Reactors in drive systems: Where do we find them?
Reactors are widely used in variable speed drive systems. They appear as sub-components inside the VFD or as external components on the grid side or machine side. In this post we look at various types of reactors, their location, function and purpose.
1. Introduction
Reactors can be found in almost every adjustable speed drive system utilizing VFDs. No matter whether the VFD is a current source inverter or a voltage source inverter, one or multiple reactors are typically required. The purpose of the reactor is versatile. Nevertheless, main function is usually to provide certain impedance in order to limit fault current, smooth the current waveform and reduce harmonics, limit the voltage or current rise time, decouple parts of a system, serve as intermediate energy storage etc.
Remark: Alternative names to reactor are choke, inductor or coil. In this article we use the term reactor as per IEC 60076-6.
2. Reactors in drive systems
Transformer, VFD and motor are obvious main system components. Besides them there are additional components as well. One category are reactors. They are used as an internal component inside the VFD or as external reactors. Most of those reactors are dry type with VPI insulation, usually with an iron core (seldom also air core) and inductance ranging from several μH up to units of mH.
At closer look we will discover many different reactors as part of drive systems. Thus, let’s classify them according to their location to grid side, VFD internal and motor side.
2.1 Grid side reactors
Reactors on the grid side mostly serve to reduce harmonics injected by the VFD into the supplying grid. A series connected reactor can be used for such purpose. Reactor can also be part of an input harmonic filter. Another application is a current limiting function which can be part of a protection concept or as for purpose of optimization of component rating.
2.1.1 Input AC reactor for harmonic reduction
Series reactance between the grid and VFD input helps to reduce harmonic distortion. An input AC reactor is frequently used in low voltage drives, especially in absence of input transformer.
Medium voltage drives almost always have an input transformer that has its short circuit impedance as specified by VFD manufacturer. Thus, an additional AC reactor is mostly not needed. However, input AC reactors may be used in “special” cases:
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2.1.2 Input AC reactor for current limitation
Input reactor inherently limits the fault current. Again, the current limiting function is normally provided by the input transformer. Nonetheless, AC reactor may be needed in following situations:
- VFD is of “direct to line” type without an input transformer
- The aim is to reduce short circuit current rating for other components, e.g. incoming switchgear or auxiliary systems supplied from the mains
The latter purpose is mainly used for system optimization. More information is shared with our premium subscribers.
2.1.3 Reactor as part of input harmonic filter
If a harmonic filter [2] is required, then an AC reactor is mostly part of it. The harmonic filter can be an integral part of the VFD (pre-engineered solution typically tuned for given frequency) or delivered as an external system (tendentially tailor made for the application or project). Harmonic filter usually provides reactive power compensation capability as well.
2.2 Reactors inside the VFD
VFD itself usually includes several reactors as well.
2.2.1 DC reactor in current source inverters
In case of a current source inverter there is a large DC reactor that decouples the grid and machine side by serving as intermediate energy storage. The inductance of such reactor depends on the design criteria and is usually in the range of few mH. The reactor transmits the full DC link current of the VFD that can reach several kA.
2.2.2 Reactors in voltage source inverters
Voltage source inverter utilizes reactors as well. They are smaller in size but larger in quantity. One use is to “soft couple” the capacitors in the dc link and limit the discharge current in case of a fault. The reactors used for soft coupling may also be utilized to create a dc filter tuned to a specific frequency.
Small AC reactors are used as part of a dv/dt filter at the inverter output. Such filter is usually part of the VFD but can possibly also the installed externally. In case of an output sine filter there is a larger reactor as part of that filter. The sine filter is usually integrated inside the VFD cabinet. However, functionally it belongs rather to the machine side category.
2.3 Machine side reactors
Various reactors can be used on the machine side of the VFD. Their function is principally same or similar to those on the grid side.
2.3.1 Output AC reactor for motor friendliness
An output reactor may be used to improve the motor friendliness. The goal is to reduce current harmonics that would otherwise cause additional losses in the motor and increase the risk of local hot spots. For voltage harmonics of higher frequencies, the reactor represents significant impedance which reduces the magnitude of corresponding current harmonics. The reactor also inherently reduces the dv/dt of a voltage source inverter thus reducing the stress of machine insulation and the risk of exciting the resonance of inverter-cable-motor system. Every series connected reactor is principally a low-pass filter.
2.3.2 Output AC reactor for decoupling impedance
This case is applicable for example for VFD soft starters [3] or drives with a bypass option. The changeover from VFD operation to direct on-line operation can be realized as “close before open” (make before break) or “open before close” (break before make).
The first option is usually preferred due to its smoothness. “Close before open” method requires a certain minimum decoupling impedance (see Figure 2b).
2.3.3 Output AC reactor for current limitation
Reactor on the machine side can be used to limit the fault current of the motor. That is applicable mainly for VFDs with fuseless protection concept. In normal situation the machine and inverter are designed to be “compatible” and the VFD can withstand the fault current of the motor. However, in some cases the short circuit withstand is not granted. An output reactor may hep to solve the issue.
2.3.4 Paralleling reactors
When multiple VFDs are paralleled to one large machine then paralleling reactors are needed. More information is shared with our premium subscribers.
2.3.5 Loading reactors
These reactors are used to load the VFD during testing (e.g. as part of factory acceptance tests). Especially in the high-power range it is normally not possible to test the VFD with a motor at full load in the test field. The common practice is to test the VFD with rated current using a reactor as a load.
3. Reactor design
3.1 Core type
Basic classification can be done according to the core type:
- Iron core reactors
- Air core reactors
Each solution has specific advantages and drawbacks [4]. Iron core reactors have major advantage in terms of space thus being suitable for most internal use. Disadvantage is the non-linear characteristic due to saturation, higher weight and noise. Air core reactors excel in linearity and are perfectly suited for current limiting function. However, their stray magnetic field and consequent magnetic clearances require significant space around the reactor.
3.2 Insulation type
Another classification can be based on insulation:
- Liquid immersed reactors
- Dry type reactors
Nevertheless, clear majority of all reactors used in variable speed dirve systems is of dry type technology.
3.3 Cooling type
Most common cooling types of reactors are:
- Natural air cooling (AN)
- Forced air cooling (AF)
- Water forced cooling (WF)
Reactors used internally inside the VFD mostly used the same cooling method as the VFD itself, i.e. reactors in air-cooled VFDs are primarily AF while reactors in liquid-cooled VFDs widely use WF cooling. Reactors as part of harmonic filters are mostly with natural air cooling.
Conclusion
Reactors are inevitable components of variable frequency drive systems. Some of them are installed internally as part of the VFD while others are used as external components. Their purpose is very versatile. Most typical function is a decoupling impedance, limitation of fault current or power quality improvement.
Summary
Here is a short summary of where we can find reactors in variable frequency drive systems:
- Grid side reactors
- Input AC reactor for harmonic reduction
- Input AC reactor for current limitation
- Reactor as part of harmonic filter
- Reactors inside the VFD or integral part of VFD
- DC reactor in current source inverters (CSI)
- Reactors in voltage source inverters (VSI)
- Soft coupling
- dv/dt filter
- Phase leg reactors (MMC)
- Machine side reactors
- Output AC reactor for motor friendliness
- Output AC reactor as decoupling impedance
- Output AC reactor for current limitation
- Reactors for paralleling of VFD banks
- Loading reactors for testing purpose
Above list may not be complete. Do you miss any application of a reactor in variable speed drive systems? Write us an email: info@mb-drive-services.com
References
[1] Trasfor – custom made reactors and dry-type transformers, https://www.trasfor.ch/
[2] Harmonic filter, https://mb-drive-services.com/network-harmonics-harmonic-filter/
[3] VFD as a soft starter, https://mb-drive-services.com/vfd-as-soft-starter/
[4] “Iron-Core Versus Air-Core Harmonic Filter reactors”, Northeast Power Systems, Inc., New York, USA
