JayBaima

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 […]

As you can imagine we get quite a few questions submitted by our website (joliettech.com & joliettech.com/blog) visitors. And as always we answer all questions. Below are just a couple questions we received this week and the answers we provided.

Q. We are a distributor in the food service industry in Atlanta, GA. We provide exhaust and supply fans ranging from 1/4 to 10 H.P., we are looking into providing Variable Frequency Drives to our customers so they can control the speed of their fans. Can you provide us with any pricing, availability, and product suggestions for our application? Thank You.

A. Thanks for contacting Joliet Technologies. Be glad to assist…

As long as the fan motors are 3-phase, the selections are numerous and drives are readily available. However, if the fan motors are single phase, I do not have a drive I can quote you. Single-phase output drives are exceedingly hard to find; I am aware of only one manufacturer, and we do not supply them because the torque output of the unit is ridiculously poor.

If […]

Motor manufacturers go to great lengths to design, build and test their motors to comply with applicable NEMA and IEC guidelines. In nearly all cases, customers can be assured that a motor provided by a name-brand manufacturer will meet their needs IF properly specified, installed, and maintained. Because applications vary so widely, sometimes that “IF” can be a big one. There are many design and installation considerations, mechanical as well as electrical, which must be clearly understood and adhered to for a successful application. Let’s examine some of the more typical mechanical considerations and how they might affect your motor application. By the way, much of which follows is referenced against NEMA guidelines; however, the same general concepts apply whether US domestic or international standards apply.

• Ambient considerations: NEMA MG 1, American National Standard for Motors and Generators, specifies usual service conditions under which motors are designed to operate. These include an upper ambient temperature rating of 40C, above which adequate cooling is difficult to provide and the motor internal components are at greater risk of exceeding operating temperature design limits. This is important because it has been estimated that for every 10C above design […]

The prevalence of adjustable speed drives and other non-linear loads in industry has heightened awareness of the effects of harmonic current distortion on utility capacities and energy costs. It is not uncommon for utility companies to demand that harmonic-mitigating strategies be applied to larger drive installations. Let’s examine drive-induced harmonics in more detail, and look at ways that the problems they might cause can be reduced or eliminated.

Harmonics arise when current and/or voltage waveforms deviate from sinusoidal. The design of the front end (rectifier) section of an AC adjustable speed drive incorporates rectifying devices (typically diodes) to convert AC to DC, which then links by bus to the output section of the drive, where it is converted to a pulse-width modulated sine-wave for supply to the load. The DC bus incorporates a capacitor to smooth out the DC signal by damping the ripple in the line resulting from the rectification process. This capacitor does not draw current continuously, but only when discharging to the output section and when the rectifier section is forward biased; i.e when the instantaneous AC voltage is higher than the DC bus voltage. Because the current draw is not continuous, […]

Output cabling from a variable frequency drive (VFD) can act as a significant source of electromagnetic emission resulting from the cables’ characteristics in response to the high-frequency voltage pulses they send to the motor. In effect, the cables serve as antennae. For drives using IGBT’s to generate the voltage pulses – the most common method used in drives today – the emission spectrum coming from the output section of an AC drive may extend to 50 MHz; it is typically strongest in the 100 kHz – 10 MHz range. Below this range, electromagnetic interference (EMI) issues are rare, but in or above it the EMI emitted and its effects on cables and connected equipment can have a negative impact on drive system controls and any adjacent, sensitive equipment. Let’s examine more closely the role that cabling plays, and ways to mitigate EMI problems through cable specification and installation practices.

At high frequencies, current will flow primarily in the outer surfaces of the conductors due to skin effects, which raises the effective impedance of the cables and increases voltage and the potential for noise. Also, “stray” or “leakage” currents can arise via capacitive coupling and, if […]