An asynchronous motor is an electric motor that rotates because the rotating magnetic field generated in the stator induces current in the rotor. No external electrical connection is made to the rotor; current is transferred magnetically, just like in a transformer. Since the rotor always rotates slightly slower than the rotating field, the motor is called "asynchronous," meaning non-synchronous. This speed difference is called slip, and the motor’s ability to produce torque depends entirely on the presence of this difference.
The vast majority of electric motors used in industry are of this type. Thanks to its simple construction, low cost, and maintenance-free rotor, asynchronous motors are used in almost every constant-speed drive, from pumps and conveyors to fans and compressors. However, the motor’s own output speed is much higher than what most machines require; therefore, in the field it is almost always paired with a gearbox. Below, we cover the motor’s operating principle, single- and three-phase types, areas of use, and its relationship with the gearbox in sequence.
What Is an Asynchronous Motor?
As its name suggests, an asynchronous motor is an AC motor whose rotor does not rotate at the same speed as the rotating magnetic field. In the literature, it is also referred to as an induction motor because the current in the rotor is not supplied externally; it is created by induction.
Its construction consists of two basic parts:
- Stator: The laminated core fixed to the housing, in which the three-phase windings are placed. This is the part supplied from the mains.
- Rotor: The part that rotates freely inside the stator. The most common type in industry is the squirrel-cage rotor: aluminum or copper bars placed in a cylindrical body are short-circuited by rings at both ends. There are no brushes, slip rings, or winding connections.
This simplicity is the motor’s greatest advantage. Because there is no brush to wear out, commutator to clean, or contact element requiring periodic replacement, maintenance on an asynchronous motor is practically limited to bearing and insulation checks. For a general comparison of motor types, you can review our motors page.
Operating Principle of an Asynchronous Motor
The operating principle can be explained by the combination of two events: the formation of a rotating magnetic field and the induction of current in the rotor.
Rotating magnetic field. Three-phase power reaches the stator windings with a 120-degree phase difference between each phase. Because the windings generate magnetic fields in sequence, the result is not a stationary field but a rotating magnetic field. The rotational speed of this field is called synchronous speed and is calculated with the following formula:
Synchronous speed (rpm) = (120 × Frequency) ÷ Number of poles
| Number of poles | Synchronous speed (50 Hz) | Typical speed under load |
|---|---|---|
| 2 poles | 3,000 rpm | 2,850 – 2,950 rpm |
| 4 poles | 1,500 rpm | 1,420 – 1,470 rpm |
| 6 poles | 1,000 rpm | 950 – 980 rpm |
| 8 poles | 750 rpm | 710 – 735 rpm |
Current induction and slip. The rotating magnetic field cuts across the bars of the stationary rotor and induces a voltage in the rotor. Because the bars are short-circuited at both ends, the induced voltage immediately becomes current. Since a current-carrying conductor in a magnetic field is subjected to force, the rotor starts to rotate and follows the field.
The key point is this: if the rotor reaches exactly the same speed as the rotating field, the change in magnetic field cutting the bars disappears. Without change, no current is induced; without current, no torque is produced. In other words, the motor loses the force needed to keep rotating the moment it reaches synchronous speed. Therefore, the rotor always lags slightly behind, and the difference between the two is the source of torque production.
The proportional expression of this difference is slip:
Slip (%) = [(Synchronous speed − Actual speed) ÷ Synchronous speed] × 100
For example, if a 4-pole motor runs at 1,440 rpm, the slip is (1,500 − 1,440) ÷ 1,500 × 100 = 4%. In a no-load motor, slip approaches nearly zero; as the load increases, the rotor slows down, slip increases, and the motor produces more torque. This mechanism is what allows the motor to adapt automatically to the load.
Types of Asynchronous Motors
Asynchronous motors are divided into two groups according to their supply configuration. Both operate on the same principle; the difference lies in how the rotating magnetic field is created.
| Comparison | Single-phase asynchronous motor | Three-phase asynchronous motor |
|---|---|---|
| Supply | 220 V, single phase | 380/400 V, three phase |
| Rotating field | Does not form by itself; an auxiliary winding and capacitor are required | Forms naturally due to the phase difference |
| Starting | Cannot start rotating on its own without an additional component | Starts directly when energized |
| Power range | Generally below 3 kW | From a few hundred watts up to the megawatt range |
| Efficiency and torque behavior | Lower efficiency, pulsating torque | High efficiency, smooth torque |
| Typical application | Workshop machines, household equipment, small pumps | Industrial plants, production lines |
Wherever a three-phase supply is available, a three-phase motor is preferred. Single-phase motors are used only at low power levels and where three-phase power is not available.
Applications of Asynchronous Motors
An asynchronous motor is the standard solution for applications where motion is continuous and at constant speed. As long as precise position control is not required, this motor type is both more economical and more durable than its alternatives.
Pump and fan groups. It is the most widely used motor type for pumping water, wastewater, and process fluids, as well as for ventilation and cooling fans. Pump and heating-cooling applications are built around drive groups using these motors.
Conveyor and material-handling systems. Belt, chain, and screw conveyors require continuous motion at a constant speed. In these applications, the motor alone is not sufficient; it must be used with a gearbox to reduce speed. For details, see our article on conveyor gearbox selection .
Agitators and mixers. Most mixing units in chemical, food, and construction-material production require high torque at low speed. Chemical and beverage-food plants commonly use this combination.
Mill, crusher, and screening groups. Cement and mining plants use high-power asynchronous motors for uninterrupted operation under heavy-duty conditions.
Compressor and machine drives. Drive groups for compressed-air production, textile machinery, and general manufacturing equipment also use this motor type.
Using an Asynchronous Motor with a Gearbox
The biggest limitation of an asynchronous motor in the field is speed. A commonly used 4-pole motor runs at approximately 1,440 rpm. By contrast, a conveyor belt may run at around 30 rpm, a mixer at 20 rpm, and a crane drum at 15 rpm. This difference cannot be eliminated electrically; it is handled mechanically with a gearbox.
While the gearbox reduces speed, it increases torque by the same ratio. A gearbox with a ratio of 50 reduces the motor speed from 1,440 rpm to about 29 rpm while increasing output torque by roughly 50 times. This is a torque level the motor could never produce on its own, and it is exactly what the machine needs.
Other benefits that the gearbox provides to the asynchronous motor include:
- Improved starting behavior: The load inertia is reflected back to the motor at a reduced value through the gear ratio, so the motor is less stressed during startup.
- Reduced motor power requirement: The same work can be done with a lower-power motor thanks to the gearbox; this reduces both investment and energy costs.
- Installation flexibility: Different mounting positions and output-shaft directions allow the motor to be placed in a suitable position on the machine. For more information, our article on gearbox mounting positions is a useful guide.
The structure in which the motor and gearbox are combined in a single unit is called a gearmotor. Compared with solutions where they are selected separately and connected by a coupling, it is more compact, produces fewer alignment errors, and is faster to install. For the technical details, selection criteria, and advantages of this combination, we recommend reviewing our article on geared AC motors ; this article discusses the motor itself, while that article covers the motor combined with the gearbox.
Two calculations are required for correct matching: the gear ratio according to the machine’s required output speed and the output torque required by the load. For these calculations, the formulas in our articles on gearbox speed calculation and torque calculation can be applied directly. Do not forget to include the service factor in the calculation; under shock loads and on lines operating long hours each day, this coefficient determines gearbox life.
Let’s Select the Right Gearbox for Your Drive Group Together
Performance in a drive group is determined not by the motor’s nameplate power alone, but by how well the motor and gearbox match the application. An incorrectly selected gear ratio continuously overloads the motor, while an underestimated service factor causes the gearbox to require overhaul much earlier than expected.
As Remak Redüktör, producing gearboxes and gear units since 1987, we can help determine the appropriate series according to your application’s speed, torque, and operating-duty values. You can review our most widely used industrial-drive family of helical gearboxes and contact us through our contact page to share the technical details of your project.
Frequently Asked Questions About Asynchronous Motors
What is the difference between an asynchronous motor and a synchronous motor?
The difference is whether the rotor rotates at the same speed as the rotating magnetic field. In a synchronous motor, the rotor runs at exactly the same speed as the field and remains at constant speed even when the load changes; this requires a permanent-magnet rotor or external excitation. In an asynchronous motor, the rotor always lags slightly behind and its speed drops somewhat as the load increases. A synchronous motor offers higher efficiency and constant speed, while an asynchronous motor is simpler, less expensive, and more robust.
Why is the starting current of an asynchronous motor high, and how can it be reduced?
At startup, the rotor is stationary, so slip is at its highest and a very large current is induced in the rotor. As a result, the motor starts by drawing several times its rated current. To prevent voltage drops in the mains and nuisance tripping of protective devices, star-delta starting, a soft starter, or a frequency inverter can be used. A gearbox also reduces starting difficulty by lowering the reflected load inertia.
How can the speed of an asynchronous motor be changed electrically?
Because speed depends on supply frequency and pole count, it cannot be changed on a fixed-frequency mains supply. The way to adjust speed is to use a frequency inverter (drive); the inverter varies the motor speed continuously by decreasing or increasing the frequency. However, at low frequency the motor’s cooling fan also slows down, increasing the risk of overheating and changing torque capacity. In applications where speed must be permanently reduced, using a gearbox instead of an inverter is both more efficient and more reliable.
How is the direction of rotation of an asynchronous motor changed?
In a three-phase motor, swapping any two phases reverses the direction of the rotating magnetic field and causes the motor to run in the opposite direction. This is done by exchanging two phase wires in the terminal box. In single-phase motors, the direction is changed by swapping the connection ends of the auxiliary winding. Before reversing direction, verify that the gearbox and driven machine are suitable for reverse operation; some systems contain a one-way lock that prevents reverse rotation.
Why does an asynchronous motor overheat?
The most common causes are: selecting a motor with insufficient power for the load, frequent starts and stops, unbalanced mains voltage or loss of one phase, a cooling-fan cover clogged with dust, and ambient temperature exceeding the nameplate value. Motors operated continuously at low speed with a frequency inverter may also require an external fan because their own fan cannot provide enough airflow. Since heat directly shortens insulation life, continuing to operate the motor without identifying the cause can result in winding burnout.
What do IE efficiency classes mean?
IE (International Efficiency) classes are an international classification showing how efficiently a motor converts electrical energy into mechanical energy. Efficiency rises as the number increases: IE1 means standard efficiency, IE2 high efficiency, IE3 premium efficiency, and IE4 super-premium efficiency. In a continuously operating motor, the purchase price is small compared with the electricity consumed over its lifetime, so a higher efficiency class often pays for itself quickly. Overall drive efficiency is determined by both the motor and gearbox efficiency; we cover this topic in detail on our efficiency and energy-saving page.
