Question Regarding Motor Starting Current and Power Source 1

[jmeyer84]

I'm working on a mining application using a 50 HP AC motor (Baldor ECR9504T) that will be powered by a 116 kW 460V generator.

The generator can source up to 145 amps. The full load amps of the motor is 61 amps, but the starting current is listed at 394 amps. I will be using a VFD in PWM mode to control the starting ramp of the motor as well as the speed. The ramp time is not critical and can be as slow as 120 seconds.

With the starting amperage exceeding the power source, will the generator be overloaded or will the PWM settings of the VFD allow for the motor to ramp up speed without issue?

 

[ScottyUK]

The VFD will start the motor nicely from the generator without the generator seeing any over-current. That's part of the magic with VFD's.

You might want to check that the generator's AVR will behave itself with a non-linear load.

 

[jmeyer84]

Thank you, Scotty. That's what I figured, but I needed a sanity check. Much appreciated!

 

[itsmoked]

You can set the starting ramp to something around 30 seconds. Then Set the display to show current. Start the motor and watch the current. With 30 seconds you'll probably see something around 90A (depending on the load it's turning). You can then dial the time back with further estimates gained from prior trials. Write everything down. Keep in mind motors that size have some starts per hour limitations. In that regard some of that is greatly relaxed by limiting the starting current but it's a bit difficult to estimate with this kind of commissioning routine, especially since the heat is mostly dumped into the un-measurable rotor. Just be aware of the motor's thermal response. The shaft will likely warm with the rotor.

Let us know how it works out so we can fine-tune our experience too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Stephen Nuchia]

Starting current is what happens when you apply full voltage to a motor with no rotor magnetization and no rotation. There's no "back EMF" (I guess that's an obsolete term now?) and it draws whatever the combined impedance of the supply and the windings will allow. Usually pretty much the same value as when it stalls.

Soft start via VFD applies a reduced voltage and so there's not so much current. Also the inverter buffers the current demand, kva in should roughly equal kva out, like a transformer. The current in the motor leads may be substantially higher than the current at the generator.

Momentary reactive overload won't kill the generator the way a real power overload would, if the load doesn't have a lot of inertia (or you have a clutch) you could probably hard-start it. But using a VFD is easier than coordinating the motor and genset controls to allow what would still be a marginal fit, and the disturbance to the voltage and frequency of the generator would be hard on anything else powered from it.

Starting (and stall) are not the worst-case current draw for an induction motor, by the way. Worst-case is when the power supply is interrupted just long enough for the slip to move the rotor 1/2 cycle out of phase when it comes back on. Modern digital motor protection relays will open the starter on undervoltage and delay reclosing long enough for the rotor field to decay. Some even sense the generated voltage on the motor leads reclose in-phase, but you usually don't see that on motors you can move with a forklift. But that situation happens only on utility power.

I'll second Scotty's concern about the genset's voltage regulator. With over half the rated output being drawn (at full load) by an inverter front-end it wouldn't surprise me at all if it gets flaky. Another concern about that front end is that they tend to have a nasty inrush as the bulk capacitors charge up. Probably won't trip your genset instantaneous but it will sag the voltage for a cycle or two for anything else on the bus.

Starting performance on big motors depends very sensitively on the load (and "big" should be understood relative to the stiffness of the supply, your 50HP motor would be small in a utility-fed factory but it's big relative to your genset-fed island). Compressors without unloading valves and large blowers tend to be among the worst. Ball mills are vicious, the load reacts chaotically as the mill begins turning. Material lift belts and chains can have highly variable pre-start weight distributions and should be tested under worst-case conditions. There is a lot more to consider than the motor nameplate.

 

[LionelHutz]

Due to the VFD being a power conversion device, watching motor current on the VFD display is rather useless to determine line power current while the VFD is ramping the motor.

 

[waross]

Note that motor starting causes both a voltage dip and a frequency dip.
The voltage dip is proportional to the motor starting KVA and the frequency dip is proportional to the motor starting kW.
I have started quite a few motors on quite a few generators.
And by the way, generator KVA and PF is more descriptive of a generators capacity than kW.
Do you have a base load in addition to the motor?
My rule of thumb for motor starting is is that the capacity available to start the motor must be three times the rated motor current.
The voltage dip will be a little greater than that allowed by the software that is supplied by the people who sell generator sets.
I have never had a customer complain about voltage dip let alone wish to spend money to reduce the starting voltage dip.
The bottom line for motor starting with no regard to frequency dip nor to voltage dip is 2.5 times motor current.
A 50 HP motor on a 116 kW, (145 KVA) generator without a base load is within the 3 times limit. (motor 50 HP @ 0.75 kW per HP, 90% efficiency and 90% PF. = 46.3 KVA 46.3 KVA x 3 = 138.9 KVA)
If there is a base load the 2.5 ratio is not usable.

Jraef, Gunnar, Scotty. Would a small 1:1 transformer feeding the AVR help to filter out most of the wave forms that are upsetting to the AVR?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill,
Just a transformer, not really, but a filter will include an isolation transformer, yes. 25 years ago when soft starters first started getting widespread use, genset mfrs discovered that the notching and distortion they created caused the AVRs to become unstable, which caused some soft starters to misfire, resulting in a "swing dance" between the genset AVR and the soft starter firing circuit until one or the other shut down. I got involved in this in Seattle when applying soft starters on fishing vessels, where having a generator go off line was extremely dangerous. Some soft starters didn't cause the problem but for a while nobody knew why, so it was the generator mfrs who came up with the fix. At first it was primarily Catepillar, so they got the lion's share of that business for years until the others caught up. All they did was come up with a filter for the AVR and for a few years afterward, you could buy a retrofit kit for older Cat gensets that didn't have it. But eventually I think they assumed that any genset that old would need to be rebuilt anyway and they stopped offering the retrofit kit because the filtering became inherent in the AVR. I checked on that for someone about 10 years ago and not only do they not offer it, I had a hard time finding anyone still at Cat who even remembered it!


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

I second Jeff, of course. But I think that an isolation transformer is worth a try. It isn't only the weird curve-forms as such that can upset the AVR. The HF common-mode voltages are also a problem for the measuring circuit of the AVR and an isolation transformer reduces them, or eliminates them completely. But most modern ones have filters that should handle that. So, an isolation transformer alone may work. But perhaps not what you find in the scrap box.

I need to say that Stephen 16 Apr 18 06:27 made me feel good. Counter EMF is still a valid concept and the fact that you mention the risk that it kills the VFD output shows your deeper understanding. The same mechanism, actually, that troubles some Y/D starters and makes the use of them questionable.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[catserveng]

It wold helpful to know a little more about the generator you are using to make a specific recommendation. There are a lot of small generators running VFD motor loads with no problems, at least in the areas I work. But problems do crop up. Some really low cost gensets may have AVR's that are not very happy about the harmonics generated by the VFD's, especially at the lower outputs when starting. In some cases sensing line filtering may help, but depending on the AVR, it may also have problems if it's seeing those harmonics on the power input.

Most "contractor" grade smaller gensets out today seem to be able to deal with VFD motor loads with no problems, at least as far as harmonic distortion affecting voltage stability. Biggest problem I see on service calls these days is someone using a VFD with power factor correction caps on the front end, causing large leading power factors at motor start, and newer generator protection systems that have reverse VAR or leading PF protections enabled.

Most of the recent problems with voltage stability and VFD's I have seen are with units that have analog AVR's with single phase direct sensing and are static excited. Digital AVR's with three phase sensing and either PMG or AREP type excitation power systems don't hardly ever show up with issues, at least from what I see.

I remember the AVR filter kits well, actually built quite a few before CAT come out with the "kit". There were actually three kits for a while. Funny how the "new and improved" regulator at that time had the problems, the regulator it replaced had been in service in mostly oil field applications running drill rig drives with very few problems for many years, but it was big, bulky and expensive.

Jraef, never heard it called a "swing dance", but sure was interesting to watch, lots of other names for it though, usually with lots of cuss words.

MikeL.