VFD Minimum Output Frequency
VFD Minimum Output Frequency
(OP)
Can typical (60 Hz input, 100 + horsepower ) VFD drives allow output at say, 1 Hz, 10 Hz, 30 Hz ? I understand that driven equipment may have problems at very low speeds, but the question here is if VFD systems usually have a "built in" minimum output frequency, or can you set the VFD at , say 2 Hz to 66 Hz on a 60 cycle system?





RE: VFD Minimum Output Frequency
Motor cooling is very often dependent on speed. So may bearing friction/greasing be. But not a problem for the VFD as such.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD Minimum Output Frequency
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Minimum Output Frequency
Thanks
RE: VFD Minimum Output Frequency
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
RE: VFD Minimum Output Frequency
Side not on the "599Hz"; that was not a random number. As of a couple of years ago, VFDs are not allowed to run at speeds over 599Hz without special permission, typically from the adept of Defence here in the U.S. and most other NATO countries. It has to do with centrifuges and weapons. So mfrs were required to gobble them with programming that cannot be changed by the user.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: VFD Minimum Output Frequency
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Minimum Output Frequency
Yes, centrifuges for uranium. Just to make it a little bit more complicated to make THE BOMB.
But making THE BOMB takes a lot of effort. And reprogramming - or build from scratch - a VFD must be one of the lesser challenges.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD Minimum Output Frequency
on topic, most VFD's have a programmable min. frequency, so the ramp up could begin at 2Hz, 4Hz, etc. But 0Hz is no issue for the drive, apart from localised IGBT heating, which can be significant. In fact, if you want todestroy a VFD, the easiest way is probably to run at high pulse frequency, slow speed, high current....
Cheers,
Mort
RE: VFD Minimum Output Frequency
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: VFD Minimum Output Frequency
My first thinking was that the heat supervision would take care about that. My second thinking is that there's just one heat sink and one sensor. So, the heat generated by four semi-conductors before the alarm goes off will be 50% higher for each IGBT. And that is probably above the limit for destruction. Good point. Will remember.
So, how come that I never experienced a problem with that? Probably because all applications where I have used "zero frequency" wasn't actually 0 Hz because the induction motor that was standing still needed a low frequency rotating field to keep the load and that low frequency was enough to even out heat between the transistors.
When Synchronous motors become more widespread, we may very well see IGBT destruction in such applications. Very interesting.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD Minimum Output Frequency
Now, you're funning me right?
Pocket?!! I gave that up long ago. I switched to the Otter Box Defender Series with Holster and couldn't be happier. Showed my son and he switched and thanks me about once a month. He now packs two phones with Otter Box defenders in holsters.
I use mine horizontally he uses his vertically. You can actually retrieve your phone while sitting which is impossible from a pocket. Also with an otter box you have zero issues with dropping it EVER. You can then go to a larger phone with no pocket involvement at all. Heck go with a phablet! LOL
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Minimum Output Frequency
Thanks for the reply. I can only speak from my personal experience with our particular products, but it is a topic I have been doing some work on in the past few years. The following refers to small (<30kW) drives.
You correctly point out that there is only 1 temperature measurement point, usually on the same substrate as the IGBTs (and the freewheel diodes, and the rectifiers). The path for heat to reach this component is long (IGBT > module plate > heatsink > along the heatsink > back to the plate > thermistor) and depends on the thermal impedances of the materials, esp. heatsink paste. For this reason, knowing the temperature of the heatsink does not tell you the temperature of the IGBT die.
The die temperature is modelled, based on power loss in the devices (from DC link volts, output current, pulse frequency, etc.) and a dynamic model of the thermal system (i.e. the system described above).
For low output frequencies, there is a multiplying factor applied to allow for local heating. at 0Hz this factor is 2.5 - i.e. the IGBT temp rise at 0Hz is 2.5x that at 50Hz, for example. This might mean that larger IGBTs are required than would be considered at first glance. The frequency at which localised heating begins to occur depends on the thermal system characteristics, and so varies from drive to drive.
Our thermal model (and hardware sizing) allows for all this, but I wouldn't like to speak for anyone else. To find out, try doing some heavy overload cycling at 0Hz, high supply voltage, and high pulse frequency. Only try if the product is under warranty!!!
Cheers,
Mort
RE: VFD Minimum Output Frequency
even more interesting. You deserve an LPS. Sorry, there are no super-sized ones...
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD Minimum Output Frequency
Secondly, why would running the inverter output at high freq near zero speed with high current be any worse than high freq at whatever speed with high current?? The VFD doesn't cool any worse putting out zero speed than any other speed.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Minimum Output Frequency
1. I was back in older days (no PWM) and simplified thinking.
2. I hoped that no-one would notice. (Silly me)
If you run at zero frequency, let's say with peak current in phase 1, then the two other phases will carry 50% each. The IGBT with maximum current will then heat more than it would if current was distributed across all three phases, as would be the case at higher frequencies.
Losses in IGBT:s are more or less current "squarish" (the constant initial voltage drop, as seen in diodes and thyristors, is quite low in an IGBT) and that makes losses in the phase 1 IGBT substantially higher at 0 Hz than at higher frequencies where load is more periodic and spread out over all thyristors. The thermal resistance between die and heat sink plays a role. More than one thinks as an everyday user.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD Minimum Output Frequency
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Minimum Output Frequency
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD Minimum Output Frequency
Cheers,
Mort
RE: VFD Minimum Output Frequency
Also, most motors have a minimum speed where a external fan is used. I thought it was around 15-20 Hz and below. This is so that the motor will not over heat. I would ask the manu of motor for that speed or hertz. I got this same advise from this website, forgot who told me that issue.
RE: VFD Minimum Output Frequency
Interestingly, this topic caused some conflict between the motor and VFD departments when developing a motor mounted drive. The motor guys work to a typical max. of 155C, but once a VFD is bolted to it, this is heavily restricted as the drive components will be generally rated to 85C.
Cheers,
Mort
RE: VFD Minimum Output Frequency
To hold a motor rated at 1760 RPM at zero speed with full torque load may require the VFD to have an output equal to 40 RPM in reverse.
Don't forget the slip frequency.
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD Minimum Output Frequency
At such low frequency, reactance are very small, and to keep flux constant, voltage must be low also, resulting an small average duty cycle; for nominal current, conduction loss are smaller but conduction time for one device may lead to a higer local temperature, depending of thermal impedance, cooling at zero speed, modulation methods.
Much worse in VFDs with buck front-end PFC, where average duty cycle may remain almost constant for entire speed range.