Induction Motor Speed Search via DC-Bus Voltage Error
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Solution Overview
Problem
Existing solutions for starting an induction motor under unknown free rotation speed, such as DC braking and mechanical braking, face issues like inadequate magnetic force, over-current conditions, increased cost, and potential system damage due to unknown load stress.
Innovation Solution
A rotation speed searching apparatus that includes a DC-bus voltage error determining module and a current error determining module to control the operation of switching units in the inverter, allowing for the identification of the rotor frequency and subsequent control of the induction motor's operation to improve the success rate of finding the correct rotation speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DC braking method is used to fix the rotor, then the rotor can be fixed according to electromagnetism, but over-current conditions may happen and the induction motor will be overheated
Solution Approach 1:
The patent applies preliminary action by performing rotation speed searching before the induction motor is driven. The control unit searches for the actual rotation speed of the rotor before applying drive voltage, and only applies voltage when the rotor speed is confirmed to be zero or below a threshold. This prevents over-current conditions and overheating by ensuring the motor is properly braked before starting, without needing excessive braking voltage.
2Reliability
If mechanical braking method is used to lock the rotor, then the rotor shaft is kept stationary, but an extra mechanical latch is needed which increases cost and the latch has to bear unknown load stress
Solution Approach 1:
The patent replaces the mechanical braking system (mechanical latch) with an electrical control system. The control unit uses electrical signals to search for rotor frequency and determine rotation speed, then controls the inverter to apply appropriate voltage. This eliminates the need for mechanical latches, reducing device complexity and cost while maintaining reliable rotor stationary control through electrical means.
3Force
If the output voltage of the inverter is increased to strengthen magnetic force for rotor fixing, then the rotor is easier to be fixed, but over-current conditions may happen
Solution Approach 1:
The control unit performs preliminary rotation speed searching before applying drive voltage to the inverter. By confirming the rotor is stationary first, the system can then apply the minimum necessary voltage to start the motor, avoiding the need for high voltage that would create excessive magnetic force and over-current conditions.
4Reliability
If the output duration of the inverter is extended to decelerate the rotor fully, then the rotor can be fixed properly, but the induction motor will be overheated
Solution Approach 1:
The control unit performs preliminary rotation speed searching to detect the actual rotor speed before starting the motor. This allows the system to determine whether braking is needed and for how long, applying voltage only when necessary and for the minimum required duration. This prevents unnecessary extended operation that would cause overheating while ensuring complete rotor deceleration when needed.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The apparatus effectively avoids the drawbacks of existing methods by reducing the risk of over-current conditions and mechanical damage, eliminating the need for additional mechanical components, and enhancing the reliability of rotation speed control.
Implementation Method 1
An input AC voltage Vin is supplied to the input side of a rectifier 104 and is converted into a DC voltage Vdc by the rectifier 104
Implementation Method 2
A capacitor Cdc 105 is located at the output end of the rectifier 104 for stabilizing the DC voltage Vdc and reducing the ripple of the DC voltage Vdc. The DC voltage Vdc is converted into an output AC voltage Vout and an AC output current Iout by an inverter 106
Implementation Method 3
The voltage modulating module 108 is configured to receive a voltage signal V and a frequency signal f to generate pulse signals PWM1-PWM4 that are used to control the operation of the switching units Q1-Q4 in the inverter 106
Implementation Method 4
The induction motor 100 is driven by the output AC voltage Vout and an AC output current Iout provided by the inverter 106
Implementation Method 5
The DC braking method is featured by applying the DC voltage generated by the variable frequency drive 101 to the inductor motor 100, and the rotor of the induction motor 100 can be fixed according to electromagnetism
Data Source
AI summary
Provided is a variable frequency drive and a rotation speed searching apparatus for an induction motor incorporated therein. The rotation speed searching apparatus is featured by scanning the rotor frequency of the induction motor and determining either the error between a detected DC-bus voltage and a set DC-bus voltage or the error between a detected output current and a set output current, so that the rotation speed of the induction motor can be searched out.