The underlying concepts shared by all motors were covered in the second part of this series. As we’ve seen, every motor either generates interaction torque, reluctance torque, or a mixture of the two. The motor’s type of torque has no bearing on the way the vehicle operates. The Torque versus Speed (T vs N) curve is the crucial factor. Figure 1 depicts a typical T versus N curve for traction motors used in EVs.
The ability of motors to deliver more power or torque than they were intended for, even for a brief period of time, is one of their special qualities. Because it affects a motor’s capacity to accelerate instantly, a motor’s overload capability is a reflection of its final output performance and is crucial in transportation applications. The stalling or breakdown torque of a motor and its operating temperature both have an impact on the magnitude and duration of the overload capacity.
The motor’s breakdown torque is comparable to an internal combustion engine’s stalling torque. Magnetic saturation and significant Ohmic and Core losses are produced when the motor is operating under an overload condition because the electromagnetic parameters are substantially altered. When the motor is overloaded, the output torque quickly reaches saturation, increasing by a factor of two even while the rated current may reach three times the rated amount.
This is because the stator core of the motor achieves saturation, and the nonlinearity of the magnetic circuit causes a reduction in the air gap magnetic density.
It is clear that a high current is required to overload the motor, which causes more copper losses or Ohmic losses. The motor’s winding heats up as a result of the larger Ohmic losses. Every time the operating temperature is raised, the efficiency decreases because as the winding’s temperature rises, the copper resistance also rises. If the copper temperature is increased from 75 degrees Celsius to 105 degrees Celsius or 150 degrees Celsius, respectively, a typical motor with 90% full-load efficiency will see a 0.5% reduction in efficiency. By raising the temperature even more, such an increase in losses is known as “temperature creep” or cumulative heating.
The motor’s overloading must be carefully considered throughout the motor design phase, and the cooling method and coolant must be properly considered.
The stator of the motors is where the majority of heat is produced, hence stator cooling needs to be done properly. Convection is taken into account in stator cooling to either directly cool the rear of the stator or to cool the housing. Direct cooling designs differ in terms of flow rate, duct shape, and whether they follow an axial or helical course. The majority of the automobile industry today uses helical cooling, but it’s vital to remember that axial cooling is still being equally explored and improved.
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