Universal and Reluctance Motors

Universal Motor

• A universal motor can work on both AC and DC power.
• These motors operate at a higher speed on DC than AC due to reactance voltage drop in AC.
• Universal motors have a high starting torque and are usually built into the devices they drive.
• Construction consists of a stator with field poles, field coils wound around the poles, and a laminated armature to reduce eddy currents in AC operation.
• The armature is made of slots with a commutator and brushes that have high resistance for better commutation on AC.
• On DC, the motor works like a series motor, with current flowing through both armature and field windings.
• On AC, the direction of current in both armature and field windings changes simultaneously, ensuring unidirectional torque despite the alternating current.

Reluctance Motor

• Reluctance motors are synchronous motors.
• The stator has a laminated construction with a winding excited by single-phase AC.
• The rotor has a specific shape that creates a variable air gap, resulting in varying reluctance between stator and rotor.
• The rotor can be a short-circuited squirrel-cage type, similar to an induction motor.
• The rotor tends to align with the stator magnetic field in a minimum reluctance position.
• When close to synchronous speed, the stator field pulls the rotor into synchronism and locks it magnetically.
• The reluctance torque, which drives the rotor, is generated by the varying reluctance.
• Advantages include: no DC supply needed for the rotor, constant speed, robust construction, and low maintenance.
• Disadvantages include: lower efficiency, poor power factor, need for low inertia, and low capacity for load driving.
• Applications: signaling devices, control apparatus, automatic regulators, recording instruments, clocks, timing devices, teleprinters, and gramophones.

Permanent Magnet Synchronous Motor (PMSM)

• PMSMs have permanent magnets in the rotor.
• Stator: Outer frame, core, and windings (usually two-phase or three-phase).
• Rotor: Located inside the stator (typically), with different designs based on the ratio of Ld/Lq (length of the field winding along the rotor axis and along a rotor radius) and its structure (e.g., axially layered or with barriers).
• Types: Surface permanent magnet (SPM) and interior permanent magnet (IPM) motors.
• Stator designs: Distributed winding or concentrated winding.
• Higher efficiency than induction motors (about 2% more) with similar stator design and variable frequency drive.
• Offers better performance in power/volume and torque/inertia compared to other electric motors.

BLDC Motor (Brushless DC Motor)

• Uses DC power supply and electronic commutation instead of mechanical brushes, resulting in higher efficiency and reduced wear.
• Consists of a rotor with permanent magnets and a stator with coils.
• Electronic controllers and sensors (Hall effect sensors) determine rotor magnet position and switch the current accordingly.
• No frictional losses due to brushes, leading to higher efficiency and less heat generation.
• Delivers high torque per volume and can operate at high speeds.
• Suitable for variable speed and high efficiency applications, such as electric vehicles, drones, appliances, and industrial machinery.
• Precise control of speed and torque is achieved through electronic control systems.
• Wider application in EVs, HVAC, computer peripherals, industrial automation, robotics, and more.