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For a voltage-source-inverter (VSI) powered PMSM, as shown in the picture below, the DC bus voltage across the DC bus capacitor Udc normally drops frequently, which will cause no harm to the cap itself. However, when the low speed command is given, the motor will decelerate because smaller speed ref leads to a smaller Iq ref value, which means less torque is generated. In this case, the capacitor voltage could often increase to be higher than the battery voltage if the battery can't be charged, which could damage the DC cap and MOSFETs. It is especially so when deceleration is high.

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I found another question Regenerated current during deceleration of PMSM motors and one of the answer mentioned that during motor deceleration, the motor and the DC cap forms a boost converter that results in the increase of cap voltage.

My question is

  1. Is the motor always in regenerative mode when the it decelerates? isn't it dynamic braking if we don't purposely implement regenerative braking? If it is dynamic braking, why DC cap voltage increases?
  2. I think the motor current is still in phase with its back-EMF when decelerating, only the current amplitude becomes smaller because PWM duty is smaller. Am I correct about this?
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Is the motor always in regenerative mode when the it decelerates?

The motor is only in regenerative mode if the deceleration rate is faster than the motor would decelerate if it were simply disconnected from power. In other words, if the losses in the motor and load use up all of the energy that must be taken from moving mass, there will be no regeneration.

If it is dynamic braking, why DC cap voltage increases?

Dynamic braking involves dissipating energy in braking resistors. If that is not taking place, it is not dynamic braking.

I think the motor current is still in phase with its back-EMF when decelerating, only the current amplitude becomes smaller because PWM duty is smaller. Am I correct about this?

The relationship between the motor current and voltage waveforms is whatever it needs to be consistent with the controller operation and the motor performance.

If the motor is "under control," it will act as a generator if it is called upon to provide braking torque. That will result in energy being pushed back into the controller regardless of whether or not the controller can dissipate the energy or return it to the source. If the controller receives energy from the motor that it can not dissipate or push back to the source, it will put it in the capacitors. That will increase the capacitor voltage until something fails. At that point, the energy will be dissipated in the resulting explosion and in the the motor or wherever else there are conductors forced to carry fault currents.

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  • \$\begingroup\$ Is dissipating energy in the motor winding itself considered as dynamic braking? \$\endgroup\$ Mar 29, 2022 at 5:28
  • \$\begingroup\$ "The motor is only in regenerative mode if the deceleration rate is faster than the motor would decelerate if it were simply disconnected from power". This is the clearest explanation I have ever read. Thumb up!! \$\endgroup\$ Mar 29, 2022 at 5:31
  • \$\begingroup\$ Dissipating energy in the motor windings is certainly very similar to dynamic braking, but it is a loss that is present during both motoring and braking like the mechanical losses in the motor and other losses in the system. \$\endgroup\$
    – user80875
    Mar 29, 2022 at 12:35
  • \$\begingroup\$ but still, i think dissipating heat in motor windings to bring motor to stop faster than free-wheeling can be categorized as dynamic braking, right? \$\endgroup\$ Mar 30, 2022 at 8:33
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    \$\begingroup\$ I suppose you could do that. \$\endgroup\$
    – user80875
    Mar 30, 2022 at 12:21
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Is the motor always in regenerative mode when the it decelerates? isn't it dynamic braking if we don't purposely implement regenerative braking? If it is dynamic braking, why DC cap voltage increases?

There's no such thing as regenerative mode on a motor. There is, however, generator mode and a motor braking is always in generator mode.

And the answer is no, because there's more than two options. If you connect up a braking resistor, that is dynamic braking. If you just short out the motor in the H-bridge that is also a form of dynamic braking. If you connect the power supply up in reverse that is regenerative braking. It's not just two options because there are more than two ways you can throw the inverter switches to brake the motor.

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