Can a motor pull more the LRC on startup?

[MrJLH]

Like the title states, can a motor pull more than LRC per the KVA Code during start up? I have a motor that keeps tripping on startup. The LRC per calculation is about 950 amps, but keeps tripping the breaker. I also did a online MCE test and measured the inrush current at about 1200A.

Some thing important to know this is a loading pump that is started at least 4 times per hour (I know this is not good, and lets not get into that)

Thanks

 

[tom baker]

If your motor is heavily loaded, and does not come up to speed, it will draw above rated current. To answer your question, yes. When a motor is at speed it generates back emf, which opposes the current, reducing current draw. Ways to re reduce inrush are SS, VFD, discharge control valve

 

[synchro]

Motors have a transient inrush current just like transformers do. The locked rotor current will continue as long as the rotor is not moving, but the inrush will die out once a back EMF is generated in the windings by the flux developed in the motor's stator. Also, the inrush current can be further increased if saturation occurs.

I just saw that Tom has mentioned the same thing.

https://www.motionindustries.com/motioncontent/knowledgehub/docs/stories/BALDOR%20-%20Manufacturing%20High%20Efficiency%20Motor%20Starting.pdf

 

[kingpb]

Like the title states, can a motor pull more than LRC per the KVA Code during start up? I have a motor that keeps tripping on startup. The LRC per calculation is about 950 amps, but keeps tripping the breaker. I also did a online MCE test and measured the inrush current at about 1200A.

Some thing important to know this is a loading pump that is started at least 4 times per hour (I know this is not good, and lets not get into that)

What is the voltage when you start? During motor start, the current is proportional to voltage so if it is higher than rated, you will see a higher measured LRC; albeit probably not the high amount you mention, unless it is higher than 10% above motor rated. Is their any type of unbalance in the voltage?

The 4 starts per hour probably means the motor is not cooling down before another start occurs thus restarting the motor before all extra heat has been dissipated to the heat already there. In that case, each subsequent start will add even more heat, raising the motor temperature until some component fails. My guess is you are starting to see failure and thus the LRA are increasing.

 

[Besoeker3]

Like the title states, can a motor pull more than LRC per the KVA Code during start up? I have a motor that keeps tripping on startup. The LRC per calculation is about 950 amps, but keeps tripping the breaker. I also did a online MCE test and measured the inrush current at about 1200A.

Some thing important to know this is a loading pump that is started at least 4 times per hour (I know this is not good, and lets not get into that)

Thanks

Locked LRC? How?

 

[Besoeker3]

Those I have experienced have been around about 5-7 times rating.

 

[MrJLH]

bly means the motor is not cooling down before another start occurs thus restarting the motor before all extra heat has been dissipated to the heat already there. In that case, each subsequent start will add even more heat, raising the motor temperature until some component fails. My guess is you are star

What is the voltage when you start? During motor start, the current is proportional to voltage so if it is higher than rated, you will see a higher measured LRC; albeit probably not the high amount you mention, unless it is higher than 10% above motor rated. Is their any type of unbalance in the voltage?

The 4 starts per hour probably means the motor is not cooling down before another start occurs thus restarting the motor before all extra heat has been dissipated to the heat already there. In that case, each subsequent start will add even more heat, raising the motor temperature until some component fails. My guess is you are starting to see failure and thus the LRA are increasing.

Motor is a 480VAC across the line starter.

The kicker is, I have 4 loading pumps in this line up, that follow the same philosophy in regards to starting method and starts per hour. The other three don't have this problem of tripping.

So far I have replaced the motor, replaced the breaker (which was an old thermal-mag), and replaced the feeder. The other motors in this pump line up, lets just say they are old.

 

[paulengr]

Motor inrush can be very high. There is a theoretical limit of 24x FLA. NEC limits breaker adjustable instantaneous to 17x. Generally high efficiency motors are very bad about this. “World” motors intended for sale in Europe are the worst. I’ve seen Toshiba and Siemens inrush hit 22xFLA.

It is generally not an issue with a thermal magnetic breaker. But electronic breakers easily trip on inrush. The solution generally is recognizing that the inrush lasts only 1-2’cycles at most, set the instantaneous delay to 1-2 cycles (16-32 milliseconds). This solves the problem. NEC does not specify maximum trip times.

 

[Besoeker3]

Motor inrush can be very high. There is a theoretical limit of 24x FLA. NEC limits breaker adjustable instantaneous to 17x. Generally high efficiency motors are very bad about this. “World” motors intended for sale in Europe are the worst. I’ve seen Toshiba and Siemens inrush hit 22xFLA.

It is generally not an issue with a thermal magnetic breaker. But electronic breakers easily trip on inrush. The solution generally is recognizing that the inrush lasts only 1-2’cycles at most, set the instantaneous delay to 1-2 cycles (16-32 milliseconds). This solves the problem. NEC does not specify maximum trip times.

But not LRC. Unless very exceptional circumstances.

 

[Besoeker3]

 

[Jraef]

I think the question is being asked BECAUSE the OP is experiencing nuisance tripping of breakers on start-up and has the breaker set for expected LRC. So it's not that the LRC itself is higher than normal, it's that there is ANOTHER component of initializing a motor that is causing the nuisance tripping.

As Pauleng said, this is a well known phenomenon of the change to "energy efficient" motors starting around 1990. Motor mfrs had to comply to new efficiency standards and did so in a variety of ways. A couple of those design changes were to take steps to reduce the resistance of the magnet wire and lower the air gap. It worked to reduce losses in the motor, it ALSO created a side problem of significantly higher MAGNETIZING inrush current; the inrush that takes place AS the magnetic fields are established, which takes just a little under 1 cycle (depending on where the sine wave was when the contacts close) and as said, can now be over 20x the motor FLA for that brief brief period. In the past, the transient was not as high and was so short in duration that MCP breaker magnetic trips would not have time to react to it. What has happened now is that the amplitude is so much higher than the effect on the mag trip elements lingers longer and occasionally trips at settings previously thought to be good.

This is why the NEC added the extension from 13x FLC as the maximum setting, to 17x FLC (if you can prove that lower settings don't work). In reality even that sometimes is not enough. We have found that changing to a THERMAL-MAG breaker that has the correct level of Magnetic adjustments tends to result in more reliable holding. The theory is that the thermal element in the breaker is adding a little more resistance to the circuit overall, and that retards the rate of rise of the magnetizing inrush just enough to not trip the breaker. So far when I have had to do that, it has always worked.

 

[kwired]

Basically LRC is what it draws when full voltage is applied, magnetic fields are established but the rotor is not turning.

When first applying power in across the line starting methods, there is no magnetic fields established yet and the motor acts like a short circuit for a short time more so than a real load. Your short circuit/ground fault protection needs to be able to handle that for a few cycles without tripping.

Things like circuit resistance and source impedance will leave same motor having a higher instantaneous current in one location but less in another. That motor may trip breaker often if there isn't much conductor between it and the source, but same motor and breaker on far end of the plant never trips - because of the resistance in conductors on the way to that motor limits that inrush enough that it don't get into the instantaneous trip range of the breaker.

 

[junkhound]

A couple of those design changes were to take steps to reduce the resistance of the magnet wire and lower the air gap. It worked to reduce losses in the motor, it ALSO created a side problem of significantly higher MAGNETIZING inrush current; the inrush that takes place AS the magnetic fields are established, which takes just a little under 1 cycle (depending on where the sine wave was when the contacts close)

And for the designs where there are short term temperature and supply voltage excursions at the design limits, AND the starting contactors clove very close to voltage zero crossing, then short term saturation produces even higher current spikes.

Plot is an example of hypothetical motor where contacts close right at voltage zero crossing vs. say peak voltage. A verbal descriptin would be that since core flux is proportional to integral of volt-seconds, if the full first half cycle has maximum volt-seconds a marginally designed motor (to maximize flux during normal operation) may touch saturation. If contact close and later than zero crossing, volt-seconds is lower and saturation does not occur and no voltage spikes.

Red trace is closing at voltage peak, blue trace at zero voltage crossing.
Sorry about the plot, cannot get PSpice to paste here in other than black background
X-axis is time, blue peak is 500 A (middle curve) decay to 20A, red peaks are 20 A, contactor close at voltage peak.
Motor core set for 1.8 Tesla saturation, LRA = 18A ; Max RMS of first 5 cycles with the 500 A spike = 14X LRA in this example.

 

[winnie]

Kwired gave the essence of LRC vs inrush. LRC is a steady state operating condition with the rotor locked and magnetic flux waveform established in the steel core of the motor.

Question for you guys who work on big motors: could a resistive pre-charge work here, where the motor is first connected to the line via resistors, then the resistors get bypassed?

I guess the problem is that there is no way to 'disconnect the load' the way one might with a transformer, so this pre-charge would need to pass lots of current, at which point you simply have a 'soft start'... which might be an option here.

Jon

 

[kwired]

Kwired gave the essence of LRC vs inrush. LRC is a steady state operating condition with the rotor locked and magnetic flux waveform established in the steel core of the motor.

Question for you guys who work on big motors: could a resistive pre-charge work here, where the motor is first connected to the line via resistors, then the resistors get bypassed?

I guess the problem is that there is no way to 'disconnect the load' the way one might with a transformer, so this pre-charge would need to pass lots of current, at which point you simply have a 'soft start'... which might be an option here.

Jon

Isn't that basically just one more possible reduced voltage starting method? Maybe more primitive than solid state soft starters, but probably somewhat same effectiveness as part winding, wye-delta or autotransformer starting methods. Guessing those other options had more advantages is why they were more common.

 

[paulengr]

Kwired gave the essence of LRC vs inrush. LRC is a steady state operating condition with the rotor locked and magnetic flux waveform established in the steel core of the motor.

Question for you guys who work on big motors: could a resistive pre-charge work here, where the motor is first connected to the line via resistors, then the resistors get bypassed?

I guess the problem is that there is no way to 'disconnect the load' the way one might with a transformer, so this pre-charge would need to pass lots of current, at which point you simply have a 'soft start'... which might be an option here.

Jon

I think of “large” as over 1000 HP. Generally speaking losses over 200-300 HP are so small to begin with that efficiency standards don’t go there. With rare exceptions we don’t get really high inrush and even LRC tends to be more in the 5-5.5x range. You’ve got to realize that often large motors have just 1 or 2 turns on each coil and use multiple parallel flat wire formed coils. There just isn’t much of a loss to start with.

As an example of a “severe” starting motor we are dealing right now with a Hyundai double cage 600 HP 460 V motor. It is a “crusher” motor. This design is going for maximum starting torque without going to a wound rotor design. It has an LRC of 8xFLA but a starting torque of 220% that only falls off to about 180-200% at the minimum. There is even an expeller fan on the end to draw air through the inner rotor. Still despite this inrush is still fairly mild although the owner starts them on a soft start.

Resistance starting is an early/small motor technique. You just have too many heat problems to scale it up very large. It was later replaced with reactance starting. Eventually wye delta and auto transformer starting displaced these before we got into the electronic age. Both techniques don’t generate very much heat. Auto transformer starting is more flexible. Very hard starting was done with wound rotor motors. A wound rotor can easily hit 275% starting torque with only 200-300% starting current. It was Tesla’s original variable torque motor. It is still used today especially over 1000 HP. Solid state soft starters are much more flexible, run cooler, and cheap. All of these reduced voltage starters (except wound rotors) also reduce starting torque. But if you don’t need to exceed name plate torque or better yet have a centrifugal load electronic soft starters are cheaper and more reliable than VFDs.

 

[winnie]

Yea, I posted that too early in the morning.

For dealing with _transformer_ inrush, if the secondary is disconnected and you are only supplying magnetizing current, a very small 'resistive pre-charge' can work to mitigate inrush current. But for motors you can't 'disconnect the secondary', so any sort of starter must supply both the magnetizing inrush and the load.

-Jon

 

[kwired]

Yea, I posted that too early in the morning.

For dealing with _transformer_ inrush, if the secondary is disconnected and you are only supplying magnetizing current, a very small 'resistive pre-charge' can work to mitigate inrush current. But for motors you can't 'disconnect the secondary', so any sort of starter must supply both the magnetizing inrush and the load.

-Jon

And load characteristics may determine what you can or can't get away with easiliy.

 

[paulengr]

Yea, I posted that too early in the morning.

For dealing with _transformer_ inrush, if the secondary is disconnected and you are only supplying magnetizing current, a very small 'resistive pre-charge' can work to mitigate inrush current. But for motors you can't 'disconnect the secondary', so any sort of starter must supply both the magnetizing inrush and the load.

-Jon

Sure you can. That’s what a wound rotor motor is all about. A motor is basically a rotating transformer. So if we put a “big” resistor on the rotor the starting curve is shifted all the way to the left and we get peak torque at zero RPM. Then by gradually removing all the resistance we accelerate at high torque/low current up to normal full speed. Try to visit a shredder or a chip mill. They use those all the time.

 

[winnie]

Changing rotor resistance still leaves 'inrush' current to deal with... but I am assuming that a wound rotor machine is not one of the 'high efficiency' motors that has problems with extra severe inrush current.....

-Jon