Restart of Motor After Stopping It

Is this a trick question? First you say, "restarting a motor right after stopping it".

Then, it's, "Though rotor speed is stil high at the point of restart".

Make up your mind. Is it stopped or just de-energized?

Try it 5,000 times and report your findings back here.

My uneducated guess is that there will be no effect.

What's your uneducated guess?

Sorry for confusing. What i meant was at the point of restart, the rotor is still turning. Hence does it mean lesser torque and amps required to run motor back to near syns speed?

Yes. Less rotor inertia to overcome.

Thanks for the reply. This is a DOL starter design. And while the contactor opened and reclosed..is there a possibility of closing 2 voltage sources of different phase? Namely one from the swgr and the other source from residual voltage of the motor? will this accelerate failure on the contacts of the contactor?

Do you mean something like:
Backfeeding - Wikipedia, the free encyclopedia

I'm taking myself out of this thread. There are many here who are far more qualified than I to speak to this subject.

Good luck.

Hi,,what i meant was closing the contactor on 2 separate voltages that is not in sync? Will it damage or burnt the contacts upon closing?

I'd worry more about the motor windings than the contactor.

Wait for the experts to get here.

Thanks lyn. For my case..the contactor was fused and burnt. Have yet to determine if the root cause was a result of this. Anyway..i thank you again

<...lesser torque...>

It is the load that presents the torque to the motor. So if it is still rotating while de-energised, as would be the case were it to be driving a humungous flywheel, the motor would experience very little torque were power to be re-applied.

It wouldn't still be rotating were it to be turning a centrifugal pump, or something similar that had very little rotational inertia. Were the power to be reapplied here, the full starting load would be applied to the incoming supply.

Nothing would see any significant distress were it to be correctly sized and rated for the application. If it were not rated for frequent multiple re-starts, then it behoves the circuit designer to introduce such (a) device(s) to the starter, either hardware or software, that would prevent the maximum permissible frequency of starting to be exceeded.

Thanks JRaef.your reply is consistent with the observation of a trend recorder which i had placed on the system. Whereby the motor drawn current was trended to be over- range at every time of the motor restart , which i was not able to explain. The motor is a 380vac, 75kw motor. Just a question, if indeed there is residual mag field in the system ..does it make semse to say that the contactor is literally closing/ making on a possible fault condition?

Just an observation, I have worked in heavy industry for over 40 years. In this time I have had in operation many thousands of electrically driven motors. Some operated by Lug-head operators who could not even tell what time of day it was and others put in by automation experts that couldn't even spell expert and have never seen a failure as such. Now I know I have not seen it all, but I have to believe that the engineering departments at the big name motor manufacturers have worked this one out long ago. So people like me don't have to worry about replacing their motors when some Lug-head miss starts a motor. Just my guess.

So having said all of that, what are you trying to do with this information?

Which country is that allowing a 75kW motor to start with DOL starter??

By the way, which country prevents it?

It is agreed that starting large motors direct-on-line causes huge voltage drops in the system due to the large starting current magnitude. But, this becomes a social issue only when the power supply for the motor is drawn from Low Voltage lines, because if you start a higher than prescribed HP motor Direct-on-Line, the LV line voltage drop might affect the loads of a neighbour connected to the same LV line. Thus, the authorities of respective provinces might impose certain limitations as to the largest capacity motor that could be started Direct-on-Line, say 7.5HP or 10HP. But, such regulations are applicable only to LV customers.

As regards a HV customer, any such DOL starting of a motor may not affect a neighbouring HV customer, as there would be a transformer absorbing the voltage drop. Also, in HV terms, the voltage drop might be insignificant to cause worries to your HV neighbour's loads. So, if you are a HV customer, it is your discretion that would limit the largest size of motor that could be started DOL.

In my experience, there are instances where I had to go for star-delta starting even for a 15HP motor and DOL starter for motors as high as 400HP.

The Motor speed is droping when disconnected and may be driven by the load inertia for some time, thus generating a voltage and current in the motor.

When reconnected, the shorter the time of disconnection, the less damage since the synchronising issue will be less significant, having an almost synchronised opposing voltage that will be quickly oercome (and not an additive current flow).

<...The Motor speed is droping when disconnected and may be driven by the load inertia for some time, thus generating a voltage and current in the motor....>

Where does the magnetic field come from with which to generate this voltage?

How does current come out of the motor if, from the moment it was disconnected to de-energise onwards, there is no connection to the supply?

Well, it depends on the type of motor, its size and any remnant magnetism that might remain ... The field might not have completely died if the re-start time is very short (?) I am not disputing the fact that if the re-connection happens after 1 second, you could assume that there will be no current flow, but action reaction of the dyeing fields could still produce some current at the moment of re-connecting.

By the way, You must have heard of what is called "Regenerative power" when a motor is driven by the load inertia after disconnection. Google it. This regenerated power is sometime sent back the mains grid. Depending on the Motor type and size, this could be significant enough when dealing with VFD's. You could also use this effect to slow or Brake the motor ...

Fine with regeneration in principle

As the thread has progressed, the motor type has been revealed as induction, which type is not renowned for use in regenerating systems. One awaits developments.

.....

Excellent explanation by the most consistent & reliable person........

You have not told us the type of motor, although everyone is assuming it is an induction motor it may not be. As Jraef has pointed out, the construction of the rotor plays a very large part in determining the strength of the magnetic field, and the resultant time it takes to decay.

Another thing to consider is the load that is being driven and its inertia. While a lightly loaded motor may not take much energy to pull up to speed, high inertia systems will require great mechanical force to do so, and that will come as a huge inrush current, sufficient to melt your contactor shut.

If this is combined with a large residual voltage that is sufficiently out of phase with the line voltage, then the contactor and motor windings may be subject to voltages that are near twice normal. This coupled with the excessive mechanical forces point to a complete inspection of the motor and its external components. Please let us know what you find.

Thank you for all your contribution. I have received good guidiance here. The motor is of imduction type and i will be following up with more trends at faster sampling rate to verify the suspects transient effect. I am not comfortable with what i have observed at site..which was to restart almist immeidately after stopping. Will be verifying the cause of this switching. Will keep you all posted on my finding. Tks

See "plugging duty" motors and starters.

I'm curious to know why someone thinks that is off topic.

It isn't now.

[Undoes 1 off-topic vote] Very little difference from if it weren't. After all, the motor doesn't care where the power comes from!

There is provisions in our VFD for "fly start". Means motor can be stsrted while it is in rotation. I thaink in that cace there shoud be a provision in VFD to match the phase angle before switching ON. ?

Yes, it matches the frequency, phase and voltage.

So, make sure that facility is programmed-in, then.

I wander how an Automatic Transfer Switch will assure induction motor operation continuity.

It won't, unless it is extremely fast, intelligent, and very expensive. Only added mechanical inertia in the form of a flywheel, or electrical inertia in the form of a continuously available bumpless transfer UPS system, will keep the machine operating. This assumes, of course, that the contactor didn't open due to a legitimate problem somewhere.

Check your starter duty specs I was always told that you will not hurt the motor but the starter may overheat with too many close excitements. Farmer---Jack of all trades and master of none.

See also:

http://www.measurlogic.com/Resources/Paper3_book.pdf

In my factory I have new big motor, the manual are inform not to start directly after stop. But wait until 15 minutes. Maybe this is for waiting the residual magnetic fade away.

Actually, this is recommended for another reason: Prevent Overheating due to starting current!

If the motor is de-energised, then no rotor current can be present.

In our plant, we have faced a similar problem.. we have 200KW compressor motor which run continuously,once it stopped and started immediately, then the motor burned.The reason is "as motor is started in "HOT CURVE" mode and it has drawn 4 to 6 times of its rated current in hot mode, so the motor failed.

So, u have to consider weather you are starting the motor in "HOT curve" or "COLD curve" mode also matters.

Most likely, it is the fact that a compressor produces pressure upstream and re-starting it immediately will mean that the motor has to overcome the esisting back pressure which did not have time to dissipate! You are supposed to start such equipment with no back pressure!

If it is to be re-started 1 sec after stopping, it may be better to avoid stopping it altogether.

So why is a 1 second stop required, please? Can it be avoided by more sensible process requirements?

Here also you are trying to tell him what to do instead of explaining why he is getting or think that he is getting whatever he stated.... Maube he is experimenting ???

Is there a upper limit for restarting just like minimum time between stop & start?.Reacceleration can be done at any time after motor stops?Generally 8sec is the maximum time set almost everywhere.

Thanks

Your question requires clarification. Are you asking if 1. there is an upper limit to the amount of time it takes to accelerate a motor , 2. if you can reaccelerate (restart) a motor any time after is de-energized, or from zero speed, 3. if 8 seconds is some kind of recommended number to get from zero speed to full speed?

If so then the answer is generally No to all of them, it is highly dependent upon the motor size and design, the type of load and its inertia, and the number of attempts and amount of time between tries. Restarting a stopped heavily loaded motor every 8 seconds will quickly overheat it unless it is specifically designed for plugging duty.

The manufacturer is the only one knowledgeable enough to answer these questions correctly.

It is not clear as to whether the restart is after motor comes to stop or while still coasting to stop.If a motor is started from rest after switching off and while in hot condition manufacturers recommendation on hot starts to be followed.Normaly up to 160 Kw (4P) 3 cold and 2 hot starts are allowed but can change depending upon severity of application.For MV motors and large LV motors manufacturers usually furnish number of hot and cold starts it can have but mostly one hot start is allowed.If restart is attempted while motor is still coasting voltage applied can be in phase with residual voltage and create mechanical forces particularly on end which are braced and motors are designed for this as now a days in many places especially in power stations auto bus transfer is allowed and motors face similar condition.