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50Hz motor, 60Hz supply revisited

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?

"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

It's hard for me to follow the whole scenario.

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

 

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

Careful now!

'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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 50Hz motor, 60Hz supply revisited

It reads like this is a Mechanical issue more than an Electrical one.
 
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

Your lawyers will want a chronological account written down in excruciating detail anyway.  Start now.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: 50Hz motor, 60Hz supply revisited

Going back to basics, I think.
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
--------------------
"Why not the best?"
Jimmy Carter

RE: 50Hz motor, 60Hz supply revisited

(OP)
"Is "marge mass" a typo for large mass? "

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  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: 50Hz motor, 60Hz supply revisited

jraef, if the motor will start across the line, then it should also start with the softstarter set to zero accel time, unless the current limit trips the starter.  I suppose you could slowly increase the accel time to try to find a point where it will still accel without tripping but that might not be possible.

If that is the case, it seems a larger starter is required to handle the high starting current.

RE: 50Hz motor, 60Hz supply revisited

I Love You Tube.

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

Quote (electricpete):

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

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).  

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

btw I'm sure Homer will be relieved that it was just a typo.

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

Quote (electricpete):

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).
It was inconclusive because it failed to start at 380vac input, 60hz.  If it has successfuly started in these conditions it would have proved that it could start at 380vac 50hz.

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.

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

electricpete, if the RVSS is set to zero accel time, then the SCR's are essentially "on" all the time and do not chop the incoming AC wave.  Set that way, the RVSS is essentially an across-the-line starter but with a very rigid current limit to protect its power circuit components.  

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

Quote (Dick):

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.  
Thanks Dick. I am familiar with the principle of a phase controlled rectifier which chops the voltage waveform.  What I wasn't sure about is whether it operates as a closed-loop / regulated power supply maintaining constant output voltage, or as an open-loop which does not adjust the firing angle.  From your comments I take it as open loop control where the firing timing is not affected by any changes in magnitude of the input and output voltages.

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?

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

Quote (electricpete):

I am familiar with the principle of a phase controlled rectifier which chops the voltage waveform.
To correct/clarify my terminology, rvss is obviously not a phase-controlled rectifier, but to my understanding it operates on roughly the same principle as a phase controlled rectifier (delay in SCR turn-on after voltage zero crossing).

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

Its been my experience if the soft start overloads on trip during ramp up, all things being ok, sized right and all.  Then its a mechanical binding issue with the belt, with whatever its driving.  I had issues like this over and over with a softstart driving misaligned and badly takeaway conveyors that had a belt.  I could get the belt to drive when I did a boost on the ramp, but this is essentially making it ATL.  And of course the belt jumps or smokes, even had sprokets tear apart, and or the softstart just OL.

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

controlsdude, if you choose a softstarter without enough shortterm overload capacity, it will trip on accel even tho the continuous amp rating was adequate for the application.  Some manufacturers even provide heavy duty softstarters with extra shortterm capacity just for this kind of situation.

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

Firing on voltage level is not anything I have seen or can imagine.  Older, analogue types always fired on delay time('millisecond based', if you prefer) and there was always a problem with changing frequency since the delay was determined by a capacitor and a control voltage. Some had to be ordered for a certain frequency and others could be adjusted or jumpered for the frequency at hand. Newer ones measure the frequency and adjust so that firing is 'degrees based'. That means that new SSes work well on 50 and 60 Hz. The other issues (speed, U/f ratio and second order effects) are the same for both types.

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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 50Hz motor, 60Hz supply revisited

(OP)
This particular soft starter cannot go "full on" initially, because it has an inherent current limit setting of 500% maximum and a initial voltage setting of 90% maximum. This is probably because the unit has integral bypass contacts and I doubt they are rated for Across-the-Line starting, so they must "soften the blow" so to speak. This may be the issue. When started ATL, there is just enough starting torque to overcome the inertia, but when reduced by the soft starter, combined with the 380V 60Hz issue, it just doesn't cut it.

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  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: 50Hz motor, 60Hz supply revisited

DickDV

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

Thanks Dick and Gunnar for explaining that to me.  In summary I guess the RVSS output voltage will be some fixed percentage of input regardless of change to input voltage magnitude and frequency (assuming there is no active current limit).

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?
http://www.sew-eurodrive-gearing.com/sew-eurodrive/index.html

If it is really spinning that fast, do you think ½" thickness is representative of the sheave? (maybe it has only spokes?).
 

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

By the way I assumed steel (cast iron) for the density. Maybe it's cast aluminum? (I haven't ever seen any aluminum sheaves, but I also haven't seen any 24" 3000-3600rpm sheaves either.)

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

Thinking about the video, I imagine the motor has got to be geared down so the disk spins much slower.  Then the belts provide a further speed reduction. Right?

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

Actually at about 2:30 into the video you can see a sheave mounted directly to the motor shaft, but the motor sheave looks far smaller than 24" diameter.  I guess it is the slow speed sheave driven by that same belt that is 24" diameter.  Small sheave goes on the fast (motor shaft), big sheave goes on the slow shaft. Duh. Never mind.

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

So:
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

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

It's all Marge's fault.... that little blue-haired vixen!

=====================================
(2B)+(2B)'  ?

RE: 50Hz motor, 60Hz supply revisited

Lisa is my favorite. Yes, I am an old male white engineer.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 50Hz motor, 60Hz supply revisited

"mall sheave which drives a slower marge mass,"

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

RE: 50Hz motor, 60Hz supply revisited

(OP)
As I said, the machine I am working on is similar, but not the same as the one in the video. Our motor is on top, there is one belt connecting it to a LARGE (not marge) sheave that is part of that large rotating mass similar to that in the video. I don't know the ratio, but if I had to guess, 6:1 maybe?

"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

6:1 with marge lass rotating slowly? Then the inertia that the motor sees is only 1/36 and then it does not seem to be a very heavy start.

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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: 50Hz motor, 60Hz supply revisited

(OP)
Update for those who are interested. We installed a slightly larger version of the soft starter, 28A instead of 23A, which means the 500% Current Limit maximum setting allowed a small amount more current to get to the motor, it started right up.

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

Congratulations Jeff, and thanks for the update.

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

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