Confusion about the rated voltage of a Permanent magnet synchronous motor

Confusion about the rated voltage of a Permanent magnet synchronous motor.

I’m currently trying to do a motor control experiment and I’m really confused by a term called “rated voltage” from the motor specifications.

Below is my Simulation scheme for the speed control of a PMSM (with sinusoidal back-emf). Under experiment circumstance, it would be something like this. The outputs of the inverter are Vao, Vbo, and Vco. The line-to-neutral voltages would be VaN=(1/3)(2*Vao-Vbo-Vco) VbN=(1/3)(2*Vbo-Vao-Vco) VcN=(1/3)(2*Vco-Vao-Vbo)

And the line-to-line voltage would be Vab=Vao-Vbo Vbc=Vbo-Vco Vca=Vco-Vao

My question is, when we mean rated voltage of a PMSM. Does this value refer to the RMS value of line-to-line voltage, the RMS value of the line-to-neutral voltage, or simply the DC bus voltage of the inverter? I want to clarify this confusion because I know that to drive a motor, the DC bus voltage for an inverter should be 1.414 times larger than the AC RMS line voltage of the Motor.

Assume that the rated voltage of a PMSM is 24V. If this rated voltage means the RMS value of the line-to-line voltage, then the required DC bus voltage (Vdc) should simply be 1.414*24=34V. If this rated voltage mean the RMS value of the line-to-neutral voltage, for a three-phase PMSM, the line-to-line voltage will be 1.732*24=41.5V. And the required DC bus voltage for the inverter will be 59V. If this rated voltage refer to the DC bus voltage of the inverter, then the DC bus voltage for the inverter will simply be 24V. And the line-to-line voltage of this PMSM under rated condition will be 24/1.414 = 17V.

Can anyone clarify this for me?

Thanks.

@ Bruce Abbott

Based on the datasheet I provided in the comment. I picked the part number DN42040S24-026 and used the following motor specs:

Stator phase resistance: 0.75ohm;
Lds & Lqs: 0.0021H; Flux linkage established by PM: 0.00477564V.s;
Inertia: 2.4e-06;
Pole pairs:4.

I have no idea about calculating the viscous damping based on the datasheet, so I assumed the viscous damping to be 0.0001N.m.s. I tried to simulate the motor’s performance at rated condition in Simulink. I initially supplied the motor with 800/3Hz three phase sinusoidal voltage inputs as (let’s call this group 1) Vao=24*1.414*cos[2*pi*(800/3)*t];
Vbo=24*1.414*cos[2*pi*(800/3)*t-2/3*pi];
Vco=24*1.414*cos[2*pi*(800/3)*t-4/3*pi].

The motor speed is around -5000rpm and have large amplitude oscillations. But when I change the supply voltage to (let’s call this group 2) Vao=24*1.414*cos[2*pi*(800/3)*t];
Vbo=24*1.414*cos[2*pi*(800/3)*t+2/3*pi];
Vco=24*1.414*cos[2*pi*(800/3)*t+4/3*pi].

The motor speed reaches a steady-state of -4000rpm. I’m confused here because I think I should have the same speed response with either group. Also, I don’t understand why I get a negative speed response at -4000rpm rather than +4000rpm. I tried to switch the phase sequences of group 2, but I get the same speed response as group 1: -5000rpm with large oscillations. I do not know what the problem is here. Any suggestions? Thank you.

• There is no neutral connection from inverter to motor in either drawing, and the half bridges operate off Vdc (a single unipolar supply) so surely Va..Vc are phase-phase voltages, not phase-neutral? Link to the specific motor's datasheet may be useful. – Brian Drummond Oct 14 '16 at 21:33
• Sorry for the confusion, I have modified the figures. The motor rating I currently used as a reference can be found on page 19 of this paper Sample PMSM datasheet. You will find that the rated voltage of this motor is 420V. Does this refer to Van (line-to-neutral), Vao, Vab(line-to-line) or simply the Vdc? – Uponmoon Oct 15 '16 at 5:21
• "I initially supplied the motor with 800/3Hz... Vao=24*1.414". First off, you don't supply a BLDC motor with a frequency - it rotates at whatever speed it wants to based on applied voltage, generated back-emf, and voltage loss in winding resistance and inductance. I don't know how your simulation software works, but perhaps that is why you get wild oscillations (motor is trying to run at a different commutation frequency than what you are supplying). Secondly, the rated voltage is 24V DC, so you should not be multiplying the phase voltages by 1.414. – Bruce Abbott Oct 15 '16 at 18:53
• Group1 and group2 are simply applying the stator voltages in opposite order (assuming angle is in radians) so the motor should run at the same rpm. Negative rpm could just mean the motor is running backwards, or it might indicate a simulation failure. Unfortunately my computer is not compatible with Simulink so I can't help much with that. – Bruce Abbott Oct 15 '16 at 19:30
• Thank you. I'll try a motor designed for sinusoidal back-emf wave and see if it works the way I expected. – Uponmoon Oct 15 '16 at 22:19