Electrical principles - Power

Relationship

 

Work
When there is movement against a resistance by using force, work is achieved. The further the movement and the greater the resistance, the more work achieved. The formula for work is:

Work = Force x Length

Work is measured in joules.

 

Energy
Energy is the ability to do work. The potential to do work is defined by the following formula:

Energy = Power x Time

The unit for energy is the joule (J).

Energy can exist in many forms:

• magnetic
• light
• nuclear
• chemical
• sound
• radiant.

Electrical energy is often large and measured in kilo watt hrs (KWh).

 

Power
Power is the rate at which work is done. It is calculated by the following formula:

Power = Work / Time

The unit for power is the watt (W).

Efficiency

The efficiency of a device relates to the performance.

The efficiency of an electrical machine (ie a motor) can be determined by comparing the input power against its output power.

No electrical device is 100% efficient and there will always be power losses. These losses include mechanical losses caused by friction and electrical losses caused by heat generated through current flow.

Mechanical efficiency can be calculated using the formula:

Efficiency = Power Output / Power Input x 100%

 

Example

What is the efficiency of a motor that consumes 800W and has losses of 200W?

Output power = 800W - 200W

Output power = 600W

 

Efficiency = Power Output / Power Input x 100%

Efficiency = 600W / 750W x 100

Efficiency = 80%