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DV DT

 

  • What are some DV/DT applications?

      The dV/dT filter reactors are specifically designed for drive/motor applications with long lead lengths (usually where the motor cable length is 100 feet and greater). They are always installed between the Variable Frequency Drive (VFD) and the motor. Typical installation applications include production process lines, conveyor systems and deep wells.

  • What are DV/DT Filter Reactors?

      The advent of pulse width modulated (PWM) inverters with IGBT high-speed transistors, has resulted in smaller more cost effective drives and increased switching speeds. A waveform with increased harmonics at higher frequencies is the result of these much faster switching devices, usually at frequencies of 10,000 to 20,000 Hertz.

      Drives and motors often need to be separated by significant distances. For deep wells or mines, the motors are usually controlled on the surface. As a result, the distance between the drive and the motor creates long motor lead lengths. In some plant applications, the motors can withstand the harsh environment but the sensitive variable frequency drive cannot. This again results in long lead lengths to the motor.

      Most manufactures of variable frequency drives will publish a recommended maximum distance between their equipment and the motor. Sometimes these recommendations create application difficulties, thus increased motor lead lengths are inevitable.

      DV/DT is explained as the steep-front voltage pulses that travel down these long leads in the circuit to the motor and subsequently reverted back in a “reflective wave”. When the conductors are long enough, usually 20 feet or more, the time for reflection matches the time for transmission resulting in a high amplitude ‘standing wave’ on the circuit. Voltage spikes of up to 2100 volts are frequently experienced for 600-volt systems, and motor winding failures are the result.

      A Filter Reactor, installed in front of the motor, combines the current limiting ability of an AC line reactor plus a resistive capacitance circuit that forms a damped, low pass filter. It provides protection for the motor by slowing the rate of voltage increase and minimizing the peak voltage that occurs at the motor terminals.  The cost of a DV/DT Filter Reactor is little more than the cost of the reactor and can be mounted next to the motor, or inside the PWM enclosure.

  • What Output Problems Can Occur with Variable Frequency Drives (VFD or VSD) and How Can You Mitigate These Issues?

      A voltage-sourced Variable Frequency Drive (VFD) uses Insulated-Gate Bipolar Transistors (IGBTs) to rapidly switch voltage on and off to form a Pulse Width Modulated (PWM) voltage source for the motor. The PWM simulates a sine wave voltage source to the motor and it operates as if it was being powered by a sine wave.  The PWM wave allows the VFD to change the fundamental frequency of the PWM waveform and simulate sine waves.  Since the speed of a motor is directly related to the fundamental frequency of the sine wave, a VFD can control speeds from a fraction of a hertz to hundreds of hertz.

      Output reactors, dV/dT filters or drive isolation transformers can be used to help mitigate some issues caused by the PWM output.  PWM outputs cause rapid switching transitions which can cause over-voltages due to parasitic capacitance and inductance in the motor’s leads. The parasitic currents and voltages can be determined by the equation of V = L × (Δi/Δt).  VFD’s switching frequencies (the amount of pulses used to simulate the sine wave) generally range from 1,000-20,000 pulses per second.  IGBT’s produce an almost perfect square wave which produces a very high Δv/Δt.   High Δv/Δt can cause higher surge currents in the leads. This then causes high voltage pulses across the parasitic inductances.  Therefore the faster the pulses switch, the greater the impact of cable capacitance and inductance. These voltage pulses stress the motor’s windings causing higher audible noise, heat and possibly premature failure of the insulation. There is also capacitance in the motor’s bearings.  The combination of lubrication and air gaps prevent direct and continuous contact of the bearings to the metal traces that contain them.  Parasitic currents [I = C × (Δv/Δt)] causes current to flow through the bearings.  The amount of current will increase as the VFD output switching speed increases. These currents can cause micro pits to form in the bearings and eventually will lead to premature bearing failure.

  • What is the typical dV/dT Filter Reactor performance?

      Category: | Frequently Asked Questions   | DV DT  

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      A dV/dT filter reactor combine appropriate values of inductance, capacitance and resistance to form a filter, which reduces dV/dT and peak voltages from the PWM (Pulse Width Modulated) voltage waveform. This will reduce motor heating harmonics and audible noise to significantly increase the life of the motor.

      Long lead lengths between the VSD and motor can experience motor terminal peak voltage spikes two or four times the DC bus voltage. Therefore motor terminal voltage peaks of 1300-2700 volts for 480V drives and 1700-3400 volts for 600V drives are not uncommon.

  • dV/dt reactor – definition