HV motor stall protection at start-up 4

[hottip]

Hello to all! I’m very glad that I discovered this excellent forum.
I have the following issue for you. I’m a maintenance electrical engineer in a plant with several HV motors 11kV/ 5.3 Mw electrical motors. The motors are protected by IMM 7960, multifunction relay (CEE relays made - UK). At start up of the motors, for rotor stall protection there is a speed sensing relay, so that if the electrical motor does not reach 60 rpm in 2 sec will be tripped by this speed sensing relay. I was reading the Protection Relay study for the plant. It is stated: ‘These motors have a run-time exceeding safe stall time so a speed switch scheme monitors the start process. The speed switch requires a speed of 60 rpm to be reached within 2s otherwise the motor is tripped.’
The too-long start protection is set in the motor protection relay IMM-7960, to allow a start current of 520% to exist up to 13 s.
On the motor label hot/cold stall time is 7s/10s. So I guess that’s why there is the speed switch relay, as the motor protection relay at starting of the motor will not trip the motor in a time less than 7s(7s is worst scenario the motor being hot)

Now the problem: when I arrived in the plant I found the speed protection relay by-passed (all of them) in the HV switchboards, so the motors at the moment are not protected in case of locked rotor at start. Nobody from the maintenance dept. could inform why the relays are by-passed. I red the motor vendor instructions and it stated that the speed sensing relay must be there.
My opinion is that the speed relay is by-passed probably from the commissioning time of the plant, and nobody discovered this issue. My duty is to rectify the problem.
Now, I have for you 3 questions:
1. The speed sensing relay must be operational or not?
2. What can happen if the motor will be stalled (at the motor start) and the relay will not operate, being by-passed?
3. You had any incident related with the speed sensing relay in your experience not operating at start up and the motor was damaged after this?

Thank you very much for your time.

 

[jraef]

Hottip,
Please post some additional information about the load that these motors are connected to.

It may be that on commissioning it was found that the motors could not accelerate the load to 60RPM within 2 seconds, so the only way they would run was to bypass that relay. If that were the case, either your motors are undersized or the application must be changed, for example to have a clutch installed to allow the motors to accelerate without being coupled to the load.

Also, what is the starting method? If the rotor heating time limit is based upon DOL stating, but you are using Reduced Voltage starting, the shorting of the speed switch may have been warranted because there may be less rotor heating over the same amount of time using RV starting, AND acceleration will be slower.

In other words, this is more complex than just yes-or-no on the validity of shorting the speed sensing relay.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[GordS]

Is it possible that the speed sensor was installed for Process reasons? Is there a need for the load to be at 60 rpm within 2 sec?

I interpret your post as the motor being capable of 7/10 sec safe stall times but your protection is set at 13 sec. If so, then I would say the motor is not protected by the relay with its existing settings. In this case, your speed switch would protect the motor but I question why the relay can't just be properly set.

 

[aolalde]

The original idea is to prevent thermal damage to the motor as result of extended time with the high inrush current.

Motor manufacturers protect themselves against warranty claims by limiting the number of starts and the time allowed with inrush current (Name Plate Limits for hot/cold starts). These limits by the other hand will allow extended life of the motor windings (rotor and stator).

Real life applications could result in stringent demands for the motor due to increased load inertia, higher load torque or voltage drop. These factors will result in increased time to accelerate the load.

My advice is to make starting tests to check the actual starting parameters. If the motors perform as required by the protection scheme, set it again as the original spec. If the starting times are longer consult the motor manufacturer to verify how the motor life will be affected if the starting protection is bypassed.

 

[hottip]

@ jraef: The starting method is DOL (there is no any possibility to adjust the speed). The motor are not undersized. (are running loaded 80-90%, some times for short period of time 105-110%) The speed sensing relay can be adjusted.
@ GordS and aolalde: The speed sensing relay was not installed for process reasons, it is there just to protect the motor. According to the protection relay setting the motor protection relay cannot protect the motor at start if the motor is stalled. The funny issue is that on the IMM-7960 the starting time is set at 13s, but if I press the button on the relay to see the last recorded starting time (the relay has this facility, I can see being between 6 and 8s (I've seen the values some time 6 or 7 or 8s)
It is possible that during the commissioning where in a hurry to hand over the plant so additional test with the motors where not carried out. The motors are driving gas commpressors.

 

[aolalde]

Hi Hottip:

The speed sensing relay in reality measures acceleration (60rpm in 2 sec) and as you stated, it protects against rotor stalling. The IMM-7960 protects against extended inrush current.

As far as I know, the protection parameters are set in such a way that nuisance relay trips are reduced but they trip before any damage is developed. That is the reason for 13 sec set in the motor protection relay IMM-7960 against 6 to 8 sec real starting time. To me 13 sec set is OK.

Totally different, the relay against stalling is an extra sophisticated protection that could safe the life of your motor, if the actual set up is tripping, the initial acceleration could be less stringent, say 30 rpm in 2 sec; the idea is to confirm the motor is accelerating as son as the voltage is applied. You never mentioned the motor speed.

 

[hottip]

Hi aolalde,
Sorry you are right , I did not mention the speed: 2980 r.p.m.
So according to what you all gentlemen say, the situation at the moment is like this: in case of the motor stall at start-up the motor will trip after 13 s (the setting from IMM-7960), because the speed sensing relay is by-passed.
In this case in the windings will be lot of heat due to inrush currents. This first occurence of this case probably will not damage the motor, as I can see from your posts. Only after a few (or considerable more???) situations like this (motor stall at start up), the motor can be damaged.

 

[aolalde]

Hi Hottip:

“in case of the motor stall at start-up the motor will trip after 13 s (the setting from IMM-7960), because the speed sensing relay is by-passed.”
This statement is Correct.

“Only after a few (or considerable more???) situations like this (motor stall at start up), the motor can be damaged.”
I am not that sure, since the motor manufacturer set maximum stall times of 7sec hot / 10 sec cold.
Remember that when the motor accelerates the current starts dropping as the motor increases speed and some heat is transferred due to the rotor and air movement. That beneficial situation is not reached with the rotor stalled.

Probably the actual set up has worked because the motor never has stalled at start.

 

[RRaghunath]

I have gone through the posts as above and would like to add as below

1) 13seconds setting: The 13 seconds is arrived at by taking the cold start time at expected low voltage (of say 80% of rated) at motor terminals and adding 2seconda for margin. May be if you look at motor data sheet for the expected starting time for 80% rated voltage, the same can be confirmed.

2) Motor is not adequately protected: When speed switch is connected to the relay, the 13seconda could / should be brought down to 6seconds as the speed switch isolates the tripping after ensuring that the motor is accelerating (and is the actual purpose of the speed switch). 13seconds setting is high and cannot protect the motor adequately (With rated voltage at motor terminals and with motor windings hot, the relay allows locked rotor condition to prolong much beyond the motor rated withstand time).

Trust the above is useful.

raghunath_n00@rediffmail.com

 

[candu6]

We have a simular set-up at our plant. Our 9000Hp motors take a considerable amount of time to run-up and the overcurrent relays could not be calibrated to allow for the extended, high current, start time. So, the motors were designed with a speed sensor and the initial motor start is monitored by an Airpax Tachometer until the motor reaches 98% full speed and then protection is passed to the GE overcurrent relay. The tachometer circuit has a couple of timer relays, which will operate and trip the motor if it does not pass certain speed thresholds in time.