# help with power consumption.

Discussion in 'General Electronics Discussion' started by zms, Jul 5, 2016.

1. ### zms

17
0
Apr 29, 2016
I need to determine the power consumed by a machine tool in kwh.

while checking out the spec-sheet of an automatic lathe i found that nowhere on the list was the mention of the power consumed in kwh, but rather it was mentioned that the lathe used a 2.5 hp motor.

so from this would it be safe to arrive upon this kind of calculation.

2.5 hp = 1.864 kw.

if the machine is run for one hour then, consumption is : 1.864 kw x 1 hr = 1.864 kwh.

Is the above calculation correct ? if so do correct me please.

2. ### zms

17
0
Apr 29, 2016
note: the 2.5 hp of the motor is the mechanical power of the motor.

3. ### Harald KappModeratorModerator

11,164
2,550
Nov 17, 2011
The electrical power will be higher due to losses in the motor. Unfortunately I don't know by how much. The most precise method wil be to use a power meter in the electrical connection and measure real power used.
Note that it is insufficient to measure the current as in AC the phase difference between voltage and current as introduced by the inductance of the motor also plays an important role. A good power meter will take this into account and show real power as well as apparent power.

4. ### Bluejets

4,614
977
Oct 5, 2014

#### Attached Files:

• ###### PowerMeter.JPG
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Last edited: Jul 5, 2016
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5. ### duke37

5,364
769
Jan 9, 2011
The power will be less than the maximum motor power, it depends on the work the lathe is doing. Is it turning balsa wood or machining stainless steel?

6. ### BobK

7,682
1,688
Jan 5, 2010
There must be a voltage and current rating for the motor. The power is the product of the two.

Bob

7. ### duke37

5,364
769
Jan 9, 2011
No, the power is the product of the voltage, current and the power factor.
The motor rating does not say what power is actually used.
The power factor will be high when the motor is heavily loaded but low when it is lightly loaded.

8. ### BobK

7,682
1,688
Jan 5, 2010
True. The product will give the max power used.

Bob