Electronic – Braking Methods of Induction Motor in VFD

brakinginduction motorvfd

As you know, we can stop an induction motor by plugging or regenerative braking. What I'd like to know is that when controlling induction motor with VFD, which braking way VFD uses? It seems to me always VFD stops motor by regenerative braking because plugging may damage motor (also enery is consumed and can't be used again) but I can't be sure.

Is it always regenerative braking? Or depending on ramp-time may it be also plugging?

Best Answer

Variable Frequency Drive

Figure 1. Variable Frequency Drive outline schematic thanks to Stephen Collings.

Yes, the VFD will continue to commutate during deceleration - this time with the frequency lower than that of the motor so that the slip is negative. The current is dumped into the DC link (C in Figure 1).

Two things can happen here:

  1. If the DC link is shared with other drives and another motor is using power then an energy saving is made and the regenerated power is consumed by the other motor.
  2. If not, then the DC link voltage starts to rise. At some point the excess energy will have to be dumped to prevent damage to the input diodes, C and the switching transistors. Usually a link voltage monitor will switch use a power MOSFET to connect a power resistor across C to divert the current. This can be PWM controlled to modulate the degree of dumping as required.

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Figure 2. A typical IGBT VFD module. Source: Fuji Electric.

The module shown in Figure 2 includes a three-phase rectifier, the P-N output to the smoothing capacitor, P1-N1 input to the IGBT section and the UVW outputs to the motor. The braking resistor will be connected between P1 and B.

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