Inverter for providing a sinusodial output having a low harmonic content

Claim 2

An inverter comprising: means for generating an alternating stepped waveform having a magnitude which varies incrementally during each half cycle, said magnitude having a first value at the beginning and end segments of each half cycle thereof and a second value greater than said first value during a segment intermediate said beginning and end segments; means for generating a rectangular wave, having a constant magnitude during each half cycle, in fixed phase relationship with and having a harmonic frequency greater than that of said alternating stepped waveform, the duration of the rectangular wave being shorter than the duration of the second value of the alternating stepped waveform during each half cycle of the alternating stepped waveform; and means for summing said alternating stepped waveform and said rectangular wave to form a composite waveform approximating a sinusoidal waveform, said composite waveform having three steps in each quarter cycle thereof.

Claim 3

The inverter of claim 1 wherein said generating means comprises a source of DC voltage providing both positive and negative voltages with respect to a neutral reference potential, a coil with an intermediate tap offset from center, and means for selectively switching said positive and negative voltages to opposite sides of said coil to generate the alternating stepped waveform at said intermediate tap, said composite waveform being developed between said intermediate tap and the neutral reference potential.

Claim 4

The inverter of claim 2 wherein said summing means includes a winding inductively coupled with the coil and means for providing a rectangular voltage waveform thereacross.

Claim 5

The inverter of claim 2 in which said switching means includes a first switch connected to one side of the coil for applying the positive voltage thereto, a second switch connected with the one side of the coil for coupling the negative voltage thereto, a third switch connected with an opposite side of the coil for coupling it to the positive voltage, and a fourth switch connected to the opposite side of the coil for coupling it with the negative voltage, and means for controlling the switches, said switch controlling means causing the first and fourth switches to assume a conductive state during the beginning and end segments, and the first and third switches to assume conductive states, during the intermediate segments, of positive half waves, and causing the second and third switches to assume conductive states during the beginning and end segments, and the second and fourth switches to assume conductive states, during the intermediate segments, of negative half waves.

Claim 6

The inverter of claim 1 in which said rectangular wave has alternating positive and negative portions with respect to a neutral reference potential, each of said portions having a duration less than that of the intermediate segment of the first waveform.

Claim 7

The inverter of claim 1 in which said rectangular wave has a frequency three times that of the alternating stepped wave.

Claim 8

The inverter of claim 1 in which said rectangular wave is a square wave.

Claim 9

The inverter of claim 7 in which said square wave has a frequency three times that of the composite waveform.

Claim 10

The inverter of claim 1 including means for generating second and third alternating stepped waveforms substantially identical to the first-mentioned alternating stepped waveform and out of phase therewith by 120° and 240°, respectively, and in which said summing means includes means for summing said rectangular wave with both the second and third alternating waveforms to form second and third composite waveforms each substantially identical to said first-mentioned composite waveform but out of phase therewith by 120° and 240°, respectively, providing a three-phase output.

Claim 11

The inverter of claim 9 in which each of said alternating stepped waveform generating means includes a coil, and means for switching a source of DC voltage across the coil during the beginning and end segments of each half cycle of the alternating waveform thereof, and said developing means includes means inductively coupled with each of the coils for developing pulses at the beginning and end segments of each of the first, second and third alternating waveforms, and means for adding all of said pulses together to form said rectangular wave.

Claim 12

The inverter of claim 10 in which each of said coils has a tap at which the one alternating waveform associated therewith is provided.

Claim 13

The inverter of claim 8 in which said first, second and third composite waveforms are developed between an output terminal unique to each of the waveform generating means and another terminal common to all the waveform generating means, said source of DC voltage has a neutral terminal at which a neutral reference potential is provided, a positive terminal at which a voltage positive with respect to the neutral reference voltage is provided and a negative terminal at which a voltage negative with respect to the neutral reference voltage is provided, and said inductively coupled means includes a circuit including three windings connected in series with one another between the neutral terminal and the common terminal which are inductively coupled with the coils of the first, second and third alternating waveform generating means, respectively.

Claim 14

The inverter of claim 12 in which the unique output terminal of each waveform generating means is connected with an intermediate tap of the coil thereof, and each waveform generating means further includes a first electronic switch connected between one end of the coil and the positive terminal, a second electronic switch connected between the one end of the coil and the negative terminal, a third electronic switch connected with another end of the coil opposite the one end and the positive terminal and a fourth switch connected between the other end of the coil and the negative terminal, and means for controlling the switches, said switch controlling means during each positive half wave causing the first and fourth electronic switches to assume conductive states during the beginning and end segments thereof and causing the first and third electronic switches to assume conductive states during the intermediate segment thereof, and during each negative half wave causing the second and third electronic switches to assume conductive states during the beginning and end segments thereof, and causing the second and fourth electronic switches to assume conductive states during the intermediate segment thereof.

Claim 15

The inverter of claim 12 wherein said rectangular wave is a square wave with a frequency three times greater than that of the alternating stepped waveforms and the three steps in each quarter cycle of each of the three composite waveforms have relative magnitudes in the ratio of 262:716:978, and the beginning and end segments of each of the alternating stepped waveforms have a duration of 30°.

Claim 16

The inverter of claim 1 in which said alternating stepped waveform, rectangular wave and said composite waveform vary above and below a reference potential during alternating half cycles.

Claim 17

The inverter of claim 1 in which each of said beginning and end segments of the alternating stepped waveform has a duration of 30°.

Claim 18

The inverter of claim 16 wherein said rectangular wave is a square wave with a frequency three times that of the alternating stepped waveform and the three steps of the composite output waveform have relative magnitudes in the ratio of 262:716:978.

Claim 19

A three-phase inverter comprising: means for generating first, second and third alternating stepped waveforms phase displaced with respect to each other by 120° and each having a magnitude that varies incrementally during each half cycle, the magnitude of each waveform having a first value at the beginning and end segments of each half cycle thereof and a second value greater than said first value during the segment intermediate said beginning and end segments; means for developing a rectangular wave in fixed phase relationship with, and having a harmonic frequency greater than that of, each of said first, second and third waveforms; and means for summing said rectangular wave with each of said first, second and third waveforms to form three composite waveforms, each of said composite waveforms approximating a sinusoidal waveform and having three incremental steps of different magnitude in each quarter cycle thereof.

Claim 20

A three-phase inverter comprising: means for generating first, second and third alternating stepped waveforms phase displaced with respect to each other by 120° and each having a magnitude that varies incrementally during each half cycle, the magnitude of each waveform having a first value at the beginning and end segments of each half cycle thereof and a second value greater than said first value during the segment intermediate said beginning and end segments; means for developing a rectangular wave in fixed phase relationship with, and having a harmonic frequency greater than that of, each of said first, second and third waveforms; and means for summing said rectangular wave with each of said first, second and third waveforms to form three composite waveforms, each of said composite waveforms approximating a sinusoidal waveform and having three incremental steps of different magnitude in each quarter cycle thereof.