Dynamic Balancing of Magnetic Levitation Motor Rotors: A Comprehensive Analysis of G1.0 And G2.5 Standards And Testing Processes

Magnetic levitation motors, with their advantages of contactless operation, zero friction, and high efficiency, are gradually replacing traditional motors as the mainstream choice for high-speed rotating machinery. However, during high-speed operation, even the slightest mass eccentricity in the magnetic levitation rotor can cause severe unbalanced vibration, directly affecting the levitation accuracy, operational stability, and service life of the motor. Therefore, dynamic balancing – a critical manufacturing process for rotating machinery – is particularly important for magnetic levitation motors.

This article systematically interprets the meanings and differences between the G1.0 and G2.5 grades in the ISO dynamic balancing standards, and provides an in-depth introduction to the dynamic balancing testing processes for magnetic levitation motor rotors.

I. Overview of the ISO Dynamic Balancing Grade System

The classification of dynamic balancing grades originates from the ISO 1940 standard (now updated to ISO 21940-11), established by the International Organization for Standardization (ISO) in 1940. This standard divides the balance quality of rigid rotors into 11 grades, ranging from the highest precision G0.4 to the lowest requirement G4000, with each grade increasing by a factor of 2.5.

The balance grade is denoted by the letter "G" followed by a number. Its physical meaning is the maximum permissible vibration velocity at the rotor surface, expressed in mm/s, and it can also be represented as the specific unbalance (g·mm/kg). The smaller the grade number, the higher the required balancing precision.

Core Calculation Formula

In the ISO standard, the relationship between the balance grade G, the rotor eccentricity e, and the angular velocity ω is:

G = ω × e / 1000

Where:

· G – balance quality grade (mm/s)

· ω – operating angular velocity (rad/s)

· e – permissible eccentricity (μm)

This formula reveals the essence of dynamic balancing: balancing precision depends not only on the magnitude of residual unbalance, but also on the rotor's operating speed. The higher the speed, the greater the vibration caused by the same unbalance, thus requiring higher balancing precision.

II. G1.0 and G2.5: Standard Interpretation and Application Scenarios

G2.5 – The "Workhorse" of Industrial Motors

G2.5 is the most commonly used balance grade in the small- and medium-sized motor industry. Its main application scenarios include:

• Gas turbines

• Machine tool spindles

• Small and medium-sized electric motors

For traditional motors, the G2.5 grade already meets the vibration and noise requirements of the vast majority of industrial applications. IEC standards explicitly state that the rotor balance grade for high-efficiency motors should reach G2.5.

G1.0 – The "High Standard" for Precision Motors

G1.0 is a high-precision balance grade, with requirements that are 2.5 times more stringent than G2.5. Typical applications include:

• Grinding machine drives

• Small precision motor rotors

• High-precision machine tool spindles

• Audio and video drive devices

How to Choose for Magnetic Levitation Motors?

Magnetic levitation motors typically operate at much higher speeds than traditional motors (tens of thousands of rpm or more), and their requirements for vibration and levitation accuracy are extremely demanding. Therefore, the selection of the dynamic balancing grade for magnetic levitation motors should consider the following factors comprehensively:

1. Operating speed: higher speeds impose stricter balance grade requirements.

2. Application scenario: precision equipment (such as magnetic levitation molecular pumps and flywheel energy storage systems) often requires G1.0 or even G0.4.

3. Rotor stiffness: flexible rotors may require higher balancing precision.

In practical engineering, the balancing requirements of many magnetic levitation motors have already surpassed G2.5 and are moving toward G1.0 or even G0.4.

III. Calculation of Permissible Residual Unbalance

After determining the balance grade G, the next step is to calculate the permissible residual unbalance of the rotor. This is the most critical quantitative indicator in dynamic balancing testing.

Calculation Formulas

According to the ISO standard, the permissible residual unbalance U_per is calculated as:

U_per = 9549 × G × m / n

or

U_per = G × M / (ω × 1000)

Where:

• U_per – permissible residual unbalance (g·mm)

• G – balance quality grade (mm/s)

• m / M – rotor mass (kg)

• n – operating speed (rpm)

• ω – operating angular velocity (rad/s)

Calculation Example

Assume a magnetic levitation motor rotor has a mass of 50 kg and an operating speed of 30,000 rpm:

• If G2.5 is selected: U_per = 9549 × 2.5 × 50 / 30000 ≈ 39.8 g·mm

• If G1.0 is selected: U_per = 9549 × 1.0 × 50 / 30000 ≈ 15.9 g·mm

It can be seen that under the same conditions, the permissible residual unbalance for G1.0 is only 40% of that for G2.5. For high-speed magnetic levitation motors, this means milligram-level weight-trimming precision.

Allocation to Correction Planes

For rotors that require two-plane balancing (length-to-diameter ratio > 0.5), the total permissible unbalance must be distributed between the two correction planes. The allocation ratio is usually calculated in inverse proportion to the distances from the correction planes to the rotor's centre of gravity.

IV. Dynamic Balancing Testing Processes for Magnetic Levitation Motor Rotors

The dynamic balancing of magnetic levitation motors differs fundamentally from that of traditional motors, due to their unique support method – contactless levitation by magnetic bearings.

1. Limitations of Traditional Balancing Methods

Traditional balancing methods require a dedicated balancing machine where the rotor is supported by mechanical bearings (rollers or V-blocks) for measurement and correction. However, the actual operating state of a magnetic levitation rotor is supported by the levitation forces of magnetic bearings. Mechanical support cannot simulate the real working conditions and may introduce additional support disturbances.

2. Balancing Methods for Magnetic Levitation Rotors

(1) On-line Balancing

On-line balancing refers to balancing operations performed while the magnetic levitation system is in normal operation. The core concept is to use the sensors inherent to the magnetic bearing system (displacement sensors, current sensors, etc.) to acquire rotor vibration signals, and then use algorithms to deduce the magnitude and phase of the unbalance, thereby determining the correction strategy.

This method eliminates the need to disassemble the rotor from the equipment, greatly improving balancing efficiency.

(2) Trial-weight-free Balancing

Traditional balancing methods require multiple trial runs (adding trial weights → measuring → adjusting) to obtain influence coefficients, which is inefficient. To address this, researchers have proposed trial-weight-free balancing methods based on the active control characteristics of magnetic bearings.

This method leverages the active controllability of magnetic bearings to generate controllable electromagnetic forces in the static levitation state, simulating the centrifugal forces that actual trial weights would produce during rotation. This allows the influence coefficients to be identified without physically installing trial weights, significantly simplifying the procedure.

(3) On-site Balancing

On-site balancing refers to balancing operations performed at the actual installation location of the equipment. Unlike traditional methods where the rotor is removed and sent to a test facility, on-site balancing takes into account the comprehensive effects of the entire system (including bearing stiffness, installation conditions, foundation vibrations, etc.), providing balancing results that better match actual operating conditions.

3. Typical Testing Procedure

Taking a magnetic levitation centrifugal blower as an example, a typical dynamic balancing testing procedure is as follows:

Step 1: Pre-inspection
Before balancing, the rotor must undergo visual and dimensional inspection to confirm there are no machining defects, burrs, or foreign matter adhering to it.

Step 2: Initial Measurement
On a dedicated balancing machine or the magnetic levitation system, perform a trial run at about 70% of the operating speed to collect vibration amplitude and phase data.

Step 3: Unbalance Identification
Through signal processing, extract the synchronous vibration component of the rotor, and combine it with the rotor dynamics model to deduce the magnitude and phase of the unbalance.

Step 4: Correction and Verification
Based on the calculation results, add or remove weight at the specified positions, then measure again to verify that the target balance grade has been achieved.

Step 5: Full-speed Range Verification
Perform vibration measurements at multiple speed points within the operating speed range (including near critical speeds) to ensure that vibration levels at all speeds meet the requirements.

4. Technical Challenges in Magnetic Levitation Balancing

Balancing magnetic levitation rotors presents several unique challenges:

• Flexible rotor effects: high-speed magnetic levitation rotors often operate above critical speeds and are flexible rotors; their unbalance distribution may vary with speed, requiring multi-modal balancing.

• Electromagnetic force interference: the electromagnetic forces of magnetic bearings may themselves introduce additional synchronous vibration forces, which must be distinguished and compensated for in the algorithms.

•  Sensor precision: the accuracy of displacement and current sensors directly affects the reliability of unbalance identification.

V. Conclusion

Dynamic balancing is the key step that takes a magnetic levitation motor from merely “being able to spin” to “spinning well”. G1.0 and G2.5 are not just two numerical labels; they represent different precision levels and engineering requirements. For magnetic levitation motors, as speeds continue to increase and applications become ever more sophisticated, the move from G2.5 toward G1.0 and even higher precision grades has become an inevitable trend.

At the same time, the unique support method of magnetic levitation motors has given rise to innovative balancing processes such as on-line balancing and trial-weight-free balancing. These technologies not only improve balancing efficiency but also transform dynamic balancing from a “pre-shipment inspection step” into a “full-life-cycle maintenance tool”.

Understanding the standards, mastering the methods, and executing with precision – this is the core essence of dynamic balancing for magnetic levitation motor rotors.