Brushless Gearless Permanent Magnet Electric Motor Low Noise PMAC Electric Motor

What applications use PMSM motors?

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Industries that use PMSM motors include Metallurgical, Ceramic, Rubber, Petroleum, Textiles, and many others. PMSM motors can be designed to operate at synchronous speed from a supply of constant voltage and frequency as well as Variable Speed Drive (VSD) applications. Due to high efficiency and power and torque densities, they are generally a superior choice in high torque applications such as mixers, grinders, pumps, fans, blowers, conveyors, and industrial applications where traditionally induction motors are found.

Advantages Of Rare-earth Permanent Magnet Motors

High efficiency: The efficiency curve of the asynchronous motor generally falls faster under 60% of the rated load, and the efficiency is very low at light load. The efficiency curve of the rare earth permanent magnet motor is high and flat, and it is in the high-efficiency area at 20%~120% of the rated load.

High power factor: The measured value of the power factor of the rare earth permanent magnet synchronous motor is close to the limit value of 1.0. The power factor curve is as high and flat as the efficiency curve. The power factor is high. Low-voltage reactive power compensation is not required and the power distribution system capacity is fully utilized.

Stator current is small: The rotor has no excitation current, the reactive power is reduced, and the stator current is significantly reduced. Compared with the asynchronous motor of the same capacity, the stator current value can be reduced by 30% to 50%. At the same time, because the stator current is greatly reduced, the motor temperature rise is reduced, and the bearing grease and bearing life are extended.

High out-of-step torque and pull-in torque: Rare earth permanent magnet synchronous motors have higher out-of-step torque and pull-in torque, which makes the motor have higher load capacity and can be smoothly pulled into synchronization.

Disadvantages Of Rare-earth Permanent Magnet Motors

High cost: Compared with the asynchronous motor of the same specification, the air gap between the stator and the rotor is smaller, and the processing accuracy of each component is high; the rotor structure is more complicated and the price of rare earth magnetic steel material is high; therefore, the motor manufacturing cost is high, which is common for asynchronous motors About 2 times.

Large impact at full power start: When starting at full pressure, the synchronous speed can be drawn in a very short time. The mechanical shock is large. The starting current is more than 10 times the rated current. The impact on the power supply system is large, requiring a large capacity of the power supply system.

Rare-earth magnet steel is easy to demagnetize: When the permanent magnet material is subjected to vibration, high temperature, and overload current, its magnetic permeability may decrease, or the demagnetization phenomenon occurs, which reduces the performance of the permanent magnet motor.

PM motor structures
PM motor structures can be separated into two categories: interior and surface. Each category has its subset of categories. A surface PM motor can have its magnets on or inset into the surface of the rotor, to increase the robustness of the design. An interior permanent magnet motor positioning and design can vary widely. The IPM motor’s magnets can be inset as a large block or staggered as they come closer to the core. Another method is to have them embedded in a spoke pattern.

PM motor inductance variation with load
Only so much flux can be linked to a piece of iron to generate torque. Eventually, the iron will saturate and no longer allow flux to link. The result is a reduction in the inductance of the path taken by a flux field. In a PM machine, the d-axis and q-axis inductance values will reduce with increases in the load current.

The d and q-axis inductances of an SPM motor are nearly identical. Because the magnet is outside of the rotor, the inductance of the q-axis will drop at the same rate as the d-axis inductance. However, the inductance of an IPM motor will reduce differently. Again, the d-axis inductance is naturally lower because the magnet is in the flux path and does not generate an inductive property. Therefore, there is less iron to saturate in the d-axis, which results in a significantly lower reduction in flux with respect to the q-axis.

Flux weakening/intensifying of PM motors
Flux in a permanent magnet motor is generated by the magnets. The flux field follows a certain path, which can be boosted or opposed. Boosting or intensifying the flux field will allow the motor to temporarily increase torque production. Opposing the flux field will negate the existing magnet field of the motor. The reduced magnet field will limit torque production, but reduce the back-emf voltage. The reduced back-emf voltage frees up the voltage to push the motor to operate at higher output speeds. Both types of operation require additional motor current. The direction of the motor current across the d-axis, provided by the motor controller, determines the desired effect.

IPM VS SPM

A permanent magnet motor (also called PM) can be separated into two main categories: Interior Permanent Magnet (IPM) and Surface Permanent Magnet (SPM). Both types generate magnetic flux by the permanent magnets affixed to or inside of the rotor.

SPM

SURFACE PERMANENT MAGNET

A type of motor in which permanent magnets are attached to the rotor circumference.

SPM motors have magnets affixed to the exterior of the rotor surface, their mechanical strength is so weaker than the IPM one. 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

INTERIOR PERMANENT MAGNET

A type of motor that has a rotor embedded with permanent magnets is called IPM.

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.

EMF and Torque Equation

In a synchronous machine, the average EMF induced per phase is called dynamic induces EMF in a synchronous motor, the flux cut by each conductor per revolution is Pϕ Weber

Then the time taken to complete one revolution is 60/N sec

The average EMF induced per conductor can be calculated by using

( PϕN / 60 ) x Zph = ( PϕN / 60 ) x 2Tph

Where Tph = Zph / 2

Therefore, the average EMF per phase is,

= 4 x ϕ x Tph x PN/120 = 4ϕfTph

Where Tph = no. Of turns connected in series per phase

ϕ = flux/pole in Weber

P= no. Of poles

F= frequency in Hz

Zph= no. Of conductors connected in series per phase. = Zph/3

The EMF equation depends on the coils and the conductors on the stator. For this motor, the distribution factor Kd and pitch factor Kp is also considered.

Hence, E = 4 x ϕ x f x Tph xKd x Kp

The torque equation of a permanent magnet synchronous motor is given as,

T = (3 x Eph x Iph x sinβ) / ωm

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.