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

11/20/2013


Most PMAC-capable VFDs employ motor current phase-advance in the constant-power operating region where motor speed exceeds the nominal speed of the motor. This operating region is typically voltage-limited; however, by employing phase advance techniques, the motor is able to operate at higher speeds without requiring additional voltage. Normally the phase of motor current and motor back-EMF are aligned resulting in maximum torque output. Advancing the current phase reduces motor torque; however, this is acceptable in the constant power region where the motor is thermally limited and higher speeds are desired.

In the case of IPM motors with sufficient magnetic saliency, VFDs equipped with modern phase-advance algorithms can operate the motor at higher speeds than typically possible with the conventional phase advance method used with SPM motors. Technical consultation with the PMAC and VFD manufacturers is necessary to determine the motor-drive phase advance capability and any hardware limitations that could impact motor or VFD reliability when operating the motor above nominal speed.

Motor back-EMF

PMAC motors generate sinusoidal backEMF voltage; the amplitude of the motor back-EMF is proportional to the rotational speed of the motor, and the slope of back-EMF amplitude versus rotational speed is termed motor Ke.

Unless the VFD is specifically pre-programmed for the PMAC motor, or provides auto-tuning capability, the motor Ke will need to be entered as a parameter in the proper units. Some VFDs require entry of back-EMF at the nominal speed of the motor, rather than a Ke entry in typical units such as mV/rpm.

PMAC motors produce back-EMF voltage when rotated in an unpowered state. It is important to address possible safety issues that could occur with unpowered motor rotation and provide warnings and/or circuit protection as appropriate.

PWM switching frequency

Motor efficiency is essentially unchanged with varying the PWM switching frequency of the VFD, however in a 3 hp system the efficiency of the VFD is reduced approximately 1.0% for an 8 kHz increase in switching frequency, as in figure 4. Courtesy: NovaTorVariable frequency drives employ pulse-width modulated (PWM) switching of dc bus voltage to generate three-phase sinusoidal current to the motor. The PWM switching frequency (sometimes referred to as carrier frequency) is normally a configurable setting of the VFD, with a typical selection range of 2-16 kHz. A low PWM switching frequency allows a high rated current output of the VFD but results in more audible noise from the motor; a high PWM switching frequency reduces the rated output capability of the VFD; however, the audible noise level of the motor is decreased. PWM switching frequency also affects VFD efficiency. VFD electrical losses increase by approximately 2-3 watts per 1 kHz PWM switching frequency in typical 3-10 hp VFDs with conventional IGBT (insulated gate bipolar transistors) power electronics.

Motor efficiency is essentially unchanged with varying the PWM switching frequency of the VFD. However, in a 3 hp system the efficiency of the VFD is reduced approximately 1.0% for an 8 kHz increase in switching frequency (Figure 4). The efficiency advantage of using a low PWM switching frequency in a given application needs to be balanced with the allowable level of audible motor noise.

Minimum current settings

Figure 5 shows the efficiency of a NovaTorque PremiumPlus+R 3 hp PMAC motor at varying torque when operated with different VFDs. Courtesy: NovaTorquePMAC motors provide high operating efficiency over a broad range of speed and torque. However, to achieve the best possible part-load efficiencies it is important to ensure that the VFD “minimum current” parameter, if provided, is not set to an unnecessarily high level. Figure 5 shows the efficiency of a high-efficiency 3 hp PMAC motor at varying torque when operated with different VFDs. All VFDs were properly configured, with the exception of VFD 4 that had minimum current set significantly higher than needed for the application. Changing the setting to match the application needs resulted in a 5%-10% motor efficiency improvement at part load.

Savings, with caution

PMAC motors offer an opportunity for significant energy savings over induction motor counterparts. However, care must be taken in the selection and configuration of an appropriate PMAC-compatible VFD to realize efficiency benefits and to ensure robust motor control.

- Kim Baker is vice president, application engineering, NovaTorque Inc. Edited by Mark T. Hoske, content manager CFE Media, Control Engineering, mhoske@cfemedia.com.

Key considerations

  • Properly configure variable frequency drives for permanent magnet alternating current (PMAC) motors for optimal system performance and desired energy savings
  • Heed safety concerns if PMAC motors rotate in an unpowered state.

 

Consider this

Get the needed help when configuring the VFD for a PMAC motor to optimize energy savings. 

ONLINE

www.controleng.com/discrete-manufacturing/motors-drives.html 

www.novatorque.com 


<< 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...
2016 Product of the Year; Diagnose bearing failures; Asset performance management; Testing dust collector performance measures
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

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 of plant safety and offers advice on best practices.
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