Motor Restart Procedure 1

[EEENGRX]

Aside Manufacturer procedure for motor re-start. Is there any general rule as to the time interval that should be allowed for re-starting a motor that has tripped? is there are such rule on IEC or NEMA etc?

 

[thermionic1]

#1 - If the MPR trips a lockout relay and the LOR trips the motor, do not start without further investigation.

 

[Gr8blu]

When a motor trips, it will typically start to slow down. As it slows, the difference between motor "apparent" voltage and bus voltage will increase in terms of amplitude differential and phase angle. If either of these gets too large, a restart is not possible without damage to the motor being started and/or to other equipment on the same bus.

If you really want to use time - the conservative approach is to take the time for the process (not just the motor) to come to a complete stop. Note that for certain applications this interval may be measured in tens of minutes, not seconds.

Converting energy to motion for more than half a century

 

[EEENGRX]

Thanks for these wonderful responses. is there any standard for determining the time interval?

 

[che12345]

EEENGRX (Electrical)(OP)26 Jun 23 21:32
"....Thanks for these wonderful responses. is there any standard for determining the time interval?"
1. You had received good learned advices already.
2. It is not up to the Standards body e.g. IEC or NEMA to dictate the conditions that varies case-to-case. You can see this that all thermal over-load protection relays do NOT reset immediately/automatically upon tripping, unless by-pass deliberately/manually.
3. It may be safe to say that a restart can be initialed after the machine had come to a full stop and with an additional prolong time the machine had cooled down to the ambient temperature. This may take a long time say one hour or more, but would be safe.
Che Kuan Yau (Singapore)

 

[waross]

It depends.
What frame class is the motor?
The U frame motors would withstand abuse (including hot starts without adequate cool down) that would destroy a T frame in a few starts.
The motor typically trips due to excess heat.
Once the motor is stopped it loses its self generated fan cooling.
Cool down may take longer on a stopped motor than on a running motor with reduced loading.
I remember a 150 HP motor that was nameplated for a maximum of 20 seconds of three starts per hour.
After a trip, it is a safe assumption that the motor was hotter than the allowed 3 starts.
Rather than a 20 minute cool down corresponding to one start, 30 minutes would be advisable.
Disclaimer: This is an example of one motor only and cannot be taken as a general rule.
Your motor may cool down faster or slower.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

is there any standard for determining the time interval?

Motor temperature.
Possibly determined by winding resistance measurements.
Take the first measurement as soon as possible after the motor has tripped.
Take measurements at intervals until the temperature drops to what you feel is a safe level for a restart.
Once you have determined the time to cool, you may use time rather than resistance in the future.
For winding resistance I recommend a four wire test set.
Roll you own: The current leads pass an accurately measured current through the winding.
The potential leads measure the voltage drop across the winding.
Do the math.
As the motor cools, the current from a stable but unregulated source will tend to increase.
Do the math each test with both voltage and current.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

For DOL starts, I recommend to my clients a winding temperature of max 50 deg C (via ETD's) for a restart.

For RVSS, 60 deg C for restarts.

For VFD starts, no limits.

Muthu

http://www.edison.co.in

 

[Gr8blu]

For squirrel cage induction and synchronous machines, the limit is NOT the stator winding temperature. It is the temperature at the rotor-bar-to-shorting-ring joint. And of course this is almost impossible to measure accurately - especially in a "on the fly" in a real life situation. With enough knowledge of the machine geometry - including number of bars, bar material and cross-section, ring material and cross-section, joint type (butt braze vs lap joint vs through joint), driven load inertia, motor rotor inertia, actual applied voltage at motor terminals during the start attempt - and so on, it is possible to mathematically determine how hot that joint is going to get.

There is no really good relationship between stator winding temperature and joint temperature either, in most cases. At least, not one that can be used as a rule-of-thumb across multiple ratings and manufacturers. This is partly due to the insulation on the stator coil (which is between the sensor and the actual conductor) and partly due to the difference in conductor cross-section. The nonlinearity gets worse when the material of the coil conductor is not the same as the bar and/or ring, too.

One other thing to note: if the machine stops turning completely, the actual temperatures are going to RISE for some period of time (even without any current flowing) because the method of cooling has suddenly become far less effective: what used to be convection plus radiation is now radiation only.

IEEE, IEC, and NEMA all say "read the motor manufacturer's directions regarding attempted starting".

Converting energy to motion for more than half a century

 

[edison123]

IEC Std. 60034-12-2007, 8.3
Motors shall be capable of withstanding two starts in succession (coasting to a rest between starts) from cold conditions and one start from hot after running at rated conditions.

So, once the stator winding is cooled down to 50 deg C, it is safe enough to assume rotor has also cooled down enough to allow a restart.

Muthu

http://www.edison.co.in

 

[Gr8blu]

The definition of a "cold start" is when all machine components are at ambient temperature. Any other condition is considered a "hot" start. So even if the machine did not get to a stable operating temperature, the start attempt (on anything other than a drive) has caused some heating of the windings and bearings primarily due to the higher inrush current - which must be eliminated to return to a "cold" condition. It takes longer to get rid of the heat if the rotor isn't turning while unpowered.

And as edison123 pointed out, most machines are - by default - required to meet the "2 cold, 1 hot" start profile.

It's also relatively important to realize that most manufacturers give a "maximum starts per day" value as well. This is applicable to anything other than a drive start, as noted previously in this post. The reason is that the mechanical stress from expansion and contraction of the bar-ring joint and relative movement between bars (they don't all heat up equally, which leads to differential axial growth and a "twist" to the ring assembly) eventually results in a fatigue failure. Most machines are designed to withstand 5000 starts over the life of the machine - which, at 6/day, translates to 833 days or about 27 months. For large industrial processes, a 1/day start cycle is a reasonable assumption - which would give a fatigue life of about 13.5 years.

Converting energy to motion for more than half a century

 

[electricpete]

This may or may not be helpful to you: faq237-1285

If you follow the link from there, it includes a detailed table for long term repetitive starting limits from NEMA MG-10, which takes into consideration speed and horsepower and specifies max connected inertia. It also gives detailed timing paramters in columns A/B/C. Oem may provide additional info, which would supersede any generic NEMA info. The long term repetitive starting limits linked above are different than infrequent acute limits. However, as mentioned in the FAQ, applying the limits / times for long term starting duty to evaluate acute starting duty would be conservative and may be the best option if you want to go beyond the one start hot / two starts cold in situations where there are no other allowances accessible to you.

 

[EEENGRX]

Thanks ya'll . i have been able to work out a schedule to adopt from your wonderful ideas.i am grateful!!!

 

[jraef]

Motors have a "thermal damage curve" and protective devices are, in theory, designed to not allow a user to go "over the curve" and cause damage to the motor. Standard Overload Relays (when properly applied) are very good at doing that for most common motors, at least those up to maybe 250HP. This is also going to be "retentive" in that if power fails, the OL relay "remembers" where it was on that cool-down time to disallow early restarting that might cause damage. That concept however is not 100% universal, there are inexpensive solid state OL relays that do not have retentive thermal memory, so watch out for that.

For larger and more critical path motors, the protection curve should be a little more tailored to the motor design, using the "cooling time constant", defined as the time taken by a motor to SHED ~63% of of the heat in it (or reach ~37% of it's steady state temperature). The determination of this on any given motor is complex, but for critical path motors (usually large ones) where one would consider using an expensive Motor Protection Relay, the motor manufacturers can usually provide it for you. Good MPRs typically have that as a programming parameter, with a default setting that is essentially the same thing a standard OL relay would provide.

You can see the formula in this older thread by the way.

https://www.eng-tips.com/viewthread.cfm?qid=262894

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