Understanding 3-Phase Induction Motors
Three Phase Squirrel-Cage Induction Motors
Three-phase induction motors are the most common AC motors around and are used in thousands of applications. They can power large machines in manufacturing and processing. The main reasons for their popularity are as follows:
Simple Construction
Relatively Low Cost
Low Maintenance
Main Components of a Three Phase (Squirrel-Cage type) Induction Motor
The following two main components work together to convert electrical energy into mechanical energy.
The “stator” is made up of a series a stacked, insulated, and compressed iron slices with cut-outs or slots through which run the stator windings. There are two sets of stator windings: the “main winding” and the “auxiliary winding.” Stacked metal slices are used to reduce electrical losses in the system. The stator windings are situated in such a way that when an alternating current flows through the stator windings, they produce a rotating magnetic field.
The “rotor” is centrally located within the stator and is a cylinder with an iron core which is also made up of laminated slices. It has conducting end-caps on each end and conducting bars running through the slots in the laminated slices that connect to the end-caps. The result is a rotating cage that looks similar to a squirrel-cage (hence, the name, squirrel-cage induction motor).
The rotor is attached to the motor shaft, and bearings support the motor shaft, allowing the shaft and rotor to rotate as it remains centrally positioned within the stator enclosure.
The shaft transports the mechanical energy created from the rotating rotor to the load. An “air-gap” between the stator and the rotor eliminates any physical contact between the two components.
The “motor frame” and “end-bells” act as a support structure and offer protection for the motor components inside. The end-bells contain bearings which allow the rotor shaft to turn freely on its axis. The type of enclosure can vary depending on the motor application.
Theories of Induction and Electromagnetism
The stator and rotor interact with each other to create electromagnetic induction.
Electromagnetic induction is a process in which current is created in a conductor by moving it through a magnetic field, or by having the magnetic field move or change around a stationary conductor. In a three-phase induction motor, the stator produces the rotating magnetic field needed to initiate and maintain the induction process.
Three-phase induction motors use balanced three-phase power. The phases are separated from each other by 120 degrees. The three-phase power is connected to the stator windings. As the stator produces a rotating magnetic field, this causes the rotor to also produce a current and rotating magnetic field which opposes (or repels) the magnetic field in the stator. Therefore, the rotor rotates around its axis in the same direction as the rotating magnetic field in the stator.
Relative motion is a calculation between the speed of one moving object relative to the speed of another moving object. In this case, the relative motion between the rotating magnetic field in the stator and the rotating rotor. The rotor rotates slower than the stator magnetic field. How much slower depends on the motor’s external load, how much energy is lost internally from friction, induction leakage, etc.
Slip
The difference between the synchronous speed of the rotating magnetic field in the stator and the mechanical speed of the rotor is called “slip.” The amount of slip is dependent upon the amount of the motor’s load. The greater the load on the motor, the slower the rotor turns in relationship to the stator’s rotating magnetic field. The more this difference increases, the more slip increases. Therefore, the increase and decrease in motor speed due to load is called slip.
Slip (S) is expressed as a percent (%) as follows:
S = ((Ns - N) / Ns) x 100
Where: S = Slip [%], Ns = Synchronous Speed [RPM], N = Rotor Speed [RPM], and (Ns - N) = the “slip speed.”
Example: Calculate the % slip of a 3-phase induction motor with a synchronous speed of 1200 RPM running at 1140 RPM. HINT: Calculate the slip speed first.
Slip Speed [RPM] = Ns - N = 1200 - 1140 = 60 RPM
S = ((60 / 1200) x 100 = 0.05 x 100 = 5 % Slip
The rotor’s speed is always slower than the speed of the rotating magnetic field of the stator in a positive torque application. If they were traveling at the same speed, there would be no induction and the rotor would not be able to create a magnetic field.
Synchronous Speed vs Rated Speed
The speed of the rotating magnetic field in the stator is called the synchronous speed. Synchronous speed (Na) is calculated by:
Na = (120 x f) / P
Where;
f = frequency, p = number of motor poles
Example: (120 x 60) / 4 = 1,800 RPM
The mechanical speed of the rotor is called the “rated speed.” A rated speed is based off a motor’s rated load. You can usually find the value for the rated speed on the motor’s nameplate indicating the general speed of the motor at the rated load.
From the previous example, the rated speed of a 4-pole motor using a 60-hertz supply would normally be between 1,725 RPM and 1,750 RPM. Therefore, we can see that the rotor always rotates slower than the speed of the magnetic field in the stator.
Effects of Stator Poles
Induction motors can be constructed to handle various loads and various speeds. One way is to change the number of poles in the stator. You can increase or decrease torque by adding or subtracting the number of poles in the stator. The more poles there are, the slower the magnetic field rotates. We can see this is true by using the “synch speed” formula:
Na = (120 x f) / P
Where: Na = Synchronous Speed, f = Frequency, and P = Number of Poles
Revolutions Per Minute (RPM) = f, which is the motor supply frequency in hertz (HZ) times 120 divided by the number of poles (P).
For a 2-pole motor powered at 60 HZ:
Na = (120 x 60) = (7,200) / 2 = 3,600 RPM
By adding more poles…say for a 6-pole motor, …the synchronous speed produced by the stator decreases to 1,200 RPM’s.
7,200 / 6 = 1,200 RPM
The chart below shows that as you increase the number of poles, the synchronous speed decreases in a linear fashion. For example, twice the poles is half the speed, three times the poles is 1/3 the speed, four times the poles is 1/4 the speed, etc.
Lower-Torque Motors = Higher Synch Speed
Higher-Torque Motors = Slower Synch Speed
The greater the number of poles, the greater the torque, and the greater the cost. So most induction motors are 2 or 4 pole configurations. If more torque is needed, most people will opt for a physically larger motor instead of a 6 or 8 pole motor.
Speed vs Torque Curve
The Speed vs Torque curve allows us to understand what takes place inside the motor windings and its general response to a variable load. As the load on a motor increases, the speed decreases. But why? When the motor is at rest, the torque is zero and slip is 100% because the synchronous speed and the rated speed are the same. When current begins to flow in the stator windings, work begins to occur and its “starting torque” rises to about 150% of its rated value. At the same time, “locked-rotor current” momentarily rises to about 6 times (600%) the motor’s rated current and decreases exponentially as motor speed increases. As the rotor speed increases, its torque decreases slightly to its “pull-up torque” (125% of rated torque), which is where the motor begins to build up torque as speed increases.
Since inertia and centripetal force aids in the motor’s rotation, less current is needed to rotate the shaft. However, this initial movement makes it easier for the motor to build up torque. Notice as the rotor speed further increases, it eventually reaches its “Breakdown Torque” (200-250% of rated torque), which is the greatest amount of torque a motor can produce.
After that, the motor torque decreases exponentially as current continues to fall exponentially, until simultaneously reaching the motor’s “rated current,” 100% full-load torque, and “rated speed.”
If the motor is properly sized for the application, it should operate at approximately 80-100% of rated current. The “no-load current” is approximately 30% of rated current and is derived simply by operating the motor without a load.
As the load increases, the amount of torque and winding current also increases while the speed decreases. If the load increases to a point which causes the motor to slow down below the “breakdown torque,” the motor will eventually stall. If this is left unattended, electrical current will continue to rise and overheat the stator windings, causing permanent damage. For this reason, over-current protection devices (such as circuit breakers, fuses, or overload relays) are used to detect and interrupt the flow of excessive current and stop the machine before damage can occur.
NEMA Motor Designs
The National Electrical Manufacturers Association (NEMA) established four design standards for electrical induction motors (A, B, C, and D). Each design type has unique torque, current, and slip characteristics, which affects how they relate to motor speed and load in their applications.
Design “A” Motors: Typical slip range of 0.5 - 5%, torque output is similar to Design “B,” not limited on starting current, lower winding impedance, lower stator resistance, and greater breakdown torque. Because of their high torque and low slip, Design “A” and “B” motors are best suited for drives, such as centrifugal fans and pumps.
Design “B” Motors: The most commonly used induction motors in the industry. Typical slip range of 0.5 - 5%, NEMA mandated limit on startup current. Design “B” motors are used in a wide variety of applications because of their ability to provide a high level of pull-up torque and withstand impact and burst loads at full speed without stalling. Because of their high torque and low slip, Design “A” and “B” motors are best suited for drives, such as centrifugal fans and pumps.
Design “C” Motors: Typical slip range of 1 - 5%. Design “C” motors are used in applications which require high breakaway torque, such as positive displacement pumps and conveyors. Breakaway torque is the rotating force required to "break" the head loose, going in the same direction as applied - tightening. This will usually give a value HIGHER than the original tightening torque because dynamic (when the bolt was tightened) is lower than static (when you try to break loose the bolt head).
Design “D” Motors: Squirrel cage motor, typical slip range of 5 - 13%, lower starting current to withstand full voltage starting, and very high locked-rotor torque. Like design “C” motors, design “D” motors typically power equipment requiring high starting torque like cranes and hoists, and high impact loads like stamping presses.
Induction Motor Nameplate - Definition of each spec.
The National Electrical Manufacturers Association (NEMA) establishes the standards for motor nameplate information, which is vital to understanding a motor’s characteristics. Typical nameplate shown below.
Horsepower: A measure of the motor’s mechanical output rating and it ability to deliver the required torque at the required load at the rated speed.
Horsepower [HP] = (RPM x Torque) / 5,252
Where: RPM = Revolutions Per Minute, Torque = [ft-lb], and 5,252 is a constant derived by 33,000 / (2 x Pi) = 5,252.
Torque: A measure of the turning or twisting force supplied by the motor to the load.
Torque [ft-lb] = (HP x 5,252) / RPM
Motor Rated Voltage: The optimal performing voltage of the motor. Since line voltage fluctuates, motors are rated with a 10% tolerance above or below the rated voltage shown on the nameplate.
Motor Rated Current: Shown on the nameplate as FLA (Full Load Amps), which is the amount of amperage needed when the motor is operating at full-load torque and horsepower.
Motor Rated Frequency [Hertz or HZ]: The frequency at which the motor is designed to operate. The frequency in North America is 60 HZ. Europe and many other countries run at 50HZ. Some motors are designed to work at different frequencies using a Variable Frequency Drive (VFD) .
Motor Rated Speed (RPM) (or Full-Load RPM): The approximate rotor RPM when the motor is operating at full load.
Motor Poles: Indicates the number of poles inside the stator of a three-phase motor.
Motor Phase: The number of AC power lines supplying the motor voltage. With a 3-phase motor, there are three power lines.
NEMA Design Letter: The motor’s NEMA design type, either A, B, C, or D, to describe the motor’s torque, current, and slip characteristics.
Insulation Class (INS): The thermal tolerance of the motor windings. The letter indicates the motor winding’s ability to withstand operating temperatures for specific lengths of time. Motors controlled with a Variable Frequency Drive (VFD) or motors than run at lower speeds usually have a higher insulation class.
Service Factor (SF): The percentage of overload a motor can handle over short periods of time while operating at rated voltage and frequency.
Frame Size: Describes the mounting dimensions, including the foot hole mounting pattern and shaft dimensions. Most of the dimensions are standard dimensions that are common to all motor manufacturers. However, the overall motor length dimension will change from one manufacturer to another.
Motor Enclosures
The types of induction motor enclosures are established by the National Electrical Manufacturers Association (NEMA) according to the motor’s use and are designated on the motor’s nameplate as ENCL.
Open Drip Proof (ODP): A motor enclosure used for indoor applications. It allows outside air to circulate over the windings while preventing any liquid from entering the enclosure within 15 degrees from vertical. Shown below.
Totally Enclosed Non-Ventilated (TENV): Uses cooling fins to dissipate heat instead of a fan or vent opening. These are designed for installation indoors or outdoors in dirty or slightly damp conditions. Shown below.
Totally Enclosed Fan Cooled (TEFC): Cooled by a motor shaft connected to an exterior fan. Although TEFC enclosures are not waterproof, they are used outdoors in dirty locations. Shown below.
Totally Enclosed Blower Cooled (TEBC): Cooled through forced convection by a rear-mounted blower. TEBC enclosures are used in both indoor and outdoor applications. Shown below.
Totally Enclosed Air Over (TEAO): Dust-tight fan and blower duty motors designed for shaft mounted fans or belt driven fans. The motor must be mounted within the airflow of the fan. Shown below.
Totally Enclosed Wash Down (TEWD): Designed to withstand high pressure wash-downs or other high humidity or wet environments. Available on TEAO, TEFC and ENV enclosures totally enclosed, hostile and severe environment motors. Shown below.
Explosion Proof (EXPL): The explosion proof motor is a totally enclosed machine and is designed to withstand an explosion of specified gas or vapor inside the motor casing and prevent the ignition outside the motor by sparks, flashing or explosion. Explosion proof motors are designed, manufactured and tested under the rigid requirements of the Underwriters Laboratories. Shown below.
Hazardous Location (HAZ): Hazardous location motor applications are classified by the type of hazardous environment present, the characteristics of the specific material creating the hazard, the probability of exposure to the environment, and the maximum temperature level that is considered safe for the substance creating the hazard. Shown below.
