MCF8315A: speed control issue

Cherry Zhou

Part Number: MCF8315A

Hi Team,

The customer is trying to set MAX_speed to 16000 and digital_speed_CTRL is also set to 16000. But the actual measured speed is only about 7000 rpm.

Could you help look into this case? Thanks.

Best Regards,

Cherry

12 months ago

0 Joshua Horlander Cruz 11 months ago

TI__Genius 13920 points

Hi Cherry,

Could the customer provide the number of pole pairs and the BEMF constant of their motor, as well as the nominal voltage of their system?

Regards,

Joshua

0 Cherry Zhou 11 months ago in reply to Joshua Horlander Cruz

TI__Mastermind 22235 points

Hi Joshua,

Thank you for the support!

The number of pole pairs is 7 pairs, the BEMF constant is 0.59, and the system voltage is 12V.

Thanks and regards,

Cherry

0 Joshua Horlander Cruz 11 months ago in reply to Cherry Zhou

TI__Genius 13920 points

Hi Cherry,

What are the units of the BEMF constant, is it in mV/Hz?

Regards,

Joshua

0 Cherry Zhou 11 months ago in reply to Joshua Horlander Cruz

TI__Mastermind 22235 points

Hi Joshua,

Joshua Horlander Cruz said:
What are the units of the BEMF constant, is it in mV/Hz?

BEMF is 0.59Vrms/k RPM.

Thanks and regards,

Cherry

0 Joshua Horlander Cruz 11 months ago in reply to Cherry Zhou

TI__Genius 13920 points

Hi Cherry,

The motor parameters, mainly the BEMF constant of the motor, will limit the max speed of the motor. To calculate the max speed, in Hz, of a motor based on its BEMF constant (mv/Hz) and the systems DC bus voltage please use the following equation:

To convert the Maximum electrical speed (Hz) to Motor mechanical speed (rpm) and to calculate the BEMF constant in mV/Hz you can use the following equations:

So even if you set MAX_SPEED to 16000 (0x3E80), which is equal to 96000 Hz or 822857.14 RPM, the motor's BEMF generation may limited to a lower speed. This limit comes from the generated BEMF eddy currents opposing the current flowing the windings. In the case of the motor parameters given the RPM limit of the motor was calculated to be roughly 8000-7000 RPM given the BEMF constant usually has a range.

Regards,

Joshua

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MCF8315A: speed control issue