Technically a soft starter is any device that will reduce the torque delivered to the power train. Mechanically, this can be a clutch, fluid drive, magnetic coupling, short coupling or any of the variety of devices that allow the motor to start-up across the line while slowly applying the shaft torque to the load to avoid “torque shock”. Electrically it can be any system that reduces the torque by virtue of reduced voltage or a change in motor connection.
Changing the motor terminal voltage reduces the torque because the motor output torque varies by the square of applied voltage. So if 50 percent voltage is applied to a motor, it will produce 25 percent of its available torque at that point.
Reduced Voltage starting can be accomplished in several ways as well, A common method is to use an auto transformer that drops the motor voltage during starting, then is switched out so that the motor gets full voltage when running. This method is called reduced voltage auto transformer starting. Similar to this are reactor and primary resistor starters which drop the voltage through those devices as well. All of the above technologies can be and often are referred to as “Soft Start” devices, but more recently this terminology has come to usually mean one specific type, the solid state reduced voltage starter.
The solid state reduced voltage starter(SSRV) uses high speed switching devices call SCRs(silicon controlled rectifiers) to switch on for only a portion of each half of the sine-wave line power. By doing so, the RMS voltage getting to the motor is reduced proportionally by the amount of time the switch is delayed. So if the SCR is not allowed to be conducting until the sine wave is already 1/2 over with, the output RMS will be 1/2 of the line voltage. By moving the gate point further back in the sine wave, the RMS voltage is increased until the SCR is being gated at zero crossing point and the motor is getting full line voltage. The speed at which the SCR gating is backed up is called the Ramp Time, and can typically be anywhere from a fraction of a second to 60 seconds. Although longer times are technically possible, most AC motors applications will not allow this because the increased current caused by the reduced voltage will begin to exceed the thermal safety limits of the motor itself, particularly the rotor. In addition the ramp time can be over ridden by a current limit setting, which determines the motor current through feedback sensors and stops the gate advancements in order to maintain the particular current setting. This feature is useful when the power system has limited delivery capabilities, such as weak utility lines or portable generators.
Finally once the motor is at full voltage the SCR firing becomes unnecessary and it is often beneficial to use a bypass contactor to shut power around SCRs. SCRs are not perfect conductors, and will reject approx 1.5 Watt of heat per running load ampere per phase. So on a phase 100A motor, the SCRs will be rejecting 450W of heat into the enclosure continuously. A bypass contactor is a good way of avoiding that heat buildup without introducing dust, moisture or other contaminants into the enclosure.
What is a BJT (Bi Polar Junction Transistor)?
A bipolar junction transistor (BJT), unlike a thyristor-like device, is a two-junction, self-controlled device where the collector current is under the control of the base drive current.
Why Darlington pair is used and what are its advantages and disadvantages?
The dc current gain of a power transistor is low and varies widely with collector current and temperature. The gain is increased to a high value in the Darlington connection,However, the disadvantages are higher leakage current. higher conduction drop. and reduced switching frequency.
How to control collector leakage current and enable base bias voltage?
The shunt resistances and diode in the base-emitter circuit help to reduce collector leakage current and establish base bias voltages.
What is avalanche breakdown (1st breakdown) and 2nd (second) breakdown in a BJT and how they destroy BJT?
Power transistors have an important property known as the second breakdown effect. This is in contrast to the avalanche breakdown effect of a junction, which is also known as first breakdown effect. When the collector current is switched on by the base drive, it tends to crowd on the base-emitter junction periphery, thus constricting the collector current in a narrow area of the reverse-biased collector junction. This tends to create a hot spot and the junction fails by thermal runaway, which is known as second breakdown. The rise in junction temperature at the hot spot accentuates the current concentration owing to the negative temperature coefficient of the drop, and this regeneration effect causes collapse of the collector voltage, thus destroying the device.
What happens when inductive load is turned off in BJT circuitry?
when an inductive load is turned off the base-emitter junction becomes reverse-biased, the collector current tends to concentrate in a narrow area of the collector junction. which causes the 2nd breakdown and BJT is destroyed after junction hotspot leading to thermal runaway of device.
What is a MOSFET, abbreviation and definition?
Unlike a BJT, a power MOSFET (metal-oxide semiconductor field-effect transistor) is a unipolar, majority carrier, "zero junction." voltage-controlled device.
Can you explain the working of a MOSFET?
If the gate voltage is positive and beyond a threshold value. an N-type conducting channel will be induced that will permit current flow by majority carrier (electrons) between the drain and the source.
What is the major advantage/disadvantage of MOSFET over BJT?
An important property of a MOSFET is that it does not have the second breakdown problem of a BJT. If localized and potentially destructive heating occurs within the device. the positive temperature coefficient effect of resistance forces local current concentration to be uniformly distributed across the total area. The conduction loss of a MOSFET is large for higher voltage devices. its turn-on and turn-off switching times are extremely small, causing low switching loss. The device does not have the minority carrier storage delay problem associated with a bipolar device.
Please name a few applications of MOSFET?
MOSFETs are extremely popular in low-voltage, low-power. and high-frequency (hundreds of kHz) switching applications. Application examples include switching mode power supplies (SMPS), brushless dc motors (BLDMs). stepper motor drives. and solid-state dc relays.
What is GTO (Gate Turn Off Thyristor)?
A gate turn-off thyristor (GTO), as the name indicates, is basically a thyristor-type device that can be turned on by a small positive gate current pulse, but in addition, has the capability of being turned off by a negative gate current pulse.
How is the enhanced capability of a GTO obtained?
The turn-off capability of a GTO is due to the diversion of P-N-P collector current by the gate, thus breaking the P-N-P / N-P-N regenerative feedback effect.
What ratio of negative gate current is required to turn off a GTO?
The turn-off current gain of a GTO, defined as the ratio of anode current prior to turn-off to the negative gate current required for turn-off, is very low, typically 4 or 5. This means that a 6000 A GTO requires as high as –1500 A gate current pulse. However, the duration of the pulsed gate current and the corresponding energy associated with it is small and can easily be supplied by low-voltage power MOSFETs.
What are Common applications of a GTO?
GTOs are used in motor drives, static VAR compensators (SVCs). and ac/dc power supplies with high power ratings. After large-power GTOs became available. they ousted the force-commutated, voltage-fed thyristor inverters.
What are series and shunt snubber and what is their application in power electronics?
The snubber consists of a turn-on component (R, L) called a series snubber and a turn-off component (R, C, and D), called a shunt snubber. This type of converter is typically used for a subway dc motor propulsion drive.
What do you mean by snubber loss?
The snubber loss may be substantial in a high-power and/or high-frequency converter. This, in turn, may reduce the converter's efficiency and put a burden on the cooling system. To combat this problem, various regenerative or energy recovery schemes have been proposed.
What are methods to cope with snubber loss
Passive energy recovery and the other is regenerative snubber.
What is Passive energy recovery method to overcome snubber loss?
When the GTO is turned off, the snubber capacitor„ charges to the full supply voltage, as usual. At the subsequent turn-on of the GTO, the stored energy is transferred to capacitor resonantly through the inductance and diode. When the GTO turns off again, the energy in capacitor is absorbed in the load and capacitor, charges again to voltage V.
What is regenerative snubber scheme to overcome snubber loss?
This scheme uses an auxiliary chopper. At GTO turn-off, the snubber capacitor, charges to supply voltage. At subsequent turn-on of the device, the energy is resonantly transferred to capacitor, as before. The energy in capacitor is then pumped to the source through a dc-to-dc boost converter.
What are common applications of regenerative snubbers and RC snubbers?
The idea of a regenerative snubber appears very attractive, but its application should be carefully weighed against the extra cost, loss, complexity, and equipment reliability. High-power GTO converters normally use regenerative snubbers. Otherwise, RC snubbers are commonly used.
Please name the most common silicon based semiconductor devices:
What are Power Diodes and their usage in electrical and electronics engineering?
Power diodes provide uncontrolled rectification of power and are used in applications such as electroplating, anodizing, battery charging, welding, power supplies (dc and ac), and variable-frequency drives. They are also used in feedback and the freewheeling functions of converters and snubbers.
What is the basic structure of a power diode?
A typical power diode has P-I-N structure, that is, it is a P-N junction with a near-intrinsic semiconductor layer (I-layer) in the middle to sustain reverse voltage.
What is cause of Avalanche Breakdown, how it can be avoided?
In the reverse-biased condition, a small leakage current flows due to minority carriers, which gradually increase with voltage. If the reverse voltage exceeds a threshold value, called the breakdown voltage, the device goes through avalanche breakdown, which is when reverse current becomes large and the diode is destroyed by heating due to large power dissipation in the junction.
What is a fast-recovery diode?
In a fast-recovery diode, as the name indicates, the recovery time t and the recovery charge Q are reduced by the minority carrier lifetime control that enhances the recombination process. However, the adverse effect is a higher conduction drop.
What is a Schottky diode, what are its advantages?
Schottky diode is basically a majority carrier diode and is formed by a metal-semiconductor junction. As a result, the diode has a lower conduction drop (typically 0.5 V) and faster switching time, but the limitations are a lower blocking voltage (typically up to 200 V) and higher leakage current.
What does the word “thyristor” means?
The term "thyristor" came from its gas tube equivalent, thyratron.
What devices belongs to Thyristor family?
That includes SCR, triac, GTO, MCT, and IGCT.
What is a thyristor and its construction and working?
Thyristors can be classified as standard, or slow phase-control-type and fast-switching, voltage-fed inverter-type. Basically. it is a three-junction P-N-P-N device, where P-N-P and N-P-N component transistors are connected in regenerative feedback mode. The device blocks voltage in both the forward and reverse directions (symmetric blocking). When the anode is positive, the device can be triggered into conduction by a short positive gate current pulse.
Does the gate of ordinary thyristor have ability to turn off?
Once the device is conducting, the gate loses its control to turn off the device.
In how many ways thyristor can be turned on?
Beside gate signal a thyristor can also turn on by excessive anode voltage, its rate of rise dv/dt, by a rise in junction temperature ( Ti), or by light shining on the junctions.
Please explain the turn on procedure of a thyristor using excess anode voltage?
If forward voltage is applied on the device, there will be a leakage current due to blocking of the middle junction. If the voltage exceeds a critical limit (breakover voltage), the device switches into conduction. With increasing magnitude of IG, the forward breakover voltage is reduced, and eventually , the device behaves like a diode with the entire forward blocking region removed.
What is latching current and holding current in a thyristor?
The device will turn on successfully if a minimum current, called a latching current, is maintained. During conduction, if the gate current is zero and the anode current falls below a critical limit, called the holding current, the device reverts to the forward blocking state.
What are Triacs?
A triac has a complex multiple-junction structure, but functionally, it is an integration of a pair of phase-controlled thyristors connected in inverse-parallel on the same chip. The three-terminal device can be triggered into conduction in both positive and negative half-cycles of supply voltage by applying gate trigger pulses.
What are advantages and disadvantages of Triac over traditional thyristor equivalent circuit?
A triac is more economical than a pair of thyristors in anti-parallel and its control is simpler, but its integrated construction has some disadvantages. The gate current sensitivity of a triac is poorer and the turn-off time is longer due to the minority carrier storage effect.For the same reason, the reapplied dv/dt rating is lower, thus making it difficult to use with inductive load. A well-designed RC snubber is essential for a triac circuit.
What are common applications of a Triac?
Triacs are used in light dimming, heating control, appliance-type motor drives, and solid-state relays with typically 50/60 Hz supply frequency.