Frankenrunner User Manual
Rev 1.0
5.7 Field Weakening for Speed Boost
Basic Setup tab
The
Frankenrunner
can boost the top speed of your motor beyond what is
normally possible from your battery voltage. This is accomplished by injecting a
field weakening current in advance of the torque producing current.
The amount of speed boost received for a given field weakening current will
depend on the winding characteristics of your particular motor. A trial and error
approach is recommended, increasing in small increments until the desired top
speed is achieved.
Increasing a motor’s top speed in this way is less efficient than using a higher
voltage pack or a faster motor winding, but for a speed boost of 15% to 20%, the
additional losses are quite reasonable.
The following graph shows a large direct drive hub motor’s rpm as a function of
field weakening current. The upper black line is the motor’s measured rpm, while
the initially lower yellow line is the no-load current draw, reflecting the amount of
extra power lost due to field weakening. We can see that at 20 amps of field
weakening, the motor speed increases from 310 rpm to 380 rpm, while the no
load current draw is still just under 3 amps.
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