (SPM) designs and interior permanent magnet (IPM) designs. IPM motors exhibit winding inductance that varies with rotor angle. Maximum motor winding inductance occurs at the quadrature-axis of rotor position and is termed q-axis inductance (Lq); minimum winding inductance occurs at the direct-axis and is termed d-axis inductance (Ld).

Figure 2 shows the winding inductance of an example IPM motor with 20% saliency. Magnetic saliency is the variance of winding inductance with rotor angle. Courtesy: NovaTorqueSPM motors have winding inductance that is nearly invariant with rotor position (Ld ≈ Lq).

The variance of winding inductance with rotor angle is termed “magnetic saliency” and may be represented as a percentage change ((Lq – Ld) / Ld) * 100.0. SPM motors have negligible saliency; IPM motors have saliency from a few percent to 100% or more depending on the design of the interior permanent magnet motor. Figure 2 shows the winding inductance of an example IPM motor with 20% saliency.

VFD control strategies differ significantly for IPM and SPM motors, particularly in the choice of suitable motor start algorithms, and in the optimization of motor speed capability in the constant power region. Figure 3 shows the regions of motor operation. For either IPM or SPM motors, an accurate configuration of VFD inductance parameters is essential for achieving optimum torque output and motor efficiency.

Motor start

Figure 3 shows the regions of motor operation. For either IPM or SPM motors, an accurate configuration of VFD inductance parameters is essential for achieving optimum torque output and motor efficiency. Courtesy: NovaTorqueMagnetic pole position must be determined in a PMAC motor before rotation can begin. The PMAC-capable VFD may provide the ability to choose between different motor start algorithms, each having a required level of magnetic saliency. An IPM motor, with a sufficient level of saliency, may allow the VFD to employ a “high-frequency injection” method where a high-frequency voltage signal is applied to the motor for a short period. The resulting current amplitude, which depends on rotor position, can be measured and used to accurately determine rotor position without shaft rotation.

SPM motors do not have magnetic saliency and require alternate methods of initial rotor position estimation; dc magnetization or similar methods may be employed to force the rotor into a known position. The acceptability of initial pole-locating rotation needs to be assessed for the intended motion control application. For most fan and pump applications, a small initial reverse rotation is likely acceptable; for other applications, such as conveyance, an initial motion in the reverse direction might not be acceptable.

Constant power region phase advance


<< First < Previous 1 2 Next > Last >>

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
World-class maintenance: The three keys to success - Deploy people, process and technology; 2016 Lubrication Guide; Why hydraulic systems get hot
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Improving flowmeter calibration; Selecting flowmeters for natural gas; Case study: Streamlining assembly systems using PC-based control; CLPM: Improving process efficiency, throughput
Putting COPS into context; Designing medium-voltage electrical systems; Planning and designing resilient, efficient data centers; The nine steps of designing generator fuel systems
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me