Like all motors, a three-phase induction motor contains a stator (the stationary part) and a rotor (which rotates). Each contains electrical windings that carry current and thus creates a magnetic field. The interaction of the magnetic fields creates the torque that rotates the rotor and the load.
Figure 1 shows a View of a three-phase induction motor. This particular machine is a “Totally Enclosed, Fan-Cooled” motor, or TEFC for short. This means the motor is sealed so that air is not exchanged between the inside and outside of the motor. The air inside is stirred by the paddles on the end of the rotor body. The fan blade is outside the motor case and pulls air across the motor case to help cool the motor.
Fig.1: Exploded view of a three-phase induction motor: 1. Fan cover, 2. Cooling fan, 3. End bell, 4. Lifting eye, 5. Nameplate, 6. Stator coils, 7. Bearing seal, 8. Ball bearing, 9. Squirrel-cage rotor, 10. Cast-iron frame, 11. Wiring box (Courtesy: Baldor Electric Company)
Stator Construction Details
Figure 2 shows a close-up View of the stator of an induction machine. The stator core is built of laminated, electrical-grade steel, and the coils are mounted in slots distributed around the circumference of the stator. The stator winding of the three-phase induction motor is very similar to the armature winding of a three-phase synchronous machine. The core and coil assembly is shown in Figure 2 is placed in a frame, as shown in Figure 1.
The frame (item 10), in this case, is cast iron and has cooling fins and mounting feet cast as part of the frame. The National Electrical Manufacturers’ Association (NEMA) defines standard frame sizes so that motors will be interchangeable from one manufacturer to another. One coil of wire occupies two slots in the stator (one slot for each side of the coil). Only the portion of the coil in the slots contributes to the magnetic field. The coil sides are connected together by the end turns, which are shown in the front of the picture. In fact, the winding for a phase of the motor is typically composed of several coils of different sizes that occupy a number of slots around the stator. There will be a set of windings for each of the three phases times the number of pole pairs in the machine. So a two-pole motor would have three-phase coils. While a four-pole motor would have six phase coils. Copper wire is used in the stator coils. The size of the wire is a function of the motor’s expected full-load current, insulation type, and rated operating temperature.
Fig.2: Stator and winding of a three-phase induction motor
Rotor Construction Details
Two types of rotors are used in induction machines: wound and squirrel cage. The wound rotor has a winding that is virtually a mirror image of the stator winding. Connections to it are made via slip rings on the shaft. The wound-rotor machine is used for special purposes, usually involving speed control.
The squirrel-cage induction motor is by far the most common type of induction motor. The rotor is built of a stack of steel laminations, such as the one shown in figure 3. The laminations are aligned and heat-shrunk onto the shaft, and then the molten aluminum is forced into the slots to form the squirrel-cage winding. It is essential that no air pockets be allowed to form as the squirrel cage is cast; otherwise, the resistance of the bar would increase, causing excessive heating. The bars are shorted together at the ends by shorting rings, which are formed at the same time as bars. The rotor winding thus appears like a squirrel or hamster cage.
Figure 4 is a picture of a small, aluminum squirrel-cage rotor, from which the rotor steel was selectively etched away to show the aluminum bars. One of the shorting rings is visible at the left end of the squirrel cage. Larger rotors usually have fins cast onto them to provide additional cooling surface and to stir the air inside the motor, as shown in figure 1.
The rotor of the machine must fit inside of the stator and be supported so that it can turn. This is done with bearings. The bearing assemblies are at the top and bottom of the rotor body in figure 1. The bearings shown there are ball bearings, which have less friction than cheaper sleeve bearings. Virtually all motors larger than one horsepower, and many smaller ones use ball bearings.
Fig.3: rotor lamination showing slots for bars of the squirrel-cage winding
Fig.4: squirrel-cage winding of a small induction motor
Types of Induction Motor Enclosures
Induction motors are used in a wide variety of operating environments, and manufacturers provide several different styles to accommodate them. Some of the more common are described next.
Totally enclosed motors allow no deliberate exchange of air between the inside and outside of the motor. There are three common types of totally enclosed motors.
Totally Enclosed, Fan-Cooled (TEFC)
The TEFC motor has an external fan mounted on the motor shaft that pulls air over the case to aid cooling, as shown in Figures 1 and 5. The fan is covered by a safety cover.
Totally Enclosed, Non-ventilated (TENV)
The TENV motor is similar to the TEFC except it does not have an external fan, as shown in Figure 6. Because the only cooling is by radiation from the case, this machine will run hotter than the TEFC. Thus, the motor must be designed for a higher operating temperature, or it may be built on a larger frame than the TEFC to provide a larger cooling surface.
Fig.5: TEFC motor. (Courtesy Baldor Electric Company)
Totally Enclosed, Air Over (TEAO)
The TEAO motor is designed to operate a fan or blower. Like the TENV, it does not have its own fan on the shaft. However, it is designed to be mounted in the airstream created by the fan or blower that it is driving (either directly or by a belt).
Open frame motors contain ventilated openings to allow air to pass through the inside of the motor for cooling.
Fig.7: ODP motor. (Courtesy Baldor Electric Company)
Open Dim-Proof (ODP)
The ODP motor is open but drops falling at no greater than 15o from vertical can’t enter. Figure 7 shows an ODP motor.
Guarded motors have mesh or wire over the openings to prevent objects being pushed (or reaching) into the motor. This also keeps small animals from getting inside larger motors.
Splash-proof motors are open, but the openings are arranged so that drops splashing at up to 100oC from vertical can’t enter.
Fig.8: Induction motor with encapsulated windings.
Encapsulated winding motors are built in an open drip-proof enclosure. The winding end turns, and perhaps the conductors in the slots, are completely encased in a heavy coating, such as epoxy. The encapsulated winding is protected against damage from vibration, dust, moisture, oil. or chemicals. Figure 8 shows a close-up view of a cutaway induction motor containing encapsulated windings. The end turns at the top right of the picture are completely encased in epoxy as are the conductors in the slot at top left.
Explosion proof motors are built to contain an internal explosion or sparking without igniting surrounding gas or vapor.