DC motors excel in precise speed control, achieving ±0.01% accuracy, ideal for robotics. AC motors, with a lifespan exceeding 20 years and 95% efficiency, dominate industrial use. DC requires regular brush replacements, while AC offers lower maintenance, reducing lifetime costs by up to 50%.
Differences in Starting and Speed Regulation
The starting torque of a DC motor is normally 150-200% of rated torque. For instance, for a DC motor with rated torque of 50 Nm, it can support 100 Nm at startup. On the other hand, the starting torque of an induction motor is usually about 75-100% of the rated torque, which may cause a loss in efficiency by about 20% on production lines.
The DC motors have a speed regulation range that can go to the extent of 1:1000, or the low speed can be as low as 0.1% of the rated speed. As a specific example, a motor with a rated speed of 3000 RPM may achieve a minimum speed accuracy of 3 RPM. In contrast, even with advanced frequency converters, the AC motors’ speed regulation range is between 1:100 and 1:300.
Generally speaking, the efficiency for small power DC motors -less than 5 kW– is comparatively high, about 85% to 90%, while similar AC motors are about 80% to 85% efficient. In contrast, in high-power applications, say above 100 kW, the latter can have an efficiency exceeding 95% or so. AC motors dominate more than 70% of the global industrial motor market.
The carbon brush replacement cycle of the DC motor is generally around 1,000-3,000 hours, accounting for about 5%-10% of the total price of the whole set of equipment. Taking a DC motor at 20,000 yuan as an example, its annual maintenance cost can reach as high as 2,000 yuan. However, the maintenance cycle for AC motors, especially for brushless permanent magnet synchronous motors, reaches between 5 to 10 years, and the maintenance cost is almost zero.
While a DC motor may have its rated current equal to 2-3 times of the starting current, AC motors have it equal to as much as 5 to 7 times its rated value. As such, an AC motor that has been rated for 50A has starting current as high as 350A.
Efficiency in high temperatures is as such that every rise of temperature by 10°C lowers the DC motor’s efficiency by 2%-3%. For example, at a high temperature of 40°C, the efficiency of a DC motor drops from 90% to 84%, while the efficiency of an AC motor decreases by only 1%-2% in high-temperature environments. A chemical plant once replaced DC motors with AC motors and reported that the annual energy savings reached about 8%, or 300,000 yuan in electricity costs.
The size of a DC motor is on average 15-30% larger compared to AC motors in the same power range. For example, a 10 kW DC motor would weigh 80 kg, whereas an AC motor can have a weight of just 65 kg with the same power.
Within the small power scope, say below 100 watts, the purchase price of a DC motor is usually lower by 10 to 20% from the AC motor. But for power above, say 10 kW, for instance, the price may be 30,000 yuan for a 20 kW DC motor, while that of an AC motor with the same specification may cost only 20,000 yuan.
Generally speaking, a DC motor will respond within 0.1-0.5 seconds from 0 to full speed, while the response of an AC motor, even with a frequency converter, is around 0.5-1 second. Such a high-speed conveyor belt driven by a DC motor can raise hourly transport efficiency by 15%-20%.
Noise and Vibration Comparison
The average noise level for a DC motor operating at rated power is about 60-70 decibels, whereas for an AC motor, it may be as high as 75-85 decibels. The noise of a 5 kW DC motor is about equal to the loudness of an ordinary conversation, while that of an AC motor of the same power may be comparable to city traffic.
In a BLDC motor, the vibration frequency is relatively stable between 10-20 Hz, but for AC motors, especially induction motors, this may be as high as 50 Hz or more. In high-precision production lines using AC motors, machining errors due to vibration can increase by 0.01 mm, while additional vibration dampening devices increase the equipment cost by some 15%-20%.
Regular replacement of carbon brushes, about every 1,000-3,000 hours can reduce the amplitude of vibration by an amount of 30%-50%. The variation in load condition of an AC motor goes up to about 20% of its capacity; the vibration amplitudes doubled, whereas it increases by about 30% in DC motors.
At a frequency of 60 Hz, an AC motor generates noise of up to 5-10 decibels. DC motors generate less electromagnetic noise. A typical noise level generated by wind turbines with DC motors is 65 decibels, whereas that for AC motors is more than 75 decibels.
It may reach an increase in noise level of about 10%-15%, with increased vibration amplitudes of up to 20%-30% for AC motors in service beyond five years. On the other hand, in the case of DC motors, where carbon brushes are replaced periodically along with bearing lubrication, the change in noise and vibration is confined only to 5-10% variation.
For instance, once an automobile manufacturing company replaced AC motors with DC motors in the paint shop, noise came down from 78 decibels to 65 decibels, the amplitude of vibration by over 50%, thereby saving them approximately 500,000 yuan per year in annual maintenance cost.
Owing to their high efficiency, from 85% to 95% of DC motors, energy waste that might further contribute to extra vibration and noise is reduced. For instance, considering a 10 kW DC motor, only 8 kWh of energy consumed per hour when working, an AC motor of the same power may consume up to 9 kWh.
The noise in DC motors increases by 5 decibels under a full load, that is, 100% of rated power, while for AC motors, it increases by about 15 decibels. The amplitude of the vibration increases twice as much in an AC motor, for which additional dampers are installed at about 200,000 yuan.
Under humid or dusty conditions, the vibration of DC motors may increase temporarily by about 15%. Under similar conditions, the vibration amplitude of AC motors may permanently increase by 30%-50%.
Suitable Application Scenarios
Industrial robots usually use brushless DC motors that can achieve high precisions of speed regulation at ±0.01%. Using a DC drive, a robotic arm may realize 500 motions in one minute with an error of just 1 mm, while a similar piece of equipment using an AC drive would show deviations of over 3 mm.
It means a 5 kW DC motor can complete a start from zero to full speed in 0.2 seconds, but an AC motor of the same specification needs 0.8 seconds or longer. In this regard, once a logistics center replaced all the motors on the sorting lines with DC motors, sorting efficiency increased by 15%, processing an additional 30,000 parcels per day and creating extra revenue of about 50 million yuan every year.
The MTBF for AC induction motors can be as high as 100,000 hours, while the MTBF for DC motors is approximately 20,000-30,000 hours.
Considering an electric vehicle fitted with a 30 kW DC motor, the peak torque can reach up to 250 Nm, while for an AC motor of the same power, it is capable of providing only about 200 Nm of torque. This allows the electric vehicle to accelerate from 0 to 50 km/h in 3 seconds.
It can be noted that DC motors applied to medical equipment produce, on average, less than 60 decibels of noise, while AC ones can easily reach more than 75 decibels. With a 15-decibel noise reduction, the patient compliance increased by 20% and the patients scanned daily increased by 30% efforts.
AC motors in mining-industry hoisting equipment with a power of 100 kW or above can reach over 95% efficiency, while the efficiency of DC motors is about 90%. A mining enterprise claims that on replacing the former with the latter, it saves about 2 million yuan electricity every year, while increasing the operational stability of the device by 30%.
AC motors are more suitable for high-humidity or high-dust working environments. For instance, AC motors in the driving devices of conveyor belts at cement factories can last up to 8 years averagely, while DC motors have a lifespan of only 5 years under the same conditions. This increases durability and reduces the frequency of equipment replacement, saving around 200,000 yuan in annual maintenance.
A DC brushless motor of 1 kW rated power weighs just 1.5 kg, but the AC motor of the same power can weigh up to 2.5 kg.
Their application in some stereo sets can vary the speed in milliseconds. It reduces audio errors less than 0.001 seconds.
Inverter-controlled AC motors can achieve a speed regulation range of 1:100 and maintain over 90% efficiency under rated load. An automotive parts company increased production efficiency by 12% with the deployment of AC motor-driven assembly lines, saving approximately 500,000 yuan annually in energy costs.
Price Differences
The current average market price for a 100-watt DC motor is around 300 to 500 yuan, with its AC equivalent reaching 400 to 600 yuan. Correspondingly, a 10 kW DC motor prices between 15,000 and 20,000 yuan, and an AC equivalent would also hover at the same price range of 12,000 to 15,000 yuan.
Most of them need to replace carbon brushes regularly, every 1,000-3,000 hours; a single replacement costs about 500 to 1,000 yuan. For devices that work 8 hours a day and need replacement every 2,000 hours, their annual maintenance cost would be as high as 5% to 10% of the price of the equipment. AC motors, especially induction motors, have very low maintenance needs. They can work for many years continuously without maintenance.
In a ten-year running life, the total cost for a 20 kW DC motor may amount to over 100,000 yuan, while over 30 percent goes to maintenance costs. The same power AC motor would be about 70,000 yuan in total cost, and less than 10 percent goes to maintenance.
The energy efficiency of DC motors is 85-90%, which is slightly lower compared to AC motors with an energy efficiency of 90-95%. Assuming a rated power of 10 kW, the power consumption of a DC motor is 11.8 kWh per hour, while for an AC motor, it will be 10.5 kWh. Assuming the cost of electricity is 0.8 yuan/kWh, the power consumption of the DC motor operating 8 hours a day amounts to 75.5 yuan/day, while the power consumption by the AC motor is 67.2 yuan/day. This will save about 3,000 yuan per year in electricity for the AC motor service.
A 5 kW brushless DC motor is about 12,000 yuan, while an AC motor with the same specification is about 8,000 yuan. Because brushless DC motors have no carbon brushes to replace, their operating cost is relatively low in the long term.
In logistics, a medium-sized courier company would need to pay an initial investment of 500,000 yuan to buy 50 2 kW motors if it chose DC, but only 400,000 yuan if it chose AC.
For instance, a 5 kW DC motor needs a DC speed controller costing about 3,000 yuan, whereas an AC motor of the same power only needs a 1,500-yuan frequency converter.
A 100 kW DC motor can cost 200,000 yuan, whereas an AC motor with the same power costs 150,000 yuan. Besides, the high maintenance cost of DC motors at about 50,000 yuan per year adds to the overall cost of the motor.
While AC motors came down in cost by some 20-30% over the last decade, the drop in the cost of DC motors was less than 10%. For wind power generation and public infrastructure projects, the market share captured by AC motors is well over 75% today.
Ease of Operation
A 5 kW DC motor has the speed range between 0-3000 RPM. Precision adjustment of speed is able to achieve by adjusting voltage from 0 to 220 volts. General response time would be within 0.1 second. Installation of a 10 kW DC motor generally requires the five steps: namely, power wiring, connecting a controller, installation of the heat dissipation device, and it generally takes about 4 hours in all. While installing an AC motor of the same specification only needs to connect in power and a frequency converter, generally, it takes 2 hours.
On an automatic production line, operators only need to adjust the current value on DC motors; for AC motors, they need to set multiple parameters on the frequency converter. Normally, the debugging time for an AC motor is 30%-50% longer than that of a DC motor.
The carbon replacement cycle for a DC motor is 1,000-3,000 hours with each replacement taking one hour on average. At the same time, AC motors-primarily their brushless versions-have maintenance cycles of more than five years, with the maintenance not taking more than 30 minutes.
Some high-end DC motors allow direct voltage adjustment or preset operation mode selection. AC motors, however, still require parameter input and mode selection on the frequency converter. For example, after introducing DC motors, a company reduced the training time for non-professional operators by approximately 50%, from the original 2 days to 1 day.
Although in a DC motor, the signal for speed control can be directly fed as a 0-10 volt analog signal, in AC motors, this needs to be converted into a PWM signal by a frequency converter. One automation company mentioned that on comparative testing, the time for system integration was less by about 30% on average for DC motors against AC motors.
Under dusty or humid conditions, cleaning of DC motors would take approximately every 3 months, which takes around 30 minutes per cleaning. Meanwhile, AC motors could extend it up to about once in a year.
Fault Localization Time: Normally, in DC motors, the time is around less than 15 minutes whereas for AC motors, it might go over 30 minutes or so.
One food processor converted the motors driving their conveyors from DC to AC and received about 30% savings in annual maintenance and training costs. The change also reduced production line downtime by about 15%.
Lifespan and Maintenance Costs
The average life of a DC motor is 10-15 years. Carbon brushes of DC motors are replaced every 1,000 to 3,000 hours, and each replacement costs about 500 to 1,000 yuan. If a DC motor runs 8 hours a day, carbon brushes need to be replaced 2 to 3 times per year, accumulating an annual cost of 1,500 to 3,000 yuan. In ten years, it will cost about 20,000 yuan to replace carbon brushes. In contrast, an AC motor can last for 15 to 20 years, especially a brushless induction motor. Some high-quality ones can run up to 25 years. Their average yearly maintenance cost is about 500-800 yuan, thus less than 10,000 yuan in ten years.
The annual failure rate for DC motors is about 5%-8%, while for AC motors, it is lower at 2%-4%. In the case of a DC motor with a lifespan of 10 years, a major repair should be done every two years, with each repair costing 3,000-5,000 yuan. On the other hand, a similarly rated AC motor may require only 1-2 repairs over 10 years, with a total cost of about 5,000-8,000 yuan.
In mining transportation equipment, DC motors usually run for about 8 to 10 years, while AC ones last over 15 years. The production loss caused by such equipment because of shutdown for maintenance amounts to approximately 1 million yuan per annum. Replacing these with AC motors reduces downtime by 50% and cuts production losses to less than 500,000 yuan annually.
A 10 kW DC motor costs 20,000 yuan in front, and the maintenance and repair cost cumulatively for about 10 years is about 30,000 yuan, totaling 50,000 yuan. While a similarly rated AC motor costs 15,000 yuan in front, and the 10-year cumulative cost is only 25,000 yuan, the total cost is 50% less than that of the DC motor. A chemical plant replaced 50 DC motors with AC motors and saved 250,000 yuan per year in maintenance cost, saving more than 2.5 million yuan in 10 years.
In humid and dusty conditions, DC motors’ service life can be reduced by up to 20%-30%. For instance, a DC motor that is supposed to serve for 15 years in ideal conditions may only serve for 10 years under high humidity. Under the same conditions, AC motors’ service life is usually reduced by no more than 10%. Once a cement plant replaced all its DC motors with AC motors, it saved 500,000 yuan every year in maintenance.
New brushless DC motors can survive 15-20 years, on average, and maintenance costs are reduced by about 50%, but their price is 30%-50% higher than traditional DC motors. So, a 10 kW brushless DC motor is about 30,000 yuan, while a traditional DC motor is only 20,000 yuan.
DC motors’ efficiency is between 85% and 90%, which enables the motor to survive longer when the working condition is optimal. However, under overload conditions (such as operating at over 10% above rated power), life may be reduced by 20 to 25%. AC motors have a higher efficiency of 90 to 95%, so their life is less affected by temperature rise. One wind power company claimed about 300,000 yuan per year in energy savings and an extension of the 5-year life of equipment using high-efficiency AC motors.
These DC motors used in MRI machines need expert calibration semi-annually; the charge for one calibration session is roughly 5,000 yuan. In all such situations, AC motors have to be calibrated just once a year with charges of 3,000 yuan per session.
New permanent magnet synchronous AC motors have an average failure rate 30% lower than traditional induction motors, with extended maintenance cycles of over 10 years. More than 85% of the industrial market is foreseen to be covered by AC motors until 2030
