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Too high current
2

Too high current

Too high current

(OP)
Hi everybody
No-load & full-load currents of a new motor are too high. What can be the reason?
Motor is: 7.5kW, 50Hz, 4 pole, full load current as per nameplate is 15A, but it is actually 24A. Measured no-load current is 10A. Motor is new. I have checked the terminal box and certainly the power cable is connected correctly.
What may be wrong?

RE: Too high current

Supply voltage too low? Load too high?

RE: Too high current

more possibilities
supply voltage too high
supply voltage unbalanced (should show as unbalanced currents)
by the way you didn't tell us the nameplate voltage

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

RE: Too high current

Supply voltage could also be too high.
Wrong V/Hz ratio set in the VFD if you're using one.

RE: Too high current

New equipment or Green site install?
Ask the mechanical engineer what size HP they selected for this part of equipment. Might be wrong size motor?
can you disconnect the mechanical load? See what the motor runs at no load without load.
Could be mechanical binding?

RE: Too high current

You have it wired for the higher voltage but you're delivering to it the lower voltage.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Too high current

Hmmm...wouldn't wiring it that way tend to lead to lower current? I can see how the reverse would...then again, maybe I should review motor theory a bit more...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Too high current

(OP)
Thank you all.
The motor and the plant are new. We are at the commissioning stage.

Nameplate voltage is 380v. But supply voltage is 400v.

Wiring is done in Y mode. As specified on the nameplate. The motor is not connected to vfd.

I will check voltage for unbalanced conditions. But currents on three phases seem to be approximately equal.

The problem cannot be due to the mechanical load. Please note that both no-load and full-load currents are high.

IMPORTANT: we have two motors which are exactly the same and are from the same manufacturer. Both of them have the same problem.

RE: Too high current

It could go either way CR.
As the voltage drops, the torque drops. The motor slides further down the torque curve. That causes it to draw more current to try to maintain the HP demanded by the load.
It depends on the actual load which we don't know. The motor may draw less current, it may draw more current, it may stall.
I favour the suggestion of unbalanced phase voltages.
Voltage? from the kW, 50 Hz and rated Amps I'll guess 400 Volts.

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

RE: Too high current

The no-load current is higher due to the higher voltage.

The load current is higher due to the load. Your motor is mismatched for whatever load it is connected to.

Muthu
www.edison.co.in

RE: Too high current

You probably have the answer, edison123.
8 or 9 Amps no load current is reasonable for a 7.5 kW motor on 380 volts. At 400 volts, 10 Amps is not unreasonable.

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

RE: Too high current

(OP)
Dear waross. I believe that since FLA is 15A, then no load current should be about 5A. By measurements it is twice !
The motor is driving a small fan.

RE: Too high current

farale,

It's not uncommon to see open shaft currents greater than 50% of FLC with a 4-pole motor. With higher pole-counts the open shaft current can be much higher. Don't forget that the high current with no load on the motor is almost all reactive.

RE: Too high current

Muthu said it.
Same motor can be used for 380V, 400V or 415V system. The ratings - kW, FLC etc. corresponding to the applicable system voltage should have been on the name plate.
You may have a data sheet which lists the motor rating at 400V applied voltage, as part of project documents.

RE: Too high current

farale

1/3rd no-load current thumb rule is not applicable in all cases, especially when you are exceeding the rated voltage. If it is a new motor, what does the factory test report say about the no-load current?

Define 'small fan'. Clearly, yours is overloading the motor. Fans' power requirements change hugely when they are not run at their design speed. You might want to check with your fan supplier as well.

As Scotty says, the no-load current is almost reactive and is not a big deal (except it would lead to a lower motor pf requiring a higher pfc capacitor, if you use such PF correction). I have seen motors with 90% no-load current and still not exceed rated current on load.

Your load current is the one you should worry about.

Muthu
www.edison.co.in

RE: Too high current

I remember this same issue happens to me somewhat per green site. Mechanicals size the motor too small for the load and most time end up going to next hp size motor.

Problem is if they do this too much in an area, it screws up your panel main feed FLA calcs and all that nonsense.

1 or 2 motors sized up are usually ok per main panel area.

RE: Too high current

My favorite chart when it comes to understanding that votlage variation in either direction can cause excessive current...


(Better version than originally posted)


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

RE: Too high current

Jeff

I don't get the +5 to -15 PF change with voltage.

Muthu
www.edison.co.in

RE: Too high current

itsmoked said it earlier, but I tend to agree.
It sounds like the motor should really be connected in delta but is in star (even if its shown on the nameplate).
The consideration that 26A is 15A * √3 tends to support it, but it might just be a coincidence.

EDMS Australia

RE: Too high current

Freddy - No, wrong advice. I doubt whether even six leads of the motor are brought out.

Muthu
www.edison.co.in

RE: Too high current

Jeff; Fun chart, but dang if it isn't missing what we're talking about! Needs one more curve that shows UNLOADED AMPS with voltage.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Too high current

If that is a centrifugal fan, restrict the discharge of the fan with a baffle plate. Been there, done that, when someone miscalculated the back pressure of a large fan.
While this may look wasteful. it is not. When you get the FLA down to the nameplate value, the motor will be doing the rated amount of work; it will be delivering the same volume of air at the same back pressure as if the correct fan had been fitted.

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

RE: Too high current

Quote:

I don't get the +5 to -15 PF change with voltage.
I guess they're saying that a motor with say pf 0.9 at rated voltage might go from pf 0.95 at 85% voltage to pf 0.75 pf at 115% voltage.

As we increase voltage, the reactive current component associated with magnetizing branch of equivalent circuit increases (just like the no-load current increases) and the real current component decreases, so the reactive current becomes a larger portion of the total current and the power factor goes down.


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

RE: Too high current

(OP)
Thank you all for such great posts.
One thing to add: six leads of the stator wires are accessible. So it is possible to verify wirings (star / delta). It is connected in star mode. If I connect it in delta mode, certainly the current will increase.

RE: Too high current

You're sure your supply is 50Hz, not 60HZ, right?

RE: Too high current

edison123, fair enough. I was thinking perhaps at that frame size it was a 380/660V motor.

EDMS Australia

RE: Too high current

The rated no-load current is already 50% of the load current, so 10 A at a higher voltage is not surprising.

What is surprising however is that such a detailed nameplate did not mention the motor connection anywhere.

Muthu
www.edison.co.in

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