Output reactor

[maypot]

Hi,

I am installing a 700 A 400 V 50 Hz VFD on a 400 k W motor located @ around 70 metres from the VFD.As a common rule , a high-frequency compensated reactor is normally installed on the load side of the drive, either at the drive end or motor end, to help protect the motor from overvoltage, stress from PWM voltage pulses, overheating and resulting stator insulation degradation, while reducing the motor whine.
Actually I have got at hand one reactor rated 594 A , 12 micro H per phase available for the particular purpose.It is also known that a reactor that is compensated for high frequencies and protected against fast rising voltage pulses is a very effective dv/dt (rate of change of voltage with respect to time) filter, and can be used on the load side of the drive (between the drive and motor).
Is my reactor suitable for this particular application ? It is quite an old reactor without any data sheet. It was formerly used for another application on a 315 k W motor and I also learnt that it was very noisy !!
Any help is most welcomed.

Bob

 

[Skogsgurra]

Hi,

I think that your reactor may have been used as a commutation reactor (at the input of a thyristor rectifier) and if so, they are always noisy because they have to handle the commutation, which is actually a short between phases.

I think that you can use the reactor for your VFD output. But you shouldn't put it close to the motor. Put it just after the VFD output instead.

There are a few things to watch for. If you are going to run the motor fully loaded, it is very plausible that the reactor gets hot. An extra fan may be needed. Also, 70 m is where resonance between cable capacitance and reactor inductance may cause ringing in the 10 - 20 kHz region. This ringing will heat the reactor core - sometimes so much that it hurts the insulation of the reactor. Damping with parallel resistors, usually ten or twenty ohms and 100 W or so, with help you get rid of that. Search for similar arrangements on the ABB homepage.


On the other hand. If the motor is a modern one with insulation that can take a VFD output I think that 70 m at 400 V is safe without reactor.

 

[mc5w]

If this reactor was used on the input to a 3-phase in DC out SCR drive it would not be rated for the VFD output. But, you would need it for the input so that the input diodes will commutate correctly. If you do not use a commutation reactor or universal harmonic filter for the VFD input the input rectifier diodes could burn out very easily.

 

[DickDV]

mc5w, I do not agree concerning the input diodes. They will be just fine without an input reactor. An input reactor may be desireable but to reduce supply side harmonics, not to protect the input diodes.

The only thing I would add to skogsurra's post is to set the drive carrier frequency as low as motor noise will permit. Lower carrier frequencies usually cause the motor to growl or sing a bit more than higher ones. I also agree that, if you have a modern motor designed for inverter power, 70 meters of motor leads is probably ok without any reactance.

Putting inductance in the motor leads can also help reduce radiated EMI/RFI if the inductor is placed at the drive, as recommended. This would be true even if the motor did not need the extra protection.

 

[mc5w]

I have seen quite a few 3-phase rectifiers burn up because there was no 3-phase inductance in the branch circuit to make up for close proximity to the busway or supply panelboard.

The 3-phase rectifier in a variable frequency drive produces 360 little short circuits per second on a 60 cycle system. This produces little surges that are very hard on the diodes what is known as line notching. Unless the branch circuit impedance or an inductance is there to control these little surges the diodes will wear out.

A 3-phase inductance in the branch circuit does very little to control harmonics if a 3-phase rectifier has a choke input filter.

For this large of a drive I would seriously think about going over to Mirus International and buy a Universal Harmonic Filter. Theirs will reduce the harmonic distortion from a 6-pulse rectifier to about the same as an 18 pulse filter. They also make a filter that will run a 3-phase input drive off of single phase power without any drive derating.