MEBS Utilities services Department of Electrical & Electronic Engineering University of Hong Kong

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Debra Bell
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1 Brief comparison of induction motors with other types of motors Electric motors exhibit wide variations of speed-torque characteristics. [Adopted from EL-SHARKAWI, Mohamed A., Fundamentals of Electric Drives.] Motors commonly used in electrical drives are: - DC motors shunt or separately excited motor speed slightly reduced when the load torque increases - DC series motors high speed at light load and low speed at high torque - DC compound motors - DC permanent magnet motors - Brushless DC motors - AC induction motors squirrel cage, wound rotor and linear complex torque speed characteristics - AC synchronous motors wound field and permanent magnet these exhibit a constant speed characteristic independent of load torque. - Switched reluctance motors these exhibit a constant speed characteristic independent of load torque - stepping motors In the past, induction and synchronous motors were employed mainly in constant speed drives. Variable speed drives consisting these machines were either too expensive or had very poor efficiency. Consequently, variable speed drive applications were dominated by DC motors in the past. Due to development of semiconductor converters employing thyristors, power transistors, IGBTs and GTOs,, at affordable costs, AC induction motors are now widely employed in variable speed drives. K.F. Chan (Mr.) Page 3 of 8 July 2010

2 Due to presence of commutator and brushes, DC motors have a number of disadvantages as compared to AC motors: Higher cost, Higher weight to power ratio, Higher volume, Higher inertia, Need for frequent maintenance, Unsuitable for explosive and contaminated environments, Restriction on maximum voltage, speed and power ratings. Wound field and permanent magnet synchronous motors have a higher full load efficiency and power factor than induction motors. Wound field motors can be designed for a higher power rating than induction motors. However, compared to squirrel cage, they have higher cost and size for the same rating and require more maintenance. The permanent magnet synchronous motors have all the advantages of squirrel cage except that they are available in lower power ratings only. (*) Wound rotor motors are more expensive than squirrel cage motors, their maintenance, though less than DC motors, are still higher than that for squirrel cage. Wound rotor motors are also available in high power ratings. Brushless DC motor is somewhat similar to a permanent magnet synchronous motor, but has lower cost and requires simpler and cheaper converter. It is being considered for low power high speed drives and for servo applications, as an alternative to dc servomotor which has been very popular so far. The DC servomotor has all the disadvantages of commutator and brushes listed above. At low power levels, the coulomb friction between the brushes and commutator is objectionable, as it adversely affects the steady state accuracy of the drive. Recently, stepper motor has become popular for position control and switched reluctance motor drive for speed control. K.F. Chan (Mr.) Page 4 of 8 July 2010

3 AC motors 3-phase HP Rotor Stator Comments and applications range Induction Squirrel cage Three Simple rugged construction; very common; fans, phase pumps Wound rotor armature s Adjustable speed using rotor resistance; cranes, hoists Higher cost and more maintenance than squirrel cage though less than DC motors. Synchronous 1-5(*) Permanent magnet Precise speed; transport sheet materials, Higher efficiency and power factor, but higher cost, larger size, more maintenance than squirrel cage DC field Large constant loads; potential for power factor 50,000 s correction Higher efficiency and power factor than squirrel cage Single phase Induction 1/3-5 Squirrel cage Main and auxiliary s Synchronous 1/10 or Reluctance or Armature less hysterisis Several types: split phase, capacitor start, capacitor run; simple and rugged; many household applications: fans, water pumps, refrigerators Low torque, fixed speed; timing applications K.F. Chan (Mr.) Page 5 of 8 July 2010

4 HP Rotor Stator Comments and applications range DC Wound Shunt Armature Field Industrial applications, grinding, machine tools, motors field hoists Series High torque at low speed; dangerous if not loaded; drills, automotive starting motors (universal motor used for single-phase ac has high power/weight ratio) Compound Can be designed to tailor torque speed characteristic; traction motors Permanent -magnet field 1/20-10 Armature Permanent magnets Servo applications, machine tools, computer peripherals, automotive fans, window motors (*) A manufacturer recently patented a new PMSM having ratings up to tens of horsepower. [Table adopted from HAMBLEY, A.R. Electrical Engineering Principles and applications,] K.F. Chan (Mr.) Page 6 of 8 July 2010

5 Squirrel cage induction motor, which costs nearly only one third of a DC motor of the same rating, is extremely rugged, requires minimal maintenance, and can be built for higher speeds, torques and power ratings. Because of the numerous advantages of AC induction motors described above, AC induction drives have succeeded in replacing DC drives in a number of variable speed applications. Machines that employ gears generally require a motor that can start at reduced speed and increase speed gradually. Induction motors controlled by frequency inverter drives are excellent choice. Machines that require a large starting period (large inertia load like flywheels or centrifuges) require a motor with large starting torque and low starting current. Squirrel cage induction motors are the prime workhorse in building services applications due to its low cost and ruggedness. (This section of text adopted from DUBEY, G.K. Fundamentals of Electrical Drives) K.F. Chan (Mr.) Page 7 of 8 July 2010

1.1 Block Diagram of Drive Components of Electric Drive & their functions. Power Processor / Modulator. Control. Unit

1.1 Block Diagram of Drive Components of Electric Drive & their functions. Power Processor / Modulator. Control. Unit Introduction Motion control is required in large number of industrial and domestic applications like transportations, rolling mills, textile machines, fans, paper machines, pumps, washing machines, robots