Where does it stop?

[ptonsparky]

A couple years ago a customer installed a 10 HP 3Ph submersible on a 1ph service. Pump installer was required by VFD mfg to over size by 3 times. My branch circuit is sized for 35 KW of the drive plus the 1.25 multiplier. They now want a standby gen which has been sized at 55 kW by supplier. With note that tractor must be rated at 110 hp.

Do you think this will be adequate?:blink:

 

[Jraef]

Overkill on top of overkill...

The drive sizing might be legit. But what might be happening is that because the drive is oversized, someone is looking at the generator size based on the DRIVE size, not the LOAD size, and that's wrong. Then because the generator is over sized, they are sizing the engine for the generator, which is wrong too.

The drive has to be over sized because of having to rectify the single phase. Nothing else needs to be over sized, it's still a 10HP load.

 

[Haji]

Nothing else needs to be over sized, it's still a 10HP load.

No. First to limit voltage drop on start of pump motor, the alternator may be sized twice i.e 20KVA which will also reduce overheating due to harmonics from the drive.
Second and similarly to prevent stalling of tractor due to peak KW during start of pump motor, the tractor engine may also sized twice i.e 20 hp or better 25hp. So my recommendation:
The alternator size:20KVA
The tractor size:25hp

 

[kwired]

No. First to limit voltage drop on start of pump motor, the alternator may be sized twice i.e 20KVA which will also reduce overheating due to harmonics from the drive.
Second and similarly to prevent stalling of tractor due to peak KW during start of pump motor, the tractor engine may also sized twice i.e 20 hp or better 25hp. So my recommendation:
The alternator size:20KVA
The tractor size:25hp

Starting a motor on a VFD doesn't result in the surge current you see when starting same motor across the line.

power factor of the motor only appears on the load side of the drive.

If this motor is only loaded to say 8 HP a generator rated at 7.5 kW will likely run it with little troubles.


Had an irrigation pump for a golf course one time where the POCO offers better rates if they can connect radio controlled load shedding equipment - which is typical for the many agriculture irrigation services in the area - they can shut down a certain number of wells during peak demand periods. If you don't want to be shut down you get a different billing rate.

When we converted this pump to the VFD, POCO said you don't have to shut it down completely when their controller is active, just make sure you are below a certain load level. So that is what we did, when the motor is at a lower speed the input power is less, and that is all they wanted.

 

[Haji]

kwired: you may take into account any need for pump operation when the VFD is in bypass mode.

 

[kwired]

kwired: you may take into account any need for pump operation when the VFD is in bypass.

The three phase motor is not going to be very effective if you bypass the drive and supply it with single phase source.

 

[Haji]

The three phase motor is not going to be very effective if you bypass the drive and supply it with single phase source.

Oh! sorry. I forgot it.

 

[Haji]

kwired: Still the alternator size should not be same as the load because of harmonics from the drive and its size be twice as much to prevent overheating on full load. Again for tractor size, the efficiency of thermal power conversion is poor, around 40%. So a hp of 25.

 

[iwire]

kwired: Still the alternator size should not be same as the load because of harmonics from the drive and its size be twice as much to prevent overheating on full load. Again for tractor size, the efficiency of thermal power conversion is poor, around 40%. So a hp of 25.

I will take Jref's word for it, he has likely forgotten more than you have ever known about motors.

But if I ever have a question about India's telephone system you are the man.

 

[iwire]

kwired: you may take into account any need for pump operation when the VFD is in bypass mode.

Who said it has a bypass?

 

[Haji]

he has likely forgotten more than you have ever known about motors.

Hey iwire that is rude as the feeling I have is same as you would have when I call you a fatty idiot.

 

[kwired]

Hey iwire that is rude as the feeling I have is same as you would have when I call you a fatty idiot.

That depends on how you read it, it could be taken as a compliment to Jraef as being an exert on motors, he probably forgot things about motors that I never knew as well.

 

[ggunn]

Hey iwire that is rude as the feeling I have is same as you would have when I call you a fatty idiot.

That is totally different; calling someone an idiot, fatty or otherwise, could get you banned from the forum.

 

[Jraef]

No. First to limit voltage drop on start of pump motor, the alternator may be sized twice i.e 20KVA which will also reduce overheating due to harmonics from the drive.
Second and similarly to prevent stalling of tractor due to peak KW during start of pump motor, the tractor engine may also sized twice i.e 20 hp or better 25hp. So my recommendation:
The alternator size:20KVA
The tractor size:25hp

Look at the load. The LOAD is a 10HP pump. Nothing about the drive, wire, alternator etc changes that. 10HP at 3 phase is still 10HP at 1 phase.

The VFD is a phase converter and is over sized to protect ITS components. The load is still 10HP. But by virtue of it being a VFD, it can allow that motor to start with FLA if necessary. So there is NO need to over size for the starting load.

True, the VFD adds harmonic current to the alternator load, but at the same time it corrects the displacement power factor of the motor to .98 which decreases it. It's not a total wash, so yes, the alternator likely still needs to be over sized a little, but my point was that this was not done correctly. Even if they used a 20kVA alternator to give it some extra fudge factor, that would NOT require a 110HP engine! My SWAG would be around 60HP at worst. 25kVA supplied by an engine conversion of 40% efficiency.