Patent Application
20130285592
The present invention relates in general to medium voltage soft starters for induction motors, and in particular, to a soft starter including a current limiting reactor.
An induction motor is an AC motor where power is supplied to the rotor by means of electromagnetic induction. Such motors are widely used in industrial drives. The process of starting an induction motor can damage the motor and influence the characteristics of the electrical power system. This is especially true in medium voltage applications. If the motor is started with a significantly higher current than rated, mechanical and thermal stress can occur to the motor or loads. Large voltage fluctuations and deep sags can occur in the electrical power system associated with the motor starting. This problem has been addressed in part by solid state soft starters, in particular medium voltage solid state soft starters. Medium voltage solid state soft starters protect the motor from failure and excessive maintenance caused by mechanical shock during starting and stopping by providing smooth, stepless acceleration and controlled deceleration.
Many such soft starters include a silicon controlled rectifier (hereinafter “SCR”) to control current. Prior Art
Soft starter 16 also includes load break switch 2 with grounding bar 18, motor fuse 3, current transformer 5, low voltage control compartment 9, isolation transformer 10, and fiber optic cable 11. With the main attention in the design, development, and application of soft starters being on motor protection, pulsating torques, harmonics, and power system voltage sags and dips, no attention has been focused on protection of the SCR. The starting process can produce electrical transients in the form of current change events that are of major concern for the SCR. These events and their value are amplified by an increase in overall system capacitance and working voltage levels.
A system's capacitance has three origins. The first capacitance origin is power factor correction capacitors. This capacitance may be easily eliminated by switching off the capacitor during the motor starting. The second origin is static VAR compensators. These line input connected capacitors typically cannot be removed during motor starting, as in many instances they are in place to support a weak power feed. The third capacitance origin is the inherent capacitance of the soft starter and power system elements, such as the power cable, contactors, transformers, etc. These are unavoidable during motor starting. Even simple measures, such as increasing cable length or quantity can affect the capacitance value. A well known characteristic of a capacitor is that voltage across the capacitor cannot be changed instantaneously. When a voltage is applied to a capacitor, therefore, a large current tries to flow instantly into the capacitor to charge it to the value of the applied voltage. This rapid current change can be greater than the capability of the SCR, leading to its failure.
The solid state soft starter adds flexibility to the system because it can be adjusted to deliver an optimal starting current profile to the induction motor. This may result in lower maintenance costs and increased lifetime of the mechanical load, as well as improved energy efficiency. Starters including SCRs typically utilize anti-parallel, or back-to-back SCRs. Their switching signals are delayed from the voltage zero crossing so that the applied motor current is significantly lower than typical starting currents. After the motor reaches full speed, the SCRs are bypassed by a contactor.
To start the induction engine, the SCRs are switched on, and there is no control on the amount of current flowing through them. The current through the SCRs is entirely dictated by the external impedance connected in the circuit and the applied voltage. The maximum rate of increase in current during the SCR switching that the SCR can tolerate is called the critical rate of rise of current for the device. The critical rate of rise is specified at maximum junction temperature. During the first few microseconds of turning on, only a small area in the gate conducts the anode current. If the current increases too fast, localized overheating will occur and the SCRs may be permanently damaged.
Now referring to Prior Art
The present invention includes a soft starter and an induction motor kit including the soft starter of the present invention.
In its most basic form, the soft starter of the present invention includes a basic soft starter configuration with an SCR and a current limiting reactor. The basic soft starter configuration uses anti-parallel SCRs and bypass and line isolation contactors, as shown in Prior Art
The current limiting reactor of the present invention provides several additional advantages. The current limiting reactor only needs to be connected to the soft starter network for the starting of the SCRs. When understacked to the SCRs, the reactor provides mechanical support for the SCRs and also provides isolation for the heat sink, allowing for the elimination of epoxy resin standoffs that are typically employed. The SCRs used with the present invention are preferably part of a heat sink assembly for the soft starter, and the understacked reactor also provides support for this entire heat sink assembly. Finally, the relatively small size of the reactor allows it to be part of the larger soft starter enclosure so that the reactor does not require its own enclosure. This saves both space and money.
Through extensive research and analysis, practical field measurements, and general experience, the inventors of the present invention have concluded that to protect the SCRs in the soft starter, the current change should not exceed 75 A/μs. This parameter dictates a reactor inductance of 50-200 μH. The inductance may be higher or lower than this range, however. These determinations were formed in part by mathematical models and Electro Magnetic Transient Program (hereinafter “EMTP”) simulations of the soft starter in order to understand the nature of the transient processes and relevant parameters that influence it. Furthermore, analytical formulae are used for the calculation of transient current without EMTP stimulations and determination of the required inductance to limit the current rise to a safe level for SCRs. The relevant equation is as follows:
L
R
=V
P/(ΔI/Δt)
Where:
LR is the required inductance to limit current use in μH;
VP is the system voltage peak in V; and
ΔI/Δt is the safe current rise for SCRs in A/μs.
In its most basic form, the induction motor kit of the present invention includes an induction motor integral to a soft starter as described above.
Therefore it is an aspect of the present invention to provide a soft starter that limits current rise during switching on so as to protect the soft starter's SCRs.
It is a further aspect of the present invention to provide a soft starter with lower maintenance and longer lifetime.
It is a further aspect of the present invention to provide a soft starter that includes a current limiting reactor with an inductance of 50-200 μH.
It is a further aspect of the present invention to provide a reactor that may be installed before or after the SCRs and inside the bypass contactor loop.
It is a further aspect of the present invention to provide a very small reactor that easily fits within the soft starter's enclosure.
It is a further aspect of the present invention to provide SCRs in combination with heat sinks for absorbing heat from the soft starter's operation.
It is a further aspect of the present invention to understack the current limiting reactor to the SCR/heat assembly to provide mechanical support for this assembly and to electrically isolate the heat sink.
It is a further aspect of the present invention to provide a current limiting reactor small enough to fit within the soft starter's enclosure.
These aspects of the present invention are not meant to be exclusive and other features, aspects, and advantages of the present invention will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description and accompanying drawings.
Referring first to
Now referring to
Now referring to
Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions would be readily apparent to those of ordinary skill in the art. Therefore, the spirit and scope of the description should not be limited to the description of the preferred versions contained herein.
