Frequent acceleration, deceleration, and changing loads place specific demands on the magnetic properties and core losses of electrical steel used in servo motor stators.

Non-grain-oriented electrical steel (NGOES) is commonly used in servo motor stators. Grade selection should account for magnetic performance and the effects of downstream processing.

1. Why Servo Motors Need Suitable Core Materials

The stator core provides the magnetic path for flux associated with the rotor magnets and energized windings. Its magnetic properties influence the current required to establish the intended flux and the losses generated during operation.

Its relatively uniform in-plane magnetic properties make NGOES suitable for servo motor stators. Thin, insulated laminations also help limit eddy-current losses.

A positioning servo may experience repeated peak torque during acceleration, while a high-speed spindle servo operates at higher electrical frequencies for extended periods.

2. Magnetic Requirements of Servo Motor Stators

Magnetization Characteristics and Flux Density

Magnetic flux is distributed unevenly across the stator teeth and yoke. Local flux density depends on the geometry and magnetic loading of these regions.

The B-H curve shows the magnetic field strength required at a given flux density. Engineers should pay particular attention to the expected operating range in heavily loaded stator regions.

Local Saturation During Peak Torque

Rapid acceleration and short overload periods can require high stator current. In these conditions, parts of the stator teeth or yoke may approach magnetic saturation.

As saturation develops, increasing the magnetizing field produces progressively less additional flux. In permanent magnet synchronous motors, this nonlinear behavior can affect inductance and the relationship between current and torque.

1793669541942-Magnetic Saturation in Servo Motor Stators.webp

B-H magnetization curve illustrating magnetic saturation alongside magnetic flux distribution in a laminated servo motor stator.

Core Loss and Heat Generation

Core loss includes hysteresis, eddy-current, and excess losses associated with changing magnetization.

Electrical steel datasheets report specific core loss in W/kg at defined frequencies and magnetic flux densities. These measurements do not capture the nonsinusoidal excitation and uneven flux distribution found in a finished motor.

In the finished motor, stator core loss contributes to heating alongside winding copper loss. Its magnitude varies with electrical frequency, magnetic flux distribution, and excitation waveform.

3. Core Loss Under Variable Speed and Load

For a synchronous motor, fundamental electrical frequency depends on mechanical speed and the number of pole pairs:

f = n × p / 60

Where:

f = fundamental electrical frequency (Hz)
n = mechanical speed (rpm)
p = number of pole pairs

An eight-pole servo motor operates at a fundamental electrical frequency of 200 Hz at 3,000 rpm and 400 Hz at 6,000 rpm. As explained in Electrical Steel Core Loss vs. Frequency, 50 Hz test data alone is insufficient for evaluating losses across this speed range.

Loss Distribution Across a Positioning Cycle

Consider a servo-driven axis moving a component between two machining positions.

During acceleration, the motor produces torque to overcome inertia and the external load. As speed rises, so does the fundamental electrical frequency. Deceleration changes the torque and current demand again.

1793669545283-Servo Motor Operating Conditions Acceleration and Deceleration.webp

Illustration of servo motor acceleration, constant-speed operation, and deceleration with corresponding speed profiles.

When the axis holds its position against a load, the motor may continue drawing substantial current at zero speed. Winding copper loss can remain significant even though fundamental rotational core loss is low.

Evaluating Material Loss Under Actual Excitation

Core-loss data at different frequencies and flux densities provides a basis for estimating material losses across the operating range.

Inside the motor, magnetic flux is distributed unevenly across the stator, while inverter-generated harmonics can introduce additional losses beyond those measured under sinusoidal laboratory conditions.

Compare candidate grades using material loss data, then assess the finished motor through efficiency and temperature-rise testing.

4. Electrical Steel Thickness and Grade Selection

Lamination thickness affects eddy-current loss. Under simplified conditions, the classical eddy-current loss per unit mass is approximately proportional to the square of sheet thickness when other material properties, frequency, and flux density remain constant.

Thinner sheets can reduce classical eddy-current losses, particularly at higher frequencies, although total core loss still varies by grade.

When comparing 0.50 mm, 0.35 mm, and 0.20 mm NGOES, manufacturers should check measured core losses across the motor's intended frequency range.

Thin laminations may require tighter control during feeding, punching, handling, and stacking, especially in high-volume production of compact stators.

5. From Electrical Steel Coil to Servo Motor Stator

Electrical steel undergoes cutting, stacking, joining, and housing assembly before becoming a finished stator core.

5.1 Cutting-Edge Damage and Local Magnetic Properties

Precision stamping is commonly used to produce servo motor laminations, while laser cutting may be employed for prototyping or specific production requirements.

Stamping introduces plastic deformation and residual stress near cut edges, while laser cutting can create heat-affected regions. Both processes can affect local permeability and increase magnetic losses in the affected areas.

The effect is particularly relevant in stators with narrow teeth, where the affected edge regions occupy a larger proportion of the magnetic cross-section.

A 2023 review published in Energies, A Review on the Effect of Electrical Steel Manufacturing Processes on the Performance of Electric Machines, examines how cutting and subsequent manufacturing processes influence electrical machine losses.

5.2 Burrs and Unwanted Interlaminar Currents

Excessive burr height can interfere with lamination alignment and create electrical contact between adjacent sheets.

Insulation coatings normally restrict current flow between laminations. Burrs or coating damage can form conductive bridges, allowing unwanted circulating currents and increasing local eddy-current losses.

Punching clearance, tool wear, burr height, and coating integrity should be monitored to reduce the risk of unwanted interlaminar electrical contact.

5.3 Stacking, Joining, and Core Construction

Servo motor laminations are assembled into stacks using methods such as interlocking, welding, or adhesive bonding.

Joining Method	Effect to Examine
Interlocking	Local deformation and residual stress around mechanical joining points
Welding	Heat-affected regions and possible conductive paths between laminations
Adhesive bonding	Bonding-layer characteristics, curing conditions, and dimensional control

Manufacturers such as Gator Lamination describe these methods in servo motor core production.

Insulation coatings and gaps between laminations reduce the stacking factor, leaving less effective steel area within a given stack volume. A lower stacking factor can increase flux density in the remaining steel for a given magnetic flux.

5.4 Mechanical Stress During Housing Assembly

Installing a stator into its housing through an interference fit introduces mechanical stress. The resulting pressure distribution depends on the stator geometry and assembly conditions.

This assembly stress can change local magnetic permeability and core losses.

An experimental study published in IEEE Transactions on Energy Conversion, Effects of Manufacturing Processes on Core Losses of Electrical Machines, reported a 23% increase in stator core losses at the rated operating condition of the motor examined.

The 23% increase was specific to the tested motor and operating conditions.

5.5 Identifying the Source of Additional Core Loss

If finished-stator losses exceed expectations, measurements at different production stages can help locate the source of additional loss.

If the supplied steel meets its magnetic specifications, test the processed laminations for cutting-related changes and the assembled stacks for losses associated with joining or interlaminar contact.

A before-and-after comparison of housing installation can help assess losses associated with assembly stress.

Use comparable test conditions when evaluating losses across production stages.

6. What Servo Motor Manufacturers Should Verify

Before approving NGOES coils for servo motor production, check the following specifications and test records:

Verification Item	What to Check
B-H Characteristics	Magnetization data across the required flux-density range
Core Loss	W/kg values at specified frequencies and flux densities
Test Method	Applicable test standard and measurement method, such as Epstein or SST
Thickness Tolerance	Nominal thickness and permitted variation
Insulation Coating	Coating properties and processing compatibility
Mechanical Properties	Properties relevant to stamping and handling
Batch Traceability	Material identification and inspection records

Comparing Magnetic Test Results

Compare core-loss values only when frequency, flux density, and test method are equivalent.

For higher-frequency applications, request loss data measured at frequencies and flux densities closer to the intended operating conditions.

Consistency Between Production Batches

For subsequent deliveries, verify the agreed specifications using batch-specific test records and material certificates.

HWHG supplies non-grain-oriented electrical steel coils for motor applications, rather than finished laminations or stator cores. For servo motor projects, buyers can inquire about available grades, magnetic test data, thicknesses, coatings, and coil specifications.

Electrical Steel

HWHG non-oriented electrical steel coils displayed alongside laminated stator cores for servo motor applications.

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