# 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.
– user16324
Oct 14, 2016 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? Oct 15, 2016 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. Oct 15, 2016 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. Oct 15, 2016 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. Oct 15, 2016 at 22:19

It depends on whether the motor is designed to be operated from an AC or DC source. If the motor is rated for 24V DC then rms doesn't apply, because rms and peak voltage are the same for DC. If it is supposed to work off AC power (either directly or through an inverter) then that AC power would normally be measured in rms.

A PMSM designed for DC operation a usually called a brushless DC motor (BLDC) while one designed for AC operation is normally described as a brushless AC motor. But these terms are not exclusive. BLDC could refer to any motor that doesn't have brushes and runs from a DC source (either directly or through a controller) - including variable reluctance, stepper, and voice coil motors - while brushless AC includes induction and hysteresis motors that don't have permanent magnets.

Most low voltage 'BLDC' motors are intended to be driven with '6 step' commutation, which produces a trapezoid drive waveform. These motors often have non-sinusoidal back-emf because they are designed to work best with trapezoid drive. Low voltage brushless motors described as 'PMSM' are usually intended to be driven with sine waves.

However both types of motor can be driven with either waveform, the only difference being that on trapezoid drive a PMSM will produce higher than rated speed with greater torque ripple, while on sine wave drive a 'trapezoid' BLDC motor will have lower than expected performance. So it is possible to describe the same motor as both BLDC and PMSM, and even rate it differently depending whether it is driven with 'AC' (sine waves) or 'DC' (6 step commutation).

• The motor I want to control is a PMSM with sinusoidal back-emf waveform. I have previously used this motor datasheet to conduct my simulation and get strange results. I build a motor model in simulink with the motor specifications provided by this datasheet and supplied it with sinusoidal voltage sources. But I got some weird results. I guess this is due to the fact that this motor is actually a BLDC motor designed to be driven by trapezoid waveform? Oct 15, 2016 at 5:35
• The datasheet says they are BLDC motors operating on 24VDC, and recommended applications include electric vehicles, scooters and pumps - all of which which suggests that they are designed for trapezoid drive. Exactly which motor did you simulate, what specs did you use, and what were the 'weird' results you got from it? Oct 15, 2016 at 6:27
• I have calculated the motor specs as Oct 15, 2016 at 14:45
• @ Bruce Abbott It's difficult to explain this in the comments. So I have added the specs and the 'weird' results I mentioned in the bottom of my original question. Thank you for your help. Oct 15, 2016 at 15:24