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January 14, 2015

Welcome to the new and improved Joliet Technologies website. This is the look of our new site.

We have converted it to a responsive site design, which means it will be viewable on all devices and the functionality and content will remain the same whatever device you are viewing it on.
It has always been our goal to have the user experience be the best possible on our website and hopefully this new design will do just that. Our new user interface allows you, in most cases and on most devices, to navigate to any page on our site with just one click.

Also we have increased the security of our site by switching to https, so whenever you are on our site anything you do is encrypted. Your connection to https://joliettech.com is encrypted with 256-bit encryption & the identity has been verified by COMODO RSA Domain Validation Secure Server CA.

All the content on our site […]

January 14, 2015

We have completed the process of re-designing our website. This will be the new look of our new site.

We have converted it to a responsive site, which means it will be viewable on all devices and the functionality will remain the same whatever device you are viewing it on.

Also we have increased the security of our site by switching to https, so whenever you are on our site anything you do is encrypted.

All the content on our site should remain the same, some pages may be updated to reflect the current status of our company and our products and services.

If you have any questions and/or comments, please feel free to contact us directly at info@joliettech.com.

The causes and effects of VFD-supplied motor shaft and bearing currents, as well as measures to counteract them, have been exhaustively covered in literature. However, as a drive and motor supplier, we frequently receive requests for information on this topic. So a summary seems in order…

Basically, shaft currents are induced because of the high frequency of the voltage pulses sent to the motor from the VFD. Recall that a pulse-width modulated VFD creates a synthesized sine wave by firing its output transistors many thousands of times per second. These pulses form high-frequency waves sent along the motor cable to the motor. Since impedance is inversely proportional to frequency, the capacitances of the cable and motor present little or no impedance to these high-frequency pulses. As a result, circulating currents can readily flow in the motor shaft. With sufficient magnitude,  and in the absence of corrective measures, these shaft currents can pass through the bearings and races to the motor frame, causing bearing pitting or “fluting” – regular, tightly spaced grooves on the bearing races – via a process referred to as Electric Discharge Machining (EDM). Ultimately, these irregularities will cause bearing failure.

Cable length and characteristics are often important factors in ensuring a high quality, dependable drive installation. Certain applications simply require a long motor lead length; moving the motors and/or drives closer to one another may be too costly or space constraints may prevent it. There are the normal cable capacitance, high-frequency pulse, and voltage drop issues, of course, but what if additional motors are added to the mix? Let’s look at one reader’s question:

Q: I intend to connect 6 fans each 1.5 kW to a single VFD unit. The largest distance between the VFD unit and a fan is 25m. However, the total length of cables connected to the 6 fans is about 120m. Is the distance from the drive unit to the most remote fan the significant parameter or is it the total length of cables that is connected to the unit?

A: The most important consideration in the case of multiple motors connected to a single drive is the length of the leads between the VFD and the point of common connection of the […]

Those of us who’ve experienced unexpected failure of variable speed drives may be interested in an analysis of the following problem:

Problem: A plant operating a large number of VFD’s is facing the problem of  spikes resulting in damaging of  the drives. Can you please suggest some means to overcome this serious problem? The plant has its own generation through its gas-operated engines and during gas shutdown the plant is supplied power through furnance oil engines. Power is generated at 11Kv and is stepped down to 400 volts through step-down transformers.

Analysis: It’s almost always advantageous in cases like this to take the “top down” approach; that is, start by investigating system elements common to all of the components being affected. If you find a problem, it may just be that correcting it will resolve all of your downstream issues. Power supply stability/quality should be checked first. If incoming sources are surging/sagging, why?  Are the power supplies stable under conditions of varying load? If not, it pays to understand whether the generators are sized properly and the controls/monitoring equipment are in good working condition. Keep in mind also that it is possible […]

I often receive questions from readers and customers regarding a wide variety of electrical/electronics concepts. The next few columns will address some of these, and hopefully provide some additional insight into both the mundane and esoteric issues associated with variable speed drive technology.

Question: What is a suppression capacitor, and is it the same as a suppression reactor?

Answer: In the drives world, suppression filters (which include capacitors) are used to reduce EMI and harmonics by shunting high frequencies to ground, since they present a much lower impedance to higher frequencies than to lower ones. A capacitor is typically inserted parallel to the load path (often as part of a typical 2- or 3-element low-pass filter) on the output side of the drive; low frequency current sees the cap as a high impedance and bypasses it, choosing instead to go through the load (i.e. the drive output section or motor, depending on where the filter is located). But high frequency (noisy/high harmonic) current sees the cap as the lower “resistance” and runs through it to the return path, rather than through the load. Most often in my experience, the term “suppression capacitor” […]

I received a question from a reader some time ago regarding an application in which a 380V/50 Hz motor was to be installed in a plant with only a 60 Hz supply. The reader intended to feed the motor with a variable speed drive and wanted to know (1) what the impact on the motor’s operation would be and (2) what the effect of over-speeding the motor would be.
The response:

Since you are planning to use a VFD anyway, there really is no problem running at 60Hz, just make sure your VFD is programmed with the proper V/Hz output for that motor. You didn’t post what your line voltage is so I can’t give you the specifics, but the following is an example.

The motor is designed for 380V 50Hz, so the V/Hz ratio is 380/50 = 7.6V/Hz. If you have 480V available, a 480V VFD will be set up to deliver 460V for the motor at 60Hz, and 460/60 = 7.67V/Hz! Close enough to not worry about the motor’s basic design. So let’s then look at the ramifications:

• Speed: your 980RPM motor will now spin at 60/50 […]

We received the following question from a customer:

Q: I have 3-phase incoming voltage measured at 505VAC on a nominal 480VAC distribution system. Will this voltage level affect my drives and motors rated for 460VAC nominal?

A: Drives are almost all rated at +10/-10% voltage input (some are +15/-10; others are +10/-15…). A typical North American drive for controlling a 460V motor (motor nameplate rating) is built to accommodate an input of 380-480V at +10/-15%, giving an allowable range of 323 – 528V input. In theory, any voltage input within this range will be taken in by the drive, converted and inverted to a set output of 460V (or 480V – you set it in drive parameters to match the motor nameplate value).

But “in theory” doesn’t much impact the plant floor. In practice, you still need to be certain of the drive’s allowable input range. Some drives rated for non- North American voltages are rated for 400V nominal; once you get up to 480V you start having problems. Such drives can sometimes be found driving equipment manufactured outside the US. What we are seeing is occasional difficulty with […]