Repeated start for testing.

[electricpete]

We are doing an electrical test which will require repeated starts of a motor. The total number of starts will be 10 or 20 or more We would like to accomplish the repeated starts in the minimum time which will not jeopardize the health of the motor (this is a critical motor and we have no spare readily available). I am trying to provide input on the duty cycle.

The motor is 60 hp, 2-pole, low-inertia load = centifugal pump.

NEMA MG-10-2001 Table 7 linked in faq237-1285 gives the following limits:
Maximum 3.2 starts per hours
Minimum 170 sec "off-time" between starts.

Although not tabulated, it is noteworthy that the motor cools faster while running between starts than while resting (since much more heat is generated during start than run, and much more cooling due to airflow occurs while running than while secured). Ideally, considering the critical nature of the motor and lack of spare, I would be comfortable with the following simple duty cycle which maximizes the run-time between starts:
* Start and run for minimum 15 minutes
* Stop and rest for minimum 5 minutes
* Repeat

However there is a problem with the above. This cycle is difficult for the fluid system to accommodate. I believe that if the system is not started up for continuous run, then the pump must be shut down in 5 minutes due to fluid system considerations. For the sake of this discussion, assume we do not want to uncoupled the motor from the pump. The cycle that appears to be needed to for consistency with plant needs is as follow:
* Start and run for 5 minutes
* Stop and rest for T minutes
* Repeat
What would you recommend for T (above) ? I think that a literal interpretation of the standard would allow T = 15 minutes, but my gut tells me we need to go much longer.

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[RRaghunath]

The motor can be started 3times in succession (without giving breaks in between starts). Considering the motor is driving a pump which is a low inertia drive, the motor can actually take more than three starts in succession.
If you have the motor data such as allowable starting time, heating and cooling time constants, it would be much helpful.
- The total no. of starts x actual starting time < The allowable starting time x 3.
- After the no. of starts as above, the motor should be allowed to cool for the time equal to motor cooling time constant as a minimum.

Motor experts on the forum may like to add.

 

[itsmoked]

I'm curious why they need to start/stop that many times in rapid succession? Is it for the plant or some reason for the motor?

Is there a way to augment the motor's cooling? Is it an open motor you can blow air thru? Is it outdoors and a TE motor you could maybe run water over? Still hard to cool the rotor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

The repeated starts were intended to verify we have resolved a problem with intermittent trips. It doesn't have to be in rapid succession, but we wanted to complete the work in a reasonable time-frame.

It turns out 10 minutes run-time was doable. We ended up doing 10 minutes run, 20 minutes off.

So, I no longer need a direct answer for this particular situation.

But I still have a general uneasiness with those NEMA guidelines. It seems to me they should address the need for a minimum run-time (rather than just max number of starts/hr and minimum "off-time") when there is a situation of repetitive starting.

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