AC Motor Braking with Resistor Bank and Contactor
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Solution Overview
Problem
Existing methods for braking AC motors, particularly with variable frequency drives, are costly and inefficient for infrequent but fast braking applications, as they require high-cost 4-quadrant drives.
Innovation Solution
A system comprising a variable frequency drive connected to a resistor bank and a three-phase contactor, where the contactor connects the resistor bank to the motor upon a speed demand reduction, allowing the resistors to absorb deceleration current and facilitate faster motor braking without the need for expensive 4-quadrant drives.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a 4-quadrant drive is used for fast braking of AC motors, then the braking speed and deceleration performance are improved, but the system cost and complexity increase significantly
Solution Approach 1:
The braking function is segmented from the main drive system. Instead of using a complex 4-quadrant drive for both motoring and braking, the patent uses a simple 2-quadrant drive for motoring and adds a separate, simple resistor bank for braking. The contactor switches between these two simplified subsystems, achieving fast braking without requiring the full complexity of a 4-quadrant drive.
Solution Approach 2:
A contactor is introduced as an intermediary switching device that connects or disconnects the resistor bank from the motor terminals. This contactor acts as a mediator that enables the resistor braking function to be added to a simple 2-quadrant VFD without requiring complex internal circuitry modifications to the drive itself.
2Loss of energy
If a 4-quadrant drive is used for regenerative braking, then energy recovery is improved, but the cost increases significantly
Solution Approach 1:
Instead of investing in an expensive 4-quadrant drive with regenerative capability, the patent uses a cheap resistor bank that dissipates the kinetic energy as heat during braking. The resistor bank is a simple, low-cost component that can be switched in only when braking is needed, providing an economical solution for applications where energy recovery is not critical.
3Device complexity
If a resistor bank is connected directly to motor terminals without switching, then the braking function is simplified, but the motor flux stability deteriorates during braking transition
Solution Approach 1:
The control system prepares for the braking transition in advance by detecting when braking is needed and pre-positioning the contactor to connect the resistor bank before the actual braking event. This preliminary action ensures that when braking begins, the resistor is already connected, avoiding flux instability that would occur with direct connection during the transition.
Solution Approach 2:
The control system uses feedback from the motor operating conditions (speed, load) to determine when to activate the braking function. By continuously monitoring motor parameters, the controller can optimally time the contactor switching to maintain flux stability while enabling effective braking 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
This solution enables efficient and cost-effective braking of AC motors by using a 2-quadrant variable frequency drive with a resistor bank to absorb deceleration current, allowing for quick deceleration and maintaining motor flux stability during the braking process, reducing the overall braking time and operational costs.
Implementation Method 1
allowing the resistors to absorb deceleration current and facilitate faster motor braking
Data Source
AI summary
A system for braking a motor. The system includes at least one resistor and a contactor connected to the at least one resistor and a motor. The system further includes a variable frequency drive electrically connected to the motor, wherein the variable frequency drive comprises a controller operably connected to the contactor, wherein at least a portion of the contactor closes connecting the at least one resistor to the motor in response to a command from the controller. The variable frequency drive is configured such that motor flux levels may be maintained at a relatively high level as motor torque current is reduced, resulting in a consistently high motor flux level as the motor speed decreases.