The speed range of the car drive motor is often relatively wide, but recently I came into contact with an engineering vehicle project and felt that the customer’s requirements were very demanding. It is not convenient to say the specific data here. Generally speaking, the rated power is several hundred kilowatts, the rated speed is n(N), and the maximum speed n(max) of constant power is about 3.6 times that of n(N); the motor is not assessed at the highest speed. power, which is not discussed in this article.
The usual way is to increase the rated speed appropriately, so that the range of constant power speed becomes smaller. The disadvantage is that the voltage at the original rated speed point decreases and the current becomes larger; however, considering that the current of the vehicle is higher at low speed and high torque, it is generally acceptable to shift the rated speed point like this. However, it may be that the motor industry is too complicated. The customer requires that the current should be basically unchanged throughout the constant power range, so we have to consider other methods.
The first thing that comes to mind is that since the output power cannot reach the rated power after exceeding the maximum speed point n(max) of constant power, then we reduce the rated power appropriately, and n(max) will increase (it feels a bit like an NBA superstar “can’t beat Just join”, or since you failed the exam with 58 points, then set the passing line at 50 points), this is to increase the capacity of the motor to improve the speeding ability. For example, if we design a 100kW motor, and then mark the rated power as 50kW, won’t the constant power range be greatly improved? If 100kW can exceed the speed by 2 times, it is no problem to exceed the speed by at least 3 times at 50kW.
Of course, this idea can only stay in the thinking stage. Everyone knows that the volume of motors used in vehicles is severely limited, and there is almost no room for high power, and cost control is also very important. So this method still can’t solve the actual problem.
Let’s seriously consider what this inflection point means. At n(max), the maximum power is the rated power, that is, the maximum torque multiple k(T)=1.0; if k(T)>1.0 at a certain speed point, it means that it has constant power expansion capability. So is it true that the larger k(T) is, the stronger the speed expansion ability is? As long as the k(T) at point n(N) of the rated speed is designed large enough, can the constant power speed regulation range of 3.6 times be satisfied?
When the voltage is determined, if the leakage reactance remains unchanged, the maximum torque is inversely proportional to the speed, and the maximum torque decreases as the speed increases; in fact, the leakage reactance also changes with the speed, which will be discussed later.
The rated power (torque) of the motor is closely related to various factors such as the insulation level and heat dissipation conditions. Generally, the maximum torque is 2~2.5 times the rated torque, that is, k(T)≈2~2.5. As the motor capacity increases, k(T) tends to decrease. When the constant power is maintained at the speed n(N)~n(max), according to T=9550*P/n, the relationship between the rated torque and the speed is also inversely proportional. So, if (note that this is the subjunctive mood) the leakage reactance does not change with the speed, the maximum torque multiple k(T) remains unchanged.
In fact, we all know that reactance is equal to the product of inductance and angular velocity. After the motor is completed, the inductance (leakage inductance) is almost unchanged; the motor speed increases, and the leakage reactance of the stator and rotor increases proportionally, so the speed at which the maximum torque decreases is faster than the rated torque. Until n(max), k(T)=1.0.
So much has been discussed above, just to explain that when the voltage is constant, the process of increasing the speed is the process of kT gradually decreasing. If you want to increase the constant power speed range, you need to increase k(T) at the rated speed. The example n(max)/n(N)=3.6 in this article does not mean that k(T)=3.6 is sufficient at the rated speed. Because the wind friction loss and iron core loss are greater at high speeds, k(T)≥3.7 is required.
The maximum torque is approximately inversely proportional to the sum of the stator and rotor leakage reactance, that is
1. Reducing the number of conductors in series for each phase of the stator or the length of the iron core is significantly effective for the leakage reactance of the stator and rotor, and should be given priority;
2. Increase the number of stator slots and reduce the specific leakage permeance of the stator slots (ends, harmonics), which is effective for the stator leakage reactance, but involves many manufacturing processes and may affect other performances, so it is recommended to be cautious;
3. For most cage-type rotors used, increasing the number of rotor slots and reducing the specific leakage permeance of the rotor (especially the specific leakage permeance of the rotor slots) is effective for the rotor leakage reactance and can be fully utilized.
For the specific calculation formula, please refer to the textbook “Motor Design”, which will not be repeated here.
Medium and high-power motors usually have fewer turns, and slight adjustments have a great impact on performance, so fine-tuning from the rotor side is more feasible. On the other hand, in order to reduce the influence of frequency increase on core loss, thinner high-grade silicon steel sheets are usually used.
According to the above idea design scheme, the calculated value has reached the customer’s technical requirements.
PS: Sorry for the official account watermark covering some letters in the formula. Fortunately, these formulas are easy to find in “Electrical Engineering” and “Motor Design”, I hope it will not affect your reading.
Post time: Mar-13-2023