Does a VFD rise the power curve?

[pietr]

Maybe it seems bizzare for some of you.
I have this situation:
A three-phase squirrel-cage 4 poles motor drives a centrifugal pump.
Motor has following parameters: 90kW nominal power, 160A nominal current, 0.86 power factor at nominal power and 0.95 efficiency at nominal power, class S1, 1500rpm.
Pump has following parameters: rated power 73kW, maximum power 90.1kW, efficiency 0.74, rated capacity 295 mc/h, total developed head 80m, speed 1485 rpm.
We started to use the pump and we expected to make the required capacity. But instead of this at aprox.157 Amps measured we receive only 80% of capacity at the pump (236 mc/h instead of 295 mc/h). In process we need the nominal capacity. But we cannot open the discharge valve more because we try to avoid motor tripping. The motor is installed in a hazardous area so we set the overload protection at nominal current as the standards says.
The designers tells us the most cheaper way to reach the nominal capacity is by installing a VFD to reduce the motor speed and release the discharge valve more.
I made some calculations and it seems it doesn't works because the torque will be higher because it flows more product in pump. The only benefit of installing a VFD is more efficiency, which means at same capacity the motor need less power, but what happens when we reduce the motor speed and increase the rotor torque (pump capacity)?
What do you think? Can a VFD solve the problem and WHY?

Thank you all!!

 

[ScottyUK]

Is your voltage low as measured at the motor terminals? The motor will, to a certain extent, behave as a constant power load so if you require 160A at nominal voltage to produce your 90kW then you will draw more than 160A if the voltage is low.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

To follow up on Scotty's post;
You can boost the voltage easily and cheaply by installing two transformers in open delta connected in autotransformer boost configuration.
Re Slowing the motor.
The designer may be correct.
On the downstream side of the discharge valve, you have 236mc/h at the utilization pressure. On the pump side of the discharge valve you have 236mc/h at a higher pressure. The difference or the pressure drop across the valve is being lost as heat to the process fluid. Given the ratio between the energy required to pump and the energy required to heat, the temperature increase is often not noticed.
However, using a VFD to run the pump at optimum pressure and flow so as to avoid the losses in the discharge valve may be a solution.
It depends. A final answer is outside the scope of this forum. The answer requires reference to the pump curves which we don't have and the designer should have.
I would look at the difference between the inlet pressure and the outlet pressure of the discharge valve.
If there seemed to be enough lost energy to be worth avoiding I would ask the supplier for a reasonable performance assurance and proceed.
It looks like you are short about 20%. That may be hard to obtain with a voltage boost unless you have very very low applied voltage.

I made some calculations and it seems it doesn't works because the torque will be higher because it flows more product in pump. The only benefit of installing a VFD is more efficiency, which means at same capacity the motor need less power, but what happens when we reduce the motor speed and increase the rotor torque (pump capacity)?

You are on the wrong track. Look at the lss across the discharge valve. Estimate it by considering the pressure drop across the valve. With the VFD installed and the valve removed you will be controlling flow by pump speed instead of throttling. I know, it sounds "crazy".

Bill
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"Why not the best?"
Jimmy Carter

 

[pietr]

Well, we also estimate the loss in the discharge valve and it's about 0.15 bar (nominal diammeter 200mm), and in my perception is insignifiant.
Because we operate with the discharge valve at about 40% opened we obtain 9.3 bar in downstream which is more than we need.
We have a well distributed power network. The transformer is at 100m distance from the motor and the power cable is copper 3x120sqmm. So voltage loss is also in limits established by standards - max 10% at start when is feeded from MCC installed in power station. In normal running the voltage loss is also insignifiant.
I will consider any suggestion you may have.
Thank you!