Motor Power

In order to gain an understanding of how much power the stepper motors consume during normal operation, some measurements of current and voltage were taken under both steady — state and peak operation of the motor. Figure 88 shows the voltage and current waveforms under steady-state operation, with no load on the motor. It shows an RMS voltage of 15 V (square wave of 50% duty cycle with 30 V peak) and RMS current of 1.2 A. This gives an RMS power consumption of 18 Watts per phase (so 36 Watts per motor), which was confirmed by using a clamp-type power meter. Under the conditions expected during normal engine operation, any movement made under these conditions (i. e. not accelerating or decelerating) can be expected to have much higher power consumption due to the pumping losses described in Section 3.1.6, which would add at least 40 Watts to the motor’s power consumption. With two motors in operation, the total consumption under these conditions is expected to be around 120 Watts, allowing a little extra for seal and bearing friction.

Motor Power

Figure 88: Motor current (blue) and voltage (red) waveforms during steady-state operation

Figure 89 shows the voltage and current waveforms under peak acceleration conditions. In this situation, the maximum possible power is delivered from the driver to the motor. Additional loading will not make a difference to the power consumption but will instead limit the peak acceleration. With an RMS voltage of approximately 15 V and an RMS current of 10 A this consumption is 150 Watts per phase, or 300 Watts per motor (600 Watts for 2 motors).

Using Figure 77 as a basis, power consumption can be estimated at different operating speeds. At 2Hz, the motors are ramping approximately 65% of the time, during which they will consume the maximum power of 600 Watts. For 15% of the time they are operating at steady state, consuming the previously estimated 120 Watts and for the remaining 20% of the time they are idle and consume no power. As a time-weighted average value, power consumption under these circumstances is 510 Watts. Similarly calculating for 1.5Hz and 1Hz operation gives 320 Watts and 175 Watts respectively.

Motor Power

Figure 89: Motor current (blue) and voltage (red) waveforms during peak acceleration operation

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