What Is a Coreless Motor?
A coreless motor is a small motor without an iron core.
It is also called an iron coreless motor. The advantage of coreless motor is that there is no loss due to iron loss because there is no iron core. There is no vibration due to cogging, and the motor operates quietly.
However, the torque is lower because the magnetic force generated is weaker than that of a normal motor.
Uses of Coreless Motors
Coreless Motors are used in vibration motors that vibrate cell phones and servo motors for radio-controlled vehicles and robots, taking advantage of their small size and low vibration, noise, and electromagnetic interference.
Because of their excellent responsiveness and controllability, they are also used in surveying instruments, camera lens motors, ultrasonic endoscopes, surgical drills, etc., making them indispensable motors, especially for medical applications. Each manufacturer is developing its own unique coil winding method and materials.
Principle of Coreless Motors
An ordinary motor consists of a coil wound around an iron core, with a magnet placed outside the coil. Electric current flows through the coil, and the magnetic field generated by electromagnetic induction is used to cause the magnet to rotate.
In a coreless motor, on the contrary, a permanent magnet is placed inside and the coil is wound around the outside of the magnet in a cup shape using resin or other material, thereby eliminating the iron core. When an electric current is applied to the coil, the coil rotates, subject to Fleming’s left-hand rule. Since the coil rotates, it is called a rotor.
1. Response to Current
Coreless motors have low inductance in their windings and are highly efficient. The electrical time constant of the motor is a parameter that indicates the characteristics of the rising current in relation to the input voltage.
It indicates the time it takes for the current to reach 63.2% of its peak value. Since the electrical time constant is relatively low for coreless motors, the current responds very quickly.
2. Acceleration/Deceleration Response
Coreless motors are motors with low inertia due to their reduced weight. Inertia refers to the moment of inertia, which increases in proportion to the mass.
Inertia is proportional to the acceleration/deceleration torque, which is the torque required to accelerate or decelerate to a given rotational speed. In other words, the acceleration/deceleration torque is also smaller in coreless motors, making them suitable for rapid acceleration and deceleration.
The use of powerful rare earth magnets, such as neodymium magnets for the internal magnets, has led to further miniaturization and increased torque. They help reduce size, thickness, and weight.
Other Information on Coreless Motors
1. Efficiency of Coreless Motor
A coreless motor does not generate iron loss. Iron loss consists mainly of hysteresis loss and eddy current loss. The energy loss caused by a change in the magnetic field report of the iron core is called hysteresis loss.
In addition, changes in the direction of the magnetic field generate electric current inside the iron core. The generated electrical energy is converted to heat and escapes to the outside, and this loss is called eddy current loss.
In the case of coreless motor, the above-mentioned iron loss does not occur, so the motor can operate at high efficiency even at high speeds. Each company’s technology is reflected in the way the coils are wound to minimize waste and increase efficiency.
2. Difference Between a Coreless Motor and a Brushless Motor
A coreless motor is a motor that does not use an iron core (core) but consists of coils and magnets. A motor with coils wound around a general iron core is called a cored motor.
A brushless motor is a motor that does not use brushes and is rotated by controlling the direction of the electric current by an electronic circuit. DC motors (direct-current motors) are classified according to whether they are controlled by electronic circuits or commutators and brushes, as they need to periodically reverse the direction of the current to keep rotating.
Coreless motors come in two types: brushed and brushless. A motor that is coreless and brushless is called a coreless brushless motor. For brushed motors, the rotor is a coil, and the stator is a permanent magnet. For brushless motors, the rotor is a permanent magnet, and the stator is a coil.
3. Advantages and Disadvantages of Coreless Motors
Advantages
- Small size and lightweight
- High-speed rotation is possible
- High efficiency and low current consumption
- Cogging-less
The iron core accounts for most of the weight of a motor. The absence of an iron core in a coreless motor allows it to be smaller and lighter. They also have a small moment of inertia (inertia), resulting in excellent responsiveness and controllability, as well as high motor efficiency and high-speed rotation.
In the case of cored motors, the iron core and magnet repeatedly move closer together and farther apart, each time causing cogging, or magnetic force trapping. Coreless motors, however, have the advantage of smooth and quiet rotation because cogging does not occur.
Disadvantages
- Low torque
- Insensitive to heat
- High price
The disadvantage of coreless motors is that they generally have low torque. The amount of torque is proportional to the current value. To achieve high torque, a large current must be applied, but because the copper wire is thin and there is no iron core, the coil is mechanically weak and a large current cannot be applied.
If a large current is applied to a coreless motor with a small allowable current value, the heat may deform the coil and lead to motor failure.