Engineers today are tasked with applying myriad motor technologies because most rotary motion is ultimately powered by electric motors. One proliferating option, 

 ac (PMAC) motors, has functionalities that partially overlap with those of both ac induction and servomotors for larger, higher-end applications requiring precisely metered torque, speed, or positioning.

In PMACs, magnets mounted on or embedded in the rotor couple with the motor's current-induced, internal magnetic fields generated by electrical input to the stator. More specifically, the rotor itself contains permanent magnets, which are either surface-mounted to the rotor lamination stack or embedded within the rotor laminations. As in common ac induction motors, electrical power is supplied through the stator windings.

Permanent-magnet fields are, by definition, constant and not subject to failure, except in extreme cases of magnet abuse and demagnetization by overheating. PMAC, PM synchronous, and brushless ac are synonymous terms.

Rare-earth elements (those 30 metals in the periodic table's two oft-omitted rows) are used in PMAC motors. Rare-earth magnets have crystalline structures with high magnetic anistropy - for one-third to two times more power than traditional ferrite magnets (generating fields up to 1.4 Tesla in some cases.)

Permanent-magnet motor technology overview

Back-electromotive force (EMF) is voltage that opposes the current that causes it. In fact, back EMF arises in any electric motor when there is relative motion between the current-carrying armature (whether rotor or stator) and the external magnetic field. As the rotor spins (with or without power applied to the windings) the mechanical rotation generates a voltage - so, in effect, becomes a generator. Typical units are (V/krpm) - volts/(1,000 rpm).

A PMAC motor has a sinusoidally distributed stator winding to produce sinusoidal back EMF waveforms.

All PMAC motors require a matched PM drive for operation; they are not designed for across-the-line starting.

PMAC-compatible drives (known as PM drives) substitute the more traditional trapezoidal waveform's flat tops with a sinusoidal waveform that matches PMAC back EMF more closely, so torque output is smoother. Each commutation of phases must overlap, selectively firing more than one pair of power-switching devices at a time. These motor-drive setups can be operated as open-loop systems in midrange performance applications requiring speed and torque control. Here, PMAC motors are placed under vector-type control.

In fact, though PMACs require a drive specifically designed for PM motors, the PM drive setup is most similar to flux vector drives for ac induction motors, in that the drive uses current-switching techniques to control motor torque - and simultaneously controls both torque and flux current via mathematically intensive transformations between one coordinate system and another. These PM drives use motor data and current measurements to calculate rotor position; the digital signal processor (DSP) calculations are quite accurate. During every sampling interval, the three-phase ac system - dependent on time and speed - is transformed into a rotating two-coordinate system in which every current is expressed and controlled as the sum of two vectors.

Force, torque, and speed

In PMAC motors, speed is a function of frequency - the same as it is with induction motors. However, PMAC motors rotate at the same speed as the magnetic field produced by the stator windings; it is a synchronous machine. Therefore, if the field is rotating at 1,800 rpm, the rotor also turns at 1,800 rpm - and the higher the input frequency from the drive, the faster the motor rotates.

Most manufacturers of synchronous motors hold pole count constant so input frequency dictates the motor's speed. For example, for a 48-frame motor with six poles, the motor's input frequency from the drive must be 90 Hz to obtain 1,800 rpm. To extract the same speed from a 10-pole 180-frame motor, input frequency must be 150 Hz. To calculate required input frequency (Hz) when the number of poles and speed are known:

PMAC motors are suitable for variable or constant-torque applications, where the drive and application parameters dictate to the motor how much torque to produce at any given speed. This flexibility also makes PMACs suitable for variable-speed operation requiring ultra-high motor efficiency.