Other Types of Motors

What was described earlier was one type of Direct Current (DC) motor. There are several types of DC motors and Alternating Current (AC) motors, all utilizing electromagnetism to generate motion.

Wound Field DC Motor – Instead of using a fixed magnet on the stator as the PMDC motor has, this is replaced by electromagnets. There are 3 types of wound field DC motor
a. Series Wound – The field winding is in series with the armature winding. This produces a high torque low speed motor
b. Shunt Wound – The field winding is in parallel with the armature winding. This produces a low torque high speed motor
c. Compound Wound – The field winding is partly in parallel and partly in series. This produces a more balanced motor of a steady speed over a range of loads.
Stepper Motors – It is an electric motor without commutators. The commutator is replaced by an external motor controller that allows the shaft to be held in a fixed position or rotated in either direction in “steps”. Typically, the construction of a stepper motor is that the windings of the motor are part of the stator and the rotor could be a permanent magnet or a magnetically soft material. Since this type of motor could be rotated to a specified position with more accuracy and ease, this is commonly used in applications that require high precision such as disk drives or camera autofocus lens. (with picture)
Brushless DC Motors (BLDC) – Like the stepper motor, the BLDC motor also does not have a commutator and brushleaf. Commutation (or switching) is produced through an external motor controller that is synchronized to the rotor’s position. The rotor position is typically determined by the hall sensors and fed back to the motor controller. Utilizing the external motor controller, variable and precise speeds can be easily produced making it suitable for applications such as disk drive spindles or blower fans. Though more expensive than a PMDC motor, the BLDC has an advantage of having longer life, higher efficiency and lower noise.
Universal Motors – As the name implies, universal motors may use single phase AC or DC input current, but generally always used AC. Closely resembles a wound field DC motor, the mechanical force generated is uni-directional (one direction). The universal motor is designed for less that 1hp operation and can run up to 10,000 – 50,000 rpm speed at no-load condition. It is suitable for power tools and food mixer applications.
Single Phase AC Synchronous Motors – Regardless of the load, this motor operates at a fixed synchronous speed directly proportional to the frequency of the input AC power. By energizing the electro-magnet in the stator, the rotor (containing a permanent magnet) will turn at a speed equivalent to the frequency. When the rotor turns it can turn in either direction. To avoid this, a mechanical device called anti-return is placed on the rotor to ensure that the rotor will always turn on the same direction. The highly accurate speed, makes this motor suitable for tape drives or mechanical clock applications.
Shaded Pole Motors – Another type of AC single-phase induction motor, this motor has one main winding and start winding being called shade pole ring. Rotation is done using a copper ring around a small portion of each motor pole. The copper ring “shades” that portion of the pole, causing the magnetic field of the pole with the copper ring to lag the field of the pole without the copper ring. This interaction between the two magnetic fields enables rotation. Because of a low starting torque, this motor is an economical and reliable solution for fan applications.