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duplicated or otherwise distributed without the express permission of Round Bank Engineering Ltd, Unit 7, Victoria
Business Park, Halifax, UK, HX1 5ND - +44 (0) 1422 647177
–
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PWM Frequency
It is critical to set the correct PWM frequency in order to obtain be best performance from the
controller.
As a rule of thumb, it is a good idea to set the PWM frequency to around 10 times the maximum
possible electrical frequency multiplied by 6.
For Example
Supply Voltage = 24V
No load motor speed @ 24V = 12000RPM
Motor Poles = 4 (2 Pole Pairs)
The no load electrical frequency = (12000 * 2) / 60 = 400Hz
From here we can calculate the commutation frequency by taking the 400 * 6 (as there are 6
commutations per electrical cycle) = 2400Hz
We need roughly 10x the commutation frequency for the PWM frequency, so 10 * 2400 = 24Khz.
If the PWM frequency is set too low, the commutation timing may be affected causing small
oscillations creating additional noise. Low PWM frequencies also create current ripple within the
motor leading to poor motor efficiency and possible overheating.
Setting the PWM frequency too high will cause additional switching losses and higher power
dissipation within the controller. Also, it is important to acknowledge that which a higher PWM
frequency the controller will lose full scale speed resolution. For example, if a frequency of 20Khz is
used, the PWM resolution will be 3000 (3000 increments between 0% up to 100% duty). A frequency
of 80Khz will provide a resolution of 750 (750 increments between 0% up to 100%).
Once the PWM Frequency has been changed, either the Reset Command (in the General Tab) must
be sent to the controller, or the controller must be power cycled in order for the new PWM
frequency to take effect.
Note
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changing the PWM frequency will effect the self calibration and the Back EMF Sum. If the
Self calibration is disabled, the Back EMF sum must be manually changed to the correct value.
Min Voltage
Minimum voltage cut-out setting. The of the supply voltage falls below this value a Min Supply
Voltage Fault will stop the motor. Units are Value / 10. For example 50 = 5.0V.
Note
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if using a PSU with a current limit, the voltage can drop very briefly and trip the min voltage
fault. The Min Voltage fault can also determine a underspecified or discharged battery, line
voltage drop.