Three-phase AC power and magnetic field generation
Transmissions with three-phase motors are driven by a cable to provide the power. There are actually three currents 120 degrees apart which generates the magnetic rotating field from the stator. Three-phase AC has a frequency usually of 50 Hz or, in the United States, 60 Hz, where the current constantly changes from +I to -I at about twice each cycle. For a motor running at 50 Hz, it has been calculated that its magnetic field rotates around to the stator pole piece with a speed of 3000 revolutions per minute (rpm). This implies such high-speed rotating magnetic fields continuously drive through rotor windings.
The voltage used in industrial control has a frequency of 380V or 400 content, and the current strength can be more than several hundred amps. A motor with an output power of 5.5kW in three carries a rated current of only 11A and can run even large machinery using its torque due to supply at a level of more than 380V. This three-phase current not only keeps steady but also provides 3 continuous electromagnetic driving forces to make the motor run for a long time.
Electromagnetic interaction between stator and rotor
A stator is a component of a three-phase motor that remains stationary and is typically made up of a laminated silicon steel core with windings embedded in the slots. The rotation generated when current flows through the stator winding creates a rotating magnetic field, which induces currents in the rotor, producing electromagnetic torque to drive their revolutions. The stator of even a typical commercial engine can operate with >96% efficiency, meaning less energy goes to waste.
The speed of the rotor is associated with the rated speed of the motor. If there is a 1500 rpm motor, its actual rotor speed would be slightly lesser than this value, for it could be at 1480 rpm; here, the difference between rated and actual rotor speed means slip as %. In asynchronous motors, the slip rate for some models is normally controlled to about 3%. The slip rate is the factor that directly influences the output torque of the motor; in the case of various load conditions, it may have different slip rates due to the different amount consumed by the rotor.
Differences in the operation of asynchronous and synchronous motors
The speed of an asynchronous motor is always less than the speed of a rotating magnetic field, and this difference in rates is called slip rate. If you have a 4-pole asynchronous motor and the speed of the rotating magnetic field is 1500 rpm, then usually with a slip rate of around 5%, the rotor’s actual speed will be approximately equal to 1425 rpm. So, this slip helps to produce the relative induced current in the rotor, through which it drives the rotation of the Rotor. Since operating at a steady speed, asynchronous motors are used for all standard industrial applications and show good efficiencies, around 93% in many cases like pumps or fans, which are very simple to construct with high reliability.
The speed of a synchronous motor’s rotor is almost entirely synchronized with the magnetic rotating field. In synchronous motors, rotor speed is related to the grid frequency. Under normal conditions of 50 Hz supply frequency, a synchronous motor with four poles will always have a speed at 1500 rpm (n s) and slip equal to zero (%). It is the design of choice in equipment requiring precision control over speed, such as governors and variable-speed motors. The efficiency of a standard synchronous motor can be as high as 98%.
The impact of rotor structure and materials on performance
The construction and material of a 3-phase motor are essential in how it will operate optimally. The squirrel-cage rotor is also the most commonly used type and has aluminum or copper bars. With a 5kW motor and an aluminum bar rotor used on it, you might turn 80% efficiency using the same size of machine parts, but by replacing that rotor with one made from copper bars, your new build can already get over this mark (85%). Although the use of copper bars increases manufacturing costs, it allows for notable benefits during heavy-load starting.
This provides better control of the efficiency in cases where adjustable starting current and torque are required. Wound rotors can be used to control the speed of induction motors, and they also provide higher starting torque with varying current flows in their windings than squirrel cage or relatively low resistance rotor(s). Take a wound rotor motor as an example; its rated current is from 5 to 7 times tiny when it just started up so these motors can start better under heavy loads like in mining devices.
The relationship between temperature, cooling, and efficiency
The corresponding temperature control for a three-phase motor directly determines the operating efficiency. The operating temperature of the motor is generally 50-80°C, and if it exceeds this scope, insulation material age as that efficiency may decline by approximately 5% to 10%. In order to operate at peak efficiency, the motor must be properly cooled, and having a well-designed cooling system is critical.
An 11kW industrial motor produces heat when it operates, and an active air-cooling or liquid-cooling system dissipates the waste heat. The cooling efficiency of air-cooling systems can usually break through to about 80%, and liquid-cooling up to more than 95% is used in most high-power motors. For motors that are tightly enclosed, common fan-cooling patterns can keep the stator and rotor very cool, which, over time, will lengthen a motor’s life.
