Direct Drive Rare Earth Permanent Magnet Gearless Motor Variable Speed

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.

The Key Features of Permanent Magnet Synchronous Motor

• Construction: PMSMs have a relatively simple mechanical design that makes them easy to maintain and repair. Their construction typically involves a fixed stator made of ferromagnetic material, permanent magnets attached to the rotor, and a set of copper windings.
• Rugged Composition: PMSMs are designed to withstand a range of environmental conditions, including temperature extremes, moisture, and vibration. This ruggedness makes them suitable for use in diverse applications, including automotive and industrial systems.
• High Reliability: A major advantage of PMSMs is their reliability, especially at high operating speeds. They have no brushes or commutators to wear out, and therefore, there is no loss of speed control due to mechanical wear.
• Variable Speed Drive: PMSMs can be driven by variable speed drives (VSDs), allowing for precise control of the motor speed. This capability makes them suitable for applications where speed needs to be continuously adjusted based on changing conditions.
• Sensorless Operation: PMSMs can operate without position or speed sensors, known as sensorless control. This removes the need for additional sensors, simplifying the system design and reducing costs.
• Precise Torque and Speed Control: PMSMs allow for precise control of torque and speed through advanced control algorithms and feedback loops. This enables precise positioning, motion control, and accurate speed regulation.
• Fast Dynamic Response: PMSMs have a fast dynamic response due to their high torque-to-inertia ratio and low moment of inertia. This makes them suitable for applications that require rapid acceleration or precise speed control.
• Smooth Torque Production: PMSMs produce a smooth and continuous torque, ensuring smooth motion and minimal vibrations in the system. This makes them suitable for precision motion control applications such as robotics or precision machining.
• Quiet Operation: PMSMs operate quietly due to their smooth torque production and lack of brushes or commutators. This makes them suitable for use in applications where noise reduction is important, such as in indoor environments or where people are present.

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);

A few small problems that are easily overlooked about the motor:

1. Why can't general motors be used in plateau areas?

Altitude has adverse effects on motor temperature rise, motor corona (high voltage motor), and commutation of DC motor. The following three aspects should be noted:

(1) The higher the altitude, the higher the temperature rise of the motor, and the lower the output power. However, when the temperature decreases with the increase of altitude enough to compensate for the influence of altitude on the temperature rise, the rated output power of the motor can remain unchanged;

(2) Anti-corona measures should be taken when the high-voltage motor is used in the plateau;

(3) The altitude is not good for the commutation of the DC motor, so pay attention to the selection of carbon brush materials.

2. Why is the motor not suitable for light load operation?

When the motor runs at a light load, it will cause:

(1) The power factor of the motor is low;

(2) The motor efficiency is low.

(3) It will cause equipment waste and uneconomical operation.

3. Why can't the motor start in a cold environment?

Excessive use of the motor in a low-temperature environment will cause:

(1) Motor insulation cracks;

(2) Bearing grease freezes;

(3) The solder powder of the wire joint is powdered.

Therefore, the motor should be heated and stored in a cold environment, and the windings and bearings should be checked before running.

4. Why can't a 60Hz motor use a 50Hz power supply?

When the motor is designed, the silicon steel sheet generally works in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency will increase the magnetic flux and the excitation current, resulting in an increase in the motor current and copper consumption, which will eventually lead to an increase in the temperature rise of the motor. In severe cases, the motor may be burned due to overheating of the coil.

5. Motor soft start

The soft start has a limited energy-saving effect, but it can reduce the impact of start-up on the power grid, and can also achieve a smooth start to protect the motor unit. According to the theory of energy conservation, due to the addition of a relatively complex control circuit, a soft start not only does not save energy, and also increases energy consumption. But it can reduce the starting current of the circuit and play a protective role.

10. When the new project is built, all the drive systems use permanent magnet synchronous motors, the project investment is basically the same as the use of asynchronous motors, and the project can continue to obtain energy-saving benefits after the project is put into operation;

In the general industrial sector, the replacement of low-voltage(380/660/1140V) high-efficiency asynchronous motors, the system saves 5% to 30% energy, and the high-voltage(6kV/10kV) high-efficiency asynchronous motors, system saves 2% to10%.