Oversized Motor

I agree that the 185kW motor will be OK.

Slightly curious about "Our original pumps are 110Kw, the new pumps are an exact match as far as hydraulic performance and have there absorbed power listed as 111.7Kw."

I assume the 110kW is motor rated (output) power, and 111.7kW is the pump shaft power. That seems a bit near the knuckle, depending whether the 111.7kW is at duty point or maximum on the pump curve.

But 185kW is plenty!

Cheers..........Codey

Along with CM's comment you lose quiye a bit on efficiency (working at approximately 60% load - it is just a bit above the permanent star value, which would have improved efficiency.

Overall I agree with CM in #1

Plese take into account, for example that the rated full load current is about 321 Amps for a 415 volts 185 KW three phase motor and about 190 Amps for a 110 kw 415 volts motor and judge for yourself the percentage increase (Note, I am not talking about the inrush it is a secondary consideration).

Not only the cables should be suitable for this increase in rated current, you should essentially take into account the suitability of magnetic contactors and thermal overload relay in the motor starter as well as increase in electricity bill.

Circuit Breaker rating in one case should be about 700 Amps and in another case 400 Amps (415 volt system).

I am not aware about all the parameters and my best advise in these circumstances is that you should check yourself the name plates of both the motors, account for the increase in rated current (FLA) as would be ascertained from the name plates.

Bottom line is that I do not agree with your philsophy of having the pumps as a set criteria to judge the electrical parameters on various accounts in the changed scenerio.

mountk2 is correct. Conductors and protection must be sized to motor nameplate information, not load. If 111.7 Kw is the maximum load on the motor, you might get away with it.

If you have to comply with codes, you may have a problem.

Previous comments regarding efficiency losses are correct- that means that the motors will draw more power than the load requires because of that issue. I recommend that you contact the motor manufacturer and get a rating for power draw with the 111.7 kW loading.

Second- ALL of the protective devices have to be set to protect the conductors. You need to verify actual conductor sizes and insulation and check the NEC to verify maximum power (amperage) that the conductors can withstand in a constant load condition- start up power does not matter to the conductors, it is only seconds befor returning to normal power.

Net effect, you can install ANY motor to ANY conductor so long as the protective devices in place protect the conductors. If the motor load will exceed conductor allowable amperage- something has to change.

NOW- If you find that the power that will be drawn from the new, bigger motors in this installation will exceed the capacity of the conductors- very possible- you have two reasonable options- short of fully installing new conductors and conduit.

First- check to see if multiple smaller conductors - say two sets of 350 MCM vs your current one set of 500 MCM will fit in the same conduit (likely) and the derating for multiple conductors will likely still let you use the 2x350. I did this once before when a chiller arrived with a rating higher than the manufacturer had provided and the motor would overload the original conductors- in my case it was 3x250 replacing 2x350.

Second option- and likely cheaper- is to use a transformer at the beginning of the feed to boost voltage- say 480 VAC, 3-phase to 600 VAC, 3 phase (dropping the net amperage and staying within the conductor insulation ratings) then install a second transformer at the motor feeds (a disconnect switch can also be the junction box).

My advice is to find out why the previous engineer ordered the motors to be bigger (185 instead of 110)! Unless he was nuts, there should be a reason: maybe he intended to increase the pumps output and found the original motor already working at maximum (you are saying that the load is now 111.7 which is the old motors full load!?).

On the protections required for the supply lines and controls, you have been answered adequately by others.

If the pumps are not going to be running for higher outputs (refer to the pump curves...) then the existing installation (cables etc) could be safe if properly adjusted so that the cables are not overloaded (as said before by Energygod...).

According to API 610 standard, permanent spare capacity requirement for motors 55 kW and above is 10 %. This means that for the declared full load of 111.7 kW selection of 125 kW motor is sensible while 185 kW is far beyond imagination.

The effect of running a motor at a lower load than optimum sizing is that power factor (electrical) goes down. The motors have to cater (electrical) iron losses and copper losses which are proportional to the capacities of the motors itself while the impact of pump (although size is unaltered) is negligible on the sizing of cables and associated switchgear.

You are correct.

For the benefit of the OP, the new pumps are already bought and installed (it seems) therefore all this is academic and instructive.

On the other Hand, while he was worried about the motors being loaded to near maximum, I would say that this is a point worth looking at: The pump Curves should be looked at and he should see what will be the maximum power draw that the pump can take when the output pressure is minimum (far right of the curve) and the volume pumped is maximum?. Can this status be achieved with his present installation?

If the motor cannot be loaded to the point where the current drawn will reach the limits of the installation design, then he is OK (the only draw back will be the extra power cost to run an oversized motor!).

You should give data of old pump to several manufacturers and get competitive quotations and select a pump of nearly same hp as the old.You must consider whether the suction/discharge pipe sizes will match the existing one and other measurements too.Otherwise you will have to spend more money in modifying them.

If the setup is controlled by an ac or dc drive with no parameter changes in the drive from what you had before,and your pump and motor are the same frame size and rpm as you were with the old setup, then you will have no problems. If the motor is mcc/breaker driven only, then the mcc/breaker will need to be modified/improved and a soft start may be needed.

The electrical side of things has been addressed by others better qualified than me. Just a comment on why such a large motor may have been specified. Could it be that you are pumping something with a low specific gravity? If so the motor may have been sized for water, either for testing purposes or for start up or flushing with water.

Seeing the situation you are in, I suggest the following line of action:

1. Check motor connecting cable size and see if it has sufficient margin on CCC. If yes, then

2. Check Circuit Breaker rating which hopefully will be 630/ 800A ACB and hence nothing to worry about.

3. Install the motor,

4. Review its protection settings vis-a-vis the existing motor settings. Prefer not to change them as new pumps are exactly same as old ones.

5. Run the motor on no load and check its performance.

6. Run it on full load briefly and see the current drawn. Hopefully it will be as per 112 Kw motor.

Your job is accomplished with only things to worry about is the constant losses and reduced pf and efficiency of the motor.

Good luck

BB Raina

I do not know so much about motors and I am not an electrical engineer, but we recently had some pumps installed and were advised that because of voltage drop in the very long cables on site, a larger motor was needed. The motor would draw much less than its rated power because the cable resistance lost some of it. The interesting thing was that because of the power drop, the cables and switchgear were not overloaded.

Run it by an electrical/switchgear/motor expert but this might be why larger than original motors were fitted.

Hello Guest

I don't think that quite holds up. The power drawn by the pump is almost constant, irrespective of the voltage drop. Power drawn would fall a little as the motor (loaded) speed falls slightly at lower terminal voltage, but unlikely to be significant.

So the current drawn rises as the terminal voltage falls. Increasing the motor size doesn't affect it directly. It might give a small further increase in current due lower efficiency, as actual load is a smaller percentage of rated power, as other posters have mentioned.

Cheers.......Codey