Control Systems Application Guide
Adjustable speed drives, both AC inverters and DC drive types; convert the AC line voltage to DC by rectification using diode bridge or SCR rectifiers. With DC drives, the converter output is connected directly to the motors and with inverters it is fed to a DC bus and inverter power section. The current created by the rectifiers is non-sinusoidal or “nonlinear” and can cause the power system voltage to become distorted. This harmonic voltage distortion can have adverse effects on other equipment connected to the same power system.
In addition, when three phase input 6-pulse rectification by SCRs is used on DC drives and on some inverters, there are six times per power line cycle when current is transferred from one phase to another and a transient “short circuit” occurs. The associated peak in current can cause “notching” in the voltage waveform which other equipment may not tolerate and may cause damaging resonant currents in the electrical system.
AC inverters and their motors create a unique situation related to the high frequency switching of transistors in the drive’s PWM (pulse width modulated) output section. The fast rise and fall times (high dvdt) of the applied voltage during each cycle can generate very high voltages on wiring to the motor and within the motor windings between phases and to ground. The problem is more pronounced with long wire runs (> 25 feet) to the motor; transient voltage levels can approach thousands of volts. This particular phenomenon is the primary reason for the development of “inverter duty motors” and inverter rated motor lead wire.
Finally, with inverters we sometimes see the effects of the high frequency PWM oscillators and switching of transistors in the form of high frequency radiated and conducted noise. With some inverters, the RFI (radio frequency interference) noise has been known to affect other circuitry even when the inverter is just powered and not operating the motor.
1.) LINE REACTORS with INVERTERS
A line reactor is a linear current-limiting inductor that is connected in series with each AC line input to a variable speed drive. Packaged as a three phase input assembly, these units do not provide isolation from the AC line. When using inverter drives, reactors can also be used between the drive and motor.
Reactors provide a two-way benefit in that they protect other line connected equipment from ill effects generated by the inverter and protect the inverter from the negative effects of other line connected equipment. Line input reactors are very useful and cost effective in reducing the effects of harmonic distortion and line notching described above. They can also protect the drive from nuisance “bus over-voltage” tripping and provide fault current limitation.
When used between the drive and motor they can filter the high transient voltages and provide motor fault current limitation. This can extend the life of the output transistors. They will reduce motor noise and operating temperature.
With inverter drives, line reactors are typically sized according to the horsepower or KVA rating and the drive/motor voltage rating. They’re available in different impedance ratings which relate to the full load voltage drop expressed as a percentage of the rated voltage. Carotron recommends 5% reactors for use on the AC line inputs and 3% units when used between the drive and motor.
2.) LINE REACTORS with DC DRIVES
DC drives also realize the two way benefit of line reactors. Line voltage distortion and notching caused by the drive is filtered from the incoming AC line supply and conversely interference imposed on the line by other devices is filtered from the drive. The available short circuit current from the line is limited.
Sizing of line reactors for DC drives may differ somewhat from inverters because the AC line current versus horsepower can be higher. Typically the primary ill affect of the drive is line notching and a 3% reactor is usually adequate. One sizing method looks at the equivalent horsepower inverter reactor size and then uses the next larger size. Another method looks at the line voltage and actual line current rating of the drive and matches to a reactor with an equivalent current rating.
3.) DRIVE ISOLATION TRANSFORMERS
An obvious difference between a transformer and a line reactor is the ability to transform the voltage level of the power system. DITs, drive isolation transformers, provide this same ability and include several differences that make them more suitable for three phase input drive applications than conventional transformers or line reactors.
We’ve discussed adjustable speed drives and their negative affects on the AC line in the form of distorted current. The severe cyclic nature of many drive processes also cause supply current surges that create mechanical stress in the transformer. The current transients that cause line notching adds to the mechanical stress. DIT type transformers can supply the distorted current loads without excess heating and are designed to deal with the mechanical stress. DITs, like reactors add reactance in the AC line that can reduce the current and voltage distortion.
Another major difference from reactors is that the secondary of a drive isolation transformer represents a separately derived power supply that is electrically isolated from the primary power source. If the secondary is wye connected, it can be grounded. Grounding prevents the transfer of common-mode noise and transients, both from the primary source to the motor drive, and from the drive to the power system.
Another benefit of a grounded wye secondary relates to the ability of motor drives to generate large induced ground currents. This is due to rapid current changes caused by diodes, SCRs, or PWM outputs that couple currents capacitively back to the source. High-frequency induced ground currents are a major cause of data disruption in digital communication and nuisance tripping of ground fault systems. Introducing a grounded, drive isolation transformer localizes the ground current effect and prevents it from extending upstream from the transformer.