What are the disadvantages of induction motors

Low starting torque

Low starting torque of induction motors is a significant issue, especially in scenarios where high load starting is required.

The torque of induction motors at starting is usually 50%-75% of the rated torque, in stark contrast to the 100%-120% of permanent magnet synchronous motors. In an actual application of mining equipment, a 75-kW induction motor could not overcome a load of more than 50 tons at start-up, resulting in a 35% probability of start-up failure. In contrast, the start-up success rate of permanent magnet synchronous motors of the same power is close to 98%.

The current at induction motor start-up is 5-7 times the rated current. In one production line, the induction motor start-up caused a 15% voltage drop, and the final downtime reached 3 hours, with a direct economic loss of about 300,000 yuan.

Among the equipment that requires frequent starting, the logistics conveyor belt, a system with a load of about 20 tons, is equipped with a 45-kW induction motor, and the number of restarts per day is as high as 8 times. This frequent restart increases the winding temperature rise of the motor by 10%, and the final life is shortened by about 20%. The cost of equipment repair and replacement increased by 100,000 yuan, accounting for 15% of the entire project budget.

In construction machinery, a crane that needs to lift 10 tons of materials has a standard induction motor with a starting torque of only 60% of the rated torque, while the actual demand is 85%. Construction efficiency has decreased by 25%.

In ship propulsion systems, a certain type of induction motor can only output 60% of the rated torque when starting, and the entire operation time is extended by 30 minutes. The cost of each operation has increased by 12%.

Adding a frequency converter can increase the starting torque of the induction motor to 80% of the rated value, but the equipment price of this solution increases by 30%-50% on average. For small and medium-sized enterprises, this investment may be unaffordable.

In the startup phase, the efficiency of induction motors is usually less than 40%, far lower than 90% when running. For example, a 55-kilowatt induction motor is started 10 times a day and runs for 5 minutes each time, and its energy waste in the startup phase is about 2 kWh.

The response speed of a robot manufacturer’s induction motor at high load startup is about 30% slower than that of a servo motor. In high-precision, high-efficiency automation scenarios, the operating efficiency of the production line is reduced by 15%.

Low operating efficiency

The average operating efficiency of induction motors is usually between 85% and 93%, while the efficiency of permanent magnet synchronous motors can be as high as 97%. This seems to be only a difference of 4% to 12%, but in large-scale industrial applications, the difference in energy consumption can reach millions of kilowatt-hours per year.

A 75-kilowatt induction motor, assuming it runs 12 hours a day and 300 days a year, consumes about 270,000 kWh of electricity per year. If a permanent magnet synchronous motor with an efficiency of 95% is replaced, the annual power consumption will be reduced by about 14,000 kWh, and based on an electricity fee of 0.8 yuan per kWh, 11,200 yuan will be saved each year.

In an induction motor, energy is transferred through the magnetic field between the stator and the rotor. Rotor losses usually account for 15%-20% of the total energy loss, while stator copper losses and iron losses account for 10%-15% respectively. Especially at partial load, the efficiency of induction motors drops more significantly, and may be less than 70%.

In agricultural irrigation systems, the average annual operating cost of a system equipped with an induction motor is about 20% higher than that of a high-efficiency motor system. A 22 kW motor used for irrigation runs 2,000 hours per year, and the power consumption of an induction motor is about 44,000 kWh, while the power consumption of a high-efficiency motor is 40,000 kWh.

When the speed of an induction motor drops to 60% of the rated speed, its efficiency may drop to 50%-60%. In contrast, the efficiency of permanent magnet motors or servo motors under the same conditions can usually be maintained above 80%.

When the ambient temperature rises to 40 degrees Celsius, the efficiency of an induction motor may decrease by 2%-3%, and its power factor will also decrease. This situation is more significant in tropical areas or high-temperature industrial environments.

Induction motors account for nearly 25% of the annual air conditioning energy consumption of a large commercial building. By switching to a high-efficiency motor system, the building’s annual electricity bill can be reduced by 5%-10%.

About 70% of industrial motors in the world are induction motors because their procurement cost is 20%-30% lower than that of permanent magnet motors. In some scenarios with limited budgets and low efficiency requirements, induction motors are still the first choice for enterprises.

In the case of frequent load changes, the efficiency fluctuation of induction motors may reach 10%-15%. On a processing line, the load fluctuation frequency reached 10 times per minute, and the average efficiency of the motor dropped by about 8%.

High energy consumption

In large-scale industrial use, about 70% of industrial electricity is consumed by motors, and the proportion of induction motors is as high as 85%.

A 75-kilowatt induction motor has an efficiency of 85% and consumes 88 kWh per hour at full load. Calculated as 10 hours per day for 300 days a year, the total power consumption is 264,000 kWh. In contrast, a permanent magnet synchronous motor with the same power but an efficiency of 95% consumes about 236,000 kWh per year. The additional electricity bill is RMB 22,400 per year.

The power factor of a standard induction motor is usually between 0.75 and 0.9, while the power factor of a high-efficiency motor is above 0.95. A 100-kW induction motor consumes 75 kVAR at a power factor of 0.8. The additional compensation equipment adds 10% to the initial investment cost.

In a high-frequency use scenario, 20 45-kW induction motors run continuously for 16 hours per day, consuming 5,256,000 kWh per year, accounting for 35% of the total factory electricity consumption. By switching to high-efficiency motors and inverters, energy consumption is reduced by 10%, saving 525,600 kWh per year and saving RMB 420,000 in electricity bills. The initial investment is about RMB 2 million, with a payback period of 4.8 years.

When the load is less than 50% of the rated load, its efficiency may drop to 60%-70%. The actual energy efficiency of a 55 kW induction motor at partial load is 15% lower than at full load.

Iron and copper losses account for about 30%-40% of the total energy loss of an induction motor. For a 75 kW motor, iron and copper losses consume about 2.25 kWh and 1.5 kWh per hour, respectively, accumulating 1,125 kWh of energy waste per year.

The efficiency of an induction motor may drop by 1%-2% for every 10 degrees Celsius increase in ambient temperature. In tropical or high-temperature industrial environments, the decline in induction motor efficiency leads to a 15% increase in annual power consumption, directly increasing operating costs by millions.

At an industrial equipment company, when the speed of an induction motor drops to 50% of the rated value, its efficiency may be only 40%-50%. The energy consumption in this case is 20%-30% higher than during normal operation.

If 10% of the electricity consumption of industrial enterprises worldwide were converted from induction motors to high-efficiency motors, carbon dioxide emissions could be reduced by about 200 million tons per year.

Limited overload capacity

The limited overload capacity of induction motors is a major drawback, especially in scenarios where they need to frequently cope with instantaneous high loads or continuous overload operation.

The overload capacity of induction motors is usually limited to 10%-15% of the rated power. A 100-kW induction motor has a maximum short-term load capacity of 110-115 kW. Permanent magnet synchronous motors can withstand 150%-200% of the rated power under the same conditions.

In the rolling mill of a steel plant, a 75-kW induction motor was overloaded for only 3 minutes when handling overloaded billets before triggering the overheating protection system, causing the entire production line to stop operating for more than 1 hour. The economic loss caused by each shutdown was about 500,000 yuan, and similar shutdown events reached 15 times this year.

The allowable temperature rise of induction motor windings is generally 105 degrees Celsius to 120 degrees Celsius, but the temperature rise may increase rapidly by 30%-50% when overloaded. In an experiment, a 55-kW induction motor ran at 125% load for 5 minutes, and the winding temperature rose to 150 degrees Celsius. The cost of repairing the motor was 12,000 yuan, and the repair cycle was as long as 2 weeks.

In lifting equipment, an induction motor used for lifting equipment has a rated power of 45 kW, but when lifting an 8-ton object, its load reached 130% of the rated power. This type of start-up failure rate is as high as 25% in similar equipment.

In a chemical plant, the ambient temperature is 40 degrees Celsius and the humidity reaches 90%, and the overload capacity of the induction motor has decreased by 20%. When dealing with emergency load increases, the equipment has overload protection many times, and the cumulative downtime throughout the year has reached 300 hours, with direct losses exceeding 1 million yuan.

A 150-kW induction motor can only withstand 110% of the rated power under experimental conditions, and the duration is less than 2 minutes. In contrast, permanent magnet synchronous motors can withstand 150% of the rated power under the same conditions, and the duration is more than 5 minutes.

In a production line, the equipment needs to cope with rapid changes in load, but the induction motor has a slow response speed and the overload capacity is only 110% of the rated power. After the company switched to servo motors, the failure rate was reduced by 50%.

When running at low frequency, the overload capacity of the induction motor may drop to 105% of the rated power. An induction motor used in a ventilation system triggered the protection system after only a 5% load increase during variable frequency operation.

In mining equipment, a 200-kilowatt induction motor has a winding life shortened from the original 10 years to 6 years due to frequent overload operation, and the cost of early replacement and maintenance exceeded 200,000 yuan.

Difficult to maintain

Induction motors are difficult to maintain. This problem is particularly prominent in industrial applications, which directly affects the service life of the equipment, operating costs, and production efficiency of the enterprise.

A 100-kilowatt induction motor needs a comprehensive inspection after 8,000 hours of continuous operation. This periodic maintenance takes 2-3 days each time, with an average cost of 30,000 yuan.

About 20% of induction motors that have been in operation for more than 5 years have experienced insulation aging or damage. For a 55-kW induction motor, insulation problems can cause winding short circuits, which costs about 12,000 yuan to repair, while the cost of replacing new windings can be as high as 20,000 yuan.

About 40% of induction motor failures are related to bearing problems. In the operating environment of a mining equipment, a 75-kW induction motor needs to replace bearings every two years, with a single replacement cost of 8,000 yuan. The annual investment in bearing maintenance alone exceeds 1 million yuan, and high-frequency bearing replacements can also cause equipment downtime.

In high-humidity or high-temperature environments, the cooling system of induction motors is prone to failure, causing internal temperatures to rise, further exacerbating winding aging and insulation damage. In a chemical plant with an ambient humidity of 85% and a temperature of more than 40 degrees Celsius, the maintenance frequency of induction motors is 30% higher than normal environments, and the annual maintenance cost has increased by 200,000 yuan.

Routine inspection and maintenance service charges for induction motors range from 3,000 yuan to 5,000 yuan, while the repair cost of sudden failures can be as high as 10,000 yuan or more. If the equipment has five sudden failures in a year, the cost of outsourcing services alone may exceed 50,000 yuan.

On a production line, a 200-kilowatt induction motor stopped due to a winding problem. The repair process took 48 hours, directly affecting 20% of the production capacity. This unexpected downtime occurs 3-4 times a year, with a cumulative loss of up to 2 million yuan.

In an industrial vibration test, the vibration amplitude of an induction motor is usually between 0.5 mm and 1 mm. An induction motor in a factory had not been maintained for a long time, and the vibration amplitude reached 1.5 mm, resulting in a smaller gap between the stator and the rotor, which eventually caused friction damage and maintenance costs of up to 50,000 yuan.

A 15-year-old induction motor had a maintenance cycle of up to 3 months due to the lack of suitable bearing replacements, resulting in a 10% reduction in power generation and direct economic losses of more than 5 million yuan.

The maintenance of induction motors requires frequent use of professional equipment, such as infrared thermal imagers and insulation resistance testers. These devices are expensive to purchase. The price of an infrared thermal imager is about 50,000 yuan, and it requires trained technicians to operate.

The maintenance of an induction motor in a food processing plant requires 24 hours of downtime. Each maintenance will affect the production plan of 5,000 tons, with a loss of about 200,000 yuan.

High noise and vibration

The noise and vibration of induction motors are high, which may lead to additional operating costs, maintenance costs, and reduced comfort of the working environment in many industrial and commercial applications.

The operating noise of induction motors is usually between 70 and 85 decibels, which is much higher than the average noise level of permanent magnet synchronous motors (50 to 65 decibels). In a large factory, a production line using 20 induction motors has a total noise level of 90 decibels, which exceeds the industrial zone working noise limit (85 decibels) stipulated by the Chinese national standard.

When a 75-kilowatt induction motor in a mine is running at full load, the vibration amplitude reaches 1.2 mm, while the normal operating vibration amplitude stipulated by the international standard should not exceed 0.7 mm. The increase in vibration has reduced the life of the motor bearings by 30%, and the bearings need to be replaced every two years, with a single cost of 8,000 yuan.

In an experiment, the efficiency of an induction motor dropped by 3% when the vibration frequency reached 60 Hz. For a 55-kW induction motor, the energy consumption increased by about 1.65 kWh per hour of efficiency decline, and the increased electricity bill was 3,960 yuan when it ran for 3,000 hours a year.

Electromagnetic noise accounts for the highest proportion, about 50% of the total noise. When the magnetic field is unbalanced, the noise level of the induction motor may increase by 10 decibels. A 100-kW induction motor of a manufacturing company has an operating noise of 95 decibels. The repair cost is as high as 80,000 yuan by re-adjusting the stator winding to reduce the noise.

In a coastal industrial park, the vibration problem caused by moisture in the induction motor caused the equipment to shut down for 30 hours, and the economic loss of each shutdown exceeded 200,000 yuan. The company invested 500,000 yuan to improve the protection measures of the motor.

A food processing plant caused residents to complain about the noise of the induction motor. The rectification plan included replacing low-noise motors and installing sound insulation equipment, with a total cost of up to 2 million yuan. It led to a 15% drop in the company’s annual profit.

Long-term exposure to an environment where the noise of induction motors exceeds 85 decibels increases the risk of hearing loss for employees by 20%. A factory equipped its employees with noise-reducing earmuffs, and the annual cost of protective equipment reached 100,000 yuan. Employees’ satisfaction with the work environment dropped by 15%.

In a logistics conveyor belt, the support structure of the conveyor belt loosened due to vibration problems, and the repair cost was as high as 300,000 yuan.

For an equipment manufacturer, adding a damper can reduce the vibration amplitude of the induction motor by 30%, but the cost of such equipment accounts for 20%-25% of the total cost of the motor.

High starting current

The starting current of the induction motor is large, and this characteristic has a significant impact on the stability of the power grid, the life of the equipment, and the overall operating cost.

The current of the induction motor at startup is usually 5 to 7 times the rated current. An induction motor with a rated current of 100 amperes may have a peak current of 500 to 700 amperes at startup. Due to the excessive starting current of the induction motor, the voltage of its distribution system dropped by 15%, causing other sensitive equipment to shut down frequently, resulting in hundreds of thousands of yuan in economic losses.

The temperature rise of the winding of a 75-kW induction motor can reach 30 degrees Celsius when starting, while it is only 10 degrees Celsius during normal operation. Frequent temperature rise will shorten the life of the insulation layer by about 20%. The failure rate of the induction motor winding used by it increased by 15% in high-frequency start-stop conditions, and the winding needs to be replaced once every two years on average, with a single maintenance cost of 20,000 yuan.

A 55-kW induction motor has a maximum current of 350 amperes at startup, which causes the company’s peak electricity rate to increase by 20%. The additional cost per year reaches 150,000 yuan, accounting for 10% of the total electricity cost.

When a mining company used a 150-kW induction motor, the starting current exceeded the carrying capacity of the original distribution transformer, causing the transformer to overheat and shut down. The transformation cost was as high as 500,000 yuan, and the economic loss during the shutdown period was about 2 million yuan.

A 45-kW induction motor started more than 50 times a day, shortened the life of the contactor by 40%, and needed to be replaced 4 times a year, with a total cost of more than 100,000 yuan. In contrast, permanent magnet synchronous motors started with frequency converters have almost no such problems, and the replacement cycle of contactors is extended to 5 years.

In a conveyor belt system, the instantaneous high torque of an induction motor at the start causes a sharp change in the tension of the transmission belt, shortening the life of the belt by 20%. The annual belt replacement cost increased by 50,000 yuan, accounting for 15% of the system maintenance cost.

When a 100-kilowatt induction motor was started, it caused the grid voltage to drop by 10%, resulting in the malfunction of the circuit breaker on the adjacent line, and the power outages caused by the starting current problem reached 15 times throughout the year.

After installing a soft starter on a 75-kilowatt induction motor, the starting current was reduced to twice the rated current, and the starting shock was reduced by 60%. However, the cost of the soft starter accounts for about 20%-30% of the total cost of the motor. For small and medium-sized enterprises, this investment may increase financial pressure.