Single phase amp draw vs convert to 3 phase

[donwynn]

Ok examples single phase motor 1.5 Hp draw 7 amp
And when I convert single phase motor 1.5 Hp to 3 phase motor 1.5 ph using VfD primary coming to is drawing 8.5 amp
and some how we are seeing significant saving using 3 phase motor ?

 

[tom baker]

VFDs are commonly used for single phase input, 3 phase output, but the VFD must be oversized
Where are you measuring the 8.5 amps and what type of meter? What does the VFD display read for amps
What type of load, water pump, fan, ???

 

[herding_cats]

Yaskawa dealer here:

First determine if the load is constant torque or variable torque. Compressor? Fan? ??

Use a line reactor in front of the drive.

In constant torque applications, you will want to double the amperage to size the drive. If the single-phase load is 7 A, use a drive rated at least to 14 A.

Need more info on the application. Thanks.

 

[winnie]

Ok examples single phase motor 1.5 Hp draw 7 amp
And when I convert single phase motor 1.5 Hp to 3 phase motor 1.5 ph using VfD primary coming to is drawing 8.5 amp
and some how we are seeing significant saving using 3 phase motor ?

You won't see large savings using the 3 phase motor vs a single phase motor.

A 3 phase motor will be slightly more efficient. A 3 phase motor doesn't have the capacitors or centrifugal switch to fail. A three phase motor likely has better power factor. If you have 3 phase power available at a location, a 3 phase motor installation is likely cheaper than a single phase motor for anything above a couple of horsepower.

A 3 phase motor draws significantly lower amps, but on significantly more wires, net result is that approximately the same power electrical is delivered to the motor in order to supply the same mechanical power to the load.

You will not see a significant energy usage savings replacing a single phase motor with a 3 phase motor running on a VFD for a constant speed application. You should only do this if you need a specific motor that is only available 3 phase or if you need speed control.

-Jon

 

[herding_cats]

If you are looking for energy savings, only by speed controlling will you get that, and maybe through demand charges from a soft start.

 

[ptonsparky]

All sorts of calculators out there.

 

[herding_cats]

This is 3 phase no drive vs. 3 phase with drive.

If you can keep the load at 40Hz and still achieve the same work then by all means yes there’s $$ to be saved.

I have around 500 HVAC units with compressors and blowers on drives. The energy savings is huge.

 

[Jraef]

Ok examples single phase motor 1.5 Hp draw 7 amp
And when I convert single phase motor 1.5 Hp to 3 phase motor 1.5 ph using VfD primary coming to is drawing 8.5 amp
and some how we are seeing significant saving using 3 phase motor ?

So are these actual measurements you have taken? 1.5HP 230V single phase motor should be around 10A, so 7A sounds like a measurement under load. Was the load exactly the same when running from the new 3 phase motor and VFD?

In general, the current for the EXACT SAME LOAD fed from a single phase source will be higher than the 3 phase current by the square root of 3 (1.732). So if the motor was drawing 7 amps as a single phase motor, then the current for running the EXACT SAME LOAD on a 3 phase motor would be LOWER by that factor, so 4A on the 3 phase motor. But the single phase current FEEDING the VFD will still be 7A, PLUS the losses inside of the VFD, roughly 3%, so roughly 7.2A. If the INPUT current of the VFD is 8.5A, there is either a significant difference in the LOAD on the motor, or a measurement error.

Also:
What is the machine?
How are you measuring current?
Who told you that switching to a 3 phase motor was going to be giving you "significant savings"? Energy savings ONLY happen when you can eliminate LOSSES in a system. If it is a centrifugal machine, like a centrifugal pump or fan, and only if variable flow is a part of the application, that can work. If you do have a centrifugal pump and you always run it at full speed with the VFD, it WILL actually consume MORE power than if you just ran it across-the-line. But if you varied the flow with a valve, and now instead you vary the flow by changing the motor speed, then the savings are potentially very significant because you are eliminating the losses associated with the valve and the motor running needlessly at full speed. So without knowing anything about how the VFD is being used, there is no way to speculate on any savings (or losses).