Engineers today are tasked with applying myriad motor technologies because most rotary motion is ultimately powered by electric motors. One proliferating option, permanent magnet 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.
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Cogging — the unwanted jerking during motor spinning from repeatedly overcoming the attraction of permanent magnets and the stator's steel structure — is often associated with PM motors. Particularly at startup, cogging arises from the interaction of the rotor magnets and stator winding when it is energized, due to harmonics. Cogging in turn causes noise, vibration, and non-uniform rotation. Many methods for reducing cogging can be leveraged to eliminate torque and speed ripple. Some PMAC motors are designed with more rotor poles than equivalent ac induction motors, which helps reduce these issues.
Closed loop functionality
In specialty cases, PMAC motors are employed in closed-loop configurations using speed feedback. Feedback allows the drive to track the exact rotor position — to provide true infinite speed range, including full torque at zero speed. The speed reference required from an external source can be an analog or encoder signal, or a serial command from a feedback device on an axis one wishes to follow. This is normally a velocity signal, sometimes further processed in the drive before it is used as a command.
Limitations and challenges
PMAC speed is limited by back EMF because the latter increases directly with motor speed. The motor is connected to the electronic drive and its electronic components are designed for a maximum voltage above rated drive voltage. Normally, the motor and controls are designed to operate well below the maximum voltage of the components. However, if motor speed exceeds the design speed range (either powered from the control or being driven by the load) it is possible to exceed the maximum voltage of the drive components — and cause failures. Note that drives are capable of limiting motor back EMF when operating properly. However, if the drive faults and loses control during overspeed, it cannot protect itself.
In addition, PMAC motor control requires some technical knowledge for implementation: All commercially available PMAC motors require a PM-compatible drive to operate, although there is ongoing research in the development of a line-start PMAC motor.
Not all ac drives are suitable for operation of PMAC motors; only drives specifically designed for permanent magnet motor compatibility are suitable. Here, a parameter in the drive programming often allows an operator to set the drive for a PM motor. Some drives not specifically designed for this use can run and control PM motors, though performance is degraded — and one can damage the motor or drive if they are mismatched.
Finally, high current or operating temperatures can cause the magnets in PMAC motors to lose their magnetic properties. Permanent magnets, once demagnetized, cannot recover, even if current or temperature returns to normal levels. PM drives reduce the risk of high-current demagnetization with over-current protection. Some motor designs further minimize the possibility of demagnetization with high-temperature magnets, integrated thermostats, and restricted motor operating temperature.
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PMAC versus servomotors
Servomotors are utilized in motion control applications where low inertia and dynamic response are important. In fact, many motors used for servo applications are similar to PMAC motors but use special controllers (amplifiers) and feedback to control position rather than just speed. However, the price for servosystems can be high — often 10 to 20 times than that of an equivalently rated induction motor. Applications requiring near-servo performance are suitable candidates for PMAC motors, benefitting from their cost-to-performance ratio. Case in point: PMACs are well suited for typical pump operations, which typically run at variable speed between 75% and 85% of maximum speed.
PMAC motors are unsuitable in typically servomotor applications approaching 10,000 rpm — out of the PMAC motor range. In addition, without feedback for the PMAC, designers can find it difficult to locate and position to the pinpoint accuracy that servomotors must often deliver.
Now compare PMAC motors to those most commonly used for servo applications — brushless dc motors. A traditional brushless-dc drive waveform is trapezoidal; here, two of the motor's three leads are used for the phases, and the third is used for hunting — so it's regularly changing fields. In contrast, the three leads of the PMAC are actively used; input waveforms are sinusoidal, to boost efficiency while minimizing noise and vibration.
As mentioned, motor stator winding patterns are typically specialized for a specific waveform shape. One cannot differentiate them by visual inspection.
A controller that produces trapezoidal waveforms is less costly than those that produce sinusoidal waveforms. However, sinusoidal controllers and motors produce more consistent shaft rotation than trapezoidal — and rotor inertia, motor rating, and specific controller characteristics magnify the difference in performance.
One caveat: In low-voltage applications (anything below 110 V), traditional brushless dc or ac induction motors are still better choices than PMAC motors — although there's work being done to address the issues that arise in these situations.
In short, brushless dc motors are commonly built for voltages down to 12 or 24 V. However, to wind a PMAC for this voltage is, in effect, taking a 200 or 300 hp and winding it for 200 V. Here, lead sizes can grow to the size of an average coffee cup (an inane result) and winding such a motor's magnet wire (with a machine or by hand) is problematic, as manufacturers in this case must redesign the stator and rotor fairly extensively to ensure that the setup is physically possible.
Induction motors versus PMAC motors
For an apples-to-apples comparison of ac induction motors to PMAC motors, we must consider both with a drive — as the latter requires a drive for operation, and cannot connect directly to supply power as typical ac motors can.
System efficiency is higher for a PMAC motor/drive setup from 40% to beyond 120% load. In addition, a PMAC motor exhibits higher power density than an equivalent induction motor: Rare-earth permanent magnets produce more flux for their physical size than the magnetic energy and resultant torque produced by an induction motor's squirrel cage rotor. In the latter, the effect of back EMF is also more pronounced: Back EMF reduces current and works to slow the motor — and gets larger as speed increases. When no load is present, it approaches the input voltage magnitude, reducing efficiency. Consider that, in general, certain PMAC motors are rated for variable or constant torque to 20:1 without feedback (open loop) or 2,000:1 for closed loop (with an encoder).
Speed (input frequency) has less effect on PMAC motor efficiency than it does on ac induction motors, which translates into energy savings at reduced speeds. PMAC motor losses (the inverse of efficiency) are 15% to 20% lower than NEMA Premium induction motors.
Depending on motor size, electric utility rate, and duty cycle, designers can realize full return on a certain PMAC motor purchases in one year. PMAC efficiency ratings are one to three indexes above NEMA Premium, which translates to 10% to 30% fewer losses than a conventional motor. Electricity is estimated to comprise approximately 95% to 97% of the total lifecycle cost of electric motors, so energy savings significantly reduce the total investment.
In short, due to their synchronous operation, PMAC motors also offer better dynamic performance and speed-control precision — a benefit in high-inertia positioning applications. Although the power factor with a drive may not be as high as a motor-only induction machine, PMAC motors generally provide higher power density due to higher magnetic flux. Therefore, more torque can be produced in a given physical size, or equal torque produced in a smaller package. Finally, PMAC motors generally operate more coolly than ac induction motors, resulting in longer bearing and insulation life.
If this excerpt was useful to you, download the entire MSD-MD eBook, Understanding AC Induction, Permanent Magnet and Servo Motor Technologies, at http://machinedesign.com/sponsor/leeson-electric-corporation-1122.