Understanding the World of Electric Motors

Electric motors are vital components in various industrial, commercial, and residential applications, powering everything from household appliances to industrial machinery. They convert electrical energy into mechanical energy, generating electric current, and allowing machines to perform various tasks efficiently and effectively. Electric motors can be classified into two different kinds according to the type of power source used – Direct Current (DC) motor and Alternating Current (AC) motor.

A DC motor uses direct current electricity to create mechanical motion, while an AC motor uses alternating current electricity for the same purpose. The frequency for DC is zero and AC has a frequency of 60Hz. The main difference between them is the direction in which the electron flows. In DC, the electrons flow in a single direction, while in AC, the electric current switches back and forth at regular intervals or cycles.

Mainly, the AC motor is classified into two types: synchronous AC motor and induction AC motor. They have the same stator configuration for producing the rotating magnetic field but different rotor constructions due to their different ways of generating the rotor magnetic field.¹⁾ AC motors are widely used in homes, HVAC systems, and commercial buildings due to their robustness, reliability, and efficiency.²⁾

Synchronous AC motors are a type of electric motor that operates at a constant speed, which is directly proportional to the frequency of the AC power supply. Therefore, they are mostly used in industrial applications where a constant speed is required such as generators, compressors, pumps, etc.³⁾ IPM motors take the concept of synchronous motors a step further. It is a type of synchronous motor where permanent magnets are inserted inside the rotor. Therefore, they generate higher torque density, efficiency, and durability.⁴⁾
Inductance plays a critical role in IPM performance because it controls the motor’s torque, and accurate parameters, such as the flux linkage of the permanent magnets and the difference between the d- and q-axis inductance are required. By adjusting the current in the q-axis windings, it can influence the overall magnetic field and manipulate the motor's torque output. A lower d-axis inductance allows for better torque control. IPMs have two key inductance values:
▶d-axis inductance : This aligns with the permanent magnets and experiences lower inductance due to the strong magnetic path provided by the magnets.
▶q-axis inductance : This is perpendicular to the d-axis and coincides with the stator windings. The air gaps between the rotor and stator create a higher reluctance (resistance to magnetic flux) in this axis, resulting in a higher q-axis inductance.

In contrast, an induction motor is an AC electric motor in which the current in the rotor that produces torque is obtained by electromagnetic induction from the magnetic field of the stator winding. Due to their robustness, efficiency, and simplicity, they are widely used in industrial machinery and household appliances.⁵⁾
Understanding the speed-torque characteristics of an induction motor is essential to use and selecting a motor that is appropriate for a particular application. The motor should be able to produce enough torque to start and run the load.
The graph below shows the relationship between the torque produced by the motor and its speed. The ‘Load Curve’ represents the torque required by the load on the motor. The point where the two curves intersect is the operating point of the motor where it is producing the torque required by the load and running at the corresponding speed.

①Starting torque: A torque generated by the motor when it begins to start from a standstill. It is also known as the Locked rotor torque or Breakaway torque.
②Pull-up torque: It is the minimum torque that the motor produces as it accelerates from a standstill to full speed.
③Breakdown torque: The motor can produce the highest torque without stalling or losing synchronous speed and typically occurs at around 80-90% of the synchronous speed.

DC motors are used in many industrial applications such as electric vehicles, electric cranes, and robotic manipulators due to their precise, wide, simple, and continuous control characteristics. The DC motor can be mainly classified into separate excited DC motors, in which the armature winding and field winding each have separate DC power supplies, and self-excited DC motors which the two windings share one DC power supply. A DC motor also has a mechanical commutation device consisting of brushes and commutators that convert DC to AC in the armature winding. ⁶⁾
Recently, the use of Brushless Direct Current (BLDC) motors is rapidly gaining popularity due to their high efficiency, higher speed ranges, noiseless operations, and many more advantages.⁷⁾ A BLDC motor is a synchronous electric motor powered by DC and has an electronically controlled commutation system, instead of a mechanical commutation system based on brushes. In such motors, current and torque, voltage, and rpm are linearly related.⁸⁾ When a BLDC motor rotates, each winding generates a voltage known as back Electromotive Force or back EMF. Electromotive force (EMF) is an energy per unit electric charge imparted by an energy source, such as an electric generator or a battery. It is commonly measured in units of volts.⁹⁾ The Back EMF system uses another motor to measure induced voltage waveform after rotating the motor to be measured at a constant speed.

The block diagram below shows a system for measuring the back EMF constant of a motor. The test motor is connected to a controller, which presumably controls the speed of the motor. The output of the test motor is connected to an oscilloscope through a voltage divider. The voltage divider scales down the voltage so that it can be measured by the oscilloscope. The oscilloscope measures the peak-to-peak voltage of the signal from the test motor. Overall, measuring a constant of back EMF is important as it can be used to calculate the torque produced by the motor.

Electric motors are integral to modern life, from their fundamental principles and diverse types to their wide-ranging applications potential. Understanding their operation, benefits, and challenges enables us to better appreciate the critical role they play in various industries and everyday devices, but also to contribute to their future development. As we strive for cleaner energy solutions and ever-more-sophisticated machines, electric motors will undoubtedly continue to evolve, driving innovation and shaping the world around us.

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