انتخاب ولتاژ مناسب برای یک موتور dc بستگی به نیازمندیهای مکانیکی پروژه از قبیل سرعت و گشتاور مورد نظر همچنین مشخصات فنی الکتروموتورهای موجود در بازار دارد. برای مثال در یک موتور dc سرعت موتور رابطه مستقیم با ولتاژ اعمالی بر آن دارد، بنابراین ولتاژ مناسب باید با توجه به سیستم ولتاژی در دسترس، سرعت مطلوب و جداول سرعت ارائه شده توسط سازنده موتور انتخاب شود. واضح است که الزامات فنی – اقتصادی کارکرد در هر رده ولتاژی ویژگیهای خاص خود را دارد که می باید در فرآیند بهتریمن انتخاب مد نظر قرار گیرد. از نظر فنی باید دید که کدام سطح ولتاژی با توجه به سرعت و گشتاور بار مورد نظر بیشترین انعطاف را در اختیار طراح قرار می دهد. مقاله زیر بطور مشروح به برخی مفاهیم مرتبط با موضوع می پردازد.
Selecting a DC motor for a particular application can be a rather involved process and should be done in close consultation with MicroMo’s applications engineers. However, it is often useful to be able to "ballpark" a motor selection on one’s own. A few rules relating to the physics and the practical application of motors should be kept in mind:
The major constraint on motor operation is thermal in nature. The heat a motor must dissipate can always be calculated as follows:
P= I 2
Heat dissipated= current through the motor squared, multiplied by the terminal resistance.
The current through a motor is solely determined by the torque the motor produces. Current and torque are related by the torque constant of the motor.
I = M o/ k M
Current through motor = torque produced divided by the torque constant
•The constant current operation of a DC motor produces constant output torque regardless of speed.
•Given a constant load (i.e. torque) the speed of a motor is solely dependent on the voltage applied to the motor.
•Power is the product of speed and torque. The maximum power of a DC motor is produced at the operating point that is defined by operation at half the no-load speed and half the stall torque. Seldom will a motor be operated at maximum output due to thermal considerations.
•The general rule of thumb for operation of a DC micromotor is to operate the motor at approximately 90% of its no-load speed and from 10% to 30% of its stall torque. This is also the motor’s most efficient area of operation.
•For use with gearing, the motor should be selected for the minimum speed practical by choosing a motor with higher voltage ratings than the available voltage supply. This will result in lower noise generation and better life characteristics.
•For DC motors operated at a constant voltage, the speed and torque produced are inversely related. The higher the torque produced, the lower the speed of the motor will be.
•Other factors, of course, enter into the selection of an appropriate motor. Such factors may include size, environmental conditions, weight, required life, etc. As an example, assume the following application parameters:
Initial Motor Choice Parameters:
•Available voltage (V) = 20 Volts DC
•Output torque required (M) = .425 oz-in
•Output speed required (n) = 5,000 rpm
•Minimum physical size is desirable
•Ambient Temperature= 22°C
Given these parameters, it is unlikely that a standard catalog motor will fulfill all parameters simultaneously since they are not independent. The selection process consists of finding the best fit.
The motor should be rated at least 1.5 to 2 times the desired output power in relation to its maximum output power (at nominal voltage). A motor with approximately 2.4 to 3.2 Watts maximum output power should suffice. Referring to the MicroMo catalog, the smallest motor to achieve this power rating is the 1331 series (13mm diameter x 31mm long). The 24 Volt version is closest to the desired operating voltage (20 volts). To find the no-load speed, a good first approximation is simply to ratio the voltages and speeds.
Since our desired speed is 5,000 rpm, this represents only 53% of the no-load speed (at 20 Volts). Unless size is of paramount importance, this motor (1331T024S) is not a great choice even though it can provide the power required. The next selection from the catalog which meets the power requirements is the series 2230 motor (22mm diameter and 30mm long) and the selection would again be the 24 Volt version (2230T024S). The pertinent data for this motor is as follows:
•n0= 9,000 rpm (at 24 Volts)
•I0= 0.005 A
•R= 50 Ohms
•KM= 3.59 oz-in/A
•Po= 2.88 Watts
The approximate no-load speed at 20 Volts would be:
The desired speed is 67% of the no-load speed, and even though its not quite 70%, it is worth continuing the selection process.
The current through the motor will be the sum of the load current and the no-load current, where:
•Im= current through the motor
•I= current due to load
•M= desired torque
•I0= no-load current
•km= torque constant
Usually a motor selection involves many more factors than are presented here. This method, however, provides the designer with a starting point for the selection of motors for a particular set of application parameters.