The supplier of the frequency converter braking unit reminds you that the frequency converter is equipped with a dynamic resistor mainly to consume a part of the energy on the DC bus capacitor through the braking resistor, in order to avoid excessive voltage of the capacitor. In theory, if a capacitor stores a lot of energy, it can be used to release it to drive a motor and avoid energy waste. However, the capacity of a capacitor is limited, and its withstand voltage is also limited. When the voltage of the bus capacitor reaches a certain level, it may damage the capacitor, and some may even damage the IGBT. Therefore, it is necessary to release the electricity through a braking resistor in a timely manner. This release is a waste of time and is an unavoidable solution.
Bus capacitor is a buffer zone that can hold limited energy
After all three-phase AC power is rectified and connected to capacitors, the normal voltage of the bus during full load operation is approximately 1.35 times, 380 * 1.35=513 volts. This voltage will naturally fluctuate in real time, but the minimum cannot be lower than 480 volts, otherwise it will trigger an undervoltage alarm protection. Bus capacitors are generally composed of two sets of 450V electrolytic capacitors connected in series, with a theoretical withstand voltage of 900V. If the bus voltage exceeds this value, the capacitor will directly explode, so the bus voltage cannot reach such a high voltage of 900V no matter what.
In fact, the withstand voltage value of IGBT with three-phase 380 volt input is 1200 volts, which often requires operation within 800 volts. Considering that if the voltage increases, there will be an inertia problem, that is, if you immediately make the braking resistor work, the bus voltage will not decrease quickly. Therefore, many frequency converters are designed to start working at around 700 volts through the braking unit to lower the bus voltage and avoid further upward charging.
So the core of designing braking resistors is to consider the voltage resistance of capacitors and IGBT modules, in order to avoid these two important components from being damaged by the high voltage of the bus. If these two types of components are damaged, the frequency converter will not work properly.
Quick parking requires a braking resistor, and instant acceleration also requires it
The reason why the bus voltage of the frequency converter increases is often due to the frequency converter causing the motor to operate in an electronic braking state, allowing the IGBT to pass through a certain conduction sequence, utilizing the large inductance current of the motor that cannot suddenly change, and instantly generating high voltage to charge the bus capacitor. At this time, the motor is quickly slowed down. If the braking resistor does not consume the energy of the bus in a timely manner at this time, the bus voltage will continue to rise, posing a threat to the safety of the frequency converter.
負荷がそれほど重くなく、急停止の必要がない場合は、制動抵抗器を使用する必要はありません。制動抵抗器を設置しても、制動ユニットの動作閾値電圧が作動せず、制動抵抗器は作動しません。
重負荷の減速状況で急速制動を行うために、制動抵抗と制動装置を増やす必要があることに加え、実際には、重負荷と非常に速い始動時間の要件を満たす場合、始動時に制動装置と制動抵抗も調整する必要があります。以前、周波数変換器を使用して特殊なパンチプレスを駆動しようとしましたが、周波数変換器の加速時間は0.1秒に設計されていました。このとき、全負荷で始動すると、負荷はそれほど重くありませんが、加速時間が短すぎるため、バス電圧の変動が非常に激しく、過電圧または過電流の状況が発生する可能性があります。その後、外部の制動装置と制動抵抗が追加され、周波数変換器は正常に動作できるようになりました。分析によると、起動時間が短すぎて、バスコンデンサの電圧が瞬時に空になるためです。整流器は瞬時に大電流を充電し、バス電圧が急上昇します。これにより、バス上で激しい電圧変動が発生し、瞬間的に700ボルトを超える場合があります。ブレーキ抵抗器を追加することで、この変動する高電圧を適時に排除し、周波数変換器を正常に動作させることができます。
ベクトル制御には、モータのトルクと速度の方向が逆の場合や、速度ゼロで100%のトルク出力で動作している場合など、特殊な状況もあります。例えば、クレーンが重い物を落下させて空中で停止する場合や、巻き戻しを行う場合など、トルク制御が必要となります。モータは発電機状態で動作させる必要があり、連続電流はバスコンデンサに逆充電されます。制動抵抗器を介してこのエネルギーを適時に消費することで、バス電圧のバランスと安定性を維持できます。
3.7KW などの小型周波数変換器の多くには、バスコンデンサの削減を考慮して、ブレーキユニットとブレーキ抵抗器が内蔵されていることが多いですが、低電力抵抗器とブレーキユニットはそれほど高価ではありません。