The Hidden Impact of Rotor Skew on Motor Efficiency: A Breakthrough in Loss Measurement

This technical summary is based on the academic paper "THE ACCURATE MEASUREMENT OF LOSSES IN SMALL CAGE INDUCTION MOTORS USING A BALANCE CALORIMETRIC METHOD" published by B.N.Shamsadeen at the University of Liverpool (May, 1990). It was analyzed and summarized for engineers and designers of electric motors by CASTMAN experts with the help of LLM AI such as Gemini, ChatGPT, and Grok.

Figure 3.2 Isometric sketch of the calorimeter

Keywords

Primary Keyword: Induction Motor Loss Measurement
Secondary Keywords: Balance Calorimetric Method, Cage Induction Motor Efficiency, Rotor Cage Skew, Stray Load Losses, Motor Air Gap Variation, Electromagnetic Losses, Motor Performance Testing

Executive Summary

(For the motor design engineer who only has 30 seconds.)

The Challenge: Standard methods for measuring losses in induction motors are often inaccurate, making it difficult to optimize designs and quantify the real-world impact of parameters like rotor skew and air gap.
The Method: A highly accurate "balance calorimetric method" was developed. It directly measures the electromagnetic losses of a motor by placing it in an insulated enclosure and balancing the heat produced with an auxiliary heater, achieving a resolution of about 9 watts.
The Key Breakthrough: Under load, motor losses show a clear dependence on rotor cage skew, reaching a maximum at a skew of 1 stator slot pitch (SSP). This contradicts theoretical models based only on the fundamental field, suggesting harmonic fields play a critical role.
The Bottom Line: The common design practice of using a 1 SSP skew may inadvertently increase motor losses and reduce efficiency. This research provides a high-precision method to quantify these trade-offs, enabling more efficient motor design.

The Challenge: Why This Research Matters for HPDC Professionals

For decades, motor designers have grappled with the challenge of accurately measuring and predicting energy losses. While methods like input-output and loss separation are widely used, they struggle to precisely quantify stray load losses, which can significantly impact overall motor efficiency and thermal performance. This uncertainty creates a major hurdle when trying to optimize critical design parameters of motor components, such as the geometry of a die-cast rotor cage or the precise air gap between the rotor and stator.

As manufacturers push for higher efficiency standards and more compact designs, the ability to predict and measure the exact losses associated with every design choice becomes paramount. Without accurate measurement, engineers are often forced to rely on empirical rules or incomplete models, potentially missing opportunities for significant performance gains. This research tackles the problem at its root by introducing a measurement technique that provides the accuracy needed to validate design choices with confidence.

The Approach: Unpacking the Methodology

To overcome the limitations of conventional methods, the researchers developed and employed a balance calorimetric method. The principle is both elegant and effective:

Motor Test: The test motor (a 5.5kW TEFV induction motor) is placed within a well-insulated enclosure, the calorimeter. It is run under the desired load conditions until a stable thermal state is reached. A constant flow of cooling air passes through the calorimeter, and the temperature difference between the inlet and outlet air is precisely measured. This temperature rise corresponds to the total heat generated by the motor's losses.
Balance Test: The motor is then disconnected from the electrical supply but is driven mechanically at the same speed to replicate friction and windage effects. A separate, adjustable resistance heater inside the calorimeter is powered up. The power to this heater is carefully adjusted until it produces the exact same temperature difference in the cooling air as was measured during the motor test.

The power delivered to the heater is therefore a direct and highly accurate measure of the motor's total electromagnetic losses (total losses less friction and windage). This "balance" approach cleverly bypasses the difficulty of measuring the specific heat and density of the air, which is a major source of error in other calorimetric methods.

The Breakthrough: Key Findings & Data

This meticulous approach yielded several crucial insights into the behavior of induction motor losses, particularly concerning the impact of rotor skew.

Finding 1: No-Load Conditions Reveal No Skew Correlation. In no-load tests, while losses varied as expected with changes in supply voltage and air gap size, the study concluded there was no meaningful correlation between losses and the amount of rotor cage skew (Abstract, Section 7.2). This is because the fundamental rotor current is almost zero under no-load conditions.
Finding 2: Load Conditions Tell a Different Story. When the motor was loaded, a clear and unexpected pattern emerged. Total electromagnetic losses increased as the skew changed from zero to 0.5 SSP, reaching a maximum value at a skew of one stator slot pitch (1 SSP). Losses then decreased as skew increased further (Abstract, Section 7.3). This is a critical finding, as a 1 SSP skew is a common design choice used to mitigate noise and torque ripple.
Finding 3: Experimental Data Challenges Theory. A theoretical model based only on the fundamental magnetic field was developed. This model suggested that losses would increase continuously with skew. The fact that the measured results show a distinct peak at 1 SSP proves that harmonic fields and other complex effects play a significant role in the dependence of losses on skew (Abstract, Section 7.9).
Finding 4: The Method is a Benchmark for Accuracy. The study repeatedly demonstrated that the balance calorimetric method is accurate, highly repeatable, and has a resolution of approximately 9.4W (Section 7.1). This establishes it as a powerful tool for validating motor designs and performance models.

Practical Implications for HPDC Products

For engineers and manufacturers involved in designing and using electric motors with die-cast components, the findings of this paper have direct, practical implications.

For Motor Design Engineers: The research strongly suggests that the conventional wisdom of using a 1 SSP rotor skew involves a direct trade-off with motor efficiency. This data empowers designers to make more informed decisions, potentially selecting a slightly different skew to optimize for lower losses and higher efficiency, especially in applications where energy consumption is critical. The findings from this research highlight the importance of considering these trade-offs during the design phase.
For Quality Control: The study noted variations in losses between supposedly identical rotors, attributing them to manufacturing tolerances and imperfections, such as the interbar impedance in the die-cast cage (Section 5.5, Section 7.3). This underscores the critical need for high-quality, consistent die-cast rotors to ensure predictable motor performance. The quality of the die casting directly impacts the final electromagnetic performance of the motor.
For R&D and Simulation Teams: The discrepancy between the fundamental field model and the experimental results provides a clear directive: accurate simulation of skewed motors must include the effects of harmonic fields. This research serves as a valuable experimental benchmark for calibrating and validating more advanced finite element analysis (FEA) and other simulation models.

Paper Details

THE ACCURATE MEASUREMENT OF LOSSES IN SMALL CAGE INDUCTION MOTORS USING A BALANCE CALORIMETRIC METHOD

1. Overview:

Title: THE ACCURATE MEASUREMENT OF LOSSES IN SMALL CAGE INDUCTION MOTORS USING A BALANCE CALORIMETRIC METHOD
Author: B.N.Shamsadeen
Year of publication: 1990
Journal/academic society of publication: University of Liverpool
Keywords: calorimeter, induction motor losses, rotor skew, air gap, balance method, stray losses

2. Abstract:

The accurate measurement of electrical machine losses using a calorimeter is described. The machine is contained within an insulated enclosure and the heat output is obtained from the temperature rise of the working fluid. For the 5.5kW TEFV induction motor tested air was used as the working fluid, the calorimeter differing significantly from that envisaged by IEC 34. It is shown how the problems associated with determining the specific heat, density and temperature of air can be overcome by employing a balance method of operation. Test results confirm that the method of loss measurement is accurate, repeatable and has a resolution of about 9W.

The calorimeter is used to investigate the variation of losses (excluding windage and friction losses) as the air gap and rotor cage skew are varied for a range of values of supply voltage and current. It is concluded that on no load the variations with voltage and air gap follow expected trends but that there is no correlation between losses and rotor cage skew. It is shown, however, that there is a dependence of losses on skew when the motor is loaded and that a skew of 1 SSP appears to produce the highest losses.

A theoretical treatment based only on the fundamental field is presented; calculations based on this model suggest that the losses will increase continuously with skew in the range considered (zero to 1.5 SSP). Comparison with the measured results which show a maximum at 1 SSP, suggests that harmonic fields and other effects must play a significant role in the dependence of losses on skew.

It is concluded that the balance calorimetric method of loss measurement is very accurate and may well make a substantial contribution to drive systems, especially those using non sinusoidal supply.

3. Introduction:

The introduction highlights the engineering trend of making electrical machines smaller and more powerful. This progress is limited by the ability to manage power losses, which generate heat. As machines shrink, thermal management becomes more critical. Therefore, having an accurate knowledge of these losses is essential for manufacturers. While prediction methods like equivalent circuits and Finite Element Methods (FEM) exist, they have limitations and cannot accurately predict total losses for all designs. The paper notes that standard measurement techniques, such as the input-output method, suffer from inaccuracies that can be significant, especially when trying to measure smaller components of loss like stray losses. The paper introduces the calorimetric method as a direct way to measure losses by measuring the heat dissipated by the machine.

4. Summary of the study:

Background of the research topic:

Accurate measurement of losses in induction motors is fundamental to improving their efficiency and performance. Existing standard methods have known limitations in accuracy, particularly for stray losses. The calorimetric method, which measures heat output directly, is recognized by standards like IEC but has been considered difficult and impractical.

Status of previous research:

Previous work by authors like Binns and Wood [14] had explored a calorimetric method different from the IEC standard, utilizing an auxiliary heater in a "balance mode." This showed promise but needed refinement. The present research builds upon that foundation to extend the method to totally enclosed machines and improve the overall system for higher accuracy.

Purpose of the study:

The primary objectives were to investigate and refine the balance calorimetric method for accurate loss measurement in small TEFV induction motors. A secondary objective was to use this highly accurate method to systematically investigate the variation in losses caused by changing key design parameters, specifically the rotor cage skew and the air gap width, under both no-load and load conditions.

Core study:

The core of the study involved designing, building, and testing a sophisticated calorimeter system. This system was then used to perform over 232 tests on a 5.5kW TEFV squirrel cage induction motor fitted with thirteen different rotors, each with a specific skew (from 0 to 1.5 SSP) and tested at various air gaps. The study meticulously segregated different components of loss and analyzed their variation with skew, voltage, and current. Finally, a theoretical model was developed to compare with the experimental results.

5. Research Methodology

Research Design:

The research was designed around a two-part experimental procedure: the "induction motor test" followed by a "balance test."

Induction Motor Test: The motor runs under a specific load and voltage inside the calorimeter until thermal equilibrium is reached. The temperature rise of the cooling air (Δt) is recorded.
Balance Test: The motor is unpowered but spun at the same speed by a DC machine. An auxiliary heater's power is adjusted until the same Δt is achieved. This heater power directly equals the motor's electromagnetic losses.
This methodology was applied across a matrix of test conditions, varying rotor skew, air gap, supply voltage, and load current.

Data Collection and Analysis Methods:

A comprehensive measurement and control system was built. This included a controlled voltage source to eliminate supply variations, a load control system to maintain constant current, and a temperature control unit for the inlet air. A microprocessor-based monitoring system logged 10 parameters every 2 minutes. Key measurements like power, voltage, current, and slip were taken with calibrated, high-accuracy instruments. DC resistance was measured by extrapolation after each test to accurately calculate stator copper losses.

Research Topics and Scope:

The research focused on a 5.5kW, 4-pole, D132S frame TEFV induction motor. The study investigated:

The effect of rotor skew (five values: 0, 0.5, 1.0, 1.28, and 1.5 SSP).
The effect of air gap (three values: nominal ± 20%, approx. 0.24, 0.30, 0.36 mm).
The effect of supply voltage at no load (rated ± 10%).
The effect of load current at load (rated ± 10%).
A comparison between single-layer and double-layer stator windings.

6. Key Results:

Key Results:

The balance calorimetric method is a highly accurate and repeatable method for measuring motor losses, with a demonstrated resolution of 9.4 W (Section 7.1).
Under no-load conditions, there is no correlation between motor losses and rotor cage skew. Losses do, however, vary with air gap and supply voltage as expected (Section 7.2).
Under load conditions, there is a distinct dependence of losses on rotor skew. The maximum losses were observed at a skew of 1 stator slot pitch (1 SSP) (Section 7.3).
For most rotors, increasing the air gap resulted in a decrease in both no-load and load losses (Section 5.4, 5.5).
A theoretical model of a skewed motor was developed. The model predicted that stator and rotor leakage reactances increase with skew, while magnetising reactance decreases. It also predicted that losses would increase continuously with skew, which conflicts with the measured results, indicating the high importance of non-fundamental field effects (Section 6.3, 7.9).
Significant variations in interbar impedance were measured between nominally identical rotors, highlighting the impact of manufacturing tolerances on motor performance (Section 5.7).

Figure Name List:

Figure 3.9- Specific heat capacity cp of air for different values of humidity and temperature.

Fig .1.1 “Open” type
Fig .1.2 “Closed” type
Figure 3.1 Overall arrangement of the calorimeter system
Figure 3.2 Isometric sketch of the calorimeter
Figure 3.3 Panel junction
Figure 3.4 Base of the calorimeter
Figure 3.5 Vertical cross section of the inlet chimney
Figure 3.6 Longitudinal and crossectional view of the duct carrying air into the calorimeter
Figure 3.7 Preheater inside the AL - box
Figure 3.8 Cooling system
Figure 3.9- Specific heat capacity cp of air for different values of humidity and temperature.
Figure 3.10- Air density depending on temperature and humidity.
Figure 3.11 Temperature measurement across the width of polystyrene at the outlet chimeny.
Figure 3.12 Position of the preheater and the main heater with the platinum resistance thermometer for measuring inlet temperature(initial design).
FIG. 4.1. LABORATORY MAINS VOLTAGE VARIATIONS
Figure 4.2 Connection diagram for the motorised buck boost system
Figure 4.3 Block diagram of the voltage regulator
FIG. 4.4. VOLTAGE VARIATIONS AT THE TEST MOTOR TERMINALS AFTER VOLTAGE REGULATOR
Figure 4.5 Schematic diagram of the load control unit.
Figure 4.6 Inlet temperature control system.
Figure 4.7 Connection diagram of the induction motor with the three watmeters and the induction regulator.
Figure 4.8 Slip measuring system
Figure 4.9 Measurement of stator winding resistance by extrapolation
Figure 4.10 DC resistance measurement
Figure 4.11 Relationship between the power input to the main heater (Pi) and temperature difference Δt
Figure 4.12 Torque measuring system
FIG. 4.13. CALIBRATION OF TORQUE MEASURING RIG
Fig 4.14 Measurement of eccentricity of rotor No.8 with stator No.1
Figure 4.15 Rotor bar and interbar resistance measurement.
FIG. 5.3 .NO LOAD LOSSES. ROTOR NO.8. SKEW=0.0 SSP
FIG. 5.18. NO LOAD LOSSES AT 90% RATED VOLTAGE
FIG. 5.19. NO LOAD LOSSES AT 100% RATED VOLTAGE
FIG. 5.20. NO LOAD LOSSES AT 110% RATED VOLTAGE
FIG. 5.35. LOAD LOSSES AT 100% RATED CURRENT

7. Conclusion:

The balance calorimetric method has been proven to be a reliable and highly accurate technique for measuring losses in small cage induction motors. The study successfully used this method to demonstrate that while rotor skew has a negligible effect on no-load losses, it has a significant and complex impact on load losses, with a peak observed at a 1 SSP skew. This finding is critical as it challenges both common design practices and simplified theoretical models. The variations in losses due to air gap and voltage followed expected trends. The research concludes that accurately measuring losses is essential for understanding the real-world effects of design changes and that the calorimetric method is a superior tool for this purpose.

8. References:

[14] - Binns, K.J., Wood, A.W."The calorimetric measurement of the losses of induction motor" Third year project, University of Southampton.
[25] - Binns, K.J., Dye, M." Effect of slot skew and iron saturation on cogging torques in induction machines" Proc. IEE, Vol. 117, No. 7, July 1970 pp 1249 to 1251.
[28] - Alger, P.L."Nature of induction motors" Gordon And Research Science Publishers 1970.
[29] - Binns, K.J., Hindmarsh,R., Short, B.B." Effect of skewing slots on flux distribution in induction machines" Proc. IEE, Vol. 118, No. 3/4, March/Appril 1971 pp 543 to 549.
[30] - Odok, A.N."Stray load losses and stray totque in induction machines" Trans. AIEE, 1958, 77,Pt. III pp 43- 53.
[32] - Waterson,D.G., Bagk,S., Diemoz,E. "Transfer of heat in electrical rotating machines, . Determination of rotor bar to bar crosspath impedance in cage induction motors " ERA Technology ,ERA report 88- 0351 issue 3, ERA Project 44-02-0213.

Expert Q&A: Your Top Questions Answered

Q1: Why is this calorimetric method better than the standard input-output method for measuring losses?
A1: The input-output method calculates losses by subtracting the measured output power from the measured input power. Because modern motors are highly efficient (e.g., >90%), the losses are a small fraction of the total power. Any small percentage error in the large input or output measurements can lead to a very large percentage error in the calculated loss value. The balance calorimetric method avoids this by measuring the losses (as heat) directly, providing a much more accurate and repeatable result with a resolution of about 9W. [Source: Abstract, Section 1.5, Section 7.1].

Q2: The study found that a rotor skew of 1 SSP produced the highest losses under load. Isn't skew supposed to be beneficial?
A2: You're right, skew is beneficial for reducing undesirable effects like cogging torque and magnetic noise. However, this research reveals a significant trade-off. While it solves some problems, a skew of 1 SSP was found to generate the highest electromagnetic losses when the motor is loaded. This suggests that designers must carefully balance the benefits of skew against the penalty of increased losses and reduced efficiency. [Source: Abstract, Section 7.3, Figure 5.35].

Q3: What is the main takeaway for me if I'm designing a new induction motor?
A3: The main takeaway is that you should not automatically assume a 1 SSP skew is the optimal choice for all designs. This research provides strong evidence that this common practice could be hurting your motor's efficiency. It highlights the importance of either using advanced simulation tools that account for harmonic fluxes or performing high-accuracy physical testing to find the optimal skew angle that balances noise, torque, and efficiency for your specific application. [Source: Section 7.3, 7.9].

Q4: The paper mentions that on no-load, there's no correlation between skew and losses. Why is the situation different under load?
A4: At no-load, the rotor current is extremely small; it's just enough to overcome friction. The magnetic field and its associated losses are almost entirely determined by the stator. Under load, a large current is induced in the rotor bars. The interaction between the stator's magnetic field and the rotor's magnetic field becomes dominant. Skewing alters this interaction along the length of the motor, creating non-uniform flux distributions and additional harmonic effects that generate extra losses which are not present at no-load. [Source: Section 7.2, 7.3].

Q5: How significant are the variations in losses between supposedly identical rotors, and what does this imply for manufacturing?
A5: The study found measurable differences in losses between rotors that were designed to be identical. For example, rotors with 1 SSP skew varied by up to 30W at the same air gap under no-load conditions (Table 5.1). The paper attributes this to manufacturing tolerances, particularly in the insulation and impedance between the die-cast rotor bars. This implies that the consistency and quality of the rotor manufacturing process, including die casting, are critical for producing motors with predictable and repeatable performance. [Source: Section 5.7, 7.3].

Conclusion & Next Steps

This research provides a valuable and precise roadmap for understanding and optimizing induction motor efficiency. The findings offer a clear, data-driven path toward improving quality by showing that established design rules, like rotor skew, have complex trade-offs that can now be accurately measured and managed.

At CASTMAN, we are dedicated to applying the latest industry research to solve our customers' most challenging die casting problems. The performance of a motor is directly linked to the precision and quality of its components, especially the die-cast rotor cage. If the issues of efficiency and performance optimization discussed in this paper resonate with your operational goals, contact our engineering team to discuss how our advanced die casting capabilities can help you implement these principles in your components.

Copyright

This material is a summary of a paper by "B.N.Shamsadeen". Based on "THE ACCURATE MEASUREMENT OF LOSSES IN SMALL CAGE INDUCTION MOTORS USING A BALANCE CALORIMETRIC METHOD".
Source of the paper: Thesis submitted to the University of Liverpool for the degree of Doctor in Philosophy, May, 1990.

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