Excessive current at motor startup
Excessive current at motor startup
(OP)
Hopefully this will be a simple one for you electrical guys:
I have a 4-pole 3.0kW motor, 380V/3-ph/50Hz, whose data sheet shows D (delta) connection and starting current of 40 Amps. The Customer reports that when they started the motor, it pulled 70 Amps which consequently caused the motor trip out on over-current.
I noticed that 70 divided by 40 is 1.75, which looks close to 1.732 (square root of 3). Then I looked at the catalog for the motor and found that the rated current for this motor in 400V Delta connection is a factor of approx. 1.73 higher than the same motor in 690V Wye connection. Also, 690 divided by 400 is 1.73.
I'm not drawing any conclusions, just providing some observations. Can anyone explain what could be causing our motor to draw approx. 1.75x its rated starting current at start-up?
I have a 4-pole 3.0kW motor, 380V/3-ph/50Hz, whose data sheet shows D (delta) connection and starting current of 40 Amps. The Customer reports that when they started the motor, it pulled 70 Amps which consequently caused the motor trip out on over-current.
I noticed that 70 divided by 40 is 1.75, which looks close to 1.732 (square root of 3). Then I looked at the catalog for the motor and found that the rated current for this motor in 400V Delta connection is a factor of approx. 1.73 higher than the same motor in 690V Wye connection. Also, 690 divided by 400 is 1.73.
I'm not drawing any conclusions, just providing some observations. Can anyone explain what could be causing our motor to draw approx. 1.75x its rated starting current at start-up?





RE: Excessive current at motor startup
xnuke
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RE: Excessive current at motor startup
RE: Excessive current at motor startup
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Excessive current at motor startup
RE: Excessive current at motor startup
Otherwise sry110, I would agree that you should not see greater than LRA. Meaning the supply is too high a voltage, the frequency is too low, or there is a wiring error. Possibly the LRA is specified for a wye connection and somehow the motor is wired delta.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Excessive current at motor startup
RE: Excessive current at motor startup
Have you confirmed the 'customers' reading of this transient event is correct?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Excessive current at motor startup
The story they gave us is this: the Customer used the 40A locked rotor current from the motor data sheet to set the current limit on their motor starter. When they started the motor they saw 70A which tripped the motor due to high current. The Customer concluded that we need to supply them a larger motor (4kW instead of the existing 3kW) so that the motor can start the load turning without encountering the high current condition.
It doesn't make sense to me. Under normal conditions, the highest current a motor can draw is its locked rotor current. Putting a higher power motor in place of the existing one will result in higher locked rotor current than the existing motor. Am I correct?
RE: Excessive current at motor startup
Did you review your load torque versus motor torque capacity? (motor torque capacity greater than the load torque). You may just be needing a change in drive sheeve diameter, imo.
RE: Excessive current at motor startup
and the start current to rated ratio it has to be 7.The voltage drop for such a small motor could be neglected.
So, the start current at 380 V/50 Hz has to be 7*7=49 A.
The load has no any influence at start. The induction motor behaves at start like a [static] transformer.
In my opinion, the actual voltage could be 420 V [400 V it is now standard in Europe +5% permissible]
and the frequency may be 48 Hz.
The new start current at 420V has to be 49*420/380=54.15 A.
IEC 60034-1 standard states:
"Unless stated otherwise, tolerances on declared values shall be as specified in Table 20”.
Table 20 Pos. 9 "Locked rotor current of cage induction motors with any specified starting apparatus =+
20%"
New starting current could be 54.15*1.2=65 A.
Another 4% for 48 Hz then finally Ist=65*1.04=67 A.
However, it is hard to say, it could be such a chain of coincidences as described above.
Nevertheless, check the actual voltage and frequency and compare with the rating plate.
RE: Excessive current at motor startup
RE: Excessive current at motor startup
@ Parchie - I should have clarified that this is a directly coupled motor, not a belt drive. Our design calculation shows that the load torque is less than the available torque from the motor. The Customer has concluded that the actual load torque is higher than calculated and exceeding the motor available torque, and the basis of their conclusion is that the motor pulled 70 Amps instead of the 40 Amps locked rotor current shown on the data sheet. What I'm trying to figure out it is this: If I have a motor capable of producing 50 N-m locked rotor torque (and assuming that locked rotor torque is higher than breakdown torque) and it comes up against a load that requires 100 N-m to move, is the motor going to produce any more than 50 N-m? And if not, then is it going to pull any higher current than the locked rotor current advertised on the data sheet (within a reasonable tolerance)?
@ 7anoter4 - Thanks for your analysis. For what it's worth, the relevant motor data (taken from the manufacturer's data sheet) is as follows:
Rated Output Pn: 3 kW
Rated voltage UN: 380 VD +/- 5% (IEC 60034-1)
Rated frequency fN: 50 Hz +/- 2% (IEC 60034-1)
Rated current IN: 6.2 A
No Load Current: 2.8 A
Starting Current IS/IN: 6.4
The motor is started using direct-on-line starter.
@ScottyUK - Unfortunately I do not have a photo of the wiring inside the terminal box, but the motor nameplate is attached below.
RE: Excessive current at motor startup
Ossilograph tests show that at the moment of energization there may be a very high and very short transient current. This transient is so short as to be ignored by most overload protection devices. The magnitude varies and depends on a combination of the residual magnetizm and the point on wave that the voltage is applied.
This transient may be several times the normal starting current.
Some digital clamp meters had a peak lock function that would detect and report this transient rather than the normal starting surge.
How is the current being measured?
What is the load?
Is the motor back spinning when the power is applied?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Excessive current at motor startup
The 3kW motor is the driver for our turning gear assembly, and the turning gear is driving a steam turbine + compressor train. Assuming no mechanical issues with the steam turbine or compressor, the turning gear will see momentary high load as it breaks the drive train away from rest and accelerates it to steady state turning speed (in this case 20 rpm). There is a 76:1 gear ratio in the turning gear between the motor and the drive train, so the drive train inertia referred to the motor is very small and therefor the motor accelerates the load to full speed very quickly - less than 2 seconds.
The motor is sized such that its locked rotor torque exceeds the required breakaway torque of the train when referred back to the motor through our gearbox. Once the shaft train is rotating the torque to keep it turning at turning gear speed is negligible (i.e. no appreciable aerodynamic load and very low bearing friction) so the load on the turning gear motor is negligible. Under normal conditions, the turning gear motor will pull close to no-load amps when turning the train continuously.
The input shaft of the turning gear, to which the 3kW motor is directly coupled, is fitted with a unidirectional overrunning clutch (a backstop clutch) which prevents reverse rotation of our system, so it is not possible that the motor is backspinning at time of energizing.
RE: Excessive current at motor startup
In NEMA land we generally use a rule of thumb that starting current is about 6 times full load current. However the smaller the HP the larger the ratio and a 3 HP motor may have a starting current of 700%. Still only about 40 Amps.
You say that the O/L is tripping on overcurrent. If that is accurate then you may be tripping on the first cycle transient. A sustained high current will trip on overload rather than over current.
If the applied voltage is somewhat above rated voltage that may be a contributing factor.
Can you supply the make and model number of the O/L device?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Excessive current at motor startup
But if I am understanding you correctly, there may be a transient current at start-up that exceeds the locked rotor current shown on the data sheet? Would this happen at in-rush, or when the motor sees the load? I only ask because in our turning gear system there is a good amount of backlash to be taken up between when the motor starts and when it actually comes up against the load, so if I were to monitor motor amps with a very fine sampling rate I would expect to see a spike at the moment the motor is energized (in-rush), then a quick drop off because there is initially no load, then another spike when the motor comes up against the stationary load.
RE: Excessive current at motor startup
The type of OL relay is relevant as well, because many people are going to Solid State OL relays now, and some of them will react in a similar way if not programmed correctly. They shouldn't, but it happens.
Still, it COULD be that even though the current is NOT really 70A, the OL is is tripping because the motor is NOT accelerating the load fast enough. That might be a number of other issues, all being masked by the initial measurement error that took everyone down a rabbit hole of "70A starting current" which you correctly identified as impossible.
One POSSIBLE issue is related to where you started this thread with, the misconnected motor. But you were thinking the wrong direction. If the motor is SUPPOSED to be connected in Delta, and they connected it in Star, then the motor would draw LESS current than normal, but also produce far less TORQUE. Think Star-Delta starter, but never coming out of Star. What would happen is that initially the current is low, but if the motor cannot fully accelerate with that severely curtailed torque, it sits in limbo at less that Locked Rotor Current, but still more that the OL curve will allow for that period of time, so the OL trips. Then because of the 70A red herring, nobody is seeing the REAL problem.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Excessive current at motor startup
RE: Excessive current at motor startup
RE: Excessive current at motor startup
RE: Excessive current at motor startup
RE: Excessive current at motor startup
On the mechanical side, what about new mounting, reduced clearances with other equipment, greater unexpected weight, more unused torque just looking to break something.
The only mechanical suggestion I'd have to to uncouple the motor and see if the problem persists. If it goes OL by itself, it's not the load. One can also verify the breakaway torque for the rest of the machine.
The thing that stands out is the motor starter is selected by the customer. Seems like a good way to have a problem.
RE: Excessive current at motor startup
If the motor turns for a few seconds or longer before an over load trip, that is a motor, connection, voltage, over load setting or load issue.
Again, how is the start current measured.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Excessive current at motor startup