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PAP (Electrical) (OP)
29 Jul 03 2:40
Dear all
 I'd like to ask for some meaning of this statement,what does it mean?
" ... permissible starting duty with motor initially at ambient temperature 40C , 3 successive start are permissible.After which motor should be allowed to cool by standing idle for 1 hour or running for 1/2 hour.  With motor initially at rated operating temp., 2 successive starts are permissible. ..."
 How the difference of meaning of cold start and hot start?
Best regards
Prasit P.
dadfap (Petroleum)
29 Jul 03 4:57
pap

The number of permissable starts in a given period is provided by the manufacturer to protect the motor.  The number of starts given is designed to allow the motor to cool sufficiently from the heating affects of the inrush current.

all the best

dadfap
Marke (Electrical)
29 Jul 03 5:09
Hello PAP

The life of a motor is dependant partially on the life of the insulation in the windings. During start, the motor is subjected to a severe overload which causes a rapid temperature rise in the windings. As the temperature of the windings rises, the life of the insulation is reduced. A typical rule of thumb is that for every ten degrees C rise, the life of the insulation is halved. If there are too many starts with insufficient cool down time, the temperature rise will cause problems.
During start, there is also a very high energy dissipation in the rotor causing the rotor bar temperature to rise dramatically. If the bars get too hot, the rotor will be damaged. If the motor is hot, the amount of energy that can be safely dissipated is reduced, therefore the starts must also be reduced.
Best regards

Mark Empson
http://www.lmphotonics.com

PAP (Electrical) (OP)
29 Jul 03 5:47
This is my interpretation:
3 successive start mean that I can start this motor 3 time continuously without waiting for cooling time.
 Is it right?
 How we determine that which condition is cold start condition ,after we start the 1st time and then stop immediately .Afterthat we start again ,is this condition  in second successive cold start ?

Please clarify.
Thankyou and best regards
PAP
Marke (Electrical)
29 Jul 03 5:56
A cold start is a start that occurs when the motor has cooled down. This can be a significant period of time after a start or run. i.e the motor is close to ambient temperature rather than running temperature.
Best regards,

Mark Empson
http://www.lmphotonics.com

PAP (Electrical) (OP)
29 Jul 03 6:27
I'm still waiting for further clarify the first statement.

Thank you for every kind and cooperation.
PAP
smms (Electrical)
29 Jul 03 6:35
PAP,
yes you can have 3 succesive starts without any period between the starts. After the last start you have to wait before you start the motor again.

Typicall number of starts:
3/2 (cold/hot)
This means, if the motor is cold you can make 3 starts, if the motor is hot (motor was running with load)you can make 2 starts. You have a cold situation typically after 5 times of the thermal time constant (you will get this value by the motor manufacturer).
 
Helpful Member!  Kiribanda (Electrical)
29 Jul 03 10:38
Hello PAP,

Ex: An induction motor 2000 kW, 6.3 kV, 225 A, Ins. Class F with Class B rise,  Rated ambient = 40 Deg C.

Cold Starts( 03 Attempts):
If the motor is at standstill, or the motor was running less than its rated load 225 A at 40 Deg C ambient, then the motor was NOT running at rated load. That means still the motor is at so called “COLD” state. Then if you are going to stop that motor and restart, then you are allowed to attempt 3 starts consecutively (one after the other) on that motor.

Hot Starts( 02 Attempts):
If the motor was running at its full load 225 A at 40 Deg C ambient and after a while when it establishes its steady state temperature, then the motor was supposed to be running at its rated conditions. That means the motor is now in its “HOT” state. Then if you are going to stop that motor and restart, then you are allowed to attempt only 2 starts consecutively (one after the other) on that motor.

“How we determine that which condition is cold start condition…. “

From my experience, it is totally dependent on the user of the motor. To decide whether the motor is at cold or hot state, you have to analyse the load in Amps, motor steady state body temp, ambient temp etc. By simply measuring the body temp and allowing a margin for the hottest point depending on the insulation class, you can decide whether the motor has reached its rated temp even at a reduced load. In any case, if the motor has attained its rated temp depending on the class of insulation, then that motor has reached its rated condition ( may be due to high ambient, bearing friction etc ) even though the load in Amps is lower than the rated 225 A. Therefore the temp is the guide.


“…after we start the 1st time and then stop immediately .Afterthat we start again ,is this condition  in second successive cold start ?”

Yes it is still a cold start since you have NOT allowed the motor to reach to its steady state temperature.

Regards!

Kiribanda











mishlove (Electrical)
29 Jul 03 15:40
Motor Thermal Curves (Thermal Damage or Capability Curve)when supplied from motor manufacturers, normally include:

Thermal heating constant running:  ie Time to reach thermal equalibrium.

Thermal cooling constant stopped:  ie Time to cool from hot condition.

Consecutive start:  ? Cold   ? Hot.

Cold is motor starting from ambient. coasting to rest between starts.
Hot is motor starting from its normal operating temp.

Ciao
jbartos (Electrical)
29 Jul 03 20:10
Suggestion: Visit
http://www.seamanship.co.uk/engineering/EKsamplepages/Motor%20starters.htm
for "fast cycling"
http://www.sea.siemens.com/speedfax/Control_Products/f_-_pp_15_48_to_15_52_Soft_Start_(SSRVS)_Control.pdf
for "frequent starts/stops rating"
etc. for more info
smms (Electrical)
30 Jul 03 0:56
To kiribandas comment:
If the motor was running with reduced power - you have not a cold situation!!!! E.g. the motor was runnning with 90% rating means that the temperature rise of the motor is appr. 80% of the maximum rise. Cold condition means that the maximum motor temperature before the first start is the max. ambient temperature!
sandipsvaid (Electrical)
30 Jul 03 1:24
at the time of start motor is like short circuited transformer when you close the breaker heavy current flows thru rotor. during start rotor conductor temp rises , in order to limit permissible rotor bar temp no. of starts limited.
jraef (Electrical)
30 Jul 03 13:19
Here is another way to look at it. The first start, when the motor is at max. ambient temperature or less, is allowed whenever you want it. After the first start it is no longer "cold", so you only get 2 more starts in that hour. After resting for an hour, you can get 2 more starts / hour but no more. After resting long enough to allow the motor to return to max. ambient, you regain that 1st (extra) start capacity again. In most cases, that wait is going to be measured in multiple hours depending on the ambient temperature.

For example, I just did a 600HP pump in Las Vegas in 40C ambient where the motor was still at 90C 6 hours after being shut down. I did a 600HP snow machine pump motor at a Ski Lodge last year where the motor temp was 40C within 3 hours of shutdown, but it was 10C outdoors that day.

Quando Omni Flunkus Moritati

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