Motor Selection

It depends upon the starting arrangements and the circuit protection arrangements, details of which have been withheld from the forum.

Consulting a qualified local Electrical Engineer is highly recommended.

...because the starting current will be somewhat larger and "upping the fuses" and "upping the overload" might not be an option with the existing cabling.

And that's why an assessment of the installation in line with the applicable national installation standard is required in order to answer the original question. The full protocols in BS7671, for example, might not be applicable as the country of installation has been withheld from the forum.

Should be fine....might start with a bit more torque, so you need to check if this will be a problem depending on starter method, and that the rpm's are the same, and the voltage rating...the amp draw should be similar under FL...You might lose some efficiency, so temperature should be monitored on startup...

https://bralpowerassociate.blogspot.com/2013/05/motor-kw-and-hp-chart-with-full-load.html

The amp draw will be similar under actual load, but not under FL (rated load) if that's what you meant.

Yes, thanks, nice catch....

The purpose of the fuses in the circuit is to protect the wiring to the motor,and the wiring should be rated at 125% of max load,minimum.

The purpose of the motor contactor,and related thermal overloads is to protect the motor.

You cannot use larger fuses unless the wiring is rated for the increased current that may be required under max load and starting current,and the thermal overloads should be sized according to the max current of the motor,including the service factor multiplier.

If all of these criteria are met,it should be alright,but I recommend consulting with a local electrical engineer to be safe.He will be familiar with local codes and regulations,to which we are not informed.

<..the wiring should be rated at 125% of max load,minimum...>

That may well vary depending on the electrical installation codes applicable in the country of installation.

Let the motor efficiency is 0.92 and power factor 0.85

The Input power of motor

= Pump power/ (motor efficiency * Pf)

=140/(0.92 *0.85)

=185 KW

185 KW motor is suitable for driving the pump of 140 KW.

* The design data may change with change in the efficiency and power factor of the motor.

Never mind the motor. The wiring may not be suitable for the increased starting current, and it is this criterion that may well determine acceptability.

Most of that is wrong, I'm sorry to say!

Input power is irrelevant to this discussion, but it = Pump power/motor efficiency, not Pump power/ (motor efficiency * Pf). Pump power/ (motor efficiency * Pf) gives the input KVA.

The fact that the KVA = 185 is irrelevant. Motor rated power is the shaft output (mechanical) power. If 140kW has been determined as adequate pump input power (taking account of variations along the pump curve, and any safety factors specified) a motor rated 140kW is suitable. A higher rated motor is also suitable, provided the precautions identified by others in this thread are followed.

Changes in the efficiency and power factor of the motor only affect the input current, to give rated (output) power.

I have some edits in my previous post:

Let the motor efficiency is 0.92 and power factor 0.85

The input KW

=140/0.92

=152 KW

The Input KVA

= Pump power/ (motor efficiency * Pf)

=140/(0.92 *0.85)

=185 KVA

The 185 KW motor is suitable for driving the pump of 140 KW. However,the frame size of the motor will get changed so base frame modification is required. The no load current will increase and motor will run at about 80 % loading in comparison to 150 KW motor. The power factor will be low and the efficiency of the motor will be low at lower loading. The cable size and voltage drop calculation needs to be checked.

* The design data may change with change in the efficiency and power factor of the motor.

You wouldn't like to acknowledge my #11 in prompting this, by any chance?

Various posters have said 185kW (not KW) motor is suitable, calculating the input kW and KVA doesn't add anything.

OP said the frame size of the 2 motors is the same, so presumably he has checked this. Incidentally, according to my data neither 140 nor 185kW is a standard rating, but maybe they are de-rated outputs due to operating conditions - ambient temperature, elevation etc.

As JRaef said, operating cost might be slightly higher with the 185kW. My guess is the effect will be small. 185kW will have lower efficiency as it operates at lower % output, but full-load efficiency of 185kW is likely to be higher than 140kW.

Thanks for information. I appreciate you.

No problem!

Assuming a centrifugal pump, the starting current characteristics of the larger motor will not be much different than your original motor, if the original starting equipment was sized properly, it will work fine, from more than just a few personal experiences.

Depending on the jurisdiction the AHJ may want the starter / switch gear and cabling sized for the motor NP.

If it is on a VFD you may want to also verify its rating.

Magnetization current may be higher also.

If in doubt, consult a qualified local Electrical Engineer.

Motor losses can be divided into two main components; fixed losses in making a motor behave like a motor at all, which account for around 25% of the losses and losses that vary with the associated load which comprise the rest. The load related losses remain the same for your pump because it is the same pump, same flow, ergo same load. The fixed losses however will be based on motor design factors so they will increase when you increase the motor size. So aside from the possible installation issues, you need to wright the slight increase in operating costs against the value of reducing the number of spares you need to carry.

It is Ok .But motor will be running at lower effiecy. Unless the motor is of energy efficient type where efficiency is constant from 100- 75% load .So if it is a standard motor it will run at lower eff and may consume more power.However you have to check your starter rating.Also starting current which is normally 6 times with 20% tolerance will be higher as it will be 6 times on full load current of 185 kw compared to 150Kw