More Output Voltage From The Same DC Bus Voltage
How to get more o/p voltage from the same DC bus voltage? :
- The inverter switches need to be rated to withstand the peak magnitude of input dc link voltage, the maximum expected load current and should be able to safely dissipate the heat generated in the switch due to conduction and switching losses.
- Because of high frequency switching, the switches in PWM inverters have significantly more switching loss than in square wave inverters. Often the switch chosen in PWM inverters is oversized, in terms of its current rating, so that the sum total of switching loss and conduction loss remains well within the heat dissipation capability of the switch and the associated heat sink.
- The VA rating of the switch, being the product of the switch voltage and current ratings. The switch cost may be roughly taken as proportional to its VA rating. The VA rating of the inverter equals the maximum VA of load power (considering only the fundamental component of output voltage and current) that the inverter may output.
- On account of higher fundamental output voltage and less switching loss, a square-wave inverter will produce a higher VA (for the given switch VA ratings) than a PWM inverter. The square wave inverter can use slower switches, requires simpler control circuit and thus the inverter cost comes further down. However due to better quality of output voltage (and hence current), PWM inverters may be unavoidable in many applications.
- For identical magnitudes of switching frequency and switch voltage stress some particular PWM techniques may allow more output voltage than other PWM techniques (in spite of comparable quality of output voltages). Sometimes the lower achievable output voltage may mean that the inverter is not suitable for given application. For example, consider a typical case where a 3-phase 400 volts rated induction motor is to be fed from a PWM inverter for a wide range of speed control.
- The dc bus voltage to the inverter is, in most cases, achieved after rectifying the 3- phase utility ac supply. Often a three-phase diode bridge rectifier followed by a large filter capacitor is used to get dc bus voltage.
- The magnitude of dc bus voltage, so achieved, may be considered close to the peak magnitude of supply line voltage. For 400 volts, 50 Hz, 3-phase supply system the dc bus voltage will be around 566 volts (i.e. 400√2 volts).
- Now using a SPWM inverter with a dc link voltage of 566 volts, one can output maximum rms line voltage of 347 volts only (= 0.612 Edc).
- The SPWM inverter will, thus, not be able to meet the rated voltage demand of 400 volts for the motor. Instead of SPWM inverter, had one used a square wave inverter, the maximum magnitude of line voltage (fundamental component) output by the inverter would have been 441 volts (= 0.78 Edc,). Thus, on account of lower output voltage a SPWM inverter may be unsuitable in certain application.
- Fortunately, there are some other PWM techniques that can output good quality line voltage waveforms (similar to SPWM inverter) and can output higher voltage. In this lesson two such popular PWM techniques namely, sine 3rd harmonic modulation and space vector modulation techniques have been described.