AC (Alternating Current) Motors
The shaft rotation speed is not constant (asynchronous) motors consist of two basic components, the stator and the rotor. The stator forms the fixed part of the motor. The stator contains the stator core and field windings. In the stators of 3-phase asynchronous motors, there is a separate winding for each 3-phase. The rotor is the rotating part of the motor. The rotors of the ring induction motors have windings, while the rotors of the squirrel-cage induction motors have short-circuited busbars. It is a squirrel-cage asynchronous motor type which is widely used in industrial applications.
In asynchronous motors, the frequency of the rotating field formed in the air gap and the rotation frequency of the rotor are not the same. For this reason, these motors are called asynchronous. The frequency of the rotating field in the air gap depends on the supply frequency of the motor and the number of poles and does not change with the load of the motor, while the rotation frequency of the rotor changes depending on the load. As the load increases in the operating region of the motor, the motor speed decreases, and as the load decreases, the speed increases. The motor speed is at its highest at idle. The speed of the asynchronous motor cannot exceed the number of revolutions of the rotating field. The number of revolutions of the rotating field is calculated as follows:
N = 120 x F/P
N: number of revolutions of the rotating field (rpm)
F: supply frequency (Hz)
P: Number of poles of the stator winding (pcs)
According to this calculation, the 2-pole motor is 3,000 rpm; 4-pole motor 1,500 rpm; The 6 pole motor is called 1,000 rpm.
In summary, AC Motors;
- A magnetic field is created by the stator windings and this field generates the current in the rotor.
- The speed of the motor depends on the mains voltage frequency and the motor pole.
- Driven by the magnetic field created, the rotor cannot move at the same speed as this field. For this reason, slip losses occur in AC motors.
- Speed control is realized with the logic of increasing the slip by regulating the voltage. The lower the voltage for speed control, the greater the losses.
- AC motors, which create a rotating field due to the sine wave form of the alternating current, are silent in terms of motor noise as they do not need other switching conductors for current direction change.
- The service life is limited by the life of the bearings and therefore the bearing used.
Direct current motors, also called DC motors, consist of stationary and rotating parts. The stationary part is called the stator, and the rotating part is called the rotor. It is based on the principle of moving with the effect of magnetic force that occurs in the opposite direction to the permanent magnets in the motor when electric current is applied to the windings in the motor. It is necessary to change the direction of this current in such a way that it constantly creates a magnetic field opposite to the permanent magnet. While this change is made by the brushes contacting the windings of the motor in brushed motors, it is made by the electronic speed control circuit in brushless motors.
In summary, DC Motor;
- The stator creates the magnetic flux.
- The rotor is a permanent magnet and rotates synchronously with the stator.
- The current direction in the stator windings is switched by the electronics according to the magnetic field position (according to the position of the rotor.
- The service life is limited by the bearing life (bearing life).
- The speed of the motor can be by changing the supply voltage or by reference signals sent to the microprocessor.
- A separate DC voltage source will be required for the use of DC motors. It is not an easy and common voltage to obtain at high powers such as the mains voltage.
The reasons why DC motors are more efficient than AC motors;
- In AC motors, the magnetic field in the stator windings and the inductive current strength created in the rotor depend on the number of windings. In DC motor, since such induction is not required on the rotor side, the design is made with less windings and the efficiency is increased.
- The losses of the induction motor are higher. Because of these losses, both energy is used inefficiently and unused energy is reflected to the environment as heat. Since DC motors are synchronous, the losses are less and therefore they have a longer life.
- With microprocessor controlled DC motors; Current speed monitoring, alarm outputs, simple speed control and soft start are some additional features that can be provided.
They are DC brushless motors controlled by an external electronic circuit board. This provides more control and higher efficiency. EC means “Electronically Commutated”, that is, DC (direct current) motor with electronic variable. What is meant to be explained here with the variable; Electronically changing the direction of electric current backward or forward. In conventional AC motors this is done mechanically. EC motors are permanent magnets and electronically adjust the direction of rotation according to the current. Therefore, it does not cause power loss in current changes as in AC (alternating current) motors. Alternating current is a curve that constantly turns from negative to positive and then back to negative. This rotation or change inside the motor creates magnetic repulsion fields that move the motor. This loss in alternating current motors causes inefficiency and heating in the motor. EC motors are more efficient as they have less copper, iron and friction losses.
EC Motor Advantages
- More efficient
- Low operating and maintenance cost
- Low sound level
- Low heat losses, less risk of overheating
- Small dimensions
- Easy control
- Compliance with the desired operating point
- Having the control unit on the motor
- Possibility of remote control
- No loss of efficiency and no increase in sound level at variable speeds
- It can be controlled with 0-10V DC.
- The EC motor is a brushless DC motor that can be driven by mains voltage. The part behind the motor, known as the integrated electronics group, is responsible for converting the alternating current, which is the mains voltage, into direct current and providing the necessary commutation for speed control.
- The voltage found in the motor coils is generally high DC voltage.
- 1-2-3 core or phase windings can be used.
The reasons why EC motors are more efficient than AC motors (similar to the advantages of DC motor);
- In AC motors, the magnetic field in the stator windings and the inductive current strength created in the rotor depend on the number of windings. In the EC motor, since such induction is not required on the rotor side, it is designed with less winding and this increases the efficiency.
- The losses of the induction motor are more and therefore it works more inefficiently. Due to these losses, both energy is used inefficiently and unused energy is reflected to the environment as heat. Since DC motors are synchronous, the losses are less and therefore they have a longer life.
- Current speed monitoring with microprocessor controlled DC motors, alarm outputs, simple speed control and soft start are some additional features that can be provided.
- 3-phase EC motor is quieter than AC motors even at low RPMs.
Other Advantages of the EC Motor
- Wide voltage supply range 1~200..277VAC or 3~380..480VAC.
- Ability to operate at 50Hz or 60Hz.
- It has a motor structure that eliminates reverse rotation in regions where the phase sequence is not known exactly, therefore the motor guarantees rotation in the right direction.
- Having facilities that reduce the number of components used in the system or application.
- Since it is the most efficient speed control and motor driving method, switching loss and heat loss are very low.
- Use with universally accepted standard analog and digital interfaces. Ability to work with constant flow or constant pressure thanks to the integrated control interface.
ERP & ECO Design
Fan motors with power between 125 Watt and 500 kW are expected to exceed a certain static efficiency stipulated by the directive. Products that cannot exceed the limit yield will not have the CE marking and their entry into the EU market will be prevented. For this reason, this regulation directly affects the fan motor manufacturers and indirectly affects the customers who use these fan motors.
The minimal efficiency difference due to the absence of some motor losses in EC motors compared to conventional motors in standard speed operation becomes more evident when operating at variable loads, that is, when speed control is performed.
Considering that speed control is made with a 0-10 VDC analog signal coming from a simple PLC or sensor, the noises due to harmonics that occur in motors driven by different speed control methods do not occur in EC technology.
In addition, operating the fan by reducing the speed for the desired capacity will reduce the energy consumption in proportion to the cube of the speed change. In applications with traditional asynchronous motors and no speed control, variable capacity needs are solved by cascade operation of the fans. Thus, if the air flow requirement is reduced by 50%, 50% of the fans are turned off and the energy consumption is reduced to half of the anticipated installed power. On the other hand, in an application with an EC fan, the desired air flow is achieved by keeping all the fans active but reducing their speed to half of the nominal speed.
When we examine the speed-controlled solution of a fan system with traditional technology; main system elements can be considered as fan, motor and inverter. Some auxiliary elements used together are sinus filter, shielded cable, external grounding or motor protection units to protect the waveform that is distorted during the driving of high capacity motors.
Reducing the spaces required for the panel in air handling units with EC motor application is another important advantage.
Source: Murat ÖZER; AC,DC ve EC Motor Tanım ve Karşılaştırmaları