What is an AC motor? What are AC motors used for?

AC motors are capable to convert electrical energy into mechanical energy, as the name indicates AC motor works on alternating current. This motor is capable to distribute alternating current over a long-range. AC motors are widely used in many industries and it is really useful for industrial operations. We can see the application of the AC induction motor in the industrial motion control system. There are many types of AC motors that are available in the market and all of them have their own applications. So the AC motor converts the current into mechanical energy and this mechanical energy is used to rotate fan, blower, or pump impeller, it is also used to lift materials and to drive compressors.

How AC motors constructed?

The three-phase AC induction motor is used for many industrial applications and it has two major parts they are stator and rotor. A stator is the stationary part of the induction motor and this part has windings. The second major part is the rotor and it is the rotating section and this part has the conductor. The stator would look like a cylinder and it has coils that are insulated and it is in the slots, so the coils and the steel core together make an electromagnet. The rotor of the motor is of two types and they are wound rotor and squirrel cage. In the wound rotor type, it has coils of wire wound in its rotor slot. While in the case of squirrel cage type it has copper or aluminum bars that will be electrically connected at each end of the conducting rings.

What are the parts of the AC motor?

  1. Enclosure – it is the outer part of the motor, it can hold parts together and can help with heat dissipation, and can protect the internal components from hazards
  • Stator – it is the stationary part also called windings it is basically an iron core which is wounded with copper wire
  • Rotor – it is the rotating part of the motor
  • Bearings – This part is to support the shaft
  • Terminal box – It is the part where electrical power is connected to the motor
  • Fan
  • Eyebolt

How does an AC motor work?

The AC motor operates by converting the alternating current into mechanical energy. If a conductor moves across a magnetic field then it will create a voltage and if this conductor is in a closed circuit then current will be created. So in an AC motor, a conductor will be placed in a magnetic field and current will be applied to the conductor and the result of this will be electromagnetism. The supply will be given to the stator of the motor, then the coils in the stator will create a magnetic flux. This rotating magnetic flux will come in contact with the rotor conductors and it will create a current. The mechanical force is created when the current interacts with the magnetic field of the stator.

What are the types of AC motors?

The AC motors are of two types and it is based on the type of rotor synchronous motor and induction motor. The induction motor is then again divided into two types they are single-phase and three-phase AC motor.

Synchronous motor

In this type, a separate DC power source supplies the magnetic field current. The rotation of the synchronous motor is related to its input current frequency. It has a constant speed and has a good power factor. The synchronous motor can provide reactive power. This type of motor has a good power factor and because of that energy costs can be reduced

Induction motor

The induction motor is also called a squirrel cage motor because it can rotate at lower speeds than its input frequency. Induction motors can be seen in many industrial applications because of its simple construction and it doesn’t need any external electrical connection to the rotor and because of this, it won’t need any slip rings and brushes.

How to control the speed of an AC motor?

The operation of the AC motor is depended on the magnetic field and the motor speed is related to the frequency of the power supply. The torque of the motor is related to the applied voltage. So if the frequency of the supply is lowered then the rotor will slow down, so we can also reduce the voltage according to the frequency then we could get a constant torque.

We can also control or vary the speed of the motor by changing the number of poles and also by changing the slip. We can also use VFD and inverters to change the motor speed.

Three-phase AC motor?

In this type of motor, it has three pairs of electromagnets and this would be connected to any of the three phases of the power supply and it could give a lot of power than a single-phase motor.

What is the major difference between DC and AC motors?

AC motors won’t use any brushes, while the DC motors have brushes in it. In an AC motor, the speed control is done by varying the frequency and it is achieved by using a VFD, while in a DC motor the speed control is done by varying the armature winding current. AC motors are used for applications where power and performance are an important factor, while the DC motors are used where the motor speed needs to be controlled.

How to check a motor before commissioning?

We must do mechanical checks like the alignment of the driving and driven equipment, flexible coupling, we must check the grease condition, we must check the air gap between the stator and rotor. We must also check the flexible coupling and also the free rotation of the rotor by hand.

We must do certain electrical checks like if all of the connections and the contacts are tightened. Check the earth connection and also check its resistance is in the normal value. Check the fuse rating and also the overload relay setting, and we also need to check the supply voltages in all phases.   

What are the losses that can be seen in a motor?

  • Fixed loss
  • Rotor and stator loss
  • Friction and rewinding
  • Stray load loss

What are the factors that must be considered while selecting a motor and how to do the motor maintenance?

  • We must understand the power requirements, the motor should not run beyond their rated load because it could be damaged
  • We must provide a proper cooling mechanism because the motor could generate a lot of heat
  • In order to prevent breakdowns, we must provide a clean and dry area for the motor
  • The motor parts like brushes, windings, etc, could wear out during time, so must replace it and we must also check the motor parts

There is a good chance that the dirt could enter the motor while providing the cooling we must take care of this, then we must clog the air gap and the cooling ducts. Insulation checking must be done periodically, to prevent insulation damage low test voltage can be applied. Check the rotors and bearings of the motor and provide lubrication to the bearing.

What are the problems that could arise in an AC motor?

  • High eddy current loss
  • Low power factor could be another problem
  • There could be sparking in the brushes
  • There could be a chance for an increase in reactance drop of armature and field winding

How to select an induction motor?

  • It must be selected according to the starting torque
  • We must consider the starting torque
  • Efficiency is an important factor
  • Pull out torque is an important factor
  • We must also consider the power factor

What are the advantages of an AC motor?

  • It has a simple design
  • Economical
  • Good power to weight ratio
  • Less maintenance
  • It can be directly connected to an AC source
  • Good power factor

What are the disadvantages of an AC motor?

  • It won’t be able to operate at low speeds
  • Positioning control is really poor
  • Speed is limited by the supply frequency
  • It has very low starting torque


post-graduate in Electronics & communication.

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