Motor start-up frequency

[mass89]

People always say how bad it is to start and stop motors frequently, question is, how bad is it and where do you draw the line.
For example, I have a 15kw pump driven from a soft starter, the pump only actually needs to do work 5 minutes every 20 minutes, does that warrant being turned off for the 15 minutes that its not actually doing work?

I tried to search the datasheet for the motor but nothing regarding start/stop intervals. This is something I frequently come across but never known the real answer.

 

[mdim]

Normally there should be classification for the duty cycle as per IEC 60034-1. You can check this: https://electrical-engineering-portal.com/10-duty-types-three-phase-asynchronous-motors
or search the net.

Normally we have our pumps at 20Hz min (out of 50). But as I see it the min. speed should be the minimum on which your pump works. I'v seen some pumps, that in the conditions they are installed don't work at 20Hz, so if you're not moving the fluid, you are also not cooling the pump, but you are putting energy in the fluid. It's dangerous.

 

[mass89]

The motor in question is S1 classification, the S1 classification doesn't indicate whether running 10 minutes on 10 minutes off is better than running 20 minutes straight (from what I am aware).

 

[OkiePC]

If it ramps smoothly, then it is probably not exceeding the duty rating of the motor and pump.

Softstarters are often rated by the number of starts per hour. 3 times per hour though should not be any trouble for it either. When you get to 10 or more starts per hour is when I usually see ratings and recommendations change for softstarters.

If you google "nema motor starts per hour" there is a pdf excerpt. Here is a brief write-up about it.

http://www.brithinee.com/technical-articles/how-often-can-a-motor-be-started/

EDIT: Nema may not apply after seeing your location.

 

[BryanG]

From another angle, how much energy (money) will you save if the pump is stopped?

 

[rupej]

From another angle, how much energy (money) will you save if the pump is stopped?

Good point. Probably enough to outweigh an ever so slightly reduced lifespan of the softstarter. Pump seals will probably last longer with less runtime. Motor probably won't care one way or the other.

 

[Tom Jenkins]

The main impact of excessive starts on a pump is heat buildup in the motor. This is principally from inrush current. If you use a soft start this heating is reduced and more starts can be tolerated.

The impact on bearings, seals, or stuffing boxes will be minimal.

This is from a pump station design guide from the Water Environment Federation a few years ago. Even across the line you should be OK with three starts per hour:

Across the line starting:
Motor hp Cycle Time, minutes Starts per hour
1 10 6
25 10 6
500 720 0.083333333

Cycle time = on time + off time
(Columns don't line up, but you get the idea.)

Have you gotten an opinion from the pump supplier?

 

[mass89]

Thanks, I haven't spoken to the supplier, I was wondering if there was something that would give you a benchmark similar to what Tom shown.