SAE J3109: A Practical Guide to Efficiency Testing for PWM HVAC Blower and BLDC Motor Controllers

Efficient HVAC blower control is an important piece of the automotive emission reduction puzzle. As regulatory bodies offer credits for technologies that cut greenhouse gases, manufacturers need a reliable, repeatable method to quantify efficiency gains. SAE J3109 (revised June 2023) fills that gap. This standard provides a unified framework for testing the efficiency of PWM (Pulse Width Modulated) HVAC blower controllers and BLDC (Brushless DC) motor controllers. In this article we break down the key parts of the standard, the required equipment, test procedures, and how to calculate the weighted average efficiency that agencies use for credit qualification.

Understanding SAE J3109 and Its Importance

The standard was created because legislation introduced credits for emission-reducing technologies, including the use of PWM blower controllers or BLDC controllers in place of older resistor or linear power module (LPM) designs. Without a uniform standard, evaluating controller efficiency and its impact on emissions was inconsistent. SAE J3109 establishes a usage-based overall efficiency measurement that reflects real-world blower operation. The result—a single weighted average efficiency—can be used by OEMs to qualify for credits under programs such as the EPA and NHTSA light-duty vehicle greenhouse gas standards.

The standard covers two controller types:

• PWM Controller with Brushed DC Motor – Uses a high-frequency switching signal to control power to a standard brushed motor.
• BLDC Controller with Brushless Motor – Uses electronic commutation; requires more sophisticated measurement due to non-sinusoidal currents and higher bandwidth needs.

Key Test Setup and Equipment Requirements

Accurate efficiency measurement starts with the right equipment and a carefully controlled setup. SAE J3109 specifies the following for both controller types:

Common Equipment

• DC power supply (0–20 V, 0–40 A, regulated)
• Signal generator or LIN-bus master (for LIN-based controllers)
• Voltage measurement equipment accurate to within 0.05 % of reading at 20 kHz square wave (average measurement)
• Current shunt: 100 A / 100 mV, ±0.1 % tolerance
• 4-wire measurement technique for shunt voltage drop

Additional Equipment for BLDC Controllers

• Oscilloscope: ≥20 MHz bandwidth, ≥10 MS/s sampling, ≥10 MS memory, ≥8-bit resolution, minimum 4 channels, with advanced math functions
• DC-capable current probes (≥1 MHz bandwidth, 30–50 A range) – ⚠️ Rogowski coils and current transformers are not allowed because they cannot measure DC components
• Differential voltage probes (≥20 MHz bandwidth, ≥20 V to ≤100 V)

Load Simulation

For PWM brushed controllers, you can use an actual HVAC blower with restricted airflow, or an inductive-resistive load simulator (inductance 70–200 µH, resistance adjustable to match target currents). For BLDC controllers, either a blower or a dynamometer is acceptable. The load must be set at 13.5 V input to the controller and adjusted until the target current or speed is reached.

Wiring and Ambient Conditions

• Use 3.0 mm² (12 AWG) wire, length 150 mm ± 10 mm from connector to connector
• Ambient temperature: 25 °C ± 5 °C
• Cooling airflow representative of production HVAC environment to stabilize thermal conditions
• Stabilize each test point for at least 10 minutes before recording data

Test Procedures and Calculation Methodology

PWM Controller Test Steps

1. Set input voltage to 13.5 V ± 0.05 V
2. Adjust input control signal (duty cycle or LIN) to achieve target output voltage (Table 1) ±2 %
3. Hold 10 minutes, record: input voltage/current, output voltage/current, setpoint
4. Repeat for all five test conditions shown below

* Hi = 1 A below the lower tolerance of the rated current (e.g., for a 28 ± 3 A device, Hi = 24 A).
** The weighting factors reflect estimated real-world usage at different blower speeds.

Efficiency calculation:

Efficiency = (Output Power) / (Input Power) = (V_out × I_out) / (V_in × I_in)

Weighted Average Efficiency

The single number used for credit qualification is the weighted average, combining efficiencies from all five test points:

Weighted Average Efficiency = 0.35 × η_Low + 0.22 × η_MedLow + 0.20 × η_Med + 0.12 × η_MedHigh + 0.10 × η_High

For BLDC controllers the test points are defined differently (scaled input powers and motor speeds), but the same principle applies: calculate efficiency at each point and apply the same weighting factors.

Engineering Design Insights and Best Practices

From a design perspective, the efficiency assessment is only as reliable as the measurement setup. Key insights from the standard:

• Shunt measurement matters: Use a 4-wire Kelvin connection to avoid lead resistance errors. The voltage drop across the shunt must be measured directly at the shunt terminals.
• Wire length is critical: The 150 mm harness is specified to control resistive losses; deviations change the apparent controller efficiency.
• Cooling is not optional: Without representative airflow, the controller may throttle or drift thermally, giving non-repeatable results.
• Stabilize before recording: A 10-minute soak at each condition ensures steady-state thermal and electrical conditions.
• BLDC testing demands high-bandwidth differential probes and DC-capable current probes – standard current clamps with AC-only response will miss the DC component and give incorrect power readings.

Frequently Asked Questions

Q: Can I test a PWM controller without an actual blower?
A: Yes, the standard allows an inductive-resistive load simulator with inductance between 70 and 200 µH. The resistance must be adjustable to hit the target currents listed in Table 1. Ensure the load’s inductance is suitable for your switching frequency to maintain continuous current.

Q: Why do BLDC tests require DC-capable current probes?
A: BLDC motor currents often contain a DC offset plus high-frequency components. Rogowski coils and current transformers inherently block DC. Using them would measure only the AC part, leading to a miscalculation of actual power and efficiency. Only Hall-effect or similar DC-capable probes should be used.

Q: How is the “Hi” current determined for a controller with a current rating tolerance?
A: Hi is defined as 1 A below the lower tolerance of the rated current. For example, if a controller is rated 28 A ± 3 A (i.e., 25 A to 31 A), the Hi current for testing is 25 A – 1 A = 24 A. This prevents operation at the absolute limit during the test and adds a safety margin.

Q: Do the weighting factors change for different vehicle classes?
A: No. The weighting factors (35, 22, 20, 12, 10) are fixed in the standard and represent an industry estimate of typical blower usage across a driving cycle. They apply to all light-duty vehicles. OEMs must use these factors when reporting efficiency for emission credits.

By following SAE J3109, engineers can produce test results that are defensible, repeatable, and accepted by regulatory agencies. Whether you are qualifying a new blower controller design or verifying supplier data, the standard provides the clear, detailed methodology needed for accurate efficiency assessment.