Portable generator concerns

Mark Kenfield · June 25, 2016

Hi guys,

I'm currently in Northern India on my second last day of pre-production for a Bollywood feature, which I'll start lensing in two days time.

This afternoon I finally managed to get a look at the generator we'll be using, which is a conventional 62.5kVA silent generator from a crowd called Jakpower, which is apparently only a month old (it does look to be in very good knick).

Now my electrical knowledge is pretty surface level, but one thing I have learnt (from this forum in particular) is that a generator putting out a pure sine wave is very important for running delicate electronics like laptops for data wrangling, and HMI ballasts.

No one at the generator supplier's knew anything about sine waves or square waves, and it was only after a great many phone calls that we managed to track down someone who could tell us that the generator puts out a sine wave, but not a 'pure' one.

So I wanted to ask if you guys think we'd be safe to run our particular package. Unfortunately, the lighting package I've been able to squeeze for this film is rather more limited than I'd like, but if we have to plug everything in at some point, we'd be running the following:

- 2x 4k HMI PARs

- 1x 1.2k HMI PAR

- 4x 4' Kinoflos

- 2x 2000w fresnels

- 2x 650w fresnels

- 4x 150w Dedolights

I'm told the HMIs are Arrisuns, but we'll see (what people say the have, very often varies from what they actually have over here). But worst case scenario, around 17kw of draw for the lights. The we'll have our laptop for the data wrangler, a small video village, an ungodly number of different battery chargers, and then unit/caterings needs.

So... am I worried about nothing, and under that sort of load, a conventional generator of that capacity will be totally fine? Or are there additional safety measures I need to take to protect the gear we're running on it?

Cheers,

Mark

JD Hartman · June 25, 2016

Don't know.....did they mean distortion of the AC sinewave output or voltage and frequency regulation? An oscilloscope or better yet a power line monitor like a Dranitz would tell the tale. It just has to fall into an acceptable range for the application. Are you getting electronic HMI ballasts or magnetic versions?

https://www.linkedin.com/company/jakpower

http://www.jakpower.com/

Mark Kenfield · June 25, 2016

No idea, "sine, but not pure" was as much information as we could get out of them. Since neither the generator operators or my lighting team on this appear to have ever even heard of a sine wave, much less an oscilloscope, I'm not really liking my chances of being able to check it.

Ballasts should be electronic.

JD Hartman · June 25, 2016

Too late to get back on a plane? Did you call the manufacturer? I've provided two links. If you're going to take other jobs like this one, I'd consider buying a solid state O'scope to add to your kit.

You might be better off renting magnetic ballasts. Higher current draw at strike, but more tolerant of dirty power. Some UPS systems/power line filters might be a good idea for your PC/data backup/chargers etc. Or all this concern could be for naught.

Edited June 25, 2016 by JD Hartman

Mark Kenfield · June 25, 2016

What we heard was from the manufacturer! ???

How large/portable is a solid-state oscilloscope? Having one would certainly lower my stress levels. I'll hopefully be able to look at the distro boxes tomorrow and see about UPS. My plan (for the first few days at least), is to keep all of our chargers, laptop etc. On mains power until we have some sense of how stable the gennie is (at the very least).

JD Hartman · June 25, 2016

What we heard was from the manufacturer!

How large/portable is a solid-state oscilloscope?

You couldn't have been talking to an engineer or even a technician. They can't be that ignorant of a product they manufacture.

How big? About the size of hardback novel and a couple of pounds.

Mains power? Good luck, from what I've read and seen in a documentary, India's power generation and distribution system sucks and subject to low voltage and multi-hour outages. If you look around, you'll see many many illegal taps which only makes a bad situation worse.

Edited June 25, 2016 by JD Hartman

Mark Kenfield · June 26, 2016

I'm not expecting great things, but the computers and battery chargers have been fine off mains so far, so that's my safety measure until we can get some sense for how stable the gennie is.

Guy Holt · June 26, 2016

So... am I worried about nothing, and under that sort of load, a conventional generator of that capacity will be totally fine? Or are there additional safety measures I need to take to protect the gear we're running on it?

To put things in perspective, no generator puts out a sinusoidal voltage waveform. A pure sinusoidal voltage, like the one represented below, is a conceptual quantity produced by an ideal AC generator built with finely distributed stator and field windings that operate in a uniform magnetic field. Since in reality neither the winding distribution nor the magnetic field can be uniform in a working AC generator (not even power plant generators), voltage waveform distortions exist, and the voltage-time relationship deviates from a pure sine function. Typically, the distortion of grid power is very small (less than 3%), but nonetheless it exists. It is an altogether different situation when it comes to generators. Since, there is a direct trade off between generator cost and quality of the power waveform, voltage distortion in the original power waveform varies greatly between types of generators. While these distortions are generally very small relative to the distortion that can be caused by non-linear loads, they may still be significant because the harmonics both sources generate will sum together in the neutral and promote high and therefore, potentially troublesome, neutral currents in distribution system conductors.

Pitch_Pure_%20Sinewave.jpg

One design factor that generator manufacturers use to control the magnitude and orders of harmonics produced in a generator is the “pitch factor” of the stator. For example, the graph below illustrates the line-to-neutral voltages of two dissimilarly pitched generators. As can be seen here, a 5/6^th pitch winding (the black line) generates a voltage with a slightly higher peak, while a 2/3^rd pitch winding (the red line) generates a somewhat flat-topped voltage waveform. The imperfect voltage waveform created by a generator may be described in terms of its fundamental frequency and the magnitude of the harmonic voltages that make it up.

The pitch of a generator is a design parameter that can therefore be used to optimize the generator waveform shape, but unfortunately at the expense of generator cost. That is because shorter pitch (lower fractional pitch ratios) use the alternator stator less effectively and require the use of more copper for the same kW output than higher pitch machines.

For example, the voltage waveform generated by a full pitch stator is fairly distorted and rich in 3rd harmonics, as can be seen in the illustration above. The waveform can be improved by making the coil pitch less (as in the illustration of a 5/6^th pitch coil below), but the emf induced in the shorter pitch coil is less than that of a full pitch coil and so requires the use of more copper for the same kW output.

For applications that involve non-linear loads, a 2/3 pitch winding can offer significant benefit – but again at a cost. As illustrated below, a 2/3 pitch winding will not generate 3rd harmonic currents. However, the use of even more copper is required for the same output of the 5/6^th or full pitch machine. In addition, the 5th and 7th harmonics generated by these machines are approximately double those generated by a standard machine. Pitch factor is an important design factor because, it can be used to reduce or eliminate specific harmonic frequencies in the generated voltage waveform, and thereby tailor the generator to specific loads.

The “Crawford” generators we use in film production are 2/3 pitch and are considered a specialty pitch winding. We use them because they dampen the generation of third harmonics and therefore are the best choice for three phase, four wire systems, powering non-linear single phase loads.

A 5/6 pitch generator, by comparison, will output a voltage with triplen harmonics. When used to power non-linear loads that generate triplen harmonics, the triplen harmonics from both sources will sum together in the neutral and promote high and therefore, potentially troublesome, neutral currents in distribution system conductors. In addition, the current harmonics they contribute create more severe voltage distortion throughout the distribution system.

A 2/3rd pitch winding on the other hand will minimize the triplen harmonic contribution from the generator, reduce voltage waveform distortion, and will not encourage an increase in the overall triplen harmonics content in the distribution system. The reason that 2/3 pitch machines are not used in industrial applications is that the higher Fifth-order harmonics (and their multiples) cause abnormal heating in rotating load devices, such as motors, because they are “negative sequence” currents. In addition, 2/3rd pitch wound machines generally have lower zero sequence reactances that can increase the single-phase fault current.

From what I saw on their website, it is likely that the Jakpower generator you will be using has a full pitch or 5/6 pitch stator. Given that likelihood, it is important that the electronic ballasts for the HMIs you use be power factor corrected (PFC). PFC ballasts will not draw 3^rd harmonic currents that will be additive with the 3^rd harmonics generated by the generator’s stator, resulting in lower voltage waveform distortion and return current on the neutral conductor of your distribution system. The top waveform in the power quality meter reading above shows voltage distortion caused by just one 4k HMI operating on a 60kVA 5/6th pitched generator. The “flat topping” you see here can cause sophisticated electronic equipment, like that used in a DIT station, to malfunction. Add a second non-PFC 4k ballasts, a couple of non-PFC 1.2k, and the non-PFC Kino ballasts, and the flat topping will be more severe. The Arrisun heads you will be renting are of the generation that they could either be power factor corrected or not, so it is worth finding out and not using them if they are not power factor corrected.

For more details on what type of generator to use with HMIs use this link for an article I wrote on the use of portable generators in motion picture production.

Harry Box, author of “The Set Lighting Technician’s Handbook” has cited my article in the Fourth Edition of the handbook. Here is what he has to say about the article:

"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."

"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before.

Guy Holt, Gaffer

ScreenLight & Grip,

Lighting Rental & Sales in Boston

Edited June 26, 2016 by Guy Holt

Mark Kenfield · June 26, 2016

Thanks Guy,

Very informative post! Checking the ballasts for PFC specs is the first thing I'll do.

Do you know if the Arri ballasts of that generation state their PFC spec? Or if only the ones that have PFC list it?

Cheers,

Mark

Guy Holt · June 26, 2016

Do you know if the Arri ballasts of that generation state their PFC spec? Or if only the ones that have PFC list it?

On every ballast there should be listed its’ electrical specifications, which if you know how to read it, will tell you if the ballast is power factor corrected. Below is the nameplate from an Arri 575/1200 Electronic Ballast with DMX Control.

Manufacturer’s nameplate from an Arri 575/1200 Electronic Ballast specifying its’ electrical characteristics (learn how to read it.)

As indicated on the nameplate, the ballast has an Apparent Power of 2290VA, which means it draws nearly twice the load of its’ 1200W output – a clear indication that it is not power factor corrected. Which explains why, according to the nameplate it will draw 18A of current ("I") at 125 Volts ("U") (2290VA/125V = 18.32A.) That translates to 19A at 120V. You will also notice that it states that the ballast has a cos@=.6 which means that the Power Factor is .6.

It is important to understand that this greater Apparent Power consists not only of high amplitude short pulses of current drawn by the ballast (the lower waveform in the power quality reading in my post above), but also harmonic currents that the ballast introduces to the distribution system (see below for the FFT graph for the same 4k ballast mentioned above.)

I won’t go into detail here how the harmonics generated by non-PFC electronic HMI ballasts can adversely effect equipment operating on it, but anyone operating HMI, Kino, and even LEDs should make themselves acquainted with harmonics (use this link for more details.)

Given the wide variety of generators manufactured for different markets, it is important to understand the benefits and drawbacks to each when it comes to their use in motion picture production. Especially, given that the increasing use of personal computers and microprocessor-controlled recording equipment in HD production has created an unprecedented demand for clean, reliable power on set at a time when the trend in lighting is toward light sources that generate dirty power. The adverse effects of the 55% total harmonic distortion exhibited in the power quality meter readings posted here, can take the form of overheating and failing equipment, efficiency losses, circuit breaker trips, excessive current on the neutral wire, and instability of the generator voltage and frequency. Harmonic noise of this magnitude can also damage HD digital cinema production equipment, create ground loops, and possibly create radio frequency (RF) interference.

Why is harmonic distortion suddenly an issue in motion picture electrical distribution systems? First, one must appreciate that the power generation and electrical distribution systems developed for motion picture production were never designed to deal with an abundance of non-linear loads like the HMI, Fluorescent, and LED lights prevalent today. In the past, when the only lights available were tungsten, attention was given to portable generator features such as automatic voltage regulation and speed regulation. But, given the increasing prevalence of harmonic currents and the problems they cause, an increasingly more important feature today is the quality of the generated power waveform and how well it interacts with today's light sources. As production gets more electronically sophisticated, a thorough understanding of the demands placed on portable generators by such production equipment is necessary in order to generate power that is clean and reliable. To generate power safely, it is important to understand the grounding requirements of the different types of portable generators.

It is the intent of the article mentioned in my previous post to establish a foundation of knowledge that will enable us to build a new production system that generates the clean stable set power capable of operating larger lights, or more smaller lights, off of portable gas generators like the Honda EU6500s or EU7000s than has ever been possible before. With this knowledge we will be able to also parallel two Honda EU6500 or EU7000 generators for an unprecedented 100A or 120A output from putt-putts.

Guy Holt

ScreenLight & Grip

Lighting Rental and Sales in Boston

Mark Kenfield · July 1, 2016

Thanks Guy,

Those specification plates appear to have been stripped from the ballasts somehow. So far we've been reasonably okay with the generator with all of our ballast-controlled lights on. I've had the producers source a small 1500kVA UPS for me to protect our data wrangling electronics.

Cheers,

Mark