Motor Power

Section 4: Motor Power Basics

The motor power output (speed times torque) is determined by the power supply voltage and the motor’s inductance. The motor’s output power is proportional to the power supply voltage divided by the square root of the motor inductance.

If one changes the power supply voltage, then a new family of speed-torque curves results. As an example, if the power supply voltage is doubled then a new curve is generated; the curve now has twice the torque at any given speed in region 2. Since power equals torque times speed, the motor now generates twice as much power as well. This is illustrated in Figure 5.



Figure 6 shows the effect of rewiring the motor from full-winding to half-winding while keeping the same power supply voltage. A half-winding connected motor delivers twice as much power as a full-winding connection at a given power supply voltage. This is because full-winding inductance is four times higher than half-winding inductance.

Also note from Figure 5 that motor output doubles when the power supply voltage is doubled for either series or parallel-wired motors. Notice that a parallel-connected motor delivers performance identical to a series-connected motor running at twice the power supply voltage.

Figure 7 shows the effect of setting the motor current to twice the rated value. This abuses the motor because it will dissipate four times as much heat as setting the current to its proper value. The actual increase in low-speed torque is considerably less than double because of the magnetic saturation of the motor iron.


What can be seen is there is no increase in the power output; the motor simply reaches its maximum power at a lower speed, all at the great expense of a four-fold increase in motor heating.

It is recommended the motor current always be set at the rated value also to get the best microstep smoothness. Setting the current higher degrades the linearity of the motor and causes microstep bunching and attendant low-speed vibration.

What comes with the increased motor power with increased power supply voltage is increased motor heating; this heating increases more rapidly than output power and ultimately sets the maximum output power from the motor. That is to say, the limiting factor in how much power a motor can deliver is ultimately determined by how much heat it can safely dissipate.

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Categories: Step Motor Basics