Softstarter DC Component Reduction via Phase Voltage Reference
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Solution Overview
Problem
Softstarters for asynchronous three-phase motors that control only two phases with two pairs of semiconductor devices face issues with undesired DC components during start and stop operations, leading to mechanical shocks, vibration, and increased maintenance costs, while also being costly to measure the third phase as a conductor.
Innovation Solution
A softstarter system that includes a second voltage measuring unit and a zero-crossing detecting unit to calculate phase delays between input voltages and semiconductor device zero-crossings, adjusting firing angles to reduce DC components by using phase-phase input voltage as a reference for timing the semiconductor devices, and estimating input voltages between phases to calculate instantaneous power and control motor torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If only two phases are controlled by semiconductor devices to reduce cost, then the softstarter becomes more cost-effective, but DC components appear in motor current during start and stop operations causing mechanical shocks and vibration
Solution Approach 1:
The patent applies preliminary action by measuring the input voltage phase positions before the start and stop operations, and using these pre-measured phase positions to calculate appropriate firing angles that compensate for the two-phase control limitation. This pre-calculation of firing angles based on pre-measured voltage phases prevents DC components from appearing in the motor current during motor starting and stopping, thereby eliminating mechanical shocks and vibration while maintaining the cost-effective two-phase control configuration
2Ease of operation
If firing angles are adjusted at zero-crossing of current through semiconductor devices, then voltage control is achieved, but timing errors occur leading to inefficient DC component reduction
Solution Approach 1:
The patent introduces an intermediary approach by using the measured input voltage phase position as an intermediate reference point, rather than directly using the zero-crossing of current through semiconductor devices. The microcontroller measures the input voltage phase, uses this intermediate measurement to calculate the firing angle, and then triggers the semiconductor devices. This intermediary voltage-based timing reference provides more accurate and stable timing compared to direct current zero-crossing detection, thereby improving measurement precision and enabling effective DC component reduction
3Measurement precision
If measurements are carried out on the third phase to improve control accuracy, then torque control precision improves, but additional measuring units and connection points increase cost
Solution Approach 1:
The patent applies the copying principle by using measurements from the first and second phases to represent and infer the conditions of the third phase. Instead of installing separate measuring units on all three phases, the system measures input voltages on two phases and uses these measurements to calculate firing angles that account for the three-phase system dynamics. This virtual copying of third-phase information from two-phase measurements achieves sufficient torque control precision without adding the complexity and cost of direct third-phase measurement hardware
Data Source
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Figure 2a
AI summary
The present invention relates to a softstarter (2) for starting and stopping an asynchronous motor (1) having three phases (L1, L2, L3), comprising two pairs (3A, 3C) of semiconductor devices of the type turning off at zero-crossing of the current therethrough, wherein each of said two pairs of semiconductor devices is connected in anti-parallel, and the first pair of the semiconductor devices is adapted to control the voltage of one of the phases of the motor and the second pair of the semiconductor devices being adapted to control the voltage of another of the phases of the motor, a DC reducing unit (16) associated with said two pairs (3A, 3C) of semiconductor devices for reducing a DC component in a load current of the said asynchronous three-phase motor during start of the motor, a first voltage measuring unit (8A, 8C) for measuring voltages across the two pairs of semiconductor devices, and a first zero-crossing detecting unit (14) configured for detecting zero-crossings of the measured voltages across the two pairs (3A, 3C) of semiconductor devices and providing zero-crossing signals (X1, X3) to the DC reducing unit.