I. Working Principle and Application Scenarios of Motor Integrated Protectors
A motor protection device is a commonly used motor protection device. Its main function is to provide comprehensive protection for the motor, such as overload, short circuit, grounding, undervoltage, overvoltage, overtemperature and other faults.
The motor integrated protector uses the pressure switch principle for protection. When the motor is running, if a fault occurs, the protector will detect parameters such as the motor's current, voltage, and temperature, transmit the fault information to the control system, and simultaneously cut off the motor's power supply to prevent the fault from escalating further.
Motor protection devices are suitable for various types of motors, such as three-phase asynchronous motors, DC motors, and AC motors, and are widely used in industries such as power, metallurgy, chemical, and mining.
II. Working Principle and Application Scenarios of Motor Phase Loss Protectors
A motor phase loss protector is an electrical device specifically designed to protect three-phase asynchronous motors. Its main function is to quickly detect, diagnose, and protect against phase loss faults during motor operation, preventing further damage to the motor.
The working principle of a motor phase loss protector is to detect the magnitude and phase of the three-phase power supply using sensors. If the current in one phase is zero or the phase difference between the currents of the other phases exceeds the normal range, a phase loss fault is identified. At this time, the protector will promptly cut off the power supply to the motor, preventing the motor from continuing to run, thereby achieving the protection effect.
Motor phase loss protectors are widely used in industrial fields such as power, petroleum, chemical, mining and water treatment, and have the advantages of easy installation, high sensitivity and reliability, and low operating cost.
III. Differences between motor integrated protectors and phase loss protectors
1. Different functions: A motor comprehensive protector can detect multiple types of faults, while a phase loss protector can only detect phase loss faults in the motor.
2. Different application scenarios: The motor comprehensive protector is suitable for all types of motors, while the phase loss protector is only suitable for three-phase asynchronous motors.
3. Price difference: Motor comprehensive protectors are relatively more expensive because they have more comprehensive functions and wider applications. Phase loss protectors, on the other hand, are less expensive because they only provide single-function protection.
In summary, both integrated motor protectors and phase loss protectors have their own advantages and disadvantages in protecting motors. Users can choose the appropriate motor protection device according to their needs and actual conditions.
Three-phase phase loss and phase sequence error protectors are designed to protect against three-phase system faults caused by phase loss or phase error in three-phase power supplies. They are mainly used in places where abnormalities in the three-phase system may cause serious accidents in industrial production, such as phase sequence errors affecting the normal operation of grid-connected inverter equipment or the reverse rotation of electrical control motors.
Motor forward and reverse rotation control principle diagram
In a three-phase AC electrical control system, under normal three-phase phase sequence conditions, the three-phase power supply voltage is ABC, with a phase voltage of 220V and a phase voltage of 380V, and an operating frequency of 50Hz. When the knife switch QF1 is closed, the system is energized. When the PLC and other controllers control contactor KM1 to engage and contactor KM2 to disengage, motor M begins to rotate in the forward direction at a frequency of 50Hz, totaling 3000 rpm. When contactor KM2 is engaged and contactor KM1 is disengaged, motor M begins to rotate in the reverse direction.
The operating control equipment can be a mine hoist, a material conveyor belt, or other equipment. When the phase sequence of the three-phase system is reversed, the hoist will run in reverse, which will inevitably endanger production safety.
Schematic diagram of grid-connected inverter
When a three-phase grid-connected inverter is connected to the grid, it is necessary to check the normal three-phase sequence of the power grid. Only when the grid phase sequence is normal can the inverter be allowed to operate and generate electricity. If the three-phase sequence is abnormal or three phases are missing, the inverter needs to stop operating. Of course, these protection functions are integrated into the inverter itself, but during the project acceptance and commissioning phases, it is necessary to check the correct grid phase sequence before power generation commissioning can proceed.
Features of three-phase phase loss protector:
1. The three-phase voltage is connected to the protector through a series resistor.
2. The operating power supply ranges from 9 to 36V;
3. When the three-phase power supply has no missing phases or the phase sequence is normal, the green light on the protector will illuminate. When a phase is missing or the phase sequence is incorrect, the red light on the protector will illuminate. This will also drive the relay to operate, providing protection access contacts to external protection devices.
4. Applicable to three-phase phase sequence detection and phase loss alarm, protecting three-phase AC motors, and used in industrial fields using three-phase AC power for phase sequence monitoring and protection.
Principle analysis of three-phase phase loss sequence protector:
Phase sequence detection principle diagram
Suppose the three-phase power supply is UA, UB, and UC, which is connected to the protector through resistor R. The protector has three components that detect the input voltage: capacitor C connected to phase A, resistor R1 connected to phase B, and resistor R2 connected to phase C.
By detecting the voltages UaNo, UbNo, and UcNo of three components, it can be determined whether a voltage phase sequence fault has occurred.
1. When a phase loss occurs, no current flows through the corresponding component. Therefore, if the component connected to the component with the lost phase is a resistor, the voltage across the resistor will be 0V. If it is a capacitor, this can be determined by detecting that the capacitor current is 0A. Thus, phase loss is a very easy fault to diagnose.
2. When a phase sequence error occurs, the terminal voltages of the three components will inevitably differ from those under normal phase sequence conditions. The phase sequence can be determined by observing the voltage changes across the components.
According to KCL and KVL theory, the voltage values of the three components can be solved:
Using the complex impedance method, the capacitance impedance is expressed as 1/Cω. Applying the nodal voltage method, the KCL equation is written:
Voltage is represented by phasors.
Let UA=220∠0 V, UB=220∠240 V, UC=220∠120 V,
Given R = 10K and 1/Cω = R1 = R2 = 1K, substitute these values into the formula to calculate Unon:
Once the voltage at the center point is known, the voltage across resistor R1 can be calculated:
The voltage across R2 can be calculated in the same way:
Here, we assume the phase connected to the capacitor is phase A. Under normal phase sequence, the voltage across resistor R1 (phase B) is 25.565V, and the voltage across resistor R2 (phase C) is 15V. It's clear that the voltage across resistor R1 is greater than the voltage across resistor R2. However, when the phase sequence is reversed, the voltage across resistor R2 will be greater than the voltage across resistor R1. This allows us to determine the voltage phase sequence using a simple RC network.
So far, we have discussed a hardware-based method for voltage phase sequence determination, which is a simple and low-cost method for detecting phase sequence. In reality, however, do all three-phase devices need to be equipped with this type of phase sequence protector?
Of course not. Some devices can use locally available materials to determine the phase sequence, such as grid-connected inverters.
Grid-connected inverters require sampling of grid-side voltage. Since the three-phase voltage magnitudes are already known through a digital signal processor (DSP), is there a method to calculate whether there is a phase loss or phase sequence error in the power grid? The following describes a software-based method for detecting voltage phase loss:
Transforming the three-phase voltage from a stationary coordinate system to a rotating coordinate system yields the DC voltage Ud+, which is the magnitude of the positive-sequence component of the three-phase AC voltage. When the three-phase voltage phase sequence is reversed, the positive-sequence DC voltage component in the rotating coordinate system is 0, while the magnitude of the negative-sequence DC voltage component Ud- is equal to the value of Ud+ when the three-phase voltage phase sequence is normal. Thus, whether a phase sequence error has occurred can be determined solely through software calculation.
What is a stationary coordinate system? Simply put, it means that the variables change over time, but the coordinate axes do not change.
Three-phase AC voltage is a physical quantity that changes sinusoidally with time and can be represented by an amplitude-phase coordinate curve. The horizontal axis is the phase angle ωt axis, which changes linearly with time, and the vertical axis is the amplitude axis, which changes sinusoidally with the angle.
If ωt is included in the coordinate axis variables, then this time coordinate with respect to ωt changes into a two-phase stationary coordinate with respect to αβ, in which αβ are both function variables with respect to ωt.
If a sinusoidal voltage quantity u is expressed in complex exponential form:
Therefore, the real part of the complex exponential voltage is the sinusoidal voltage quantity, and the imaginary part is the voltage quantity that is 90 degrees to the sinusoidal voltage quantity. Defining the horizontal axis as the α axis to represent the real part and the vertical axis as the β axis to represent the imaginary part, this new coordinate system is the two-phase stationary coordinate system. Since the amplitude of Ua only varies along the α axis, if we also represent the two voltages Ub and Uc on the same coordinate system, we obtain the two-phase stationary coordinate representation of the three-phase voltage.
Assuming the initial phase angle of Ua is 0 degrees, the initial phase angle of Ub lags Ua by 120 degrees, and the initial phase angle of Uc leads Ua by 120 degrees, then they are arranged clockwise in the two-phase stationary coordinate space. At any given time, the three-phase voltages can be synthesized into a voltage vector, and the synthesized voltage is also a phasor rotating counterclockwise.
If we take this composite voltage vector as the d-axis of the rotating coordinate system and the axis perpendicular to the d-axis as the q-axis, then we obtain the transformation from two-phase stationary coordinates to rotating coordinates. Clearly, Ud is stationary relative to the d-axis of the rotating coordinate system, while the d-axis of the rotating coordinate system rotates relative to the α-axis; that is, the rotating coordinate system involves the rotation of the coordinate axes.
Therefore, if there is no positive sequence component rotating in the coordinate system, and only the negative sequence component rotates, then Ud+=0, and there is only the negative sequence rotating voltage Ud-.
By calculating Ud- through the program, it can be determined whether a phase sequence reversal has occurred.
