A normal ac inverter has three parts:
1. An input converter to rectify ac power to dc power. It is normally called the source bridge.
2. An energy storage device which separates the input from the output and allows each to operate independently from the other. It is usually called a link filter.
3. A dc-to-ac inverter in the output stage. It is called an inverter. It generates the desired ac output voltage and frequency.
Pulse-Width-Modulated Inverters (PWM) is referred to as time ratio control. From a constant dc input voltage, we get a variable output voltage and frequency by varying the percentage of time that the power control switch is closed. The output voltage will increase by increasing the percentage of time the switch is closed. The switch is either open or closed. There is no power dissipation across the switch in both states.
Below figure illustrates a typical two-level PWM inverter circuit. It is similar to the SCR bridge, but it uses IGBTs for the switching devices. The energy storage capacitor is denoted by C. The motor connections are a, b, and c. The inverter operation is as follows:
Once the output frequency required to satisfy the speed regulator is given to the control system, it calculates the three-phase voltage commands. A triangle voltage waveform is generated and synchronized with the desired IGBT switching frequency and phase. This is the PWM carrier waveform that sets the basic inverter switching frequency. The average width of the PWM waveforms generated approximates the sine wave reference. The inductances average and smooth the resulting waveform.
The IGBT is similar to the power transistor, except that it is controlled by the voltage applied to its gate rather than the current flowing into its base, as in power transistors. The current flowing in the gate of an IGBT is extremely small because the impedance of the control gate is very high. This device is equivalent to the combination of a metal-oxide semiconductor field effect transistor (MOSFET) and a power transistor (below Figure).
Since the current required to control an IGBT is very small, it can be switched much more quickly than a power transistor. The IGBTs are normally used in high-power, high-frequency applications.
These bipolar power transistors are driven by an insulated gate metal-oxide transistor. A relatively simple 15-V gate driver signal is used to control the resulting high-current power transistor. The IGBT is a four-layer semiconductor similar to the SCR. Its main features are that
1. It has very fast switching on the order of 100 to 150 ns and resulting high-voltage transients dV/dt of 5000 to 10,000 V/μs.
2. The IGBT chips are soldered in place and connected with discrete bond wires. They are very weak when it comes to thermal fatigue problems. The IGBT modules have significantly lower thermal fatigue capability than other semiconductors. The high dV/dt generated leads to problems with bearing currents and the insulation system.
Although thyristors, diodes, and IGBTs are solid-state devices, they have wear out mechanisms just as insulation and other mechanical parts do. The wear out and failure rates of these devices can be calculated.
Common Failure Modes
Differential Expansion (Mechanical Fatigue).
This failure mode is mechanical fatigue or wear out caused by the difference in expansion rates as the temperature of the device changes. As the temperature of the device changes, different parts expand at different rates. These are the expansion coefficients for materials used in semiconductors:
Thus, the parts slide over each other, causing mechanical wear out. This failure is common to all semiconductors. It normally occurs at the end of life of these devices.
Below figure illustrates the general failure rate curve of SCRs, diodes, and IGBTs. The initial high failure rate is caused by manufacturing defects, application problems, and drive start-up stresses and lasts a few weeks. The high failure rate at the ends indicates the end of the life for the devices. In general, the lifetime of a device becomes shorter when it is operated harder and closer to its voltage rating.
Fault Current Limit.
This mode of failure is not applicable to IGBTs because they are not able to conduct currents in excess of their ratings. It is only applicable to thyristors and diodes GTOs. The junction temperature increases when the fault current increases. The maximum surge current that can be tolerated results in junction temperature excursion Tj of 300°C. This temperature excursion can occur once in the lifetime of the equipment because it would have been damaged (maimed) by the high temperature. The number of current surges that can be tolerated increases rapidly if the peak current level (peak junction temperature) is reduced. The number of surges can be approximated by
This failure mode is normally caused by incorrect application. The designer of the system must ensure that the maximum fault current in the bridge cannot exceed these limits. This failure mode occurs normally during commissioning. However, it can occur at random intervals during the lifetime of the device.
Device Explosion Rating.
This failure mechanism can occur in any of the power devices. Thyristors can break down in the reverse direction due to a fault. This is usually followed by a large surge current. The resulting arc at the edge of the device could be strong enough to blow open the ceramic housing. The explosion rating for a thyristor is normally 50 to 100 percent above the surge current rating. This type of failure can have serious consequences because conductive plasma is vented from the failed cell into the bridge, resulting in extensive arcing and destruction throughout the whole bridge. These failures are normally caused by inadequate fault coordination (design deficiency). They usually occur during commissioning or at any time during the lifetime of the device.
These failure modes normally occur in the middle region (slowly increasing failure rate) which extends over several years of operation. The failure rate depends on these application factors:
- Type of device application
- Voltage applied (as percentage of PIV)
- Junction temperature (at normal running load)
In general, the lifetimes of all semiconductors decrease when the applied voltage or temperature (as a percentage of the rating) increases.
IGBT SWITCHING TRANSIENTS
Voltage transients are generated due to the switching in the PWM inverter. They propagate down the power cables to the motor. If the motor cables are not terminated properly, the switching waves will be reflected when they reach the motor. They will be transiently increased or decreased depending on the relative impedance between the line and the load. An alternative solution for this problem would be to slow down the rate of voltage rise.
If the connecting cable is long, the mismatch of impedances will generate a voltage reflection at the point where the line impedance changes (at the motor terminals). A transient rise in voltage of the wave will occur at the motor terminals due to the voltage reflection.
For a given switching rise time, the voltage rise at the terminals will increase with the length of the connecting cable. The wave front is reflected back to the inverter. If no mitigating actions are taken, the transient voltage at the motor terminals can double that of the inverter in the worst case. In most cases, the reflection problem increases gradually with the length of the motor cable. The rise in transient voltage is important for the following reasons.
Insulation Voltage Stress
The increase in voltage stress in the motor connecting cables and the motor insulation will shorten the lifetime of the insulation. If the peaks of any of the voltage transients exceed the insulation corona discharge level (partial discharge level), the insulation will degrade with each voltage pulse. It will eventually fail. The new “inverter-rated” motors have triple-layer insulation. They have a 1600- to 2000-V partial discharge level. This allows them to withstand double the voltage peak transient from a 600-V inverter. Since most “standard” induction motors can withstand about 1200 V, its use in this application will shorten its life drastically. It can be as low as a few hours only.
Motor Winding Voltage Distribution
All high-frequency transient voltages tend to be unevenly distributed across the motor windings. The high frequencies develop greater voltages across the first windings rather than being evenly distributed across the whole length of the windings. The effect of high frequency transients tends to be accentuated on the first few windings. This is where most motor insulation failures occur. The problem becomes worse when the frequency of the transients (i.e., the IGBT switching speed) is higher. This problem does not occur with drives using older transistors or GTOs because the switching speeds are much lower (2 to 4 μs). When the drive uses IGBTs with switching speeds of 50 to 150 ns, the motor should be connected to the inverter by a cable shorter than 20 ft, or special precautions should be taken.
This problem can be solved by adding a filter to the output of the inverter to slow down the IGBT switching transients. When the inverter switching speed is reduced, the length of the connecting motor cable can be increased.
1. Unbalanced ac input voltages to a diode input source cause a current unbalance up to 20 times the voltage unbalance.
2. The power factor of a diode source bridge without a dc link reactor can be as low as 50 to 60 percent.
3. Older transistors and GTOs are much slower (50 to 20 μs) than IGBTs (100 to 150 ns).
4. a. Fast transient voltages can generate reflections that increase the voltage stress of the insulation system.
b. The voltage reflections increase with the length of the motor cable and the switching speed.
5. a. The reflections can be stopped by a motor terminal (matching) filter. However, this filter retains the switching speed.
b. The switching speeds are reduced by an inverter filter. This is the preferred solution.
c. The motor and cable insulation voltage should be increased if neither filter is used.
6. EMI radiation is generated from the motor and ac line cables of all PWM inverters, especially the ones having IGBTs.
7. a. The motor connecting cables (including ground conductors) for PWM inverters having
IGBTs must be symmetric.
b. The best cable for these applications has continuous aluminum sheath, three conductors, and three grounds.
c. The bearing currents and radiated EMI are reduced by the cable symmetry and continuous outer sheath.
8. The damage to the motor bearings is reduced by an inverter output filter or an electrostatic shield