50Hz motor, 60Hz supply revisited
50Hz motor, 60Hz supply revisited
(OP)
OK, I know I contributed to Keith's (itsmoked) FAQ on this subject, but believe it or not I need help.
The situation background is that I helped with a machine control system that is being built here in the US to be sent to Europe. The supply is going to be 380V 50Hz (reportedly). The motor is an SEW Eurodrive 2 pole 15HP (11kW) that is 380-420V/50Hz Delta, 690-720V Star, connected in Delta; 19.5A FLC on the nameplate. The original control panel had a Siemens 3RW30 soft start controller feeding the motor, rated 22A. This motor drives a marge mass, a 24" dia. sheave that is connected by belts to several other rotating elements for the purpose of making cans. It takes approx. 7 seconds to accelerate Across-the-Line (DOL). Starting current ATL is about 170A, unloaded running current is only 7A.
I got called in because the soft start fried; the bypass contact welded the first time they started it for testing. The had customer complained that the motor itself smelled "like burnt insulation" when they first opened the crate, before ever being energized. The motor supplier "tested" it and declared it OK, I since discovered he tested it with a VOM only. I provided a replacement soft starter, this time a full starter (3RW40), but insisted they get a "real" test done on the motor (I didn't have my megger with me) before energizing it. They did, apparently everything is OK with that motor.
When they started it with the new soft starter, rated 24A, the motor never finishes accelerating before the soft starter trips on OL. They have tried various settings, nothing works. They put in an ATL starter (hence the data on starting) and the motor works fine. I have not been able to witness the start-up, there are mechanical problems every time I visit.
They seem to think that in order to properly test this motor, they needed to use a transformer to step down to 380V. But because it's just a transformer, it's now 380V 60Hz. They have created a V/Hz ratio of 6.33 for a motor designed for 7.6, so they have essentially weakened the motor. I contend that this is extending the acceleration time out too far when the starting torque is reduced by the soft starter, any soft starter. They seem to believe it's a problem with the starter and are installing a Y-Delta to replace it. I think this is a failure in the making. I suggested bypassing the transformer for motor testing at 480V, the motor supplier is telling them this will damage the motor, I disagree but I cannot prevail. Here is my concern;
At a 17% decrease in the V/Hz ratio, I believe they have reduced the available motor break down torque by .832 or 69% of rated BDT, and this is causing the motor acceleration time to extend beyond the thermal limit of a Class 20 OL setting. Is there something else about applying the higher frequency at the design voltage that I'm missing here?
The situation background is that I helped with a machine control system that is being built here in the US to be sent to Europe. The supply is going to be 380V 50Hz (reportedly). The motor is an SEW Eurodrive 2 pole 15HP (11kW) that is 380-420V/50Hz Delta, 690-720V Star, connected in Delta; 19.5A FLC on the nameplate. The original control panel had a Siemens 3RW30 soft start controller feeding the motor, rated 22A. This motor drives a marge mass, a 24" dia. sheave that is connected by belts to several other rotating elements for the purpose of making cans. It takes approx. 7 seconds to accelerate Across-the-Line (DOL). Starting current ATL is about 170A, unloaded running current is only 7A.
I got called in because the soft start fried; the bypass contact welded the first time they started it for testing. The had customer complained that the motor itself smelled "like burnt insulation" when they first opened the crate, before ever being energized. The motor supplier "tested" it and declared it OK, I since discovered he tested it with a VOM only. I provided a replacement soft starter, this time a full starter (3RW40), but insisted they get a "real" test done on the motor (I didn't have my megger with me) before energizing it. They did, apparently everything is OK with that motor.
When they started it with the new soft starter, rated 24A, the motor never finishes accelerating before the soft starter trips on OL. They have tried various settings, nothing works. They put in an ATL starter (hence the data on starting) and the motor works fine. I have not been able to witness the start-up, there are mechanical problems every time I visit.
They seem to think that in order to properly test this motor, they needed to use a transformer to step down to 380V. But because it's just a transformer, it's now 380V 60Hz. They have created a V/Hz ratio of 6.33 for a motor designed for 7.6, so they have essentially weakened the motor. I contend that this is extending the acceleration time out too far when the starting torque is reduced by the soft starter, any soft starter. They seem to believe it's a problem with the starter and are installing a Y-Delta to replace it. I think this is a failure in the making. I suggested bypassing the transformer for motor testing at 480V, the motor supplier is telling them this will damage the motor, I disagree but I cannot prevail. Here is my concern;
At a 17% decrease in the V/Hz ratio, I believe they have reduced the available motor break down torque by .832 or 69% of rated BDT, and this is causing the motor acceleration time to extend beyond the thermal limit of a Class 20 OL setting. Is there something else about applying the higher frequency at the design voltage that I'm missing here?
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: 50Hz motor, 60Hz supply revisited
Just to throw out some random aspects that I don't think were mentioned in the FAQ:
1 - If you could get the exact same same torque vs slip curve for a 50-hz and 60hz supply, it would take 20% longer to accelerate at 60hz because it has to accelerate thru a wider speed range.
2 - The starting torque may depend not only on volt-hz but also simply on hz.... because the deep bar effect used to boost starting torque is frequency dependent. So if you take a 60hz torque speed curve and translate it to 50hz using Torque~(volts/hz)^2 correction, the resulting Torque vs slip curve may be higher than actual T vs s curve in the range of low-speed/hi-slipwhere the actual 50hz start does not benefit as much from deep bar effect
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
'380' usually is 400 V nowadays. At least in EU - but not always in the UK.
Also, if there is an acrid smell, it may very well come from a winding with internal shorts. That will not show when meggering. But it will surely reduce torque and prolong starting.
What country is it being exported to?
I do not envy you at all.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 50Hz motor, 60Hz supply revisited
The 24 inch diameter at 2 pole for the given horsepower stands out to me.
These two comments in the original post also highlight this factor:
1)"This motor drives a marge mass, a 24" dia. sheave that is connected by belts to several other rotating elements..."
Is "marge mass" a typo for large mass?
2)"...I have not been able to witness the start-up, there are mechanical problems every time I visit."
One can dance with settings, swap out different drives, reconnect for different voltages, etc.
Yet if the motor type, and it's mechanical arrangement is flawed, it's difficult to imagine anything you do electrically is going to solve motor acceleration.
It as electricpete indicates as well:
"It's hard for me to follow the whole scenario."
John
RE: 50Hz motor, 60Hz supply revisited
Mike Halloran
Pembroke Pines, FL, USA
RE: 50Hz motor, 60Hz supply revisited
As I understand motors, a motor should develop the same torque as long as the V/H ratio is the same. As the frequency increases the impedance increases and higher voltage is needed to drive the same current.
I would say that the 400V/50H motor should be tested at 480V if testing is done at 60 Hz. If the expected voltage is actually 380V then you may want to test at 456V to maintain an exact V/Hz ratio, but no lower.
There may be some issues with the higher speed at 60Hz such as a longer acceleration time, particularly with a highly inertial load.
When I was younger I would argue these issues fiercely, now I am more inclined to say something like
"Wow, you've told me things about motors that I never suspected, even though I have worked with motor problems for several years. You really don't need me here. I should leave."
However, if you are representing a supplier and must make nice, don't. Nice guys finish last. Void their warranty unless and until they rent a generator and test the equipment at the design voltage and frequency. Don't waste your time arguing. Walk away and send a memo voiding the warranty.
By the way, I agree with you and sympathize with you.
Bill
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"Why not the best?"
Jimmy Carter
RE: 50Hz motor, 60Hz supply revisited
LOL, yes. Marge mass I supposed would be a tumor on Mrs. Simpson...
It's going to the Netherlands. I too questioned the 380V vs 400V issue, apparently at the site where this machine is going, they confirmed it is 380V.
My communication skills are slipping, too much holiday cheer I suspect.
11kW motor is designed for 380V 50Hz.
Test setup is supplying 380V 60Hz, even though I told them to use 460V 60Hz.
Soft starter failed to accelerate the motor, but it will accelerate DOL. Unloaded, but it has to accelerate a large mass, i.e. flywheel of sorts.
Very similar machine:
http://www.youtube.com/watch?v=Noe2uRdKcx8
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: 50Hz motor, 60Hz supply revisited
If that is the case, it seems a larger starter is required to handle the high starting current.
RE: 50Hz motor, 60Hz supply revisited
Thanks for posting that URL jraef.
The particular motor on the machine shown in the video looks to be running more as a 4 pole unit. (I realize it's not the same machine we're discussing.)
If the motor is indeed starting across the line UNDER LOAD,
then it seems reasonable to echo what DickDV has mentioned.
There remains a tangled set of "info" in this sentence:
"...Soft starter failed to accelerate the motor, but it will accelerate DOL. Unloaded, but it has to accelerate a large mass, i.e. flywheel of sorts."
It still reads like the motor for the machine "we" are discussing is incorrectly sized for the required load to be driven.
Enjoying the Forum,
John
RE: 50Hz motor, 60Hz supply revisited
To elaborate a little bit:
#1 obviously provides a disadvantage to the 60hz starting scenario.
#2 describes a minor correction to the thumbrule Torque(s) ~ (Volts/hz)^2. The correction arises because the thumbrule is based on assumption of constant equivalent circuit parameters which is not completely true when deep bar effect is present. The correction provides an advantage to the 60hz starting scenario as compared to what we would predict using the thumbrule. But it is a 2nd order correction to the main effect ... and the main effect is of course that lower volts/hz leads to lower accelerating torque which is a disadvantage in the 60hz lower volt/hz starting scenario.
So far we talked about the voltage at the input to the soft starter. What is completely unknown to me is how the soft-starter output voltage varies with changes of the input voltage and frequency. Is it a closed loop control of the output voltage to some setpoint or a simple fixed-angle scheme? I'm sure Jeff and others can answer that question but it's a mystery to me.
The assumption seems to be that the soft-starter simply passes along an output voltage which is some fixed fraction of the input voltage (and the fraction doesn't change with changes of input voltage and frequency). If that is the case then the 60hz lower voltage test is certainly inconclusive... it doesn't prove that the soft starter won't start successfully with 380V 50Hz (nor does it prove that it will).
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
Likewise if you persuaded them to run a test at 460 or 480vac 60hz, a successful start would prove it'll work at 380vac 50hz, but an unsuccessful start would remain inconclusive due to factor #1.
If we know the fundamental voltage on the RVSS output (I assume you do... and I assume it is not affected by complication like current limit), and if we furhter assume the machine represents primarily an inertia with no load torque, then we could guesstimate your 60hz reduced voltage starting time from your 60hz DOL start starting time:
Treducedvoltage = Tdol *(Vdol/Vreduced)^2 = 7sec * (Vdol/Vreduced)^2
The above assumed no load torque. To the extent that the machine provides some load torque during start, the above equation will underpredict the true starting time. It also reminds me that if load torque varies with speed then it is another disadvantage for the 60hz start (it will see higher load torque at full speed).
All of this is nothing new to you I'm sure....just talking through some ideas. I'll be interested to learn for my own curiosity how the rvss starter output varies with change in input frequency and voltage.
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
With any kind of accel time, the SCR's delay their conduction part way into the sine wave so the motor sees the same sine wave as on the input except that the leading portion is missing. For example, at 50% voltage, the front half of each half wave would be missing. That would be from zero degrees to 90 degrees and from 180 degrees to 270 degrees. At three quarters voltage, the conduction would be missing from zero to 45 degrees and from 180 to 225 degrees.
RE: 50Hz motor, 60Hz supply revisited
But even in the case of open loop control, there is still another question: we have a fixed/specified interval between voltage zero crossing and SCR turn-on... is that interval specified in "degrees" or in "milliseconds"? It would make a difference when we vary the input frequency. It strikes me that the simplest circuit to design would be based on time and in that case the output voltage as fraction of input voltage would vary with frequency. From your comments I take it that it is specified in degrees...i.e. the machine is smart enough to sense the input power frequency and adjust the firing delay accordingly?
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
I had to stand there with the mechanical guy to get them to adjust and adjust on one unit to convince them that it was mechanical. Of course it was not 1 unit but 100 to get thru. Turns into a pointing fingers game. Seems like you have to do the mechanicals job sometimes, irritating.
RE: 50Hz motor, 60Hz supply revisited
electricpete, in the brands that I am familiar with, the SCR firing was done at a voltage level, I believe. This rather than time or phase angle, as you mention.
That would make the input frequency issue somewhat smaller, it seems to me.
Someone familiar with the internal design of softstarters may well correct me on this but, at least with Benshaw, I believe voltage level firing is done.
RE: 50Hz motor, 60Hz supply revisited
Tangent: Talking about old analogue things, I just came home from a lathe with a servo about my age. Not quite, but not far from.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 50Hz motor, 60Hz supply revisited
I'm going back out on Tuesday morning, I'm going to try again to convince them to allow me to bypass that transformer that's stepping down to 380V and give 480V to that motor. I really think that is the culprit.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: 50Hz motor, 60Hz supply revisited
Ok, so how does jraef know that the mechanical load is going to be a high inrush load?
Only mechanical would tell him that would he not? Otherwise its a binding issue with the mechanics.
RE: 50Hz motor, 60Hz supply revisited
Jeff - If you re-run the test at 60hz and 480vac (approximately 460vac) and it passes, then you have proved it will work for 380vac 50hz. But if it fails, then that doesn't prove that it won't work at at 380vac 50hz... because there are still two starting disadvantages for the 60hz/480vac start:
1 – It has to accelerate to a 20% higher speed at 60hz.
3 – If any speed-dependent load torque is present, it will be higher during the last 20% of 60hz acceleration than any torque ever seen during the 50hz start.
So it'll be a useful test but you still might want to warn them ahead of time that the 60hz 480vac test will validate the design if it succeeds but will be inconclusive if it fails.
I think ControlDude has a reasonable question in the big scheme of things although it may not be the focus of Jeff's efforts.
I was thinking about this sheave / disk and how much inertia it presents:
OD = 2 Feet
Let's say thickness is just 0.5" (that sounds on the low end for a 24" diameter sheave)
Thickness = 0.041666667 Feet
rho = 490 lbm/ft^3
J = DiskInertia = pi rho L D^4/32 = 32 lbm-ft^2
That is about twice the allowed inertia for DOL start of a 2-pole 60hz motor per NEMA MG1. I can see it would be quite a starting challenge if that disk gets up to full speed.
Jeff - Is the 24" disk really rotating at motor speed or are there internal gears with the disk spinning at a lower speed?
Maybe the internal gears similar to those shown here?
h
If it is really spinning that fast, do you think ½" thickness is representative of the sheave? (maybe it has only spokes?).
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
This motor drives a marge mass, a 24" dia. sheave that is connected by belts to several other rotating elements
should've been:
This motor drives a mall sheave which drives a slower marge mass, a 24" dia. sheave that is connected by belts to several other rotating elements
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
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(2B)+(2B)' ?
RE: 50Hz motor, 60Hz supply revisited
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 50Hz motor, 60Hz supply revisited
So Marge, who is quite slow owing to her mass, was shopping at the mall when she was driven by a sheave wielded by a mall rat. It happened in front of a "Transformers" exhibit.
Her last words were;
"It Hertz!"
Sorry, the devil made me do it.
Bill
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"Why not the best?"
Jimmy Carter
RE: 50Hz motor, 60Hz supply revisited
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 50Hz motor, 60Hz supply revisited
I think someone made a mistake of some sort, realized it, corrected it and is now trying to get away with it by involving someone else and not telling the whole story.
Been there quite a few times. It doesn't seem to be much to spill sweat or blood over.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 50Hz motor, 60Hz supply revisited
They now believe me.
New plan: when they build the next machine, we are going to function test everything for using 480V 60Hz, then do one final run at 380V 50Hz by renting a genset for a day when the customer arrives for final acceptance testing.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 50Hz motor, 60Hz supply revisited
Bill
--------------------
"Why not the best?"
Jimmy Carter