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LAN_403

HV Enthusiast

Wed Apr 17 2013, 05:06PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Hello,

I've recently purchased several large 3-phase motors, and while doing testing, I'm find that the inrush current for these motors is about 180A for a duration of 100ms before settling down to about 60A which is the advertised datasheet inrush (lock rotor current) number.

I'm not an expert with motors, but is there typically an additional short term inrush (perhaps magnetizing current) for these motors thats above and beyond the typical inrush numbers which are advertised on the datasheet?

These are 440VAC, three phase motors and full load current is approx. 8A per phase, and locked rotor current of about 60A and advertised inrush of 63A.

But for every motor i've tested, i'm getting 180A PER PHASE for about 100ms on start-up. This current also occurs if the motor is just depowered for a second and is still rotating at a high speed.

Any ideas?

Thanks

BigBad

Wed Apr 17 2013, 10:36PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

I speculate that it might be thermally limiting; the coils get hot, and then resistively limit. You would get that kind of behaviour.

It wouldn't matter in a grid-powered situation, but if you're driving it electronically it could be problematic.

doctor electrons

Wed Apr 17 2013, 10:55PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

What is the horsepower of the motors? With that i can probably answer all of the questions. i Assume they are not
squirrel cage type motors. The reason you still see the current for a short time after killing power is that you effectively
have a generator at that point. The magnetizing current of your motors is typically less than 25% of their full load current.
So it should be less than 2 amps / phase. If the motor is starting under heavy load the inrush will usually be higher than the advertised (no load)
inrush. In industry, VFD's are usually used to control 3 phase motors, in addition line reactors are used to keep unwanted noise from
feeding back into the line. VFD's use pulse width modulation and dc currents to accurately control the motor.

klugesmith

Wed Apr 17 2013, 11:09PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Good mystery.
It's hard to imagine conductor temperature accounting for more than 50% increase in cold start current, with respect to hot start current.
And I never heard of three-phase motors with start windings or starter switches, centrifugal or thermal.

Can you reproduce the inordinate inrush current behavior in a single-phase induction motor?
I bet they are found more frequently than three-phase induction motors in the labs of 4hv-ers.

Might the cause be like the occasional 10x inrush current in power transformers,
which depend on inductance rather than back EMF to limit the current?
Zero voltage is the -worst- phase to close the switch.
To inductively buck the first half-cycle of voltage, we would need the normal peak-to-peak flux swing.
Often impossible if flux starts at zero.

With little personal experience with three-phase motors, I wonder how much skew there is between three sets of switch contacts. How are you measuring the inrush currents? Can you record all three phase currents in a single start event?

Avalanche

Wed Apr 17 2013, 11:26PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Is the datasheet figure for inrush current based on 'star' rather than 'delta' configuration?

Induction motors in kit I've dealt with have a contactor to start the motor in star then switch to delta. As you're inrush current is about 3 times the rated, that implies 3 times the torque - which I think is also the difference in maximum torque between the star and delta configurations...

klugesmith

Thu Apr 18 2013, 12:03AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

New thing I learned today: Star Delta Starters. Thanks, Avalanche!
One reference said they are being phased out (no pun intended) in favor of electronic soft starters.

Found this amusing exchange on a 3 phase forum thread.
Q:

HV Enthusiast

Thu Apr 18 2013, 01:28AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

The motor is a WEG W22 7.5hp motor.

It is wired for DELTA and powered with a DELTA 440VAC source. I wonder if this makes a difference vs. a star (wye) wired motor. As a delta wired motor as the full 440VAC voltage across each of the three windings whearas in a wye configuration, the voltage is distributed across two adjacent windings. And perhaps the advertised datasheet inrush current is actually for a wye motor.

2Spoons

Thu Apr 18 2013, 02:40AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

It has more to do with phase relationships in an R/L circuit. Normally if you drive an R/L circuit with a sine wave, and then plot V against I you get an ellipse centered on zero (its a circle for zero resistance), as V and I are out of phase. This is the "steady state" situation.
On initial power up something else happens - at the start V and I are actually in phase, even though both are zero. Don't believe me? Ask yourself what the current is when V is zero in the steady state situation I first described - its definitely not zero!
So somehow there is a transition from the initial state to the steady state, and this is where the inrush current surge comes from.
It is most easily seen by plotting V against I for the initial few cycles. You can see the phasor plot shifted to one side, giving an initial current peak up to double the steady state value - see attached plot derived from a SPICE simulation.

It gets worse. In order to save weight and cost most motors and transformers have just enough iron to avoid saturation when running. The initial surge can easily saturate the iron, leading to a current peak several times that of the normal running condition.

Simulation of initial excitation of an R/L circuit :

It took me a while to get my head around this phenomenon when it was first explained to me.

Thinking about this some more this would be one reason why you'd start a motor in wye then switch to delta - as has been said, in wye connection the windings see a lower voltage, which would result in a lower inrush.

Steve Conner

Thu Apr 18 2013, 09:24AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What 2spoons said, the locked rotor current is a steady-state value. There is no back-EMF from a spinning rotor, but the magnetic circuit has got over its inrush and settled to a steady state.

I think of the initial transient as a "DC offset" because that's what it looks like when you plot current vs. time.

This source suggests an inrush current of >20x full load for the first half cycle, then 4-8x full load for several seconds until the rotor gets up to speed. That agrees with your measurements.

The advertised inrush current may have had some sort of marketing factor applied to it. Maybe they just mean that it won't pop a 63A Type-D breaker.

doctor electrons

Thu Apr 18 2013, 09:53PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Though the above answers are correct this doesn't happen very often. I talked to the resident genius
engineer i work with and he believes you may have something else wrong. Can you post a snapshot of the
nameplate on the motor? There are a few different connection options with that motor. Low and high, the genius informed
me that if you feed the motor high voltage when it is wired for low or vice versa you will get exactly what you are seeing.
He also said that there isn't a soft start or overload that could handle that type of inrush. You may be seeing the locked rotor current when the motor settles out because you have two windings wired so their fields are opposing. With no load on that motor you should not have a normal running current of 60 amps.
here is a typical wiring scheme for a 460vac/480vac motor.

Tie together, 4&7 , 5&8 , 6&9
Connect the lines, L1 to 1 , L2 to 2 , and L3 to 3

This is for high voltage connection, not low. Also for WYE

How many leads are coming out of the box, aka "peckerhead"??
You may have it wired for delta when it should be wye or the opposite.

If it is rated 460 and you give it 440 you will also see a bit more current.

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