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I plan to use a BLDC motor of around 1/3 hp supplied by a DC rail that is generated from AC mains. If the AC voltage is from 100 to 250Vac, I would have a rectified DC supply of from 141Vdc to 352Vdc.

The speed of the motor will be controlled by a purpose-built 3 phase electronic speed controller. In order to be able to use a single model of DC motor at any mains voltage I intend to use PWM to control the motor DC voltage to be about 140 VDC. ie the purpose of the variable voltage control is to keep the DC rail at about 140 VDC when the AC mains voltage varies between about 100 to 250 VAC.

For example, when VDC=141V I will apply 100% duty PWM
and when VDC=353V I will apply 40% PWM.

Question:
Is this voltage control system adequate?
Can a single motor model be used this way.

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Notes:

I have a single DC bulk voltage generated from mains, this is the dc voltage that would be commuted by the 3 pairs of high and low side IGBTs, not filtered, its 120 degree control where PWM only sets voltage.

The DC bus voltage ripple would be very low because I would use a pair of 470uF x 450V capacitors on the DC rail.

EDIT:

I will shown my current schematics regarding the BLDC motor, if that helps.

enter image description here enter image description here enter image description here

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    \$\begingroup\$ BLDC motors require a controller that provide appropriate speed control signals. Varying supply voltage does not alter speed. If the PWMd supply is filtered to DC and only used to supply operating voltage them all may be OK. Speed control would need to be via a purpose designed controller. A BRUSHED DC motor can be speed controlled by varying supply voltage. \$\endgroup\$
    – Russell McMahon ♦
    Jun 25, 2020 at 10:42
  • \$\begingroup\$ @Russell McMahon Thanks! I have already designed the controller circuit so thats not my doubt. My doubt is about the motor itself not the circuit. I want to have a single model of bldc motor that could operate from the entire voltage range, that is, the higher the input voltage, the lower the pwm's duty cycle, always trying to keep the same mean voltage applied to the BLDC. And I would vary only the switching frequency from 50 to 200hz to vary speed of bldc. Is this ok? \$\endgroup\$
    – abomin3v3l
    Jun 25, 2020 at 11:15
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    \$\begingroup\$ Yes - PWM for voltage control only is OK - provided you filter the PWM to provide adequately smooth DC. If Dc is not smooth enough you MAY get interaction between the BLDC sped control and the supply ripple. \$\endgroup\$
    – Russell McMahon ♦
    Jun 25, 2020 at 11:29
  • \$\begingroup\$ @Russell McMahon. I have a single DC bulk voltage generated from mains, this is the dc voltage that would be commuted by the 3 pairs of high and low side IGBTs, not filtered, its 120 degree control where pwm only sets voltage and frequency would set speed. I want to apply square waves anyway. And the bulk voltage ripple would be very low because I would use apair of 470uF x 450V capacitors on the DC rail \$\endgroup\$
    – abomin3v3l
    Jun 25, 2020 at 11:44
  • \$\begingroup\$ @Russell McMahon I did not understood so well your last comment. And also my bldc would be at maximum a 1/2 hp which is only 350W. And also I have in series with the dc bulk a component called ICL - inrush current limiter \$\endgroup\$
    – abomin3v3l
    Jun 25, 2020 at 12:04

1 Answer 1

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Yes, it is technically acceptable to use simple PWM control to provide a constant voltage DC bus and to then use separate 3 phase variable-frequency switching to control motor speed.

It is also possible if desired to combine these two functions on a single switching operation where a DC bus is provided that varies in voltage with mains voltage and the 3 phase motor speed control then varies the effective voltage of the resultant three phase waveform by varying the duty cycle of the final switching waveform.

Both approaches have good and bad points.

  • Having separate voltage control and speed control switches allows lower DC bus voltages but care needs to be taken that the bus voltage never exceeds component ratings during start up, shut down and transient conditions.

  • Having combined speed and voltage control functionality allows elimination of the separate voltage control switching stage at the expense of some control complexity.

Your proposed method should work acceptably well.

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  • \$\begingroup\$ Thanks man! My doubt was, regarding to the bldc, if it would be the same thing to apply a continuous 100vdc (100% duty) or a 200vdc with 50% pwm duty. This should be ok according to your answer. If this is ok, I coud drive the same bldc from a 110 or 220vac network input \$\endgroup\$
    – abomin3v3l
    Jun 26, 2020 at 9:57
  • \$\begingroup\$ You can check my edited topic. I have added my current schematics regarding the BLDC circuit \$\endgroup\$
    – abomin3v3l
    Jun 26, 2020 at 10:28
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    \$\begingroup\$ You can quite possibly drive the motor from the higher voltage at lower duty cycles PROVIDED THAT it tolerates the higher absolute applied voltages. This should be covered in the motor specification or, if not, the motor manufacturer may be willing to comment. \$\endgroup\$
    – Russell McMahon ♦
    Jun 26, 2020 at 10:42
  • \$\begingroup\$ Let me see if I understood well. Doing the way I want, consider a AC network of 250VAC. DC bulk voltage should be something like 353Vdc. The motor should accept that voltage (353) (in terms of instantaneous voltage)? \$\endgroup\$
    – abomin3v3l
    Jun 26, 2020 at 10:49
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    \$\begingroup\$ @abomin3v3l Yes. Should :-). Insulation must withstand the higher voltage. If you PWM a DC voltage at half the duty cycle (about) (say 10% or whatever) you get the equivalent of half the voltage for twice the rime (say 20% PWM) . You can fine tune for various factors. BUT at HV the motor is being exposed to higher voltage stress and the insulation must be specified to resist this. Some motors will be rated with this in mine. Others MAY not. The spec sheet MAY say and the manufacturer will know. \$\endgroup\$
    – Russell McMahon ♦
    Jun 26, 2020 at 12:44

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