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# Plunger Triplex Pump- Stiff Shaft cause high current

## Plunger Triplex Pump- Stiff Shaft cause high current

(OP)
Hi,

We have triplex pump running for sometime with VFD of 20% of 50 HZ

Cat Pump

11 KW 4 Pole motor 400 V
Maximum current 415 V is 20.2 Ampere.

We see the running current at Discharge Pressure of 51 Bar is 20.6 Ampere and at 0 Bar is 11 Ampere.

At 0 Bar to 45 Bar, the motor was running okay, no trip. Until it reaches 20.6 Ampere of 51 Bar it tripz.

some doubts to clarify

1. We find the plungsr is tight if hand rotate by shaft when motor was coupled.

2. At 51 Bar, it is trip after 15 minutes of running due to Motor ovwrload.

3. From the Cat website, this formula is the motor sizing

HP= flow in GPM x PSI / 1450

The model ia 1541
Max Flow 19.3 gpm / 73 lpm
Max Pressure 1200 PSI
RPM 1180

Is recalculating the power at VFD speed would be as followed.

10 PSI case of 20% speed

HP= ( 0.2 of 19.3) x 10 PSI / 290 RPM = 0.134 HP
We found that it did not correspond to running current which show 11 Amperes which used up around 50 % of motor rated current of 20.3 Ampere at pressure.

If we used the running current of the pump at 10 PSi.

Converting to power = 400 V x 11 =4400 Watt or 0.44 KW or 0.54 HP. This seemed fsr off from the manufacturer calculated formula when run ay 20% of speed.

4. What could be the cause of overload, is there a way to calculate the real running current using VFD theory at 20% OF 50HZ.

5. Any experience the piston pump has mechanical failure and caused pump shaft to stiffly rotated so the running current is higher.

### RE: Plunger Triplex Pump- Stiff Shaft cause high current

I think you're mixing up a lot of data.

The voltage applied at 20% ( very low) frequency won't be 400V.

Investigate the output or measure the voltage on the motor terminals as well as the current.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Plunger Triplex Pump- Stiff Shaft cause high current

(OP)
Littleinch,

Yes would do, i measured from incoming, when run on VFD, i did not measure the voltage at outgoing.

Ok would do check on this terminal phased as well.

### RE: Plunger Triplex Pump- Stiff Shaft cause high current

I think it depends on how the VFD is set up but I think a lot of them with on a fixed V /htz ratio.

Please let us know how you get on.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Plunger Triplex Pump- Stiff Shaft cause high current

Is that phase to phase or phase to ground?

Either way a lot less voltage than 400V, so a lot less power being used.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Plunger Triplex Pump- Stiff Shaft cause high current

(OP)
Hi Littleinch,

It is Phased to Phased
UV
UW
VW

However, wondering why the current is exceeding rated current of the motor ? .

Is it normal at VFD 20%, generating flow 18 LPM 51 Bar, the power usage is greater than 11 KW ?

Using the manufacturer formula

HP= flow in GPM x PSI / 1460

At 20 % VFD 290 RPM 4.8 GPM, HP = 4.8 GPM x 740 PSI/ 1460 = 2. 4 HP

or

At 20 % VFD 290 RPM 4.8 GPM, HP = 4.8 GPM x 740 PSI/ 290 = 12. 2 HP

I am not sure if we shall considered the speed of 1460 or 290 RPM as from the example from the website, it uses 1460 to get the Power Hp at 55 Bar for model 1541 pg 1, 9.9 HP at 68 LPM, Pump RPM 1100 RPM.

At 1199 RPM 18 GPM, HP = 18 GPM x 800 PSI/ 1460 =9.9 HP

Some doubts, if we used direct drive motor inverter 4 Pole 50 Hz, to bring down the speed of 11 KW motor at 20% or 50% at 55 Bar, would it be sufficient ?

I changed

### RE: Plunger Triplex Pump- Stiff Shaft cause high current

The calculation provided is based on water and a fixed efficiency of the pump (about 85%) so the 1460 us a constant and not related to rpm.

I put this through Eng tool box and got about 2.1hp https://www.engineeringtoolbox.com/pumps-power-d_5...

However you're still mixing up lots of different bits of data and I can't work out which case corresponds to which speed or current.

Like you show when you use a VFD, the voltage changes.

You also need to note that a PD pump in VFD is great at changing the flowrate, but the pressure is dictated by the downstream system.

So you always need to find out at the different speeds, what is the flowrate, what is the speed, what is the line to line voltage at that speed, what is the current and what is the differential pressure (outlet - inlet).

Only then can you start to make any sense of this.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Plunger Triplex Pump- Stiff Shaft cause high current

(OP)
Hi Littleinch,

Issue is my system pressure is throttled not gdnerated by systsm at 51 bar and the motor is overload whixh is not supposed to be.

Littleinch, i managed bring down the pump to my setup system however i am using a different 15 KW motor to test this pump actual power consumption.

I surprised 15 KW is not able to run at more than 25 bar. it was overload. I am using gate valve to throttle it near the discharge.

https://1drv.ms/f/s!AshU1On9Vy6fiv0vcygvzRgH9yYR1g
Fron understanding tbe power, torque speed

[quote]
Based on the relationship, torque must double if HP is to remain constant when speed is reduced by one half. To produce the same HP at the lower speed, a motor has to do twice as much work per rotation, which requires twice as much torque. That is why the shaft and frame of a 900 rpm motor are usually larger than those of an 1,800-rpm motor of the same HP.

When the VFD reduces frequency and motor speed, it also reduces voltage to keep the volts/hertz ratio constant. Torque remains at 100 percent, but HP is reduced in direct proportion to the change in speed.

At 30 hertz, the HP is just 50 percent of the 60-hertz HP.
[End]

In other words, if my motor 11 KW at 20% of VFD it is only have 2.2 HP while keeping the full load torque constant.

Is 2.2 HP sufficient to drive this pump at 51 Bar 20% speed ? .

From my experiment in my 15 KW setup, it was overloaded at 25 bar.

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