kwpitt67
Member
- Location
- emporia virginia
How does running a variable speed drive at more than 60hz effect the wires and motor?
Effect of variable speed drives
- Thread starter kwpitt67
- Start date
- Status
kwpitt67
Member
- Location
- emporia virginia
I am wondering about this because a rendering plant I sometimes work at has a lot of problems with 12 awg thhn going bad, then having to pull new wires to fix the problem. they are using inverter rated motors. Just wondering how running the vsd at 90 to 120 hertz will effect the motor and wires.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The problem is more likely to be standing waves in the wiring between VFD and motor. This can cause high voltages to be present at localized points along the wire and degrade the insulation.
I suspect that this is ordinary building wire rather than VFD cable, yes? Not a good idea.
junkhound
Senior Member
- Location
- Renton, WA
- Occupation
- EE, power electronics specialty
12 awg thhn going bad
Betcha you have 460V motors?
The 20 or 50 kHz components of the VFD drive would be what is eating away the insulation - if you observe a very fine pebbled surface on areas of the insulation near where failures occur, you are likely seeing partial discharge (aka corona) etching away the insulation. The air gaps between conductors arc, each little arc eats away an infinitesimal amount of insulation. With a VFD, it is the kH vs. just 120 Hz that accelerates the etching.
Solution, like GD says is VFD rated insulation - made to resist partial discharge by various techniques, including semiconductiog layers and different dielectric constant insulation layers. Complex subject, not easy to describe in verbage. Suffice it to say THHN cannot withstand the constant very tiny arcing of partial discharge air breakdown.
Betcha you have 460V motors?
The 20 or 50 kHz components of the VFD drive would be what is eating away the insulation - if you observe a very fine pebbled surface on areas of the insulation near where failures occur, you are likely seeing partial discharge (aka corona) etching away the insulation. The air gaps between conductors arc, each little arc eats away an infinitesimal amount of insulation. With a VFD, it is the kH vs. just 120 Hz that accelerates the etching.
Solution, like GD says is VFD rated insulation - made to resist partial discharge by various techniques, including semiconductiog layers and different dielectric constant insulation layers. Complex subject, not easy to describe in verbage. Suffice it to say THHN cannot withstand the constant very tiny arcing of partial discharge air breakdown.
Aleman
Senior Member
- Location
- Southern Ca, USA
I haven't seen any evidence of failures of this type where I work. A lot of the VFD cabling is SO in the field and THHN in the panels. We have used
VFD cable on our more recent installations but still have THHN in the control panels. Some of these systems have been around for several years and seem ok. We have
one system built by another company that uses line reactors both on the line and load side of all VFD's. Then the cabling to the motors is just SO cable. I do believe everything
I have read about the subject as far as noise and spikes, discharging etc. But how bad is it really? Some of this might be pure marketing.
What would be a better choice for hookup wire in the cabinets?
VFD cable on our more recent installations but still have THHN in the control panels. Some of these systems have been around for several years and seem ok. We have
one system built by another company that uses line reactors both on the line and load side of all VFD's. Then the cabling to the motors is just SO cable. I do believe everything
I have read about the subject as far as noise and spikes, discharging etc. But how bad is it really? Some of this might be pure marketing.
What would be a better choice for hookup wire in the cabinets?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
A critical factor in possible insulation damage is just how fast the drive pulse edges are. To increase efficiency and reduce the stress on the drive transistors, the VFD will try to make the switching transition as fast as possible. But that is also what causes the largest reflections and the greatest insulation damage.
The problem will, I believe, be worse for longer runs of wire.
If you have mostly or entirely short runs or older VFDs with slower transitions, you may have avoided the potential problems. Lower motor voltages relative to the insulation voltage will also minimize the problems.
Tapatalk!
The problem will, I believe, be worse for longer runs of wire.
If you have mostly or entirely short runs or older VFDs with slower transitions, you may have avoided the potential problems. Lower motor voltages relative to the insulation voltage will also minimize the problems.
Tapatalk!
Shorter runs - higher dv/dt.
Longer higher peak volts.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
This is the general rule even for 60Hz max operation, keep the runs short, 480 volt operation will have higher peak voltage than 208 or 240 volt and may push the insulation limits of either conductors or the motor itself - especially if not a motor rated for use with a VFD, and the use of line reactors will smooth out the wave some.
I'm guessing OP maybe is using 480 volt system and may experience less insulation failure if it were a 208 or 240 volt system with everything else remaining the same simply because the peak voltage will be lower.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Hookup wire inside of a cabinet is usually so short as to be somewhat irrelevant and you will not likely see insulation damage there. But are you running output wires inside the cabinet to something else besides the motor? If its just the short wires from the VFD terminals to a load reactor or a field wiring TB, I wouldn't get too excited about it.
I hope you realize however that using unshielded SO cable for connecting your motors to VFDs is blasting radio frequency interference all around your plant and likely degrading other electronics near by at an accelerated rate by making their filtering systems work under constant stress. Of all the choices you could make for that wiring, SO cord is likely the worst, although that's a tossup against THHN in PVC conduit.
Think about what a VFD is doing; modulating frequency. Now think about the definition of "FM radio". Without shielding of some sort, be it shielded VFD cable or building wire in grounded ferrous metal (not aluminum) conduit, those conductors are becoming your own private local radio station. I learned this the hard way on a Nuke storage site in Nevada where I built a VFD controlled concrete conveyor that moved, so we used SO cord. Scientists (with armed secutity guards) from 25+ miles away converged on us by triangulating on our radio signals, not knowing what the heck was going on. They said it was messing up their instrumentation!
Re: GoldDiggers mention of voids in the PVC insulation of THHN, he his dead on. It may be fine for a while, but most responsible VFD mfrs are telling people to NOT use THHN if it can be avoided and use XLP insulated wire instead. All of the "VFD cable" mfrs I have looked at are using XLP, most are rated at 1000V, and of course they all include shielding.
Yes. I have posted on this before. Here (UK) almost all power cables used in industry are SWAPVC armoured XLPE.
I've been in this game since before the Dead Sea reported sick I haven't (yet) seen any cable problems but I been asked a few times to investigate motor insulation failures.
I think the installation method is probably to a large extent why we don't see bearing failures. Or at least I haven't.
Aleman
Senior Member
- Location
- Southern Ca, USA
The THHN runs we use are short, just to a field wiring TB, maybe 8-10' at most. I'll look into getting some XLP wire for this application in the future.
One thing we are doing now on a new build is using tray cable size 12-4 and 10-4. This is new to me but I like the small diameter and it's easy to pull
in trays. No shielding however. Probably 90% of our motors run at 60HZ or less. Will this minimize noise issues? We don't seem to get any strange
noise effects with our systems. All of our controls are 24VDC and all use ethernet. No weird crashes etc.
One thing we are doing now on a new build is using tray cable size 12-4 and 10-4. This is new to me but I like the small diameter and it's easy to pull
in trays. No shielding however. Probably 90% of our motors run at 60HZ or less. Will this minimize noise issues? We don't seem to get any strange
noise effects with our systems. All of our controls are 24VDC and all use ethernet. No weird crashes etc.
As others have pointed out, provided the motor is operating within it's current rating and is mechanically suitable for the 90/120Hz operation, that ought not to be an issue.
The VFD switching frequency can be. At any speed. Very sharp edges can do, and do do nasty things to insulation.
Nasties like this......
.....can hit the insulation several thousand times a second.
It's an actual measurement on a site where motors were failing like flies in a frost.
Note the time frame for the horizontal axis.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I would love to take credit for that, but it was junkhound that went into that level of detail.
mike_kilroy
Senior Member
- Location
- United States
hee hee.. all the replies so far are generic and are applicable at ANY FREQUENCY: none have answered the OP question.....
So to the OP ACTUAL question: not at all, other than motor goes faster.....
Last edited:
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If you count the OP's first two posts (before any answers were given), then the replies are pretty relevant:
And the answer to that is that running the VFD at 90 to 100Hz will not affect the wires any more than running the same VFD and motor at 60Hz or less as long as the voltage is not raised above the nominal voltage when the VFD goes to higher than nominal speeds.
BUT the VFD at any speed can have a bad effect on the wires, as discussed in great detail.
- Location
- Illinois
- Occupation
- retired electrician
I keep reading about this issue, but the plant I often work at has a couple of hundred VFDs all installed using THWN, with a lengths well over 100'...it often takes over 75' of wire just to get out of the MCC room. We don't have any of those issues. These are all 3 phase 480 volt motors from 1/2 to 500 hp.
The only drives we have used 1,000 volt rated wire on are the large DC drives, but the armature voltage on those is 700 volts, so we can't use 600 volt rated wire.
The only drives we have used 1,000 volt rated wire on are the large DC drives, but the armature voltage on those is 700 volts, so we can't use 600 volt rated wire.
In many neon advertisement boards people use ordinary house wires to supply neon lamps from transformer MV output. The installation works for some time. It is because dielectric breakdown is a statistical process. Out of large number neon advertisement boards with household wires, some may continue to operate indefinitely without any breakdown.
Perhaps the same mechanism at work in Don's case?
Perhaps the same mechanism at work in Don's case?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I'm also guessing most or even all of them have line reactors on the output and it will help smooth the voltage peaks.
Or similar.
The PWM waveform can cause partial discharge (PD) even when the applied voltage is lower than the insulation breakdown voltage.
It is a cumulative effect. When the insulation is subjected to continual partial discharge pulses, it progressively degrades. How long it takes depends on a number of factors not least among them being the PWM inverter switching frequency.
This from a report I did a few years when users were less aware of such potential problems than now:
Voltage Overshoot
At the points it was measured the voltage overshoot is within the limits of Technical Specification IEC TS60034-17 third edition. The limit given for the switching times measured is about 1200V. Using figures from the Gambica Technical Report No1, (second edition) the peak voltages recorded give a very low probability for partial discharge and even if it did cause PD, the number of pulses to breakdown would exceed 1013. Bearing in mind that the switching frequency is below 1 kHz the motor will never see that number of pulses.
- Status