In brief
An induction motor is a three-phase motor whose rotor turns slightly slower than the rotating field; it is the most widely used industrial drive.
Contents
What is Induction motor?
In the stator of an induction motor, the three-phase current generates a rotating magnetic field. This induces currents in the rotor, whose magnetic field produces torque. The rotor turns slightly slower than the rotating field; this difference is called slip. The most common design is the squirrel-cage rotor with short-circuited conductor bars. It is robust, brushless and low-maintenance. Slip-ring rotors are found in special applications.
Key ratings are rated power, voltage and connection (star or delta), frequency, rated speed, number of poles, rated current, power factor and efficiency class to IEC 60034-30-1. Mechanically, shaft height or frame size, mounting arrangement (foot, flange or both), shaft dimensions, degree of protection, cooling method and insulation class all count. Speed is determined by the number of poles or by a frequency inverter.
Induction motors drive pumps, fans, compressors, conveyor belts and agitators and are therefore present in every maintenance department. Common causes of failure are bearing damage, moisture in the winding, overload, phase loss and overheating due to dirty cooling fins. Vibration measurement, insulation resistance testing and thermography help to detect damage before the motor fails. Repairs usually involve replacing the bearings or rewinding.
In practice: maintenance and procurement
For a replacement, power, voltage, speed, frame size, mounting arrangement, degree of protection and shaft dimensions are needed from the nameplate; IEC standard motors of the same frame size are usually interchangeable in terms of mounting. It should also be checked whether a higher efficiency class is mandatory for the replacement motor and whether this changes the overall length or starting current.
Synonyms and abbreviations: three-phase induction motor · asynchronous motor · squirrel-cage motor · three-phase motor · IEC standard motor
Frequently asked questions
What does slip mean in an induction motor?
Slip is the relative difference between the speed of the rotating field and the actual rotor speed. Without this difference, no voltage would be induced in the rotor and no torque would be produced. As the load increases, slip increases and speed drops slightly. This is why the rated speed is always somewhat below the synchronous speed.
What is the difference between star and delta connection?
In star connection, each winding receives a lower voltage than in delta connection. Which connection is correct depends on the supply voltage and the voltage stated on the nameplate. Star-delta starting uses the changeover to limit the starting current. An incorrect connection leads to loss of power or overheating.
What information is needed to replace an induction motor?
From the nameplate, power, voltage and connection, frequency, speed, current, frame size, mounting arrangement, degree of protection and efficiency class are needed. Shaft diameter and length, flange size, terminal box position and special versions such as forced ventilation, brake, encoder or explosion protection are also important. A photo of the nameplate is usually the quickest starting point.
What do IE2, IE3 and IE4 mean?
These are efficiency classes to IEC 60034-30-1, rising from IE1 to IE4. A higher class means lower losses at the same output. In the EU, an ecodesign regulation sets minimum requirements for placing many motors on the market. Which class applies to a specific motor depends on power, number of poles and design and must be checked case by case.