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Old October 21st, 2018, 08:01 PM   #1
NevisGroup
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GS4 VFD and Motor draw EXCESSIVE current

Hello all,

I just replaced a dead Yaskawa F7 drive with an Automation Direct GS4
This is driving a Roots screw type vacuum pump with an internal motor.
The motor nameplate says 75 Volts at 60Hz max 50 Amps
I have been fiddling with the Volts/Hertz settings in the drive but I am not having any luck. I don't have any reason to thing the motor is bad. (The drive was damaged when it was wired incorrectly)
So at this point I have the max voltage turned down to 20 volts. The motor will start to spin up but the current starts to climb with the voltage until it is 70 amps and the drive shuts down on overload.


This is actually the second time that I have run into this. I had another motor that did the same thing on a mill. We just replaced the motor and called it a day.


What am I missing about this motor?
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Old October 21st, 2018, 10:56 PM   #2
Saffa
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75 volts sounds like a very odd rated voltage. Could you post a nameplate photo?

I thought you guys over there in the states generally used 460V 3 phase motors
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Old October 21st, 2018, 11:59 PM   #3
jraef
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Quote:
Originally Posted by Saffa View Post
75 volts sounds like a very odd rated voltage. Could you post a nameplate photo?

I thought you guys over there in the states generally used 460V 3 phase motors
Well, 460 or 230V, but not 75V!

Sometimes though, OEMs will have custom motors made for some specific reason and without knowing what that reason is, we can only guess. It might be wrong information, it might be that custom motor situation. We can’t tell without more info and yes, a photo would be great.

One such reason is that the intent was to run the motor at higher than line frequency via the VFD. So for example building the motor as 75V 60 Hz gives it a V/Hz ratio of 1.25 so if you feed the VFD with 240V, you can run that motor at 300Hz without losing torque.
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Old October 22nd, 2018, 10:18 PM   #4
NevisGroup
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I can't find any info on the web about this particular motor.
It spins freely and I don't have a reason to believe that the motor is damaged (but it may very well be)


One thing that I found was that it looked this motor does not create back EMF ( is that even possible?). I can get it to spin if I limit the applied voltage low enough that the current is limited by the winding resistance. (We are talking 8-10 volts here) but it will draw 50 amps. The windings all draw the same current.
Unfortunately the original Yaskawa F7 drive is dead so I dont have the parameters out of it.


This is the second time that I have run across this. I had a 3hp mill that the drive smoked. When I put a new drive in it all it would do is draw max amps and turn slowly. We ordered a new motor from the original manufacturer (out of china) and the new motor did the same thing. I put an off the shelf 3hp motor on it instead and all is well. I suppose they could have sent me a bad motor but this one is now the third... what are the odds
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Old October 22nd, 2018, 11:29 PM   #5
GaryS
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This may sound very strange but try to restrict the intake of the vacuum pump.
If I remember correctly years ago when I was doing a lot of work on printing presses they use vacuum pumps to handle the paper.
Anyway air is exactly the same as fluid except reverse in vacuum pump when you first start the pump you are trying to move the full volume that it can pull in, and as the vacuum level goes up or gage goes down whatever you define it the actual load on the motor goes down (reverse of a fluid pump) by restricting the pump intake you limit the amount of fluid you are moving and lower the load.
To a vacuum pump or a fan air is just a fluid the more you have to move the more power is required.
At high vacuum you are moving very little fluid less current

the name plate and the VFD look good constant HP between 60 and 120 HZ voltage can be set to whatever the manufacture wants where it is made would determine the voltage.
post back and let us know how you make out

Last edited by GaryS; October 22nd, 2018 at 11:32 PM.
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Old October 24th, 2018, 06:33 PM   #6
NevisGroup
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Quote:
Originally Posted by GaryS View Post
This may sound very strange but try to restrict the intake of the vacuum pump.
We tried that early on. We pulled it down with the roughing pump but no luck.
The motor itself is not overloaded, when the drive faults the motor will coast to a stop over a few seconds. So there is really no load on the motor.


Is there such a thing as a constant current motor?
Where we feed it a constant 50amps and adjust the volts as needed?
(The exact opposite of a standard squirrel cage motor)


There may well be something wrong with the motor. But the guys swear up and down it was working before the drive whent up in smoke.



I checked the winding resistance (with a cheap meter) and all three check the same around 0.5 ohms.
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Old October 25th, 2018, 01:53 AM   #7
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A few thoughts
Remember when dealing with fluids it think the formula is the load goes up by the square of the speed
You also have to keep in mind that the load will only go up if you are actually moving the fluid so if you are not moving the fluid the load will be minimum .
To answer the question about a constant 50 amp output on an ac inverter. I have never heard of one, it could be done with a dc drive but depending on the resistance you will need a very high voltage power supply. That said with most vfd’s you can limit the output current to some value less than the rated output. The vfd would then limit both the output voltage and output frequency at the current limit based on the v/f settings. This would limit the power delivered to the motor and accordingly to the pump. By lowering the frequency you lower the motor speed and accordingly you would lower the volume of the fluid pumped. It’s all interactive. You say you pumped a vacuum on the system with another before you tried this pump why not just use a restrictor plate similar to what they use on a NASCAR.
You say the motor is not overloaded. How do you know this did you measure it. The current leaving the vfd must go someplace out one motor lest end returns to the vfd through one of the other leads
Remember all current I any circuit must return to the same source ac or dc both work the same. If the vfd is putting out 50 amps the motor is getting 50 amps. If you are not pulling more than that them you are overloaded if not the you are within the ratings of the vfd.
Another thing I noticed is your pump is a vane type pump with an oil level. Are you sure you didn’t run it dry and burn the bearings that will cause you pull high amps.
As I said vacuum pumps don’t act like any other loads they do look strange at times.
As for the statement this motor dose not create back emf. All motors generated back emf some call it counter emf it’s generated when a conductor passes through a magnetic field. Some vfd’s can’t read it or use it but that really has nothing to do the drive faulting.
20volt limit is way to low you may not have the torque to handle the load.
Try setting you base frequency at 120hz with a motor voltage at 150 that should give you the 75volts at 60 hz v/f
Those setting are all doable in a Ysakawa vfd

Why not just use an A1000 drive F7 is obsolete.
The lower cost drive are not capable of the finer detail settings
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Old October 25th, 2018, 07:30 AM   #8
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Hi, well not to sound goofy, but looking at the nameplate data, it looks like to me it is a dual voltage motor. 75 V and 150 V. Is a single phase motor? If so, can it be run on 120 VAC? Just an observation and I could be way off. If so, sorry for the bother.
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Last edited by saultgeorge; October 25th, 2018 at 07:31 AM. Reason: Typo
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Old October 25th, 2018, 11:01 AM   #9
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I did a bunch of searching and came up with pretty much nothing on that vacuum pump, other than they were used on lasers. Can't find any reference to the LB series on Ulvac's web pages.

Strange. Proprietary stuff?
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Last edited by KB1GNI; October 25th, 2018 at 11:18 AM.
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Old October 25th, 2018, 03:16 PM   #10
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Thanks for the thoughts GaryS:
So I don't believe the motor is over loaded because with enough fiddling with the drive, I can get the motor to spin at a few hundred RPM.
But it is still drawing an absurd amount of current. Once the drive trips out the motor coasts back to a stop over a few seconds. If there were any load on the motor I would expect it to stop quickly as opposed to coast.
BTW, it is not a vane pump, it is a screw pump. But the oil is quite thin and does not appear to offer much resistance to the screws.
Measured with a crappy volt meter, the winding resistance is about 0.5 ohms. So I only need about 25 volts to get my 50 amps. This matches what I see with the drive. If limit the max volts I can manage the current.


My question about back EMF is because if I fiddle enough with the drive settings I can get the motor to spin a a few hundred RPM. I would expect the current to fall off if there was back EMF. It is not.
Amps~(Applied Volts - Back EMF)/Winding resistance.
So if I limit the volts to 20 I get ~40amps into the motor. It will start to spin and get up to a few hundred RPM. But I don't see the current drop.

At this point we are only putting 400 watts into a 5KW motor so in theory I could leave it like this all day long. (Not a recommended idea!!)


The original F7 drive has been sent out to a repair center to see if they can get the parameters out of it. I am hoping that we are just missing some simple setting.


I have check for stray fault currents but everything that goes out comes back. There is 0 net current on the leads to the motor.
So no ground faults or such.


I almost wonder if it is a PM synchro motor. I don't know how to check for that. The motor is sealed inside the pump so I cant spin it externally. (Even if if was you would think that it would develop some back EMF.)





KB1GNI,
Yep this is in a Panasonic laser on a Mazak cutting center.




It is a big $$$$ project to pull this pump out of the system. It is an even bigger $$$$ project to have it rebuilt. So I am really reaching for anything before we pull it out.


Key thing to remember is that it was running fine before the original drive got smoked. The drive was smoked because they put 460 on a 230 drive. When they cooked the drive it did not draw enough current to pop the fuses or the breakers. So my gut feel is that the drive did not take out the motor.


But once you eliminate the impossible, what ever is left, no matter how improbable must be the answer. ( Yeah, I don't remember who I stole that from
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Old October 25th, 2018, 03:44 PM   #11
GaryS
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You are right I don't think the drive failure hit the motor the drive would have t turn on for that. It didn't have enough time before it went.
you are calculating the motor current wrong volts / resistance is for DC AC motor use the reactance of the motor.
I don't think the motor is a PM type other wise the label would state that.
as far as costing when it stops at low (stopping ) the load would be very little, with a pump you don't see much load until you reach 30% of base speed.
the label dos not state the motor base rpm's or the number of motor poles so we really don't know what the speed is.
Try setting the drive if it's possible base frequency of 120 hz and baase voltage of 150 volts. that should give you linear v/f you should set the current limit on the drive to max 55 amps that should get it running and keep it from tripping it would also allow you to see at what frequency the drive will reach

if you can't set the base frequency to 120 set it to 60 with the base voltage at 75 set max frequency to 120 it would give you same results
from the label it looks like the pump is designed to run from a VFD certainly 3Ph not single phase as was subjected. and it was made for use in the US most of the rest of the world uses 50hz standard
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Old October 25th, 2018, 05:25 PM   #12
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GaryS,


As far as reactance vs dc resistance.

At a few hertz they are close enough for rough guess. The readings from the drive bear that out. When I apply 20 VAC to the motor at 20 hz it draws about 40 amps. If anything, it should draw less because current because of the reactance should go up with freq ( to a point anyway)


That's the part that has me confused.


It is acting like there are no windings. Even a locked rotor would show some reactance at 20hz. This thing is pretty much looking like a .5 ohm resistor.


I have fiddled with most of what you suggested. I can see the end of one of the screws through the oil site glass so that's how we know the speed (strobeoscope)
There is nothing it indicate that the motor is mechanically overloaded.
It is just that no matter what it just wants to draw huge current.



This is the third supposedly good motor that I have run across this behavior. The other two were 3hp motors on a mill. I just put an off the shelf motor in the mill and forgot about it.
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Old October 25th, 2018, 07:19 PM   #13
Mike Nash
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If you haven't already, I suggest using V/Hz (P2.11=0) rather than sensorless since this motor is not your typical ratings.

I would not let it Auto Tune (P0.05=0), at least until you can get it close to runnable.

V/Hz should be Custom (P2.00=3) and drop the voltages (but not frequencies) to a ratio of 75/230 of the default values as a starting point. For example P2.09 default of 7V set to 7*75/230=2.3V.

If this gets you close but you want more speed, double the voltage and frequency settings for anything to do with that V/Hz curve from what you have at 75Hz.

If this doesn't get you any closer, then I've got nothing. I do suspect you may have too much voltage for the frequency when it is starting, which may put you into stall prevention and you just get stuck there.
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Old October 25th, 2018, 10:37 PM   #14
GaryS
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Strange problem
it would be interesting to see what happens at low speed
set the VFD up with normal v/f 120hz 150 volts
then run it at 7 hz only don't take the frequency above that and see how it runs. that speed should be low enough to keep the volume down so the load should stay down.

Are you actually measuring the output current to the motor I know you said that net sum of the output current is 0 but did you actual measure each motor lead sometimes the sensors in the vfd don't work that well. also a standard clamp on amp meter will not work you need one designed to work with the vfd output wave form.
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Old October 26th, 2018, 04:54 AM   #15
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Yes, I think an A1000 would be a great choice.


And, as long as the main board did not fail on the old F7 VFD. It is easy to hook it up and get the parameters out of it.


Do you know what died on the old VFD that started this whole thing? Was an overload fault or a CPU fault?


Even if you can not get the old parameter settings. The A1000 would have enough adjustment to work with this oddball motor.
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