When choosing a DC motor, you need to determine it based on the load requirements, working environment, and power supply conditions. For example, the voltage range is usually 12-48 volts, and the power selection must meet the load requirements. The current calculation formula is power/voltage. When the load is large, choosing a 48-volt motor can reduce energy loss and improve efficiency by about 15%.
Determining Speed
In industrial production lines, conveyor belts are usually required to maintain a running speed of 2 meters per second, and the motor speed is usually set at 1500 to 3000 revolutions per minute. If a production line needs to run continuously for 16 hours a day, and a 1500RPM motor is selected, its power consumption may be 2.5 kilowatts, and the annual operating cost is about RMB 21,900 (calculated at RMB 0.6/kWh). Selecting a 3000RPM motor and adjusting the transmission ratio can save nearly 10% of electricity costs per year.
If a 50 kg load needs to be driven, the output torque of the motor must reach the appropriate value. A motor rated at 500 watts has a rated output torque of about 3.18Nm at 1500RPM; when the speed drops to 750RPM, the torque can increase to 6.36Nm. 75% of equipment does not correctly match the load characteristics when selecting the motor speed, resulting in performance problems or high maintenance costs.
Brushless DC motors can typically achieve higher speeds, up to 6000RPM, while maintaining up to 85% efficiency, while brushed motors experience significantly increased brush wear above 3000RPM, resulting in a shortened lifespan. Under continuous high-load use, the average lifespan of a brushed motor is only 2000 hours, while a brushless motor can reach more than 10,000 hours.
In mines, equipment needs to operate in conditions of high humidity (above 90%) and high dust. It is recommended to use a low-speed motor with a speed of 750 to 1000RPM, with an additional dust seal design. Its failure rate is proportional to speed, with a 12% increase in failure probability for every 500RPM increase in speed.
In industrial robotic applications, the speed of the end effector usually needs to be kept in the range of 100 to 300 RPM to ensure that the position error does not exceed 0.1 mm. Some high-end brushless motors can control speed fluctuations within ±0.05% through closed-loop control technology.
A certain device needs to achieve an output speed of 50RPM, while the rated speed of the motor itself is 3000RPM, and a 60:1 reduction ratio transmission device needs to be designed. If a higher speed motor (such as 6000RPM) is selected, the complexity and cost of the transmission device will increase significantly.
A factory that produces electronic components replaced a higher-speed motor and increased production efficiency by 15% by optimizing the transmission system, thereby increasing annual profits by 120,000 yuan. The initial investment increased by 30,000 yuan, and the payback period was about 3 months.
When the elevator is running, the motor needs to switch smoothly between low-speed starting, high-speed operation, and low-speed stopping. Some modern elevator systems use brushless DC motors with a speed control range of 0 to 1500RPM and an accuracy of up to 0.1RPM. Each elevator can save about 5,000 yuan in electricity bills each year.
Determining Torque
Torque is usually measured in Newton meters (Nm). A motor with a rated torque of 2Nm drives a piece of machinery that requires 1.5Nm. If the equipment requires more torque than the rated value, reaching 2.5Nm, the motor may be overloaded and its operating efficiency may be reduced by more than 30%.
In actual applications, a machine that needs to lift a 100kg load at a height of 2 meters can calculate the required torque using the formula: Torque = Force × Radius. Assuming the roller diameter of the lifting device is 0.1m, the required torque is 98.1Nm.
In the logistics industry, conveyor belt systems need to operate continuously to carry packages weighing up to 50kg, and the torque required is generally 5-15Nm. If peak loads are not considered in the system design, the motor may stall during peak traffic. Its failure rate is more than 40% higher than normal design, and the repair cost is higher.
Starting torque is the minimum torque required for the motor to start running from a standstill, which is usually greater than the rated torque. A motor with a rated torque of 10Nm may need to have a starting torque of 20Nm. The torque requirement at startup may be two or three times that of normal operation.
A robot arm that needs to lift a 5kg object may have a torque requirement that increases from 15Nm at rest to 30Nm when in motion, and then falls back to 10Nm when unloaded.
Some DC motors are rated at 1.5kW, and the relationship between their torque and speed follows the formula: power = torque × angular velocity. If the speed is 1500RPM, the torque is about 9.55Nm. If a higher torque of 15Nm is required, the speed must be reduced to 1000RPM, or a higher power motor must be selected.
Many companies use redundant design and select motors with a rated torque 20%-30% higher than the actual demand. In the machining center of the manufacturing industry, a motor with a rated torque of 100Nm is often used to drive a device with a demand of 70Nm, which can extend the life of the motor by an average of more than 40% and reduce maintenance costs by 20%.
In a motor operating in a low temperature environment (below -20°C), the lubrication system may increase friction due to the temperature drop, thereby increasing the required torque. Conversely, in a high temperature environment (above 50°C), the expansion of the material may cause the mechanical parts to have greater resistance to operation.
In one study, two motor configurations were compared, one was a motor running at full load (close to 90% of the rated torque) and the other was a motor running at light load (load below 50% of the rated torque). The motor running at full load was more efficient and consumed about 15% less energy.
Determining Voltage
The rated voltage of DC motors ranges from 12V to 48V, and the specific choice depends on the design requirements of the device. Small portable devices such as electric scooters often use 24V motors. A 24V, 250W motor with a 10Ah lithium battery can support the scooter to travel about 30 kilometers at a speed of 20 km/h.
48V or even higher DC motors are often used in industrial scenarios. A 1500W motor uses 31.25A at 48V, while the current rises to 62.5A at 24V. Using a 48V system can reduce power loss by about 15% per year compared to a 24V system, while reducing cable procurement costs by 20%.
In the automotive industry, many automotive motors such as wiper motors and seat adjustment motors are designed with 12V. The typical power of a wiper motor is 50W, and the current is about 4.2A at 12V. If it is replaced with a 24-volt motor, additional changes to the design of the vehicle’s electrical system are required, and the cost may increase by more than 30%.
Mobile platforms in the robotics industry usually use 24-volt motors, but when the load demand is large, the voltage is increased to 48 volts to meet the heavier load demand. When the load of the mobile robot increases from 50 kg to 100 kg, the 48-volt motor can maintain efficiency above 80%, while the efficiency of the 24-volt motor will drop to 65%.
A motor with a rated voltage of 24 volts and a rated speed of 3000 RPM will drop to about 2250 RPM if the voltage is reduced to 18 volts. In cutting equipment in the construction industry, the use of 48-volt motors can increase cutting efficiency by 20% and reduce material waste.
Motors below 50 volts are considered to be in the safe voltage range, so many household and portable devices such as power tools and children’s toy electric cars use 12-volt or 24-volt motors. About 70% of home users prefer low-voltage power tools.
In industrial equipment, when using 96-volt or higher DC motors, additional overvoltage protection devices and isolation switches are usually required. These additional devices may increase the overall system cost by 10% to 20%.
For equipment operating at extremely low temperatures, the voltage of lithium batteries may drop by 10%-15% in an environment of -20 degrees Celsius. Many polar equipment will choose motors with higher rated voltages, 72-volt or 96-volt motors, to ensure stable power output even when the battery voltage fluctuates.
Some smart agricultural vehicles use 24V motors. The speed of a 24V motor can be increased to 3200 RPM to enhance the operating ability of a robot in weeding or harvesting tasks. This increase in speed can improve efficiency by about 15% in these applications.
Consider size
The size of the motor is determined by its outer diameter, length and weight. A medium-sized DC motor with an outer diameter of 80 mm and a length of 150 mm, which is usually suitable for robotic platforms or medium-sized mechanical equipment, can be rated at 750 watts and weigh about 3.5 kg.
In industrial assembly lines, equipment designs usually reserve specific installation spaces. A motor with a diameter of 120 mm requires a larger installation space than a motor with a diameter of 90 mm, which may result in an increase in the device housing by more than 10% and an increase in material costs by about 15%.
Electric toothbrushes use small DC motors with a diameter of less than 30 mm and a length of no more than 50 mm, which usually weigh less than 150 grams. Such motors usually have a power of about 2 watts and a speed of up to 30,000 rpm. If the application scenario requires high power and torque of the motor, small motors may not meet the needs.
A motor with a length of 300 mm may have a surface area of 0.1 square meters, which is about 25% higher than the heat dissipation capacity of a motor with a length of 150 mm. In high-power application scenarios, industrial compressors or electric vehicle drive systems, motors that are too small may cause inefficiency due to insufficient heat dissipation, or even overheating failure.
In the drone industry, a common drone DC motor has an outer diameter of 22 mm, a length of 20 mm, and a weight of only 50 grams. Its rated power can reach 150 watts and its efficiency exceeds 85%. It can provide about 3 watts of output power per gram of weight, which is very suitable for aviation applications.
In industrial robot applications, joint drive motors need to withstand loads of up to 50 kg, with a diameter of 150 mm and a length of 300 mm. The motor has a rated torque of up to 100 Nm and weighs more than 10 kg.
In high humidity or dusty environments, it may be more appropriate to choose an enclosed motor. Such motors are usually larger in size and require additional housing and sealing design. A motor suitable for mines may be 20%-30% larger than an ordinary industrial motor and weigh about 5 kg more. Extend motor life by about 50%.
In large industrial equipment, larger motors are usually designed with easily removable housings and larger terminals for easy repair and replacement of parts. The average motor downtime can be reduced by 30%, saving the company about 50,000 yuan in maintenance costs each year.
A motor with a diameter of 100 mm costs about 20%-30% more than a motor with a diameter of 70 mm. Larger motors generally have higher efficiency and longer life. If the application scenario has high requirements for efficiency and reliability, choosing a slightly larger motor may be a more cost-effective solution.
Performance considerations
A motor rated at 1000 watts can convert 1 kilowatt of electrical energy into mechanical energy per hour when fully loaded. If the load demand is 1200 watts and the selected motor can only provide 1000 watts, it will cause overload operation, a 10%-20% drop in efficiency, and may even cause motor damage. It is generally recommended to choose a power margin that is 20%-30% higher than the actual demand.
A motor with an efficiency of 90% consumes only 1.11 kilowatts of electrical energy when outputting 1 kilowatt of power, while a motor with an efficiency of 80% consumes 1.25 kilowatts of electrical energy. For example, if it runs for 8 hours a day and 300 days a year, a high-efficiency motor can save about 400 yuan in electricity bills each year (calculated at 0.6 yuan per kilowatt-hour of electricity). The saved electricity bills can recover the initial investment within 2 to 3 years.
A motor with a torque of 10 Newton meters can drive a load equivalent to 100 kilograms at rated speed. If the load demand is 15 Nm and the motor cannot provide enough torque, the device may freeze or stop running. More than 25% of device failures are caused by insufficient torque performance of the motor.
Robot motors need to respond quickly to signals and complete acceleration or deceleration operations in a short period of time. Some high-performance DC motors can accelerate from zero to 2000 rpm in 0.2 seconds, while ordinary motors may take 0.5 seconds or even longer.
A motor with 85% efficiency usually has a temperature rise of about 40 degrees Celsius, while a motor with 90% efficiency only rises to 30 degrees Celsius. For every 10 degrees Celsius increase in temperature, the insulation life of the motor may be shortened by 50%.
In medical devices or laboratory instruments, the noise of the motor should be controlled below 50 decibels. Using a brushless DC motor can reduce noise by about 15 decibels and reduce vibration amplitude by about 20%, significantly improving the user experience.
A motor that supports regenerative braking can recover 20%-30% of the kinetic energy as electrical energy when decelerating, which can be used to recharge or drive other devices. An electric car can recover about 5% of the energy each time it brakes.
Some high-performance motors are designed to last up to 20,000 hours, while ordinary motors usually have a lifespan of only 8,000-10,000 hours. For industrial assembly lines or server cooling systems, choose equipment that runs 24 hours a day, and motors with longer life can reduce maintenance downtime by more than 50%.
Standard industrial motors can usually withstand 150% overload, while some high-performance motors can have an overload capacity of up to 200%. This design allows the motor to handle excess loads for a short period of time without causing damage or overheating, and is particularly suitable for applications such as cranes and heavy machinery.
In the aerospace field, motors need to have high power density and high temperature resistance. A high-performance motor can output about 2 kilowatts of power per kilogram of weight, while ordinary motors have a power density of only 1 kilowatt/kilogram.
Duty cycle
The duty cycle directly affects the life and performance of the motor. A motor that runs for 2 minutes and then rests for 8 minutes has a duty cycle of 20%. Such low duty cycle motors are usually suitable for intermittent working scenarios, such as small motors in access control systems or vending machines.
Industrial assembly lines usually require a duty cycle close to 100%. A motor designed for continuous operation has a more durable structure and uses higher quality materials inside to reduce heat and wear generated by long-term operation. Low duty cycle motors have a failure rate that increases by more than 50% under continuous operation conditions.
A 5 kW DC motor may have a duty cycle limited to 50%. If a higher duty cycle is required, additional heat dissipation devices such as fans or liquid cooling systems must be added. Increasing the motor’s duty cycle to more than 80% is suitable for high-load, high-frequency use scenarios.
A micro DC motor used in medical equipment may have a duty cycle of 10 seconds running and 50 seconds rest, with a duty cycle of only 16.7%. If the working time is increased under the same load conditions, the temperature rise of the motor may exceed its design range, resulting in performance degradation or permanent damage.
The drive motor of an electric vehicle has a higher workload when accelerating and climbing, and a lower load when cruising on a flat road. This variable load condition causes the actual duty cycle of the motor to be less than 100%, usually between 60% and 80%. By optimizing the duty cycle of the motor, the efficiency can be improved by about 10% and the driving range can be extended by 5%-8%.
Motors with low duty cycles are usually smaller, lighter and less expensive. For example, a motor with a design duty cycle of 25% may cost 30%-50% less than a motor of the same specification with a 100% duty cycle.
In high temperature environments, the heat dissipation efficiency of the motor decreases and the duty cycle may need to be reduced accordingly. A motor with a rated duty cycle of 50% operating in an environment of 40 degrees Celsius may have an actual duty cycle of 35%-40%.
In industrial hydraulic pump systems, motors need to drive loads of up to 500 Nm and are usually designed for a duty cycle of 75%-100%. If a motor with insufficient duty cycle is selected, it may overheat under high load conditions. Failures caused by incorrect duty cycle selection account for more than 20% of the total equipment failures.
If the actual operating conditions of a motor with a design duty cycle of 50% require a duty cycle close to 100%, its maintenance frequency will increase significantly. The standard maintenance cycle is once every 1000 hours of operation, but under overload conditions, it may need to be shortened to once every 500 hours of operation.
Some equipment is designed with a low load at the beginning, and the duty cycle only needs 50%, and the equipment may need to run continuously for a long time. If the motor selected at the beginning cannot support a high duty cycle, the entire motor system may need to be replaced later, resulting in an additional cost increase of more than 30%.
