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It's said by some experts that harmonics can significantly impact the motors, basically through voltage distortion. This voltage distortion at the motor's terminals translates into harmonic fluxes within the motor. These harmonic fluxes don't contribute much to motor torque. However they do rotate at a frequency different than the rotor synchronous frequency. In turn it induces high-frequency currents in the rotor.  

One way of looking at this is shown in Figigue 2 below, we see a power factor (PF) improvement curve superimposed over a harmonic resonance curve. 

Note that the 5th and 7th order harmonics are shown interfering across a wide area of capacitor addition, specifically in the high power factor (PF) area, in which you would normally want to have their performance characteristic. As you can see from the figure, the interference is significant. 

Power Factor improvement vs. harmonic resonance 

The figure below shows a basic 3-phase, 6-pulse, pulse-width modulation (PWM) drive (VSD). 

All pulse-width modulation (PWM) drives contain the following main parts, with subtle differences in hardware and software components. The input section of the pulse-width modulation (PWM) is the converter, and it contains six diodes (arranged in an electrical bridge). These diodes convert AC power to DC power.  

Compounding this problem is the fact that these nonlinear loads typically have a low power factor (PF), forcing many industrial facilities to use capacitor banks to improve power factor (PF) to avoid utility penalty charges.  

We know that DC motors, unlike AC motors, act like a electrical generator when the motor has been disconnected from the power source. The DC motor functions like a generator due to the DC motors permanent magnet. The resistance to rotation causes the dynamic braking. The voltage and current produced by the DC motor is directed into the dynamic braking resistor to dissipate the heat energy.  

 When the induction motor's rotor is turning faster than the synchronous speed set by the drive's output power, the motor is transforming mechanical energy available at the drive shaft of the motor into electrical energy. This electrical energy can be transferred back to the drive. This process is called as regeneration. Thus if the motor is regenerating, the DC bus voltage will increase, thus causing an over voltage fault at the drive. 

Generally the larger the better. In fact larger inductances at low DC resistances mean larger chokes, which cost more money. Typically, 5-20 Henries is a good choice with the standard 32-50uF electrolytic capacitors.  

DC resistance 

If you are using a choke-input filter, the choke (inductor) must be capable of handling the entire current of the output tubes as well as the preamp section.