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Low Vibration And Noise High Power Density PM Motor Permanent Magnet Motor
What Is The Permanent Magnet Synchronous Motor?
The Permanent Magnet Synchronous Motor (PMSM) is an AC synchronous motor whose field excitation is provided by permanent magnets and has a sinusoidal back EMF waveform. The PMSM is a cross between an induction motor and a brushless DC motor. Like a brushless DC motor, it has a permanent magnet rotor and windings on the stator. However, the stator structure with windings constructed to produce a sinusoidal flux density in the air gap of the machine resembles that of an induction motor. Its power density is higher than induction motors with the same ratings since there is no stator power dedicated to magnetic field production.
With permanent magnets the PMSM can generate torque at zero speed,
it requires a digitally controlled inverter for operations. PMSMs
are typically used for high-performance and high-efficiency motor
drives. High-performance motor control is characterized by smooth
rotation over the entire speed range of the motor, full torque
control at zero speed, and fast acceleration and deceleration.
To achieve such control, vector control techniques are used for
PMSM. The vector control techniques are usually also referred to as
field-oriented control (FOC). The basic idea of the vector control
algorithm is to decompose a stator current into a magnetic
field-generating part and a torque-generating part. Both components
can be controlled separately after decomposition.
Working of Permanent Magnet Synchronous Motor
First, the permanent magnet synchronous motor needs to establish the main magnetic field, and the excitation winding is passed through the DC excitation current to establish the excitation magnetic field between polarities;
then the three-phase symmetrical armature winding is used as the power winding, which becomes the carrier of the induced electric potential or induced current;
in the prime mover When the rotor is dragged to rotate, the excitation magnetic field between the polarities rotates with the shaft and sequentially cuts the stator phase windings.
Therefore, the armature winding will induce a three-phase symmetrical alternating potential whose size and direction change periodically.
Through the lead wire, AC power can be provided. Due to the symmetry of the armature winding, the three-phase symmetry of the induced potential is guaranteed.
Analysis of the principle of the technical advantages of permanent
magnet motor
The principle of a permanent magnet synchronous motor is as
follows: In the motor's stator winding into the three-phase
current, after the pass-in current, it will form a rotating
magnetic field for the motor's stator winding. Because the rotor is
installed with the permanent magnet, the permanent magnet's
magnetic pole is fixed, according to the principle of magnetic
poles of the same phase attracting different repulsion, the
rotating magnetic field generated in the stator will drive the
rotor to rotate, The rotation speed of the rotor is equal to the
speed of the rotating pole produced in the stator.
Back-emf waveform:
Back emf is short for back electromotive force but is also known as the counter-electromotive force. The back electromotive force is the voltage that occurs in electric motors when there is a relative motion between the stator windings and the rotor’s magnetic field. The geometric properties of the rotor will determine the shape of the back-emf waveform. These waveforms can be sinusoidal, trapezoidal, triangular, or something in between.
Both induction and PM machines generate back-emf waveforms. In an
induction machine, the back-emf waveform will decay as the residual
rotor field slowly decays because of the lack of a stator field.
However, with a PM machine, the rotor generates its own magnetic
field. Therefore, a voltage can be induced in the stator windings
whenever the rotor is in motion. Back-emf voltage will rise
linearly with speed and is a crucial factor in determining maximum
operating speed.
Permanent magnet AC (PMAC) motors have a wide range of applications
including:
Permanent magnet synchronous motors can be combined with frequency converters to form the best open-loop steppless speed control system, which has been widely used for speed control transmission equipment in petrochemical, chemical fiber, textile, machinery, electronics, glass, rubber, packaging, printing, paper making, printing and dyeing, metallurgy and other industries.
A PM motor can be separated into two main categories: surface permanent magnet motors (SPM) and interior permanent magnet motors (IPM). Neither motor design type contains rotor bars. Both types generate magnetic flux by the permanent magnets affixed to or inside of the rotor.
SPM motors have magnets affixed to the exterior of the rotor surface. Because of this mechanical mounting, their mechanical strength is weaker than that of IPM motors. The weakened mechanical strength limits the motor’s maximum safe mechanical speed. In addition, these motors exhibit very limited magnetic saliency (Ld ≈ Lq).
Inductance values measured at the rotor terminals are consistent regardless of the rotor position. Because of the near unity saliency ratio, SPM motor designs rely significantly, if not completely, on the magnetic torque component to produce torque.
IPM motors have a permanent magnet embedded into the rotor itself. Unlike their SPM counterparts, the location of the permanent magnets makes IPM motors very mechanically sound, and suitable for operating at very high speeds. These motors also are defined by their relatively high magnetic saliency ratio (Lq > Ld). Due to their magnetic saliency, an IPM motor has the ability to generate torque by taking advantage of both the magnetic and reluctance torque components of the motor.
The development trend of rare earth permanent magnet motors
Rare earth permanent magnet motors are developing towards high power (high speed, high torque), high functionality and miniaturization, and are constantly expanding new motor varieties and application fields, and the application prospects are very optimistic. In order to meet the needs, the design and manufacturing process of rare earth permanent magnet motors still need to be continuously innovated, the electromagnetic structure will be more complex, the calculation structure will be more accurate, and the manufacturing process will be more advanced and applicable.
Application of rare earth permanent magnet motor
Due to the superiority of rare earth permanent magnet motors, their applications are becoming more and more extensive. The main application areas are as follows:
Focus on the high efficiency and energy saving of rare earth permanent magnet motors. The main application objects are large power consumers, such as rare earth permanent magnet synchronous motors for textile and chemical fiber industries, rare earth permanent magnet synchronous motors for various mining and transportation machinery used in oil fields and coal mines, and rare earth permanent magnet synchronous motors for driving various pumps and fans.
Sensorless Control
The rotor position information is needed to efficiently perform the
control of the PMS motor, but a rotor position sensor on the shaft
decreases the robustness and reliability of the overall system in
some applications. Therefore, the aim is not to use this mechanical
sensor to measure the position directly but instead employ some
indirect techniques to estimate the rotor position. These
estimations techniques differ greatly in approach for estimating
the position or the type of motor to which they can be applied. At
low speeds, special techniques like high-frequency injection or
open-loop start-up (not very efficient) are needed to spin the
motor over the speed where BEMF is sufficiently high for the BEMF
observer. Usually, 5 percent of the base speed is enough for proper
operation in sensorless mode.
At medium/high speed, a BEMF observer in the d/q reference frame is used. The PWM frequency and control loop must be sufficiently high to get a reasonable number of samples of phase current and DC bus voltage.
Flux weakening/intensifying of PM motors
The operation beyond the machine base speed requires the PWM
inverter to provide output voltages higher than its output
capability limited by its DC link voltage. To overcome the base
speed limitation, a field-weakening algorithm can be implemented. A
negative d-axis required current will increase the speed range, but
the applied torque is reduced because of a stator current limit.
Manipulating the d-axis current into the machine has the desired
effect of weakening the rotor field, which decreases the BEMF
voltage, allowing the higher stator current to flow into the motor
with the same voltage limit given by the DC link voltage.
What applications use PMSM motors?
Permanent magnet synchronous motors have the advantages of simple
structure, small size, high efficiency, and high power factor. It
has been widely used in the metallurgical industry (ironmaking
plant and sintering plant, etc.), ceramic industry (ball mill),
rubber industry (internal mixer), petroleum industry (pumping
unit), textile industry (double twist machine, spinning frame) and
other industries in the medium and low voltage motor.
Why you should choose an IPM motor instead of an SPM?
1. High torque is achieved by using reluctance torque in addition to magnetic torque.
2. IPM motors consume up to 30% less power compared to conventional electric motors.
3. Mechanical safety is improved as, unlike in an SPM, the magnet will not detach due to centrifugal force.
4. It can respond to high-speed motor rotation by controlling the two types of torque using vector control.
How to improve the efficiency of the motor?
To improve the efficiency of the motor, the essence is to reduce the loss of the motor. The loss of the motor is divided into mechanical loss and electromagnetic loss. For example, for an AC asynchronous motor, the current passes through the stator and rotor windings, which will produce copper loss and conductor loss, while the magnetic field is in the iron. It will cause eddy currents to bring about hysteresis loss, high harmonics of the air magnetic field will generate stray losses on the load, and there will be wear losses during the rotation of bearings and fans.
To reduce the loss of the rotor, you can reduce the resistance of the rotor winding, use a relatively thick wire with low resistivity, or increase the cross-sectional area of the rotor slot. Of course, the material is very important. Conditional production of copper rotors will reduce losses by about 15%. The current asynchronous motors are basically aluminum rotors, so the efficiency is not so high.
Similarly, there is copper loss on the stator, which can increase the slot face of the stator, increase the full slot ratio of the stator slot, and shorten the end length of the stator winding. If a permanent magnet is used to replace the stator winding, there is no need to pass the current. Of course, the efficiency can be obviously improved, which is the fundamental reason why the synchronous motor is more efficient than the asynchronous motor.
For the iron loss of the motor, high-quality silicon steel sheets can be used to reduce the loss of the hysteresis or the length of the iron core can be lengthened, which can reduce the magnetic flux density, and can also increase the insulating coating. In addition, the heat treatment process is also critical.
The ventilation performance of the motor is more important. When the temperature is high, the loss will of course be large. The corresponding cooling structure or additional cooling method can be used to reduce friction loss.
High-order harmonics will produce stray losses in the winding and iron core, which can improve the stator winding and reduce the generation of high-order harmonics. Insulation treatment can also be performed on the surface of the rotor slot, and magnetic slot mud can be used to reduce the magnetic slot effect.
