AC motors are electric motors driven by alternating current (AC). AC motors are widely used in industry, primarily due to their highefficiency, and their ability to produce constant torque up to the rated speed.
AC Motor Types:-
The two most widely used types of AC motors are induction motors and synchronous motors.
How AC Motors Work:-
The two basic parts of an AC motor are the stator (the stationary outer drum) and the rotor; the rotating inner portion of the motor which is attached to (and drives) the motor shaft. Both the stator and the rotor produce rotating magnetic fields. In the windings of the stator,this rotating field is provided inherently by the sinusoidal nature of alternating current. In the rotor, the magnetic field is created by permanent magnets, reluctance saliency, or by additional electrical windings.
Synchronous motors operate in lock step with the frequency of the supply current because their rotors have either permanent magnets or electromagnets generating the rotating electromagnetic field.
In an induction motor, the magnetic field in the windings of the rotor is “induced” by the magnetic field of the stator. In order for this induction to produce torque, the speed of the rotor’s field must lag the field of the stator’s magnetic field. This speed differential is known as “slip”, and is the reason that induction motors will have a “Nameplate RPM” rating that is about 5% less than their synchronous speed.
For example, an Iron horse model MTRP-001-3DB18 (1hp, three phase, four pole, AC induction motor) has a synchronous speed rating of1800 RPM (assuming 60hz power), but the “Nameplate RPM” rating is 1760. This motor shaft will turn at 1760 RPM when powered directly with the US standard of 60 Hz three-phase power.
Differences Compared to DC Motors industrial DC motors have historically been of the brush type. DC motors with brushes and commutators have a number of drawbacks when compared to AC motors: added maintenance (brush replacement), limited speed ranges and overall life expectancy is shorter.
AC induction motors have no brushes and have a much longer life expectancy.DC motor speed is controlled by varying the armature current, while AC motor speed control is achieved by varying the frequency of the alter-nating current, often with a variable frequency drive (VFD).
Brushless DC motors have become available over the last several decades, primarily as a result of the advent of the semiconductor control circuit required to operate them, and the availability of high-quality permanent magnets. Brushless DC motors require no brushes or physical commutator and thus have increased service life. They also overcome the speed limitations of the brushed versions.
AC Motor Control:-
When simple on/off control is required, contactors or manual motor starters are often employed. Contactors (large, three-phase relays) allow a PLC or other controller to toggle power to an AC motor. Reversing motor starters are specialized versions with two contactors wired such that they also allow reversing of the direction of spin of the motor shaft. Manual motor starters include a manually operated switch or knob that allows the operator to switch the power. All of these types are known as “across-the-line” control – as the motor is wired directly to the incoming power “line” (through the contactor or motor starter).
Soft Starters:-
Soft starters are more complex motor controls that enable acceleration and deceleration ramps for stopping and starting the motors more smoothly than is possible with across-the-line control. Soft-starters typically use silicon control rectifiers (SCR control) to gradually increase or decrease the firing angle to slowly increase or decrease the amount of energy used, and give the application a softer start or ramp down than an across-the-line motor starter. Soft starters reduce wear and tear on the motor and any connected mechanical devices, and they also drastically decrease the inrush current required to start the motor. For large motors, this can have major implications for reducing utility costs.
Speed Control (VFDs)
Three-phase AC induction motors are sometimes powered by Variable Frequency Drives (VFDs) which, as their name suggests, varythe frequency of the power to the motor in order to vary the speed of the motor. These devices accept standard 60hz input power(single- or three-phase), rectify it to DC, and then use pulse width modulation (PWM) to create simulated AC power at whatever frequency is required to spin the motor at the target speed.
Single-Phase Operation:-
Single-phase AC induction motors are also available. These motors require special circuitry to start (starting capacitors, and centrifugal switches) but operate identically to their three-phase counterparts once they are spinning. Single-phase AC induction motors are not compatible with VFDs, and can result in higher utility costs due to their inherently unbalanced load on the power grid.
Specifying AC Motors:-
If you are specifying a motor for a new application, start by determining the voltage, speed and horsepower required along with the application type .. If you are replacing a properly-sized motor in an existing application, you can find all the required information on the motor nameplate of the existing motor. If you are considering rewinding the motor in lieu of replacing,
