Motor distance from VFD
- Thread starter Bvess
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
In general there is a disconnect within 50' and within site so then VFD would generally be within 50'
- Location
- Illinois
- Occupation
- retired electrician
The code does not care how far the motor is from the VFD. Most VFD manufacturer's cite a number in their instructions.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The longer the distance, the more you must contend yourself with capacitive coupling in the conductors, which then contributes to standing wave creation and motor EDM bearing damage, etc. etc. How long that distance is becomes a variable because of multiple factors that go into the equations. I have seen 15kM done with MV drives successfully with no motor problems, I've seen LV motors damaged from 25ft of improperly installed cables.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You must always get the easy installs with no challenges, like the motor being 100 feet above grade but the motor control room is on grade and out of the dust, rain, whatever.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
as far as the code goes, no.
in practice, there are a number of factors that might tend to limit that distance.
Electric-Light
Senior Member
ASD in theory behaves like a set of very large synchronous alternator driven by a synchronous motor through a hydrostatic CVT. Such a system produces clean sine wave that behaves no different from ordinary power but ASDs in practice do not provide sine wave output. This is where capacitance and cable distance issues come into play. A very large scale drive is more able to afford getting closer to real sine wave.
Many of which can be overcome by design but that involves additional cost. Filters, cable size, motor winding design, and so on. Borehole pumps obviously have to be located at the borehole and that can be some distance from the sub/switch-room where the drives are located. A few hundred metres in some cases. As I recall, the longest run we did was about 300 metres (1,000 feet) but is was only 75kW (100 HP) as I recall.
Then we had one site where the motors were less than 50 metres from the drives. But their windings were standard and not designed for PWM waveforms. Random wound rather than mesh. They started failing about a month after commissioning and had to be rewound. Expensive embarrassment for the motor designers/manufacturers. Four off 760kW pump drives. And a steep learning curve....
Touching on a post in another thread, buck/boost transformers with VSDs.
The pumping station supply was 3,300V. The VSDs were rated for 500V operation so step down input transformers were required. The motors were 690V units so step up output transformers were needed.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I am surprised that a step up transformer between the PWM output and the motor can perform well. Are they custom transformers for the application?
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Open Neutral
Senior Member
- Location
- Inside the Beltway
- Occupation
- Engineer
If you have load side inductors/filters, which problems are reduced and which are not?
Obviously no one wants to spend $$$ on transformer iron & copper windings, but.....
Yes, they were gapped autotransformers. The gap was to prevent saturation owing to any non-symmetrical component in the PWM waveform.
Had we been able to get a 690V VSD at that time, we would have used them. You can now so we would use that and obviate the need for an output transformer.
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