Inverter-motor system with different control characteristics for inverter voltage and frequency

Claim 2

A control system for an inverter connected to drive an electric motor, the inverter providing an a-c voltage over an output line to the motor upon receipt of a d-c voltage over a d-c bus, and a controllable d-c bridge connected to supply the d-c voltage to the d-c bus, which control system comprises: circuit means connected to provide a control signal varying as a function of the level of the motor current; and a constant voltage circuit, coupled to the circuit means, connected to provide a modified control signal for regulating the operation of the d-c bridge and thus regulating the amplitude of the d-c bus voltage, to regulate the amplitude of the inverter a-c output voltage in a manner such that as the inverter is reduced from 100% load to a reduced operating level substantially below 100% load, the frequency of the inverter a-c output voltage is gradually reduced but the amplitude of the inverter a-c output voltage remains substantially constant.

Claim 3

A control system as claimed in claim 1 in which said reduced operating level is 80% or less of 100% load.

Claim 4

A control system as claimed in claim 1 in which said reduced operating level is 70% of 100% load.

Claim 5

A control system as claimed in claim 1, and in which said circuit means comprises a motor current sensing stage, a current transformer coupled to said inverter output line for providing a signal varying as the motor current to the motor current sensing stage, and a function generator circuit coupled between the motor current sensing stage and the constant voltage circuit, to provide said control signal for regulating the inverter operating frequency.

Claim 6

A control system for a power transfer system in which an electric motor is connected to a load, an inverter is connected over an output line to the motor to energize the motor with an a-c voltage of variable amplitude and variable frequency, a d-c bridge is coupled between an a-c input line and a d-c bus supplying the inverter to provide an adjustable d-c voltage which determines the amplitude of the inverter a-c output voltage, a d-c bus controller connected to regulate operation of the d-c bridge and thus regulate the adjustable d-c voltage, frequency control means connected to sense the level of current supplied to the motor and to provide a control signal for regulating the frequency of the inverter output voltage in accordance with a first function, and a constant voltage circuit, connected between the frequency control means and the d-c bus controller, operative to modify operation of the d-c bus controller in the region below 100% inverter load so that the d-c bus controller produces a modified control signal for application to regulate the inverter output voltage amplitude in accordance with a second function differing from the first function by which the inverter output frequency is regulated until the inverter load reaches a predetermined reduced load level, and as the inverter load is further reduced below the reduced load level, both the amplitude and the frequency of the inverter a-c output voltage are gradually reduced.

Claim 7

A control system for a power transfer system in which an electric motor is connected to a load, an inverter is connected over an output line to the motor to energize the motor with an a-c voltage of variable amplitude and variable frequency, a d-c bridge is coupled between an a-c input line and a d-c bus supplying the inverter to provide an adjustable d-c voltage which determines the amplitude of the inverter a-c output voltage, a d-c bus controller connected to regulate operation of the d-c bridge and thus regulate the adjustable d-c voltage, frequency control means connected to sense the level of current supplied to the motor and to provide a control signal for regulating the frequency of the inverter output voltage in accordance with a first function, and a constant voltage circuit, connected between the frequency control means and the d-c bus controller, operative to modify operation of the d-c bus controller in the region below 100% inverter load so that the d-c bus controller produces a modified control signal for application to regulate the inverter output voltage amplitude in accordance with a second function differing from the first function by which the inverter output frequency is regulated until the inverter load reaches a predetermined reduced load level, and as the inverter load is further reduced below the reduced load level, both the amplitude and the frequency of the inverter a-c output voltage are gradually reduced.

Claim 8

A control system as claimed in claim 5, in which said first function is a virtually linear relationship between the inverter output voltage frequency and the inverter load as the load is reduced from 100% to said reduced load level substantially below 100%, and said second function exhibits virtually no change in the inverter output voltage amplitude between 100% load and said reduced load level.

Claim 9

A control system as claimed in claim 6, in which said reduced load level is substantially 70% of full load.

Claim 12

A control system as claimed in claim 1, in which the constant voltage circuit includes a component which is easily removed and, upon such removal, operation of the constant voltage circuit is modified to produce a concomitant decrease of both the amplitude and the frequency of the inverter a-c output voltage as the inverter operating level is reduced below 100% load.

Claim 13

A control system as claimed in claim 5, in which the constant voltage circuit includes a component which is easily removed and, upon such removal, the constant voltage circuit produces an output signal virtually identical to its input signal, thus providing constant volts/cycle system operation as the inverter load decreases below 100% load.