At present, the research more high power inverter circuit are

At present, the research more high power inverter circuit are:
Multilevel voltage inverter
Transformer coupling of multiple pulse inverter
Hand over frequency converter
Doubly-fed ac frequency conversion speed control system
(1) level voltage inverter
Japan nagaoka university of science and technology, A.N abae in 1980 in IAS annual meeting for the first time put forward three

level inverter, also called Neutral clamp type (Neutral Point Clamped) inverter. It is the emergence of high pressure and large

capacity
rotating joints voltage inverter research opens up a new way of thinking.
Multilevel voltage inverter and ordinary double level compared to the inverter has the following advantages:
1. The more suitable for large capacity, high voltage occasions.
2. Can produce M layer trapezoidal output voltage, the staircase to modulation can get very good approximate sine wave,

theoretically improve level number can be close to pure sine wave type, harmonic content is small.
3. Electromagnetic interference (EMI) problem is reduced greatly, because the switch element a movement of the dv/dt usually

only traditional double level 1 / (M - 1).
4. High efficiency, eliminate the same harmonic, the double level using PWM control switch frequency is high, the loss is big,

and multilevel inverter can be used low frequency switch movement, the switching frequency is low, the loss is small, the

efficiency improving.
(2) transformer coupling of multiple pulse inverter
Transformer coupling of multiple pulse inverter three level circuit, to rotary union get a more level will only each phase series unit

inverter bridge the same number can be increased. Its advantages are:
1. Don't exist voltage balance problems. No clamp diode or capacitance, suitable for speed regulation control;
2. Modular degree good, easy maintenance,
3. To the same level as the number, the number of devices at least;
4. No clamp diode or capacity restriction, can realize more level, the higher the voltage, achieve more low harmonic,
5. Control method is relatively simple, can to each level respectively for PWM control, then waveform restructuring.
This, of course, the shortage of the structure in need a lot of isolated dc power supply, the application of limited to some

extent.
(3) hand over frequency converter
Hand over converter using thyristor as the main power device, in the mill and mine hoist drive has a lot of demand. Thyristor

the biggest advantage is to fiber optic rotary joint switch the power (up to 5000 v / 5000 a), suitable for large capacity ac motor speed control system.

At the same time, the high efficiency crystal thyratron production and technical function technique is quite mature, through

with the modern ac motor control theory of digital combination, will have strong competitiveness. But hand over frequency

converter, there exist some inherent defect: speed range is small, when the power supply is 50 hz, maximum output frequency not

more than 20 hz; On the other hand, the power factor is low, the harmonic pollution is big, therefore need at the same time of

the reactive power compensation and harmonic management.
(4) doubly-fed ac frequency conversion speed control system
Doubly-fed ac frequency conversion speed control system of the frequency converter power of small, power factor adjustable,

system reliability is higher, so recently received the attention of many researchers. Because of the frequency converter power

account for only 25% of the motor capacity, so can greatly reduce the cost of the system. But, doubly-fed ac frequency

conversion speed regulating system of motor need special design, can't use common asynchronous motor; And by the converter

capacity and speed range restriction, does not have the ability of soft starter.