Variable frequency drive configuration, high-efficiency operation, and permanent magnet motors
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.
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, firstname.lastname@example.org.
- 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.
Get the needed help when configuring the VFD for a PMAC motor to optimize energy savings.
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