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Electric Starting Motor Test Procedure – SAE J544 Guide
SAE J544 is the definitive standard for testing electric starting motor output performance and plotting its characteristic curves. This recommended practice also provides a graphical method to determine engine cranking speed using the motor performance curve and an engine demand curve. Developed and maintained by the SAE Cranking Motor Standards Committee, the standard is essential for engineers validating starter motor applications or designing new systems.
Testing Methods and Setup
The starting motor must be mounted in a test stand as shown in the standard. Torque can be measured either directly at the motor axis (using a special test end frame for larger motors) or at the torque-loading point. It is critical to note on the performance curve which method was used and whether inertia effects have been accounted for.
Two test methods are recognized:
- Method A (Discrete Points): The motor is run at several discrete torque loads. Voltage, torque, current, and speed are recorded at each point. Sufficient cooling intervals between points must be allowed to ensure temperature changes remain negligible. The ambient test temperature must be noted on the curve.
- Method B (Continuous Recording): The motor operates continuously while torque is increased from no load to stall. Plotting equipment records the parameters automatically. The loading rate must be slow enough to avoid significant temperature change.
⚠️ Common Mistake: Skipping or shortening cooling intervals in Method A can introduce temperature-related errors, resulting in performance curves that do not reflect actual motor behavior.
Instrumentation must meet these accuracy requirements:
- Voltmeter and ammeter: ±1% of full scale (full scale not to exceed max reading by more than 50%)
- Speed and torque sensors: ±1% of actual reading
Terminal Voltage Curves and Selection
Proper selection of the terminal voltage curve is vital. The standard provides recommended curves for different battery voltages and duty ratings. The selected curve must not exceed the motor manufacturer’s recommendation.
| Curve Name | Open Circuit Voltage (V) | Voltage at Specified Current |
|---|---|---|
| 24V Heavy-Duty | 24 | 16 V at 1000 A |
| 24V Standard-Duty | 24 | 12 V at 600 A |
| 12V High Output | 12 | 10 V at 1000 A |
| 12V Extra Heavy-Duty | 12 | 8 V at 1000 A |
| 12V Heavy-Duty | 12 | 6 V at 1000 A |
| 12V Medium-Duty | 12 | 6 V at 600 A |
| 12V Standard-Duty | 12 | 6 V at 400 A |
| 6V Standard-Duty | 6 | 2 V at 800 A |
If a solenoid is used for meshing, the applied voltage and total current draw must include the solenoid and be noted on the performance curve.
Determining Engine Cranking Speed
Once the motor performance curve is obtained, it can be combined with an engine demand curve (per SAE J1253) to estimate the cranking speed using a graphical overlay. The intersection of the motor torque-speed curve and the engine demand curve indicates the expected cranking speed.
🔍 Engineering Design Insight: For accurate application-level predictions, correct the motor performance curve for the actual operating temperature and terminal voltage expected in the vehicle. Always consult the motor manufacturer for correction factors.
Temperature and voltage corrections are necessary because motor performance varies significantly under low-temperature cranking or voltage drop conditions.
Frequently Asked Questions
What are the two test methods in SAE J544?
Method A uses discrete torque points with cooling intervals, while Method B uses continuous loading from no load to stall with automated recording. The choice depends on available equipment and desired data resolution.
How do I select the proper terminal voltage curve?
Choose the curve from Table 1 that matches your system voltage (6 V, 12 V, or 24 V) and the motor’s duty rating (e.g., heavy‑duty, standard‑duty). Ensure the selected curve does not exceed the manufacturer’s maximum recommended voltage.
Why is temperature control important during testing?
Motor performance is temperature‑sensitive. Uncontrolled heating during testing can skew data, leading to inaccurate torque and speed readings. Proper cooling intervals (Method A) or slow loading rates (Method B) prevent this.
How is cranking speed determined using performance curves?
Plot the starting motor’s torque‑speed curve and the engine’s demand curve (from SAE J1253) on the same axes. The intersection point gives the expected cranking speed under the tested conditions.
