VFD to motor connection via the motor terminal block assembly

[Simonsglwc2]

Good Morning everyone. I have read many of the threads on this forum and was looking for some experienced opinions on the titled topic.
We fully understand the benefits and possible side effects of using VFD units. Particularly the issues of reflected waves. Got it. we understand the electrical theory. The issue we have is the issue of the motor terminals themselves. The terminal block is an insulation block with metallic threaded stud of either mild steel or brass and an assortment of nuts and washers attached to the cable lugs that are attached to the VFD output cable. Connecting the cable lug to the terminal block entails placing the cable lug over the stud and securing in place with nuts and washers to provive a secure electrical connection. Our question is , has any one ever documented the actual values seen at the motor terminal as a study?
The use of the motor teminal assembly and the cable lug with its through hole that may or may not fit snugly arond the stud seems to rely on the surface area of the cable lug flat pad and the surface area of the nuts If we add in the mix of dissimilar material , steel , brass and copper , and not perfectly fitting components as well as factoring in the copper condcutor in the cable and its grade and stranding is this a part of the problem, that is stated in white papers on these issues?
The next question is would a purpose designed connection, all copper ,or all brass with a matched fit be beneficial, to reducing the reflections back fom the motor to drive? Essentiallly getting as close as we can to an uninterrupted run of cable from the motor to the drivem which is unrealistic in reality.
It just seems that the existing termination method is an old solution suited to 60 Hz sinusoidal systems and not neccessarily best suited to modern power electronic driven systems with in field termination requirements.
Looking for some feedback if anyone has time
Thank you Have a good day everyone

 

[jim dungar]

The connection is from the compression lug to the bus bar. The screw, washers, and bolt are not intended to carry current, so dissimilar materials and 'fit' are not concerns. The choice of materials is about them being able to be torqued properly. Another area of concern is being able to disassemble the connection, so corrosion of the threads needs to be considered.

I have not heard of motors supplied by VFDs having a high amount of failure at the motor leads.

 

[garbo]

Not a great answer but here goes. I worked at a large candy plant that most equipment had IEC Motors. In areas were they pressured washed every night the IEC motors 5 HP & under had too small of a pecker head that always had water inside. Between the flimsy six studs the diameter of a 6/32 screw. Often found the metric studs not fully torqued so retorqed them. Too many would short out so I removed the studs and wired nutted connections then wrapped with quality tape down at least 3/4" onto insulation. As far as studs where brass or plated studs having a steel flat washer & or steel lock washers usually never caused a problem. Spent 40 years troubleshooting. replacing & installing company that I worked at VFD'S and never read anything about your concerns or while a total of three weeks in drive classes. I always installed quality flat washers on both sides of a lug followed with a new lock washer onto studs. Never used the inferior paper thin flat washers that graingers sold that where made in cheating lack of quality control Communist china. Even if you two of these cheating Chinese flat washers they bent. Had a lot if VFD'S connected to motor studs running trouble free for over ten years & 75,000 running hours.

 

[milmat1]

You may be way over thinking this. I haven't seen a lot (any that I recall) issues with the terminals at the motor.
SEW eurodrive motors all use the terminal type you described and the biggest problem with them is the ceramic being broken during termination by the electricians over-tightening the connection. ( Every Time !)

As far as the dissimilar metals, I guess you could argue that as current passes through them there will be a very small voltage develop. But it's so small and the size is such a tiny fraction of wavelength at these freqs, that I would think it would be negligable. I have never noticed corrosion on the terminals. But then I never really looked for it either....

 

[garbo]

You may be way over thinking this. I haven't seen a lot (any that I recall) issues with the terminals at the motor.
SEW eurodrive motors all use the terminal type you described and the biggest problem with them is the ceramic being broken during termination by the electricians over-tightening the connection. ( Every Time !)

As far as the dissimilar metals, I guess you could argue that as current passes through them there will be a very small voltage develop. But it's so small and the size is such a tiny fraction of wavelength at these freqs, that I would think it would be negligable. I have never noticed corrosion on the terminals. But then I never really looked for it either....

Any time we had a SEW eurodrive motor rewound we told the motor repair shop to only bring out three motor leads to make it easier,faster & more reliable connection. If it was a pump motor or a motor in a daily wash down area asked the motor shop to epoxy the motor leads so water would not travel from so many partial water filled pecker heads into inside of motors.

 

[Jraef]

A part of the reflected wave phenomenon is due to the change in impedance taking place in the conduction path from drive to motor. Higher or lower is not as important as “different”. So regardless of the materials used at the terminal block, there will be an impedance change there and some of the standing wave will go through, some will reflect back to the drive and meet up with the new waves coming in to increase the amplitude, then hit the impedance vhsnge at the drive and add more going back the other way again. More or less impedance (via resistance st the terminals) only affects the short term issue of the initial reflection, the rest is mostly dependent on the length of the conductors as to the ultimate voltage peaks, so you get to the same place eventually.

The only thing that would change this is if you ran the motor magnet wire all the way back from the stator to the VFD. That was the idea behind mounting the VFD directly to the motor, which failed in its first iteration due to motor heat getting into the drive, but has recently resurfaced as “ECMs” and some incoming new products you will see hitting the market soon.