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2013-11-20   Control Engineering

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.


By: Kim Baker

Permanent 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).

SPM 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

Magnetic 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

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

Variable 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

PMAC 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. 

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