Brake Chopper Synchronization for Accurate Current Measurement
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Solution Overview
Problem
The structure of frequency converters with DC current measurement from the DC intermediate circuit is problematic due to current flowing through the brake chopper being summed with motor current, leading to inaccurate phase current measurements and potential saturation of the current measurement device.
Innovation Solution
Synchronizing the use of the brake chopper with current measurement by determining specific time instants within the control period when the brake chopper is disabled, ensuring accurate phase current sampling by preventing the brake chopper's operation during current sampling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the brake chopper is used to dissipate regenerated energy in frequency converters, then the voltage of the intermediate circuit can be controlled within acceptable limits, but the current measurement device may become saturated due to summed current from brake chopper and motor
Solution Approach 1:
The brake chopper operation is synchronized with the control period of the frequency converter. The control method determines specific time instants within each control period when the brake chopper is disabled, creating a periodic pattern where the chopper operates only during intervals when current measurement is not performed. This periodic action allows the system to maintain voltage control reliability while eliminating measurement saturation and improving current measurement accuracy.
2Ease of manufacture
If DC current measurement is performed from the DC intermediate circuit, then cost-effective current sensing is achieved, but the measurement includes summed current from both brake chopper and motor phases
Solution Approach 1:
The invention maintains the cost-effective DC current measurement approach but introduces periodic disabling of the brake chopper during specific time instants within control periods. This creates measurement intervals where only motor phase current flows through the measurement device, eliminating the summed current issue while preserving the economical single-sensor architecture.
Solution Approach 2:
The invention extracts the brake chopper current component from the total measured current by temporarily removing (disabling) the brake chopper during measurement intervals. This allows the measurement device to capture only the motor phase current, separating the two current components in time rather than requiring complex simultaneous measurement and subtraction.
3Device complexity
If the brake chopper operates independently based on voltage measurement, then voltage control is simplified, but accurate current sampling cannot be performed when brake chopper is active
Solution Approach 1:
The invention merges the voltage control function and current measurement function by coordinating their operation within the same control period. The brake chopper's independent voltage control is combined with periodic current measurement requirements, creating a unified control strategy where both functions operate harmoniously without compromising either voltage regulation or current sampling accuracy.
Solution Approach 2:
The control method implements periodic disabling of the brake chopper at specific instants within control periods, synchronizing it with the current measurement timing. This periodic coordination allows the independently operating voltage control to pause briefly for accurate current sampling, then resume, maintaining both voltage regulation and measurement accuracy.
Data Source
AI summary
A method of controlling a brake chopper and a control arrangement, the brake chopper being connected between the positive and the negative rails of an intermediate voltage circuit feeding an inverter bridge, wherein current measurement means are arranged between a DC voltage source and the brake chopper and adapted to measure DC current flowing in the intermediate voltage circuit, which method comprises steps of forming with the inverter bridge an output voltage to a load connected to the output of the inverter bridge, determining a time instant when the DC current of the intermediate circuit is sampled, and preventing the use of the brake chopper at the determined time instant.