Load Reactors (Motor Choke)
 
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In long lead application, GR Motor chokes located between the drive and motor help reduce dv/dt and motor terminals peak voltages. The use of the motor choke also helps protect drive from surge currents caused by rapid changes in the load.

Benefits:

  • Protect motor from long lead effects up to 150 meters
  • Reduce output voltage dv/dt below 500V/ sec
  • Reduce motor terminal peak voltage below 1000V
  • Reduce surge currents
  • Extend semiconductor life
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With Multiple Motors:

When more than one motor is controlled by a single drive, a single motor choke can typically be used between drive and all motors. Size the choke based on total motor horse power.

The choke performance is ensured by not exceeding the cable length between motor and drive. For an application with several motors connected in parallel, the cable length must include all cabling. If a cable longer than that recommended, the motor choke may over heat.

 
  Description of reflected wave phenomenon:
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The inverter section of a drive does not produce sinusoidal voltage, but rather a series pulses created from DC bus. Voltage wave reflection is a function of the voltage rise time (dv/dt) and the length of the motor cables. The impedance on either end of the cable run (cable-to-inverter cable-to-motor) does not match, causing voltage pulse to be reflected back in the direction from which it arrived. As these reflected waves encountered other waves, their value adds, causing higher peak voltage. As wire length or carrier frequency increases, the over shoot peak voltage also increase. Peak voltage on a 460V system can reach 1200 to 1600V, causing rapid breakdown of motor insulation, and leading to motor failure.

The same peak voltage that damage motor can also damage the cable. Multiple drive output wires in a single conduit or wire tray further increase output wire voltage stress between multi-drive output wires that are touching. Drive 1 may have (+) 2 DC bus voltages while Drive 2 may simultaneously have a (-) 2 DC bus voltage.

 
  Voltage Rise Time:

While peak voltage can reach 1600V or more, it is important to note that these same spikes can have a rise time, dv/dt, in excess of 7500V/Ás. such high rise time can cause significant damage to the motor windings and the insulating systems, resulting in premature motor failure. The life of the motor can be greatly extended by limiting both magnitudes of the voltage spikes to levels below 1000V and the dv/dt at the motor terminals to levels less than 500V/Ás on 480V systems.

 
  MOTOR TEMPERATURE REDUCTION:
Motors operated on a VFD tend to run warmer than when they are operated on pure 50 Hz, such as in an across-the-line stator application. The reason is that the output waveform of the VFD is not pure 50 Hz, but rather it contains harmonics which are currents flowing at higher frequencies. The higher frequencies cause additional watts loss and heat to be dissipated by the iron of the motor, while the higher currents cause additional watts loss and heat to be dissipated by the copper windings of the motor. Typically the larger horsepower motors (lower inductance motors) will experience the greatest heating when operated on a VFD.
Reactors installed on the output of a VFD will reduce the motor operating temperature by actually reducing the harmonic content in the output waveform. A five percent impedance, harmonic compensated reactor will typically reduce the motor temperature by 20 degrees Celsius or more. If we consider that the typical motor insulation system has a "Ten Degree C Half Life" (Continual operation at 10 degrees C above rated temperature results in one half expected motor life), then we can see that motor life in VFD applications can easily be doubled. Harmonic compensated reactors are actually designed for the harmonic currents and frequencies whereas the motor is not.
 
  MOTOR NOISE:

Because the carrier frequency and harmonic spectrum of many Pulse Width Modulated (PWM) drives is in the human audible range, we can actually hear the higher frequencies in motors which are being operated by these drives. A five percent impedance harmonic compensated reactor will virtually eliminate the higher order harmonics (11th & up) and will substantially reduce the lower order harmonics (5th & 7th). By reducing these harmonics, the presence of higher frequencies is diminished and thus the audible noise is reduced. Depending on motor size, load, speed, and construction the audible noise can typically be reduced from 3 - 6 dB when a five percent impedance harmonic compensated reactor is installed on the output of a PWM drive. Because we humans hear logarithmically, every 3dB cuts the noise in half to our ears. This means the motor is quieter and the remaining noise will not travel as far.

 
  MOTOR EFFICIENCY:
 

Because harmonic currents and frequencies cause additional watts loss in both the copper windings and the iron of a motor, the actual mechanical ability of the motor is reduced. These watts are expended as heat instead of as mechanical power. When a harmonic compensated reactor is added to the VFD output, harmonics are reduced, causing motor watts loss to be reduced. The motor is able to deliver more power to the load at greater efficiency. Utility tests conducted on VFD's with and without output reactors have documented efficiency increases of as much as eight percent (at 75% load) when the harmonic compensated reactors were used. Even greater efficiency improvements are realized as the load is increased.

 
  SHORT CIRCUIT PROTECTION:
When a short circuit is experienced at the motor, very often VFD transistors are damaged. Although VFD's typically have over correct protection built-in, the short circuit current can be very severe and its rise time can be so rapid that damage can occur before the drive circuitry can properly react. A harmonic compensated reactor (3% impedance is typically sufficient) will provide current limiting to safer values, and will also slow down the short circuit current rise time. The drive is allowed more time to react and to safely shut the system down. You still have to repair the motor but you save the drive transistors.
 
  OTHER APPLICATIONS:
Output reactors solve other problems on the load side of VFD's in specialized applications also. Some of these include: Motor protection in IGBT drive installations with long lead lengths between the drive and motor, Drive tripping when a second motor is switched onto the drive output while another motor is already running, and Drive tripping due to current surges from either a rapid increase or decrease in the load.
 
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