Variable frequency, variable voltage power supply

#This invention relates to static inverters for supplying variable frequency and variable magnitude voltage from a unidirectional source to a load such as an induction motor.

#Different types of variable frequency, variable voltage power supplies are known, but they all have singular inherent disadvantages, particularly in cost and in size. For example, the response to transients by variable voltage link inverters with phase controlled rectifiers and choppers is unsuitable for some applications and also such inverters require an expensive DC filter. Pulse width modulated (PWM) inverters of the SCR type have difficulty in maintaining acceptably low motor current ripple and necessitate reactive enegy storage and expensive commutation circuits. The SCR's of known PWM inverters must be of the high-speed, and thus expensive, type because they switch at carrier frequency. Further, known inverters of the SCR type having a voltage-controlling transistor chopper commutate the SCR's off while they are conducting load current and thus require large and expensive inductive and capacitive elements in the commutation circuits. The transistor switches coupled to the high voltage bus of PWM transistor inverters are very expensive because they require high power, continuous, isolated drive signals which make the interface circuits thereto costly.

#It is an object of the invention to provide an improved variable voltage, variable frequency AC power supply which eliminates the above disadvantages of, and is cheaper in cost than, prior art devices. Another object is to provide such a variable voltage, variable frequency power supply which does not require an expensive DC filter or commutation circuit or expensive switch interface circuits and still has improved transient response and lower motor ripple than prior art apparatus. Still another object is to provide an improved PWM inverter having SCR'S for supplying multi-step wave forms made up of variable width carrier frequency pulses to a motor load and wherein the SCR's which at modulation frequency and may thus be of low-speed inexpensive type. A further object is to provide an improved inverter of the SCR type having a voltage-controlling transistor chopper and wherein the SCR's are commutated while the chopper is turned off and load current is not flowing through the SCR's, thereby eliminating large and expensive inductive and capacitve commutation elements. A further object is to provide an improved variable voltage, variable frequency powder supply which has a unique configuration of semiconductor switches in the inverter which permits the switches to be low in cost and inexpensively driven and to provide low impedance, free-wheeling, out-of-phase paths for reactive current that result in low motor ripple. Still another object is to provide such an improved variable frequency, variable voltage AC power supply for a polyphase motor load which has a sufficiently high constant carrier frequency for the motor to do the filtering.

#The above and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered together with the accompanying drawing wherein:

#FIG. 1 is a schematic diagram of a preferred embodiment of the variable frequency, variable magnitude voltage supply of the invention with the control circuit shown in block form;

#FIG. 2 is a schematic circuit diagram of the control circuit of FIG. 1; and

#FIG. 1 is a schematic block diagram of a preferred embodiment of the invention adapted to provide variable frequency and variable voltage power from a unidirectional source 10 to a load such as an induction motor M to control motor speed over a wide range. Source 10 may supply 300 volt DC power through a voltage controlling transistor chopper CH to the inverter semiconductor switches and may in known manner comprise a three phase, full wave bridge rectifier having three diode rectifiers 11 with commoned cathodes connected to a positive bus + and three diode rectifiers 12 having commoned anodes connected to the negative bus - and a smoothing shunt capacitor 15. The inverter is shown as a series bridge INV which has three phase legs with a controlled rectifier, or SCR TA in the phase A leg having its anode coupled to the emitter of transistor chopper CH and in series with a transistor switch QA having its emitter coupled to the negative bus and the node therebetween consitituing the phase A output terminal at adapted to be connected to phase winding M.sub.A of motor M; a controlled rectifier TB in series with a transistor switch QB in the phase B leg and the node therebetween constituting the phase B output terminal bt adapted to be connected to motor phase winding M.sub.B ; and a controlled rectifier TC in series with a transistor switch QC in the phase C leg and the node therebetween constituting the phase C output terminal ct adapted to be connected to motor phase winding M.sub.C. Feedback diodes DA1, DB1, DC1 may be connected in inverse polarity shunt relationship with SCR's TA, TB, TC respectively between the corresponding phase terminals at, bt, ct and the positive bus + at the input to chopper transistor CH, and feedback diodes DA2, DB2, DC2 may similarly be connected in inverse polarity shunt relation to transistor switches QA, QB, QC respectively. A separate voltage source BA for forward biasing chopper CH and for reverse biasing SCR's TA, TB, TC is shown in a battery BA and may have its negative terminal referenced to the output of chopper CH and its positive terminals coupled to the collector of a bias-applying transistor Q1 whose emitter is coupled through diodes D1, D2, D3 respectively to the corresponding phase output terminals, at, bt, ct. The positive terminal of source BA is also coupled to the emitter of a chopper driven transistor Q2.

#SCR's TA, TB, TC are turned on sequentially by trains of gate pulses (See FIGS. 3h and 3i) as described hereinafter, and turning on transistor Q1 will apply bias source BA across the anode-cathode paths of SCR's TA, TB, TC to reverse bias them and commutate them off when chopper CH is turned off and they are only conducting low magnitude current. The reactive current paths through feedback diodes DA1, DB1, DC1 and also through feedback diodes DA2, DB2, DC2 are in inverse polarity to the unidirectional power source 10 so that reactive current flow therethrough does not interfere with commutation of SCR's TA, TB, TC. It will be noted that the disclosed arrangement commutates SCR's TA, TB, TC off while transistor chopper CH is turned off and only low magnitude current is flowing therethrough and thus eliminates the conventional means such as auxiliary thyristors and large and expensive commutation inductors and capactitors which are utilized in known force-commutated SCR inverters to commutate off the load current carrying switches.

#Transistor switches QA, QB, QC are controlled to provide, in conjunction with the feedback diodes, out-of-phase, low impedance free-wheeling paths for reactive motor current during the off-time of SCR's TA, TB, TC. The transistor switches QA, QB, QC may be driven inexpensively because they have the common emitter potential of the negative bus, and such reactive motor current paths may be easily altered because of the inherent turn-off ability of transistor switches QA, QB, QC.