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I would like to ask the following question.

How can I calculate the base speed of a Synchronous motor for variable speed applications controlled in dq frame? I want to calculate the base speed for different Vbus and I suppose phase voltage is half Vbus.

Thank you in advance.

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  • \$\begingroup\$ For the most part, I don't know what you are asking. But a synchronous motor should have a well defined relationship between input frequency and rotation speed. There is no complicated math. The number is fixed. For example, if it is 3600 RPM at 60 Hz, it will be 3000 RPM at 50 Hz, etc. \$\endgroup\$ – mkeith May 21 '15 at 18:16
  • \$\begingroup\$ Hi, I am talking about the case of a Syncr. machine supplied by a dc source followed by an inverter. The inverter's duty cycle determines how much power is transferred to the load. The ac voltage that is created after the inverter has variable frequency and amplitude according the speed reference in the load. I am looking for the relation between load speed and Vdc. \$\endgroup\$ – Dbine May 22 '15 at 12:26
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If you are using an inverter, there is no relationship between the DC Buss and the speed except that at sufficient buss voltage, it will work. Below sufficient buss voltage, it won't.

The electronics in drives are very particular about their internal operation.

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  • \$\begingroup\$ Hi, Depending on the modulation method (the algorithm used to open and close the switches in the inverter) there is a variation to the max usable Vbus (from 0.5Vbus to 0.57Vbus). \$\endgroup\$ – Dbine May 29 '15 at 9:30
  • \$\begingroup\$ Hi, Depending on the modulation method (the algorithm used to open and close the switches in the inverter) there is a variation to the max usable Vbus (from 0.5Vbus to 0.57Vbus). This has an impact also to the maximum speed that can be achieved. I am just searching how much.. \$\endgroup\$ – Dbine May 29 '15 at 9:40
  • \$\begingroup\$ The only thing that affects the speed of an induction motor (synchronous or not), is the applied frequency. If you want to operate in the constant torque curve of the motor, you must maintain the nameplate ratio of V/Hz (much more complicated in Vector drives). A 480VAC 60HZ base speed motor requires 8 volts per HZ for constant torque. Above base speed (60 HZ), the motor will continue to accelerate if you increase only frequency, but will then be operating in a constant HP curve (Torque drops off as speed increases). If VBus is too low, it just won't work. \$\endgroup\$ – R Drast May 29 '15 at 10:43
  • \$\begingroup\$ Hi, as it is mentioned in the initial question as I am talking about vector control(dq frame). You are talking about scalar control that is not indicated for variable speed applications. In dq frame the equations of SM are similar to a dc machine and there is a relation between voltage and speed. \$\endgroup\$ – Dbine May 29 '15 at 15:40
  • \$\begingroup\$ No, even in vector mode, the rotating field generates speed, not voltage. I can set the effective voltage to whatever I want on a vector drive, only the frequency controls speed. \$\endgroup\$ – R Drast May 29 '15 at 15:43

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