Flying start on 3phase induction motor 4

[GGPIE]

Someone has told me that when a 3phase induction motor trips but continues to spin for some period (e.g. 60 seconds) due to a high inertia load and then power is reapplied while the motor is still spinning then a massive inrush current will occur. Can anyone back-up this statement and/or explain why this phenomenon ocurs?

 

[busbar]

In most cases, 60-second reclose would be of no significant concern.

Reference: thread237-11584 thread237-9710 thread237-14629 thread237-20953 thread237-11584

 

[RAMConsult]

It depends upon what your friend defines as massive. As the threads suggested by busbar indicate, there is no problem after an induction motor has been de-energized for more than 10 cycles; however, if the high inertia load has coasted down significantly and is re-energized there will be an inrush current that is some multiple of normal running current but less than locked rotor amps while the load is accelerating back to normal speed. As one of the threads points out, this can be a problem if there are many other large motors that are also simultaneously re-energized. Depending upon how robust your supply is there may be a significant voltage drop on the bus that causes high current due to significantly longer acceleration times, it may even be possible for the motor to stall at some subsynchronous speed.

All bets are off if this is a synchronous motor and the field is still energized. The motor is then acting as a generator and will attempt to transfer all the remaining kinetic energy in the load back to the system unless the speed, voltage and phasing are exactly matched.

 

[don01]

ggpie
It's possible your contact is refering to either:-

a) changing direction while the motor is running down. if you try to reverse the direction while the motor has an inertia then your start or inrush current is going to be heaps higher than planned. This is why zero speed switches or time delays are often fitted to direction changer ccts.

b) too many starts per hour. when you start the motor you draw a high current and heat up the motor do it too often then you burn out the motor.

sorry if all that sounds like I'm teaching my grandmother how to suck eggs but I hope it helps

Don

 

[jbartos]

Suggestion: Reference:
IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power System Analysis, (Brown Book).
Chapter 9 Motor-Starting Studies
When the induction motor loses power supply, it will keep on spinning for some period of time depending on the load torque-speed curve. It will generate terminal voltage that opposes the power supply voltage when applied. This will create large inrush current if the voltage generated by the spinning motor is still high. Also, t x I^2 motor curve should be taken into consideration for motors with large inertia to prevent them from overheating and thermal damages.

 

[RAMConsult]

jb, please provide the specific documentation from the Brown Book that says that the voltage will still be high enough to cause damage after 60 seconds as the original poster questioned.

 

[GGPIE]

My aploigies. My original question was not written clearly. I am actually interested in the starting current if the power is reapplied before the motor actually stops spinning. Thank-you to all who have responded.

 

[electricpete]

I think it was understood by all that the high inertia load has not stopped spinning after 60 seconds, otherwise certainly no reason to ask any question about higher inrush upon reenergization.

There is a residual voltage generated while the rotor spins. That residual voltage can increase the inrush current.

Residual voltage is proportional to the product of flux times speed.

Speed in this case decays very slowly.
However (with exception of effects of very small remnant magnetism), the flux should decay away in accordance with the motor "open-circuit ac time constant" which is defined in NEMA MG-1.Section 1.60.1 - as
= (Xm+X2)/(2pi*f*r2) where Xm, X2, r2 are equivalent circuit parameters (per-phase), f =power frequency.

If you looked at equivalent circuit with supply branch open-circuited, this is the L/R of the curent circulating through the magnetizing branch and rotor branch.

Rough numbers for open circuit time constant are in the neighborhood of 6 cycles. Generally we want at least 1.5 open circuit time constants (~ 10 cycles) to the flux to decay to 33% or less of rated.

Flux decays even faster than than voltage due to speed decay. But even ignoring speed decay (taking credit only for flux decay) after 1 second the voltage should be far less than 1% of rated.

All of the above seems in accordance with NEMA MG-1 which relies very heavily on the open circuit time constant to determine whether residual voltage has decayed far enough. I have heard several people on this board tell me they can measure substantial voltage if they use induction motor as a generator... drive the shaft by a combustion engine and measure voltage at the terminals... with no capacitors connected to the machine! They argue that this would suggest that effects of residual magnetism may be too great to be ignored. But I suspect if any load were applied to that machine the voltage would immediately disappear. And therefore would not contribute to any increase in inrush current.

In summary, I agree with Ramconsult.

 

[GordS]

The flux generated by a motor upon disconnection can be greater than 10 cycles.A paper entitled "Automated motor Bus Transfer Theory and Application" measured the voltage on a high inertia 6000 HP induction motor running at 25% load before being disconnected - it took 60 cycles to drop to 40% magnitude and for the phase to change by 340 degrees. A 960 HP induction motor on a pump took 60 cycles to drop to 5% magnitude; the phase changed by 360 degrees in about 8 cycles.

 

[mindspan]

Rather than get all scientific and throw a bunch of formulas around, think of it this way...

Once power is removed that motor is now a generator.

That generator is NOT going to be synchronized with the power system.

When power is reapplied while the motor(now generator) is still spinning you are essentially crashing the power supply field into the field created by the motor.

The motor current now tries to overcome the out of sync voltages AND the coupled load.

Not only do you get off the chart currents but you get bizarre physical forces at the motor because it is confused.

I have witnessed this (by accident of course) for a 4160V, 2000HP pump. We updated a PLC program that required a momentary shutdown. The time-delay relays shown in the motor schematics had been bypassed. We cut power to the pump and five seconds later power was reapplied because the motor starter had been comprimised. It shook the whole building as the motor 'realigned' its generating field with the power supply field. It scared the everliving $*&% out of me.

All bets are off unless the motor starts from zero.

 

[electricpete]

One thing I should have highlighted more is the role of caps. If caps remain connected to the motor they will sustain the voltage for a much longer period. I would be interested to know if Gord and mindspan had caps attached (also mindspan was it an induciton motor?)

If no caps, the NEMA standard seems to makes it clear you should have no transient concerns whatsoever for closing after several seconds. Don't take my word for it. The following quote from NEMA MG-1 Section 20 (Large Induction Motors)

"20.33 BUS TRANSFER OR RECLOSING
Induction machines are inherently capable of developing transient current and torque considerably in
excess of rated current and torque when exposed to an out-of-phase bus transfer or momentary voltage
interruption and reclosing on the same power supply. The magnitude of this transient torque may range
from 2 to 20 times rated torque and is a function of the machine, operating conditions, switching time,
rotating system inertias and torsional spring constants, number of motors on the bus, etc.
20.33.1 Slow Transfer or Reclosing
A slow transfer or reclosing is defined as one in which the length of time between disconnection of the
motor from the power supply and reclosing onto the same or another power supply is equal to or greater
than one and a half motor open-circuit alternating-current time constants (see 1.60).

It is recommended that slow transfer or reclosing be used so as to limit the possibility of damaging the motor or driven (or driving) equipment or both. This time delay permits a sufficient decay in rotor flux linkages so that the transient current and torque associated with the bus transfer or reclosing will remain within acceptable levels. When several motors are involved, the time delay should be based on one and a half times the longest open-circuit time constant of any motor on the system being transferred or reclosed."

 

[jbartos]

Comment on RAMConsult (Electrical) Dec 29, 2003 marked ///\\jb, please provide the specific documentation from the Brown Book that says that the voltage will still be high enough to cause damage after 60 seconds as the original poster questioned.
///The original posting does not state specific values of parameters to provide such answer. However, the cited reference has more than enough theoretical and practical background on the motor starting and transients to be able to calculate or graphically solve the problem.
There is also Chapter 11 Switching Transient Studies.
As examples, capacitors and transformer switched transient are presented. However, the motors were not included. Maybe because the transformer is a special case of induction motor and somewhat simpler to use as an example. There is definitely the better background for the motor analysis and software references than NEMA MG-1 offers. The rotating induction motor when turned off becomes an asynchronous generator. If unloaded, the terminal voltage will be dependent on the motor rotating dynamic energy, which will be discharged into a power supply when the motor starter, breaker, and switch become turned on. Certain type of motor-load sets may be accelerating for tens of seconds, e.g. 20 seconds. Such set will have very high terminal voltage, when turned off, for many seconds. \\

 

[electricpete]

I looked at one of those linked threads and now I remember we have one Westinghouse 7000hp 3600RPM induction motor with ~ 2.5 sec open circuit time constant listed on the OEM motor data sheet. That particular motor was purchased from OEM with term box including surge caps. I don't know if those caps are big enough to affect the time constant. Also as was mentioned I believe high-speed, high horsepower motor is expected to have the highest time constant. For that particular motor would require <1.5*2.5 ~ 4 sec off-time before reclosing to get to 33% remaining voltage to avoid possible damaging transient torques. For smaller/slower motors and without caps, minimum required off-time to prevent damaging transients would be lower.

The 6 cycle number I believe was intended to be typical of motors <200hp with no caps.

 

[RAMConsult]

jb, sorry, the original poster provided enough information by stating 'induction motor trips' and '60 seconds'. Based upon these criteria and the input from other posters, there is no analysis that will support your contention.

The main reason that Chapter 11 does not include motors has to do with the fact that capacitors and transformers are passive devices having no energy storage beyond a rapidly decaying electrostatic or electromagnetic field whereas a motor is an active device with energy stored in the form of a rotating mass. Furthermore, the terminal voltage of an unloaded induction motor is dependent upon the rotor speed and the magnetic flux in the air gap, NOT the motor rotating dynamic energy as you state. Again, all bets are off if there are capacitors on the motor terminals.

 

[jbartos]

Comment on the previous posting marked ///\\RAMConsult (Electrical) Dec 30, 2003
jb, sorry, the original poster provided enough information by stating 'induction motor trips' and '60 seconds'.
///I did not consider the following, posted in the original posting, sufficient:
&quot;&quot;&quot;...when a 3phase induction motor trips but continues to spin for some period (e.g. 60 seconds) due to a high inertia load and then power is reapplied while the motor is still spinning then a massive inrush current will occur.&quot;&quot;&quot;
I had a problem with &quot;&quot;&quot;...and then power is reapplied while the motor is still spinning then a massive inrush current will occur.&quot;&quot;&quot; since the power may be reapplied at any instant within e.g. 60seconds. This instant is important to address since it may be at the beginning when the electromotive force is high or at the end when the electromotive force is negligible (very slow spinning).\\
Based upon these criteria and the input from other posters, there is no analysis that will support your contention.
///I do not see it this way. In fact, there is supposed to be analysis and software for literary everything. Obviously, not everything is in an explicit form.\\The main reason that Chapter 11 does not include motors has to do with the fact that capacitors and transformers are passive devices having no energy storage beyond a rapidly decaying electrostatic or electromagnetic field whereas a motor is an active device with energy stored in the form of a rotating mass.
///I disagree, since for example SMET is designed for energy storage. It can be design to store very large inductive energy. Visit

http://www.itee.uq.edu.au/~aupec/aupec01/122_CAUSLEY_AUPEC01paper.pdf

http://www.ifp.fzk.de/hotline/Hottline/jul00/hot_jul00sec.PDF

etc. for more info on SMES.\\
Furthermore, the terminal voltage of an unloaded induction motor is dependent upon the rotor speed and the magnetic flux in the air gap, NOT the motor rotating dynamic energy as you state.
///Disagree. If the machine comes to standstill, then the dynamic energy will be zero and there will be zero voltage (if the capacitive charge voltage is neglected).\\
Again, all bets are off if there are capacitors on the motor terminals.
///Good to know. However, this feature is not mentioned in the original posting.\\\