Electronics Weekly Electronics Design & Components Tech News
The choice of battery greatly affects the design of an electric two-wheeler (E2W). Lithium-iron-phosphate (LiFePO4) batteries provide a stable chemical and thermal chemistry, better safety (no thermal runaway) and lower manufacturing cost compared to Li-ion batteries. The energy density (Wh/kg) in Li-ion batteries, however, is higher than in LiFePO4 batteries, which encourages the use of Li-ion batteries in size- and weight-constrained applications. While an E2W battery typically employs a cooling method, a Li-ion battery requires more thermal management than other battery technologies because of the higher discharge rate.
Battery cooling methods
Heat generated across a battery pack is directly proportional to the discharge rate of the battery. For safe operation a cooling system must maintain external battery-pack temperature at approximately 20˚C to 40˚C and a maximum internal temperature variation of no more than 5˚C. Various cooling methods are available, including fin, air and liquid cooling.
Cooling fins work on the principle of higher power dissipation through higher surface area to increase the rate of heat transfer. The heat transfer process occurs simultaneously from battery pack to fin (conduction) and from fin to air (convection). As the surface area of the fins increases, the weight of the fins also increases. They are widely used in lower power electronics components, but can be impractical for battery-cooling applications.
In the air-cooling method the battery-to-air heat transfer occurs through convection. As air runs over the surface heat emitted by the battery pack is carried away from it. Air cooling is simple but not very efficient and it is relatively crude compared to liquid cooling. When feasible, it is preferred because it is relatively simple to implement in lower-powered electric vehicles, specifically, two-wheelers.
Liquids have higher heat conductivity and capacity than air, which makes liquid cooling efficient. Indirect liquid cooling uses a system of pipes to circulate a high heat capacity coolant and direct cooling submerges the entire battery in a very low conductivity coolant.
All methods have advantages and disadvantages. Although direct cooling achieves the best cooling performance, vehicle safety concerns have prevented its adoption, making indirect cooling the most acceptable liquid-cooling solution. Air cooling requires a high-speed fan that implements air circulation and indirect cooling requires a coolant pump driven by a high-speed motor with additional cooling fans (optional) for the cooling mechanism.
Motors for cooling
Practical battery-cooling solutions require motors to drive fans or pumps that deliver air or liquid for cooling. The performance of a motor driver for this application is characterised by noise, efficiency and startup speed. Brushless DC (BLDC) motors are generally preferred for their high efficiency, high-power density, low maintenance requirements, long lifespan and wide range of speed controls. Performance, reliability and cost factors must be weighed when choosing a BLDC motor.
During cold startup, the thermal management of the battery is critical due to higher thermal resistance, which demands fast startup motor drivers. The performance achieved by many commutation algorithms must be measured to ensure the desired response speed, power, noise and efficiency are met at all stages of operation. Protection features must be included in the motor drivers and gate controller to protect the motor and driver/controller in the event of a fault.
The noise performance of the motor depends on the commutation technique – trapezoidal can provide greater efficiency at high speeds, but sinusoidal has superior noise performance, while field-oriented control (FOC) algorithms are the fastest and most accurate method.
The overall cost and size of a system depends on the number of external passive components required for the motor driver operation. Many implementations of motor drivers require the addition of an external capacitor, which creates EMI in the form of high frequency switching noise. Mitigating EMI can require additional components and thermal management, with corresponding development time and cost penalties.
Motor driver algorithms
Sensorless startup algorithms can drive a BLDC motor from startup to full speed in typically 50ms. This ensures the fan (air cooling) or pump (indirect liquid cooling) starts quickly and receives the necessary cooling during cold crank.
Two-pulse initial position detection (IPD) provides an accurate method for fast startup of the BLDC motor in sensorless operation. The IPD algorithm ensures reliable and accurate initial position detection, requires less resolution (30°) and less detection time than other startup methods, and assists in reducing the overall startup time of the BLDC motor.
The FOC algorithm ensures a unity power factor to achieve the highest motor efficiency and low noise operation. Closed-loop speed control within the controller ensures operation at a steady speed regardless of any voltage variations in the load or line.
Motor drivers for battery-cooling fans and pumps are available with integrated control including options for an integrated power stage or a gate driver stage. Components needed for full-speed operation can be very minimal (see Figure 1).
Figure 1: Components required for the A898303 and A89307
The A89303 is a three-phase sensorless pump driver for integrated motor drivers with fast startup, which includes sensorless startup and two-pulse IPD algorithms. It can be driven at full speed with 100% duty input on the PWM pin.
The A89307 automotive FOC BLDC motor controller integrates code-free algorithms and application-specific parameters, for example, rated speed, current, speed profile and resistance.
Housing the capacitor of the internal regulator within the device bypasses the high frequency switching noise of internal digital circuitry. A spread spectrum clock further helps to spread the emission. Both can reduce the development time needed to achieve compliance with EMC standards with minimal additional components.
To protect the device and connected systems in the event of a fault protection features such as current limit, overcurrent protection/drain-to-source sensing, programmable safe braking thresholds, undervoltage lockout, overvoltage protection, lock detection, open phase protection, vibration lock detection, overspeed protection and thermal shutdown can be integrated.
Cool down, hot rods
To provide the power demanded in today’s rising E2W market designers are obliged to use Li-ion batteries because they have higher energy density. The cooling requirements that come with these batteries place a greater burden of performance on the motor drivers for battery-cooling pumps and fans. Devices that integrate purpose-built, proprietary algorithms can provide efficiency and noise performance, accurate and fast startup with protection for fast, reliable, efficient cooling systems that meet the demands of Li-ion batteries.
