Expansive speed control Not able to operate at low speed Are there anymore Disadvantages?
Creates nasty harmonic voltages in the building's power system.
Speed Control The main disadvantage of AC motors, when compared with DC motors, is that their speed is more difficult to control. AC motors can be equipped with variable frequency drives, which provide smooth turning moment, or torque, at low speeds and complete control over the speed of the motor up to its rated value. Variable frequency drives improve speed control, but do create losses with reduced power quality. Control Complexity The device used to control an AC motor, known as an AC drive, is typically more complex than that used to control a DC motor. An AC motor controller must convert AC mains electricity to DC, and convert DC back again to an adjustable frequency and voltage output to the AC motor. By contrast, a DC motor has a separate field, or excitation, and power circuits, requiring just a single power conversion from AC to DC -- so the circuitry of a DC motor controller is simpler. Power Torque Historically, another disadvantage of AC motors has been that they develop less power, or less torque, per frame size. Research conducted by Rockwell Automation compared typical, commercial AC and DC motors in industry standard National Electrical Manufacturers Association (NEMA) frame sizes, and found this to be the case. However, AC and DC motors employ different classes of insulation and have different service factors -- the overload capacity at which the motor can operate without damage -- so direct comparison is not always straightforward. Other Disadvantages AC motors are typically more expensive than DC motors for most horsepower ratings. If the rotor of an AC motor rotates faster than the speed command of the AC drive, the motor acts as a generator and pumps, or regenerates, energy into the drive. If the drive cannot absorb this energy, the drive will switch off, or trip, with a fault. AC drives capable of regeneration are more complex and more expensive than DC regenerative drives. AC motors may also produce more audible noise than DC motors.
