2011年9月23日星期五

electric bicycle motor efficiency

For a long time, the electric bike motor efficiency and power as "not tell" problem, whether it is the standard narrative, or samples of goods, nameplate; whether professionals or sales, purchasing, electric bicycles and motor efficiency Power has never acknowledged and a clear definition. So to re-discuss the electric bicycle motor efficiency and power is very necessary.
Industry-standard motor design, in general there are two principle categories:
1. Fever principles:
Bldc Motor windings, permanent magnet material or conductive parts, the main structural elements (such as bearings) in the economic life period (15-20 years of industrial motors, brush allows the periodic replacement) to allow safe operation within the limits of temperature. The lesions were generally clear for the temperature (or temperature) limit, different materials have different allowable limits of temperature. Polyester fibers such as polyester film paper for the slot insulation and high-intensity polyester enameled wire electrical system is composed of Class B insulation. Allow continuous operation of the winding temperature rise limit of 80K (with resistance assay).
2 Performance principles:
Performance principles including electrical, mechanical and other properties. Usually refers to electrical power index performance (such as efficiency, power factor), speed, rate of change of speed, torque, short-term overload, change to the other. Shape and mechanical properties are generally installed size limit (as in the axial or radial size is restricted), moment of inertia, material, limit speed. Other properties are generally noise, vibration, reliability, performance / price ratio, the special environment uses.
Depending on the application, the motor can be roughly divided into two categories. One for the driver to use, and the other for the control of use. Obviously, the electric bicycle motor, should be classified as a drive motor. In the long-term practice, with the motor drive industry standards, skillfully blend these two principles as a whole. If the temperature rise and efficiency of AC motors are actually very close to the standard limit, you can hardly say that it is "heat principle" design or "performance principle" design. Temperature rise and efficiency while meeting the standard limit value of motor efficiency is usually not high. There is also a "high efficiency" motors, motor efficiency is usually higher than normal 4-7% (with power, speed and so on), its temperature is very low, a "performance principle" design. For short-term use (such as valve motor, sometimes days or even once a year to run) motor, the efficiency is usually not considered necessary to ensure the basic performance requirements of the conditions, should be used "heat principle" design. Conversely we can also say, a motor rated power is uncertain, according to "hot" or "performance" to determine, the same motor rated power to a large extent is a variable value.
Electric bicycle because of its special nature of the energy, the motor should be designed using "performance principle" design, that is designed to be as much as possible to higher motor efficiency. Temperature rise of high-efficiency motors typically not a problem.
I believe many people will say: "Then we put the design of high efficiency motors, not become it?." No! Because efficiency is an effective material (copper, magnetic materials, magnetic materials) to pay the cost, that is, the higher the efficiency, material consumption, the more the higher the cost, more severe motor. Traditional motor design has a classic theory that efficiency by 1%, effective materials to consume 10%. For the electric bicycle motor, you want to significantly improve efficiency is not merely a material cost, vehicle weight and size probably are not allowed.
We said above, the electric bicycle motor should be designed using "performance principle" design, then how to determine a standard performance?
State parameters of the body electric bike (tire tread, size, inflated), and riding the state parameters (riding speed, road conditions) complex, also can not use a standard parameter to describe the riding condition. In general, to 20Km / h constant ground speed riding, the standard load mass (75 kg) and no wind conditions, the electric bicycle power consumption 95-115W, the average power of 105W, we can consider this Electric bicycle is the "standard riding state" when the motor power. Taking into account the continuity of a weak wind and non-small slope can be riding the next (allowing reduced rate), and to have a certain dynamic performance (acceleration), the motor power 150-180W is enough. Electric bicycle ride in the city are often run on start - acceleration - constant speed - deceleration - braking, constant states are often very short. During acceleration, the motor torque or power output limit (more precisely should be the torque, because the power is also related with the speed) depends on the controller's current limit. 3 times rated current (the current standard when riding state 105W, 36V at about 3.6-4.0A, and the efficiency of the motor) to 12A (24V to 18A), can get about 3 times the rated torque. If you want to get better dynamic performance and climbing performance, requires the motor rated power of 200 W or more, this time the motor in the "standard riding state" run, may not be the most energy efficient.
A large number of calculations and practice has shown that the electric bicycle motor, the performance difference is mainly the motor speed, rather than a brush or brushless motors. Of the high-speed motor with a 2-3 level reduction agencies (commonly known as the tooth motor) and low-speed gear box with no direct drive motor (commonly known as motor without gear) to compare and use a statistical curve (20km / h, 610mm bike ) to describe (Fig. 1).
Can be seen from the curves, low-speed motor (gearless electric wheel hub) in the A and B near the high efficiency, generally up to 82-73%. In C (running speed) efficiency performance of the poor. Slow down the performance of the motor to the contrary, that in area B (standard cycling state) the efficiency of the gradual upward trend, the overall average is lower than the low-speed motor, generally between 72-78% (with reduction gear related). Performance than the low speed zone in a good motor, maximum efficiency even in C, D,. In D, the motor speed reducer motor much worse than the performance, but non-limiting area work area, so there is no practical significance. Single indicators of efficiency, low-speed electric motors compared to the wheels and slow, are not overwhelming, can have their advantages and disadvantages, is currently impossible to reject out of any party.
As the special bike run with a single "standard riding state" is not entirely reasonable. If using a statistical weighting factor to correct the efficiency curve, and take the A, B, C three under actual operating range of the envelope curve of the area to determine the "equivalent" of efficiency, it is more objective, but this would allow data treatment becomes particularly complex, difficult to operate, especially in the electric bicycle industry. Therefore, we believe that the introduction of the "standard riding state" is necessary.
Specifically, the "standard riding status" is
Power P2 = 105W (can be determined through further certification is more appropriate value)
Speed ​​V = 20km / h
V = π * D * N * 60 * 10-6 = 1.885 * N * D * 10-4 km / h
D - Wheel diameter calculation, mm (actual diameter of the tires when riding no-load than smaller diameter tires)
N - rotational speed, r / min
To 610mm electric bicycles, for example, electric wheel speed is
N = 106103.3 / D = 106103.3/610 = 174 r / min
Torque M = 9.55 * P2 / N = 9.55 * 105/174 = 5.76 Nm
The reason why the introduction of the concept of power rather than torque, because in a certain speed, different diameter wheel torque difference, while the power is basically the same.
P2 = M * N/9.55 = D / 2 * F * N/9.55 = K * F * V = constant (V is constant)
F - electric bicycle driving force (horizontal component)
M = D / 2 * F
If the standard also provides the "standard riding status" 105W and C the middle point, about 150-180W the efficiency of the two states (or the efficiency of the average) can regulate the actual operational performance of electric bicycles. 105W when the power is called the "standard riding power"; 150-180W of output power can be defined as "power rating."
There are many companies with the highest point of efficiency curve as a product of the "nominal" state is wrong. Because both the low-speed electric motor hub or deceleration, the "operating point" is not the highest point in the efficiency curve. Furthermore, the electric bicycle is a section of the run rather than a point.
Another misunderstanding is the business trying to label their products power very large, sometimes even more than the maximum possible motor power (perhaps also with the sales price is proportional to the power of it!).
As the motor output power
P2 = M * N/9.55
With the increase of the torque M (proportional to the current increase), while the rotational speed N is decreasing, so P2 has a maximum torque when the motor is less than the load torque, the motor will brake the (N = 0) , then the maximum motor torque (current maximum is also known as short-circuit), while the output power P2 = 0.
For low-speed electric wheel hub, the maximum power generally do not. For example, the rated speed of 180 r / min motor, the maximum power of 210W on the good. The higher the speed, the greater the maximum power. Rated speed 2000 r / min above the motor, the maximum power of 400W is not surprising.
The motor rated power problems. YAMAHA think that they will be set at the motor rated power of 235W (our national standard set at 240W Shipi reference to the Japanese technology Tiaojian do not know), because they may be running electric bicycles in Japan, hills, this is their national conditions. YAMAHA motor, without exception, high-speed motor (brushless brush all this) 235W rated power is not difficult to achieve. We use domestic axis drive motor (speed about 2000r/min) can reach this level, even power index is also higher than Japan. Should be noted that high-speed motor (4000r/min or so) might want to use 3 or 2 gear friction reducer (YAMAHA and the domestic production of a mini-hub has been used), the motor no-load current is high, the highest point of efficiency may exceed the actual operating area, and enter the restricted flow area, the actual power consumption of electric bicycles large, not necessarily energy efficient.

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