Pump cyclic applications

[Roger00]

Hi All,

I would like to know the impacts to life time of a centrifugal horizontal pump and motor with flow 80 m3/h, Head 100 meter, fluid water, motor 75 kW , insulation F, duty type S1 with cyclic operation around 24 cycles per day, each cycle is 6 minutes.

is the S1 motor duty and pump are suitable for cyclic operation?

thanks

 

[Artisi]

Do not understand that your problem is - 1 start per hour and the pump will run for 6 minutes duration. I would hardly consider that any sort of a problem.

Want to expand your question?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Roger00]

Hi thanks for responding , basically im asking can i run a pump cyclic with the conditions and specifications bove without really affect the pump motor life time?

I see examples in industries that service water from service water tank that supply to service water pump and directed to the hydrostatic tank (with compressed air to maintain pressure) for consumers that consume water intermitently. When the selection of hydrostatic tank becomes beneficial than just to have pump that will operate cyclic ?

 

[LittleInch]

I had to go look up electric motor duty cycles, but it looks like an S2 duty cycle ( short time duty) would be more suitable. S1 is quoted as continuous with enough time to reach temperature equilibrium. Six minutes per hour is probably not enough time to stabilise the motor temperature, but you may be better asking the same question on an electric forum to see if this sort of duty would actually result in the motor failing prematurely. The pump is no problem operating like that. Repeated start stop of more than 4 or 5 an hour are not great for motors due to overheating due to high start currents unless they are designed for it.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LittleInch]

Actually an s3 or s4 motor may be better. That's why you put the type of operation on the data sheet you give to the vendor. Have you asked the motor vendor about this motor?

See

http://www.engineeringtoolbox.com/iec-duty-cucles-d_739.html

for a description of motor types.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ScottyUK]

The problem with multiple starts is that it puts a lot of heat into the motor, especially into the rotor, and the shaft-driven fan does not have a lot of time to get rid of the heat. The rotor in particular needs the internal fan to transfer heat to the core and then the casing in order to cool itself, and it can only do this effectively when spinning.

The heat energy into the rotor is governed principally by the acceleration time, so starting a centrifugal pump will put far less energy into the rotor than starting a laden conveyor. You may well find that the motor is OK with this duty cycle unless it is operating at the standard 40°C ambient, is loaded to 100% of capability, starts against an open valve, and everything else goes against you.

 

[LittleInch]

The other thing which helps a lot is that from your description the shaft power is 35kW based on a 0.6 efficiency. Therefore your 75kW motor looks quite oversized, but then well able to handle this sort of on off operation.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

Come on guys, lets not go overboard - we are talking about 1 start/hour with a run time of 6 minutes, hardly enough time to get the motor up to normal operating temp.
That's unless of course we only have half a story which is not unusual.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Roger00]

Thank you all for sharing your opinions.. I think the motor S1 duty type is suitable for this kind of operation. I see same examples and the motor are still in good condition.

 

[bimr]

Standard NEMA motors are rated for continuous duty (IEC S1) and run 24/7 at rated load, temperature, etc. Other motors are designed to run for short periods of time and start and stop as needed. NEMA motors have a Time value on the nameplate from 5 min to 60 min. EIC motors specify a Duty Cycle from S1 - S8. This is the maximum time, specified and tested by the OEM for safe operation without overheating or otherwise damaging the motor.

A cause of overheating is the number of start-stop cycles per hour. While starting, a typical motor draws five to six times the rated running current. This starting current accelerates heating dramatically. Most continuous-duty motor designs are intended to do just that, operate continuously. Though various provisions are made relative to loading and off-time, NEMA essentially limits a three phase continuous-duty motor to two starts in succession before allowing sufficient time for motor to stablize to its maximum continuous operating temperature. This is highly application-dependent, so its best to check with your motor manufacturer if you face a high cycle application. A custom design may be required.

Here is how NEMA addresses starting duty for standard motors. That will exclude motors which are required to drive more than the standard inertia.
NEMA MG 10-2001 Table 7 gives parameters labeled "A", "B", "C", as follows:
A - Max # of starts per hour, regardless of inertia WK^2
B - Max Product of [starts-per-hour] times [inertia=WK^2 in lb-ft^2].
C - Minimum time between starts (in seconds) to allow motor to cool sufficiently to allow another safe start.
Items A and B are used to attempt to limit long-term degradation due to cumulative damage from repeated starts with adequate cooling between starts, and item C ensures the motor is cooled sufficiently for another start (prevent short-term abuse which will cause immediate damage)

http://www.conae.gob.mx/woruk/sites/CONAE/resources/LocalContent/2391/1/images/MG10.pdf

For a 75 kw motor, NEMA is showing that the motor can be started 5.2 times per hour without effecting the service life of the motor.

 

[itsmoked]

The smaller the motor the more starts and hour it will tolerate. If you indeed need only 35kW then you should use a smaller motor than 75kW as it will be much happier with frequent starts and short running times. In this case 'bigger' is definitely not 'better'.

Also a smaller motor will be more efficient and likely will have a higher power factor which is important these days for keeping your entire facility's energy costs down.

Keith Cress
kcress -

http://www.flaminsystems.com