VFD on 1PH

[ptonsparky]

Trying to get an idea of what the input current of a 30hp 22kw vfd will be when connected to 230v single phase. Motor is a 15hp 11 kw 3ph pump. Pump installer is supplying the drive, I just need to communicate the power requirement to the POCO.

My first thought is 22000/230=95.6 but I only have 11kw load. 11000/230=47.8. That simple? What do I size the feeder at? 95.6 is my shot.

 

[mike_kilroy]

11000/230=47.8. That simple?

almost. you forgot to multiply by 1.73 since you no longer have 3ph; so it can draw at full motor load 83 amps.

but best to do the math using the motor's nameplate current rating to be sure; some motors have higher than nominal current rating for various reasons.

 

[ptonsparky]

Not sure cause I am getting older, but I believe I started with the single phase current. Dividing that answer (47.8/1.73=current on 3ph) Ignoring PF.

 

[Besoeker]

Trying to get an idea of what the input current of a 30hp 22kw vfd will be when connected to 230v single phase. Motor is a 15hp 11 kw 3ph pump. Pump installer is supplying the drive, I just need to communicate the power requirement to the POCO.

My first thought is 22000/230=95.6 but I only have 11kw load. 11000/230=47.8. That simple? What do I size the feeder at? 95.6 is my shot.

In principle but you also need to take into account of the motor and the VFD.
The 15hp or 11kW is the motor output. I don't know what the efficiency of a 15ho motor is but I'd guess no better than 90%. The VFD possibly 95% which then gives you a total input power of something over 13kW. Then there is the effect of input current that will increase the value of the RMS input current significantly. Can the VFD manufacturer give you that?

 

[sgunsel]

Table 430.250 (3 phase) lists 42 amps for 3 phase 15 HP.
Table 430.248 (1 phase) lists 55 amps for 10 HP - as high as it goes.
So it looks like 80-90 amps should be in the right ballpark.
Make sure the drive will handle the amps.

 

[T.M.Haja Sahib]

If the POCO base their connection charges also on the hp/KW of the motor size, it is better to communicate the same to them instead of telling them what the input hp/KW of the motor is, which would definitely increase their charges.

 

[Besoeker]

almost. you forgot to multiply by 1.73 since you no longer have 3ph;

Since it's single phase power that's being calculated, the sqrt(3) doesn't come into it. The supply doesn't know that the VSD output is feeding a 3-phase motor.

so it can draw at full motor load 83 amps.

The VSD input current isn't the same as the motor current.
For example, imagine the motor running on light load. A 15 hp motor is likely to take in the region of 30% of full load current at no load.
All the VFD has to supply is the losses so, under those circumstances, the VFD current would be very much lower than the motor current.

 

[bob]

I believe the code requires the circuit to have the ampacity of 125% of the drive rating.
Does not matter what size the motor is. Correct me if this is incorrect.

 

[Jraef]

I believe the code requires the circuit to have the ampacity of 125% of the drive rating.
Does not matter what size the motor is. Correct me if this is incorrect.

Absolutely correct (for us here in the US). Article 430.122 I believe.

So here is how it really works: To power a 15HP motor from a 230V single phase source, you will need a 30HP VFD. A typical 30HP 230V VFD is going to be rated for 120A (based on A-B). That means your circuit must be NO SMALLER THAN 150A.

This little "gotcha" has burned many a contractor because the user or OEM selects the components and doesn't know what the code is going to require, but the contractor is the one responsible for that aspect.