motor rotor speed lower than pump speed

[fe2655847]

Hi experts,

Can a motor with 10HP, rotor speed of 1765 rpm having synchronous speed of 1800 rpm drive a centrifugal pump with 1770 rpm and rated power of 7.3HP? Motor rotor speed is less than pump speed but can the motor accelerate the pump to its rated speed? Or motor rotor speed and pump speed
should be the same? to meet the pump requirements at that pump's rated speed. Thanks in advance

 

[LittleInch]

In order of your questions,
Yes
Yes. This depends on how the motor starts, i.e. DOL our some sort of soft start is what affects the ability to start. Most pumps like to start against a closed valve, but it's not vital.
They are the same within normal limits. This sounds like a 60htz, 4 pole motor. You will find the variance on incoming frequency gives you more change in rpm than this and the pump will not be affected by this very minor gap in rpm.

My motto: Learn something new every day

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

 

[Artisi]

In my part of the world frequency is guaranteed, 50 hertz is 50 hertz, voltage may vary slightly but is usually of no consequence.

If the pump curve is drawn at 1770 rpm and the motor speed at the fullload motor speed is 1765 rpm then the flow and head would be slightly lower but hardly noticeable on a pump of this size. However, at the duty point of the pump system the motor speed could well be above 1765 rpm but would have to be measured to confirm this.

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.)

 

[LittleInch]

I just looked up the frequency requirement here and it's +/- 1% and suspect it's the same everywhere else. There is no such thing as "fixed" or "guarenteed" when it comes to utility supplies... guarenteed to be within 1% maybe, but not 0. Given the variability of loads 0 would be physically impossible.

That's +/- 17.5 rpm based on 1750rpm, so 5 rpm is negligible.

My motto: Learn something new every day

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

 

[BigInch]

Yes. There is a speed range.
1800 is the no load speed
it slows down a bit when the pump is loaded

Approximate Electrical Motor Speed (RPM)
........Speed with-----Synchronous Speed with
......... Rated Load ..... No Load
Poles.....60 hz 50 hz ... 60 hz 50 hz
2........3450 2850......3600 3000
4........1725 1425.....1800 1500
6........1140 950....... 1200 1000
8.........850 700 ........ 900 750

Independent events are seldomly independent.

 

[Pumpsonly]

The 1765 RPM is the motor RPM at rated full load of 10HP. WHen the motor is not required to deliver full load, the RPM will be higher. Since your pump rated power is 7.3 HP, the motor will run at higher than 1765 RPM.
The pump curves are usually drawn or published with speed between 1750 to 1780 for 4 pole 60Hz operation.
Induction motor speed varies with the load.
Even for the same power supply frequency and No of pole, the full load speed varies with the size of the motor.
The larger kW motor will have speed closer to the synchronous speed and vise vesa.
For induction motor,the No load speed will still be below the synchronous speed by a few RPM.

 

[ozmosis]

If your motor is an asynchronous motor/induction motor (IM), this is called slip.
Slip is basically the difference between synchronous speed and the nameplate rated speed when under full load (current).
Your nameplate will show a slip speed that equates to the rated current that, given you are on the rated voltage, will deliver the nameplate power for the motor.
The closer the rotor speed to synchronous, the less load/current is used. If an IM is operating DOL and operates at a slower rated rotor speed than the nameplate, it will be drawing excess current and going into overload. Not good if operating like this for long periods of time and your safety overloads should protect the motor from overheating.

 

[fe2655847]

Thanks guys..I have never seen a clause in API or other standars that requires motor rotor speed and pump speed should be the same. I think as long as the motor is sized correctly to drive the pump regardless of gap in speeds, the pump/motor combination will be working fine. Please correct me if I'm wrong.

 

[Artisi]

For a direct coupled pump/motor, the pump speed can only be the same as the motor speed. To consider anything else is nonsense. Cannot understand why any spec. would make this statement unless it is trying to say no speed increaser or decreaser between the motor / pump is allowed.

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.)

 

[ozmosis]

I agree with Artisi, I cannot really understand the focus on pump speed unless there is an actual device to control speed. The motor slip has an obvious (small) effect on speed but this is completely related to load. For an asynchronous motor, you will always have slip but it is the load/current you monitor usually, not speed.
You are talking about a -2% deviation as slip speed from synchronous and, unless you have an additional device that is uncontrolled, I cannot see why this is being requested unless it is a safety function to feedback that the motor is actually running when commanded and, for example, a belt has not broken between motor and pump.
However, this would be a comparator rather than actual measurement of speed and again, usually this is load being measured (or no load in this case).

 

[electricpete]

I think op is referring to nameplate speeds.


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