When buying a motor, it is often asked which technology is better, alternating current or direct current, but it actually depends on the application and cost.
1. AC Motors
AC motors are highly flexible in many functions, including speed control (VSD-Variable Speed Drive). Compared with DC motors, they have a larger installation base. The key advantages are:
Low power demand start
Controlled acceleration
Adjustable operating speed
Controlled starting current
Adjustable torque limit
Reduce power line interference
The current trend of VSD is to add more functions and programmable logic control (PLC) functions, which is beneficial to experienced users, but requires more technical expertise during maintenance.
The types of AC motors include:
(1) Synchronous
In this type of motor, the rotation of the rotor is synchronized with the frequency of the power supply current, and the speed remains constant under varying loads, so it is ideal for driving equipment at a constant speed, and is used for high-precision positioning equipment, For example, robots, instrumentation, machine and process control.
(2) Induction (Asynchronous)
This type of motor uses electromagnetic induction from the magnetic field of the stator windings to generate current in the rotor, thereby generating torque. These are the most common types of AC motors. Because of their load capacity, they are important in industry. Single-phase induction motors are mainly used for smaller loads, such as household appliances, while three-phase induction motors are more used in industry. Applications include compressors, pumps, conveying systems and lifting devices.
2. DC Motors
DC motors are the first type of motors that are widely used. The initial cost of the system (motor and drive) is generally less than the low-power unit of the AC system, but the greater the power, the higher the overall maintenance cost, which needs to be considered. The speed of the DC motor can be controlled by changing the power supply voltage, and can be used in a wide voltage range, but the most popular types are 12 and 24V. Among them, the advantages are:
Easy installation
Wide speed control range
Quick start, stop, reverse and accelerate
High starting torque
Linear speed-torque curve
DC motors are widely used, ranging from small tools and appliances to electric cars, elevators and cranes.
The two common types are:
(1) Brushed
These are more traditional types of motors, usually used in cost-sensitive applications where the control system is relatively simple. For example, in consumer applications and more basic industrial equipment, these types of motors can be broken down into:
Series Wound-This is the case where the field winding is connected in series with the rotor winding, and the speed control is by changing the power supply voltage. However, this type of speed control is poor. As the motor torque increases, the speed drops. Applications include automobiles, cranes, cranes and cranes because of its high starting torque.
Shunt Wound-This type has a power supply, and the field winding is connected in parallel with the rotor winding, which can provide greater torque without reducing the speed by increasing the motor current. It has a moderate starting torque at a uniform speed and is suitable for lathes, vacuum cleaners, conveyors and grinders.
Compound Wound-This is the accumulation of series and shunt, where the polarity of the shunt winding increases it to the series electric field. This type of starting torque is very high. If the load changes little, it can run smoothly. It is used to drive compressors, variable diameter centrifugal pumps, rotary presses, circular saws, shears, elevators and continuous conveyors.
Permanent Magnet-As the name suggests, it is not an electromagnet, but a permanent magnet, which is used for precise control and low torque applications, such as robots and servo systems.
(2) Brushless
Brushless motors alleviate some of the problems associated with more common brushed motors (short life with high service life) and simpler mechanical design (without brushes). The motor controller uses a Hall-effect sensor to detect the rotor position, and uses the controller to accurately control the motor through the current in the rotor coil) to adjust the speed. The advantages of this technology are long life, less maintenance, and high efficiency (85-90%), but the disadvantage is that the initial cost is higher and the controller is more complicated. This type of motor is usually used in applications that require reliability and robustness, such as fans, pumps, and compressors, as well as speed and position control.
A stepper motor is an example of a brushless design, mainly used for open loop position control, from printers to industrial applications such as high-speed pick and place equipment.