What is a Motor?
A motor converts electrical energy into mechanical energy. It is estimated that nearly half of the world's energy consumption is consumed by motors. Therefore, increasing motor efficiency is expected to have a significant impact on the global energy crisis. Here we will explain the operating principle of motors, beginning with the basics.
Motor Types.
Motors that operate through rotary motion by receiving power when current flows within a magnetic field are common. However, a wide range of motor types exist, including ones that operate through linear motion. Motors are broadly classified by the type of drive power used (source): AC or DC.
They can also be further divided by the rotational principle, as shown in the chart below.
How Does a Motor Work?
Motor Rotation Using a Magnet/Magnetic Force
The basic operating principle of a motor is as follows.
Around a permanent magnet having a rotational axis:
① When the outer magnets rotate (referred to as a rotating magnetic field),② The N and S poles attract and repel each other, ③ Causing the magnet with the rotational axis (center) to turn.
Alternatively, supplying current to a conductor generates a magnetic field around, creating a rotational magnetic field (magnetic force). This results in the same effect as rotating a magnet.
If we wind a conductive wire into a coil the magnetic force is combined, generating a large magnetic flux along with North and South poles.
Actual Motor Operation
Here we see an actual method used for rotating a motor by generating a rotational magnetic field from a 3-phase AC source and conductive coils (3-phase AC is an AC signal with phases shifted 120°).
The synthetic magnetic field in ① above is shown in ① below
The synthetic magnetic field in ② above is shown in ② below
The synthetic magnetic field in ③ above is shown in ③ below
As mentioned above, the 3 phase coils (U, V, and W) are wrapped around an iron core, positioned 120° apart, with the higher voltage generated at the North pole(N) and the lower voltage at the South pole(S). Since each phase varies sinusoidal, the pole (N/S) generated from each coil will continue to change, along with the magnetic field.If we look at just the North pole phase, it switches in order from the U coil → V coil → W coil → U coil, enabling rotation.
What is the difference between an AC motor and a DC motor?
While both A.C. and D.C. motors serve the same function of converting electrical energy into mechanical energy, they are powered, constructed and controlled differently. 1 The most basic difference is the power source. A.C. motors are powered from alternating current (A.C.) while D.C. motors are powered from direct current (D.C.), such as batteries, D.C. power supplies or an AC-to-DC power converter. D.C wound field motors are constructed with brushes and a commutator, which add to the maintenance, limit the speed and usually reduce the life expectancy of brushed D.C. motors. A.C. induction motors do not use brushes; they are very rugged and have long life expectancy. The final basic difference is speed control. The speed of a D.C. motor is controlled by varying the armature winding’s current while the speed of an A.C. motor is controlled by varying the frequency, which is commonly done with an adjustable frequency drive control.
Different types of motors and their use.
Different types of motors and their use
When purchasing a motor, it’s often asked which technology is better, AC or DC, but the fact is that it is application and cost dependent.
AC Motors
AC motors are highly flexible in many features including speed control and have a much larger installed base compared to DC motors, some of the key advantages are:
Low power demand on start
Controlled acceleration
Adjustable operational speed
Controlled starting current
Adjustable torque limit
Reduced power line disturbances
The current trend for VSD is to add more features and programmable logic control (PLC) functionality, which are advantages for the experienced used, but require greater technical expertise during maintenance.
Types of AC motor include:
Synchronous
In this type of motor, the rotation of the rotor is synchronized with the frequency of the supply current and the speed remains constant under varying loads, so is ideal for driving equipment at a constant speed and are used in high precision positioning devices like robots, instrumentation, machines and process control
Induction (Asynchronous)
This type of motor uses electromagnetic induction from the magnetic field of the stator winding to produce an electric current in the rotor and hence Torque. These are the most common type of AC motor and important in industry due to their load capacity with Single-Phase induction motors being used mainly for smaller loads, like used in house hold appliances whereas Three-Phase induction motors are used more in industrial applications including like compressors, pumps, conveyor systems and lifting gear.
DC Motors
DC motors were the first type of motor widely used and the systems (motors and drive) initial costs tend to be typically less than AC systems for low power units, but with higher power the overall maintenance costs increase and would need to be taken into consideration. The DC Motors speed can be controlled by varying the supply voltage and are available in a wide range of voltages, however the most popular type is 12 & 24V, with some of the advantages being:
Easy installation
Speed control over a wide range
Quick Starting, Stopping, Reversing and Acceleration
High Starting Torque
Linear speed-torque curve
DC motors are widely used and can be used from small tools and appliances, through to electric vehicles, lifts & hoists
The two common types are:
Brushed
These are the more traditional type of motor and are typically used in cost-sensitive applications, where the control system is relatively simple, such as in consumer applications and more basic industrial equipment, these types of motors can be broken down as:
Series Wound – This is where the field winding is connected in series with rotor winding and speed control is by varying the supply voltage, however this type offers poor speed control and as the torque to the motor increase, then the speed falls. Applications include automotive, hoists, lifts and cranes as it has a high starting torque.
Shunt Wound – This type has one voltage supply and the field winding is connected in parallel with the rotor winding and can deliver increased torque, without a reduction in speed by increasing the motor current. It has medium level of starting torque with constant speed, so suitable for applications include lathes, vacuum cleaners, conveyors & grinders.
Compound Wound – This is a cumulative of Series and Shunt, where the polarity of the shunt winding is such that it adds to the series fields. This type has a high starting torque and run smoothly if the load varies slightly and is used for driving compressors, variable-head centrifugal pumps, rotary presses, circular saws, shearing machines, elevators and continuous conveyors
Permanent Magnet – As the name suggests rather than electromagnet a permanent magnet is used and are used in applications where precise control and low torque, such as in robotics, servo systems.
Brush-less motors alleviate some of the issues associated with the more common brushed motors (short life span for high use applications) and are mechanically much simpler in design (not having brushes). The motor controller uses Hall Effect sensors to detect the rotors position and using this the controller can accurately control the motor via current in the rotor coils) to regulate the speed. The advantages of this technology is the long life, little maintenance and high efficiency (85-90%), whereas the disadvantages are higher initial costs and more complicated controllers. These types of motors are generally used in speed and positional control with applications such as fans, pumps and compressors, where reliability and ruggedness are required.
An example of brush less design are in Stepper Motors, which are primarily used in open-loop position control, with
uses from printers through to industrial applications such as high speed pick
and place equipment.
