Two apparatuses are disclosed for controlling the speed of an AC synchronous motor-pump utilizing a series of stepped voltage pulses at the driving frequency. The first apparatus is an isolated variable frequency drive comprising step-down transformer 100, full wave rectifier and filter 101, micro-controller 108, driving-voltage array generator 102, gate driver 107, inverter bridge 103 and step-up transformer 104. The second apparatus is a non-isolated high voltage variable frequency drive comprising full wave rectifier and filter 201, low voltage, dual output power supply 205, micro-controller 208, driving-voltage array generator 202, gate driver 207 and inverter bridge 203. Methods are given to determine voltage array values and pulse times to generate a stepped voltage approximation of a sine wave driving waveform.