Drives and motors are becoming more compact and robust in design and energy efficient as well. By Avi Chavan

Improving the efficiency and design of electric motors and the equipment they drive can save energy, reduce operating costs, and improve nation’s productivity. Major types of electric motors manufactured in big volumes include single phase fractional horse power (FHP) motors ranging from between 0.18 to 0.75KW, and three phase squirrel cage motors up to 5HP (3.7KW). Three phase squirrel cage motors are used in pump sets, compressors, blowers, exhaust fans and conveyors. Manufacturers offer drive solutions for ratings up to 5MW and beyond. The types of drives offered by them range from the simple variable frequency controlled drives to complex flux vector control and special purpose crane and elevator specific drives. Drive solutions are used by all major industry segments like oil & gas, water and wastewater, HVACR, material handling and transportation. They are also used for standalone applications like fans and pumps as well as co-ordinated systems like rolling mills.

Basically, electric motors can be divided into alternating current (AC) motors, direct current (DC) motors, and universal motors. A universal motor will run with either AC or DC current. AC electric motors are further subdivided into single phase and three phase motors, with former applicable to homes and the latter only to factory settings. DC electric motors are further split into brush motors, brushless motors, and stepper motors. Of these types, brush electric motors are the most common. They are easy to build and very cost effective. Their only drawback is that they use carbon brushes to transfer electrical current to the rotating part, so these brushes wear over time and eventually result in the failure of the electric motor. The DC brushless motor eliminates the brushes, but is more costly and requires much more complicated drive electronics to operate.

A stepper motor is a special type of brushless motor that is used primarily in automation systems and has a special type of construction that allows a computerised control system to “step” the rotation of the motor. Their best known application is in controlling a robotic arm. For instance, when you wish to move a specific distance as directed by a procedure in a programme on the computer, a stepper motor may be the best choice.

Universal motors tend to have many features in common with DC electric motors, especially with brush motors. They are called series-wound motors with major applications in household appliances that run very fast for a short period of time, such as in food processors, blenders, and vacuum cleaners.

All electric motors usually come in different horsepower depending on their application. The most common sizes are in fractional horsepower, i.e. 1/2 horsepower or 1/4 horsepower. Larger motors can range in size to thousands of horsepower.

Electric motors also come with various speed ratings. Speed is usually specified as rotations per minute (RPM) at no load condition. As the motor is loaded down, the speed will slow down. If the electric motor is loaded too heavily, the motor shaft will stop. This is known as the stall speed and should be avoided.

Emerging technologies

For automation applications, the latest technology available in low voltage and medium voltage AC drives are the multi level output stages as well as active frontend or 36 pulse input stages. These have a near zero harmonic pollution and unity power factor. The latest generation of AC drives offer control performance at par with DC drives with a better starting torque and speed holding accuracy.

According to major motor manufacturers, most customers prefer inbuilt control capability in the drives and hence the latest generation of drives have a dedicated memory which is available to the user for customising as per their individual control requirements. For example, Siemens has come up with a uniform platform of Sinamics for all low voltage and medium voltage drives. This is a great advantage to all the drive customers as they do not have to learn new platforms for various power and voltage ratings.

The new concepts are:

  • Modularity
  • Drives supporting green technology, with features like low harmonics and regeneration
  • Flash card for data portability
  • Safety integrated functions

In control technology, motors have vector control for better dynamic speed and torque control. Most of these drives are now provided with multiple protocols for networking. Frequency drives are being used for applications that were the domain of DC drives through the incorporation of latest in flux vector control and on-line auto tuning for mapping the motor parameters accurately inside the drive software, using advanced processors in the control circuit to perform these calculations accurately and in a very short time.

Since energy savings have now assumed paramount importance, user industries have now started appreciating the use of regenerative front end. This allows savings of power which otherwise would have been wasted in resistor banks. Regenerative front end also helps in lowering the total current and voltage harmonic distortion to well within the limits specified by IEEE 519.

An important trend in control technology is its modularity and the concept of same control philosophy for all ratings. Servo motors now feature power saving circuits with high density power magnets in the rotors to manage better current to torque ratios. Another trend for the future is ‘Matrix Convertor Topology,’ where there will be direct AC to AC conversion without the DC link, with built in regenerative facility which improves efficiency as well as harmonics.

Power devices are now shifting towards higher voltage and current handling capability of IGBT, which will help lessen the number of power devices for a given voltage level and power rating. This will in turn improve the reliability and efficiency of motors. In power devices, IGBTs are available with higher voltages and new improved IGBTs with low loss are available for building up power stacks.

Control circuits are now provided with multiple protocol interfacing, multi level switching, and auto tuning. An important development in rotor resistance control is the new generation IGBT-based Slip Power Recovery System with unity power factor and negligible harmonics for advanced slip ring motor control. This system has high tolerance to supply voltage disturbances.

As power circuits and other electronics get sophisticated, motors are becoming more robust and usable in extreme weather conditions and drives are getting smarter allowing for better control of the motor.

Concept of variable speed drives

Most types of variable speed electrical drive systems comprise of the following components – an electronic actuator as the controller, an electrical motor and the driven machine (the load) such as a pump, fan, blower or a compressor. The task of a variable speed electrical drive is to convert the electrical power supplied by the mains into mechanical power with a minimum loss. To achieve an optimum technological process, the drive must be variable in speed. This will steplessly adjust the speed of the driven machine. This is ensured by the low loss control using solid state technology in electronic controllers. The solid state devices, which convert the AC supply to DC supply, were first used as variable speed devices in DC technology. Using these devices the armature voltage of a DC motor and therefore the speed can be adjusted, almost without losses and over a wide range of speeds.

Using these features the drive can be designed to start smoothly and jerk-free. This helps to maintain the desired selected speed, independently of the load and operate with good dynamic response. The DC drive needs special consideration in some applications. For example in hazardous atmosphere, with vibrations and higher speeds the usage of AC motor with squirrel-cage rotor is advantageous. Variable frequency inverters supply power to AC motors allowing a new orientation to enable handling variable speeds during operation. Every standard AC motor can be fitted with a variable speed drive using a frequency inverter. Frequency and voltage of the single – phase or three – phase mains are varied by the frequency inverter, such that the motor can be operated with varying speeds over large range settings.

Motor maintenance

A motor’s purchase price is only two per cent of its total life-cycle cost. The key to maximising the life of electric motors is to study the failures and document them. Then, take actions to prevent these failures from happening to your other motors in similar applications. This may involve changing your maintenance procedures. Perhaps you need to grease more or less, or use more compatible grease. Perhaps you need to identify voltage supply problems. Check to see if contamination and water are getting into the motors. If either one of these is an issue, check if you have the right enclosure, and whether it needs a higher level of environmental protection. If so, look into severe duty, in IEEE 841 or wash down motors. Also, focus on proper lubrication and temperature. In this context, bearings must receive not only a good quality lubricant but the correct quantity at proper intervals in order to optimise its life and reliability. Under or over greasing will be detrimental to reliability. Under-greasing does not provide the lubricant at the time it is needed, resulting in bearing wear or heat damage. Over-greasing can damage shields or significantly increase operating temperatures due to fluid shear friction. This reduces the grease’s lubricating capability. Oil-lubricated bearings must have the correct type and viscosity of oil. Also follow manufacturer’s recommendations for lubrication types, amounts and schedules.

Temperature is the biggest hazard for electric motors. Overload, undervoltage, over-voltage, unbalanced voltage and improper ventilation can all work to increase the motor’s operating temperature. The rule of thumb is that motor life is cut in half for every increase of 10 degrees Celsius. Focus on mitigating the operating temperature will result in increased life and reliability. Also, never make the mistake of increasing the rated capacity of a motor applied in a high ambient temperature environment to accommodate the winding temperature increase.

The author is a freelance technical writer based in Mumbai, India. Email: cpi.industry@gmail.com