Brushless Gearless PMSM PMM Permanent Magnet Synchronous Motor

The defining feature of PMACMs – the permanent magnets within their rotor – are acted upon by the rotating magnetic field (RMF) of the stator windings, and are repelled into rotational motion. This is a deviation from other rotors, where the magnetic force must be induced or generated in the rotor housing, requiring more current. This means that PMACMs are generally more efficient than induction motors, as the rotor’s magnetic field is permanent and does not need a source of power to be used for its generation. This also means that they require a variable frequency drive (VFD, or PM drive) to operate, which is a control system that smooths out the torque produced by these motors. By switching the current on and off to the stator windings at certain stages of rotor rotation, the PM drive simultaneously controls torque and current and uses this data to calculate rotor position, and therefore the speed of the shaft output. They are synchronous machines, as their rotational speed matches the speed of the RMF. These machines are relatively new and are still being optimized, so the specific operation of any one PMACM is, for now, essentially unique to each design.

Why choose permanent magnet ac motors?

Permanent magnet AC (PMAC) motors offer several advantages over other types of motors, including:

High Efficiency: PMAC motors are highly efficient due to the absence of rotor copper losses and reduced winding losses. They can achieve efficiencies of up to 97%, resulting in significant energy savings.

High Power Density: PMAC motors have a higher power density compared to other motor types, which means they can produce more power per unit of size and weight. This makes them ideal for applications where space is limited.

High Torque Density: PMAC motors have a high torque density, which means they can produce more torque per unit of size and weight. This makes them ideal for applications where high torque is required.

Reduced Maintenance: Since PMAC motors have no brushes, they require less maintenance and have a longer lifespan than other motor types.

Improved Control: PMAC motors have better speed and torque control compared to other motor types, making them ideal for applications where precise control is required.

Environmentally Friendly: PMAC motors are more environmentally friendly than other motor types since they use rare earth metals, which are easier to recycle and produce less waste compared to other motor types.

Overall, the advantages of PMAC motors make them an excellent choice for a wide range of applications, including electric vehicles, industrial machinery, and renewable energy systems.

SPM versus IPM

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.

Self-sensing versus closed-loop operation

Recent advances in drive technology allow standard ac drives to “self-detect” and track the motor magnet position. A closed-loop system typically uses the z-pulse channel to optimize performance. Through certain routines, the drive knows the exact position of the motor magnet by tracking the A/B channels and correcting for errors with the z-channel. Knowing the exact position of the magnet allows for optimum torque production resulting in optimum efficiency.

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.

The permanent magnet synchronous motor has the following characteristics:

1. Rated efficiency is 2% to 5% higher than normal asynchronous motors;

2. The efficiency rises rapidly with the increase of the load. When the load changes within the range of 25% to 120%, it maintains high efficiency. The high-efficiency operating range is much higher than that of ordinary asynchronous motors. Light-load, variable-load, and full-load all have significant energy-saving effects;

3. Power factors up to 0.95 and above, no reactive compensation required;

4. The power factor is greatly improved. Compared with asynchronous motors, the running current is reduced by more than 10%. Due to the decrease in operating current and system losses, energy-saving effects of about 1% can be achieved.

5. Low-temperature rise, high power density: 20K lower than three-phase asynchronous motor temperature rise, the design temperature rise is the same and can be made into a smaller volume, saving more effective materials;

6. High starting torque and high overload capacity: according to requirements, it can be designed with high starting torque (3-5 times) and high overload capacity;

7. The variable frequency speed control system is used, which is better in dynamic response and better than that of asynchronous motors.

8. The installation dimensions are the same as the asynchronous motors currently widely used, and the design and selection are very convenient.

9. Due to the increase in power factor, the visual power of the power supply system transformer is greatly reduced, which improves the power supply capacity of the transformer, and can also greatly reduce the cost of the system cable (new project).