Motor starting issue

[Ahm96]

Dears
I have motors (11KV-1.8MW-DOL)
The name plate mentions that the jogging factor (Cold/Hot) :2/2
Meaning that the number of consecutive starts Cold/Hot is 2/2
My question is that incase of failure of the two start attempts (Trip or Shutdown reason),What could be the waiting time before the third attempt?
The manufacturer didn't mention that.
Regards

 

[edison123]

Motors of that MW range are bound to have stator winding RTD's wired to a temperature scanner/controller or DCS. Wait till the winding temperatures come down nearly to ambient, which could take hours.

More importantly, why did the motor fail to start two times?

Muthu

http://www.edison.co.in

 

[Ahm96]

Waiting the winding temp.to decrease to ambient takes more than 12 hours.
We can't wait this time due to process production issue.
- The start can fail due to reasons concern the load (gas compressor) not the motor itself.

 

[waross]

The first guess would be starts per hour, based starting limitations on other motors.
eg: Max 3 starts per hour, or Max x number of starting seconds per hour.
But really, for a 1.8 MW It may not be safe to depend on free advice from strangers on the internet.
Ask the manufacturer, in writing, and wait for a reply in writing.
Don't trust a verbal answer from a sales tech.
Some sales techs have a tendency to make it up as they go along, and conveniently "forget" the conversation if things go bad.
If a sales tech is able to direct you to written guidelines in company literature, that may be acceptable.
Be sure that it applies to your motor.
If two starts are allowed, it should be safe to wait until the motor cools to the temperature encountered after one start, before one more start is allowed.
Second guess: It may be safe to start after an hour.
But, why the repeated starts? If the motor was unable to turn and was locked rotor during a DOL start attempt, you may have already used up your thermal allowance with the first start attempt.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

The manufacturer didn't mention that.

And hence the safe room temp suggestion.

Google I2t. There is a hot reason why the no. of motor starts is restricted.

If you have frequent problems starting your gas compressor, may be it's time to assess the load torque requirement correctly and go for a high torque, higher HP motor.

Muthu

http://www.edison.co.in

 

[panter]

Rotor temperature and cage condition may be conditional on a new motor start. For high voltage motors of this size, manufacturer usually recommends just one warm start. If you have higher technological requirements, installation of compressed air or nitrogen might help to cool the engine a little faster by forced ventilation. The condition for compressed air is to be very dry .
Good luck !

 

[waross]

Ambient temp T[tt]a[/tt]
Motor Normal Running temp T_n
Motor Temp after 1 hot start T_h1
Motor Temp after 2 hot starts T_h2
If two hot starts are allowed, you should be safe to start when the Motor temp drops to T_h1
Probably 1 to 2 hours.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Hope you have heating and cooling time constant data available. With this data and others such as full load current and hot & cold withstand times, the motor protection relay will be able to calculate the thermal content in the motor.
You can set in the relay the cold start level as 45%. Thus once the thermal content in the motor comes down below 45%, the protection will allow another two starts to be made.
Hope motor starting time is well below the motor hot withstand time. It is important as compressor tend to take long time to start.
If not, you require speed feedback to be wired to relay so as not to stress the motor, if stalled during start.

 

[Ahm96]

Unfortunately I don't have any data regarding the motor heating/cooling time constants.
Safe stalling time (cold/hot) :25/10 seconds.
The motor starting time is around 5 seconds.

 

[waross]

I had a problem with some 40 HP wood planer motors.
If a piece of wood jammed in the planer, the heat developed by the would at times cause the moisture in the wood to flash to steam, exploding the wood and stalling the motor.
We would occasionally lose a motor before the overloads could react.
We solved this with a zero speed switch, that would instantly disconnect the motor when it stalled.
There are now speed switches that may be set to any speed and are able to detect when a motor is turning but not coming up to speed.
One speed switch was developed to detect and warn about Vee belt slipping.
This was after a major multiple explosion and fire had destroyed a major grain terminal at the port of Vancouver.
The original cause?
A slipping Vee belt started to throw off burning belt particles which started the original fire.
I lived about a mile from the fellow who developed the switch that was widely used after the disaster.
I used some of his switches to prove rotation of the main fan on some large gas burners.
It was a trouble free replacement to the differential pressure switches that were previously used.
The diff switches were notoriously trouble prone.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Safe stall time hot) of 10s with stating time of 5s shows good margin and there is no need of speed feedback to motor protection relay.
You can look at the motor protection settings - what thermal content threshold the motor is allowed to restart (after completing one sequence) and restart inhibit time settings.

 

[waross]

My point that if a speed detection system sees the motor hanging up somewhere past the breakdown torque it may be kinder to the motor to trip it off rather than wait for the thermal trips to take it off line.
Heating will be less with an earlier trip and a restart may be safely initiated sooner.
With a history of failed starts, an additional low speed initiated trip may be kinder to the motor and will allow a restart sooner.
Not needed, but nice to have.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Ask the manufacturer if possible.

 

[edison123]

EASA article on safe starting of motors (pdf link)

https://www.google.com/url?sa=t&rct...cumentid=583&usg=AOvVaw1OVNoIg7sGPbVp9m9aBudd

Muthu

http://www.edison.co.in

 

[7anoter4]

If you intend to cool the motor for a single new start you may start from the hot first start temperature. In my opinion this temperature could be 60oC [ambient temperature 40oC ]
and cooled in standstill condition it will reach this temperature into approximate 35 minutes.
For calculation I followed this article:

https://www.gegridsolutions.com/multilin/family/motors/motor_thermal_model.pdf

The motor data 10 kV 1800 kW 1500 rpm [AMA 500 L type] according to ABB Catalogue:
ABB/ BU Machines / HV Induction motors IEC catalogue / Engineered motors EN 12-2007

 

[LionelHutz]

The start isn't what heats the motor, it's time and current. An unloaded start is easier on the motor then a start when the rated load inertia is coupled to it.

If this motor can be stalled for 10 seconds when hot, I wouldn't be too concerned about starting it twice with 5 second starts.

 

[7anoter4]

In my opinion, this motor [presented in the catalogue] on its own inertia start in less than 1 second. For 5 seconds the load inertia has to be 10 time more.