Large starting current
The starting current of an induction motor is 5 to 7 times its rated current. When the rated current is 40 amperes, the starting current may be as high as about 200 amperes.
In some large factories, induction motors are often used to drive heavy equipment. The starting current of such motors may even reach more than 8 times the rated current. When a 500-kilowatt motor used in a chemical plant was started, the current once surged to 3,000 amperes. This caused the voltage nearby to drop by 5% instantly.
Excessive current at each start will accelerate the aging of the motor coil and armature. If a motor is started frequently (for example, more than 10 times a week), its service life may be 50% shorter than normal.
When several induction motors are started at the same time, the transformer load increases, causing the transformer temperature to rise and the electrical equipment to overheat. Due to the excessive starting current of the motor, the power company had to replace the equipment twice in just one week, with a total cost of about 500,000 yuan.
In some areas, power companies determine the charging standards based on the maximum demand current of users. If the current at the start of the motor exceeds the rated current many times, the power company may increase the price based on these surge currents. If the starting current of a motor exceeds its rated value by more than 5 times, the power company may charge a 15% surcharge.
In the case of excessive starting current, the voltage fluctuation can be as high as 8%, which may cause instantaneous shutdown of production equipment. A precision instrument is very sensitive to voltage fluctuations. Every time the voltage fluctuation exceeds 3%, the equipment will automatically shut down.
Many companies have begun to use equipment such as soft starters or frequency converters. Effectively smooth the increase of current and avoid the impact of instantaneous large currents. By using a soft starter, the starting current of a 100-kilowatt motor can be controlled between 2 and 3 times the rated current. It was found that the voltage fluctuation of the power system has dropped from the original 5% to less than 2%. The initial investment is about 1 million yuan.
Poor speed regulation performance
In traditional induction motors, the speed of the motor is usually determined by the grid frequency. Once the grid frequency is fixed, the speed of the motor is also locked. To adjust the speed of the motor, the frequency of the grid must be changed.
The rated speed of some motors is 1500 rpm, and the speed of these motors often cannot be strictly maintained at 1500 rpm. When the grid frequency fluctuates by 0.5%, the motor speed may change by up to 75 rpm.
Although the voltage regulation method using a transformer can affect the motor speed to some extent, its speed regulation range is usually very limited. Moreover, the use of transformers may also cause power loss, further affecting the overall efficiency of the system. The efficiency loss of speed control by voltage regulation can be as high as 10%.
The inverter controls the speed of the motor by changing the frequency of the power supply. The motor with the inverter can adjust the speed over a wide range, from 50% to 120% of the rated speed. A 30 kW motor equipped with an inverter costs about twice as much as the motor itself.
With the inverter, the efficiency of the induction motor is still low at low speed. Especially when running at low load, the efficiency of the motor drops significantly, even as low as about 40%. In contrast, the efficiency of the permanent magnet synchronous motor (PMSM) at low speed can usually be maintained above 80%.
In rolling mills in steel production and CNC machine tools in precision machining, the speed of the motor needs to be very precise and stable. The speed deviation of rolling mills using traditional induction motors can reach ±3% in actual operation, which is completely unacceptable in precision machining.
When the load changes, the speed and output power of induction motors change slowly, resulting in reduced production efficiency. In the case of a sudden increase in load, the response delay time of induction motors can reach more than 10 seconds, while the response time of synchronous motors can usually be shortened to less than 2 seconds under the same load conditions.
Noise is more obvious
The noise level of a standard industrial induction motor during operation is usually between 70 and 85 decibels, and some large motors can even reach more than 90 decibels. This noise level is almost equivalent to the noise of a high-speed car or a busy street.
The noise level of an induction motor rated at 75 kilowatts is usually as high as 80 decibels during operation. Long-term exposure to a noise environment above 85 decibels may cause hearing loss and other health problems.
During the operation of the motor, when the current passes through the stator coil, a rotating magnetic field is generated. The interaction between this magnetic field and the rotor causes the rotor to generate electromagnetic vibrations, thereby emitting a certain amount of noise. The electromagnetic noise of the motor rotor accounts for about 30% of the total noise of the motor.
In some high-power motors, the fan noise may account for more than 40% of the total noise. For example, a 200-kilowatt induction motor can have a fan noise of 45 decibels.
Some companies have begun to adopt closed motor designs or silencers. By wrapping the motor housing with soundproofing materials, the noise of the motor can be reduced by 10 to 15 decibels. Modern low-noise motor designs usually keep the noise below 70 decibels.
About 40% of heavy machinery and manufacturing equipment worldwide still use traditional induction motors. As a result, operators working in high-noise environments for a long time may suffer from health problems such as hearing loss and mental fatigue.
The noise generated by induction motors used in an industrial area has led to a 25% increase in the complaint rate of residents within a 500-meter radius. Some cities have issued relevant policies requiring industrial enterprises to take noise prevention and control measures to avoid excessive noise pollution.
In high-noise environments, employees’ work efficiency has dropped by an average of 15%. This impact is mainly reflected in the distraction of employees and increased fatigue, which leads to problems such as reduced work quality and delayed production schedules.
Regular maintenance is required
The regular maintenance cycle of induction motors is once every 6 months to 1 year, depending on the motor’s workload and operating environment. For some high-load motors, the maintenance cycle may even need to be shortened to once every 3 to 6 months.
The maintenance of induction motors primarily consists of checking the insulation status of the motor, cleaning the fan and radiator, checking the bearing lubrication, etc. For a 150-kilowatt motor, inspection and replacement of bearings at regular intervals can increase the service life by at least 20%.
An inspection of the electrical system of the motor is also very important—from cables to connectors and brushes—regularly. Aging or improper contact of its electrical system may account for 30% of the failures in motors. Operating under humid or hot conditions will make electrical short circuit or insulation aging more probable.
The motor operating expense allocated to maintenance annually is from 5% to 10% and involves some 300,000 yuan as a single-time repair cost. On average, the expenses to be recovered from when its maintenance was to be held and thus deferred were estimated as almost three to five times compared to a usual maintenance expense for that specific motor.
About 20% of production lines are forced to halt production because of motor failure. Such emergencies result in bearing failure of the motor because of the absence of regular maintenance of the motor, and the downtime is as long as 72 hours, directly losing more than 1 million yuan in production revenue.
Motors in precision machine tools are very sensitive to speed control and load changes. A minor failure may cause the processing accuracy to decline. Annual motor maintenance can also ensure stability in the accuracy of equipment produced and decrease rework rates by 10%.
Without cleaning, it may increase its temperature rise above 15%. Regular cleaning of the motor will improve the working efficiency of the motor with reduced energy consumption and chances of overheating failures.
In heavy industrial fields such as metallurgy and mining, the load of the motor is usually heavy, and the running time may reach 24 hours a day. The maintenance of this type of motor requires not only checking the wear of mechanical parts but also checking the contact and insulation status of the electrical parts. About 25% of motors need to be inspected and maintained at least once a month.
Control Complexity
The complexity in the control of induction motors is much greater compared to that of the DC motor, especially with high precision speeds and torques. The only parameter that really influences the induction motor speed, as far as its grid is concerned, is the grid frequency. This happens to be the fixed value as determined by the utility. Speed control of induction motors is usually done by controlling the power frequency supplied to it.
A 75-kilowatt induction motor typically requires a VFD to control its speed efficiently and precisely. The range of adjustment for the variable frequency drive ranges from 30% to 150% of rated speed over a wide range by adjusting the power supply frequency to the motor. For example, variable frequency drives, like most high-power induction motors, are used to save energy.
Some industrial automation equipment needs dynamic load control during its operation in order to ensure that the motor can vary the torque output in time as the load changes. As such, high-precision sensors must be installed in the control system, and a corresponding closed-loop control system should be designed.
Induction motors draw a high starting current during start-up. Therefore, soft starters or star-delta starting are used for such motors in most cases. A soft starter controls the rate and peak of acceleration of the current with the help of a microprocessor coupled with a sensor. During a motor’s start-up operation, the torque produced by the motor’s starting torque may not be sufficient to operate some heavy loads.
In some high-load application scenarios, the control system of the motor should involve multiple aspects related to motor cooling, overload protection, and high accuracy of regulation of the rotational speed. Considering a 500-kilowatt motor for application in a power plant, motor control systems ensure the accurate controlling of speed as well as the torque with appropriate monitoring of temperatures, vibrations, and other parameters with real-time capability.
The operating condition of the inverter directly influences the performance of the motor. In case of inappropriate debugging or timely maintenance, this may cause damage to the motor, overheating, and more. The average failure rate of the inverter is 5%-8%. These failures can lead to stagnation in production and increase maintenance and replacement costs.
The control of the induction motor remains slightly crude compared to the DC motor. The induction motor cannot achieve the control like the DC motor where perfect control is obtained by adjusting the current, and thus in some applications that have high demands, its performance is inferior to that of the DC motor.
For example, in large companies, the cost of training on the motor control system often goes to about 15% to 20% of the technical costs of an enterprise for the entire year. These costs cover only the charges mainly used in the training of engineers on hardware and software-related control system operations for the early and prompt identification of faults that have occurred within the control system.
High installation cost
The installation cost of induction motors is usually higher than other types of motors, and this is mostly reflected in the price of the motor itself, the cost of additional equipment, and the labor cost during installation and commissioning. In most cases, the installation cost of a 100-kilowatt induction motor is normally 20% to 30% higher compared to other types of motors.
Induction motors that have an output of about 100 kW in general cost RMB 20,000; the price for a DC or permanent magnet synchronous motor of comparable power is basically the same. A manufacturing enterprise with an annual output value greater than RMB 100 million can spend millions to install induction motors on all the production lines that it needs to configure.
Besides the purchase cost of the motor itself, the company will have to purchase additional inverters, soft starters, and other power control equipment. The inverter cost usually makes up 30% to 40% of the motor installation cost. In the process of installing inverters for multiple induction motors, the price of one inverter is roughly RMB 60,000, and installation and commissioning costs represent about 50% of the motor’s price itself.
In some cases, the cost of cables and laying them could be 15% to 20% of the motor price. In a large mechanical installation project, because the original factory building’s electric facilities were very old, during the renovation process, the cost of electric equipment and wiring reached more than RMB 500,000.
Usually, the labor cost in installation and commissioning induction motors takes 20% to 30% of the total installation cost. In some large projects, with a long commissioning time, the labor cost can be as high as tens of thousands of RMB.
In some mining and oil extraction equipment, motors sometimes have to be installed in environments that are more specific, and so explosion-proof housing may be especially required and supplemented by additional protection devices. An example is: A coal-mining project specially customized and installed the equipment by adding an additional RMB 300,000.
As the size and weight of motors in transport and handling are too huge, special equipment and vehicles need to be used in some big motor projects for large motors. Generally, a high-power induction motor transportation will cost around RMB 20,000, and it will be much more expensive in some remote areas.
Some of the induction motors operating at high loads require annual maintenance, and the cost of maintenance usually makes up 5% to 10% of the installation cost of a motor. As the service life increases, so does the maintenance cost of the motor, especially in the case of equipment operated under harsh conditions.
Performance degradation at high temperatures
Induction motors are usually ideal for operating temperatures below 40°C, but when the temperature exceeds 50°C, the efficiency and performance of the motor begin to be affected. When the operating environment temperature of the motor reaches 60°C, the efficiency of the motor may decrease by 5% to 10%.
The life of the insulation material of the induction motor is reduced by about half for every 10°C increase in ambient temperature. A motor may have a service life of 10 years at normal temperature, but if it is operated in an environment of 50°C, its service life may be shortened to 5 years.
In high-temperature environments, the heat dissipation effect of the motor is often not effectively guaranteed, causing the motor temperature to continue to rise. Once the temperature of the motor exceeds the rated operating temperature range, its efficiency will be significantly reduced, and it may even cause the motor to burn out. When the motor temperature rises above 120°C, the insulation performance of the motor will drop sharply and may even cause the motor to fail completely.
Enterprises usually need to add heat dissipation devices for the motor. Use stronger cooling fans, strengthen the design of the radiator, and even use liquid cooling systems in some cases. The additional investment in the cooling system of each motor is about 200,000 yuan, and a maintenance fee of 5% is required each year.
In some steel plants and metallurgical industries, steel plants need to monitor the temperature of the motor in real time during the operation of the motor and reduce the load of the motor through automatic adjustment systems. The failure rate of motors using temperature control systems in steel plants in high temperature environments is about 30% lower than that of equipment that does not use the system.
The starting current of some motors at high temperatures is about 50% higher than that at normal temperatures, which may cause the motor to overload at startup. In a thermal power plant, when the outside temperature reaches above 40°C, the failure rate of the motor starts increases by 15%, and the current impact of the motor during the startup process also increases the burden on the power grid.
High temperatures may also cause vibration problems in the motor. The amplitude of motor vibration usually increases by 20% to 30% at high temperatures. Long-term high temperature vibration not only increases the wear of the motor, but may also cause the motor components to loosen, ultimately affecting the running stability of the motor.
High temperature will reduce the viscosity of the cooling oil and lubricating oil of the induction motor, thereby affecting the lubrication effect of the motor. The quality of the lubricating oil directly affects the operation of the motor bearing. If the lubrication is insufficient, the bearing wear will increase. In high-temperature environments, the number of motor failures caused by insufficient lubrication increases by about 20% each year.
Some companies configure their motors with special insulating materials to improve their stability at high temperatures. The price of such high temperature adaptable motors is usually 30% to 40% more expensive than ordinary motors.
