between 360 v at 45 hz and 500 v at 75 hz the principles by which inverters operate are well known a typical inverter uses pulse width modulation u201c pwm u201d to control the voltage and frequency of the ac output the inverter 34 includes a bank of electronic switches that turn on and off in response to control signals from an inverter controller by electronic switching pulses of varying amplitude and duration are produced the output is a three phase ac waveform in many inverters the ac power is filtered to remove high frequency harmonics and to produce a sine wave output fig4 5 show additional preferred embodiments of the present inventive transport refrigeration system an input inverter bridge with a dc bus voltage control 33 replaces the rectifier 31 and elevator chopper 32 to produce the regulated dc power signal the input inverter bridge 33 is an inverter with the output terminal coupled to the alternator 22 the inverters 34 36 provide numerous advantages to the system described above first the transport refrigeration system 14 is more compact because a second prime mover independent from engine 20 need not be included in the system 14 the electrical operation of the system 14 and the absence of a second prime mover.