- Power is a rate of work done with respect to time.
- In other form, Power is the flow of energy with respect to time.
- Power is a product of force & velocity.
- Electrical power is product of voltage & current.
Term Power holds association of two or more quantities ! In a traction system, power is defined by two quantities. This includes drag or force on its wheels and its velocity.
A cycle & a truck are moving at the same velocity. But a truck is having high power due to its heavy engine force or drag. Both drag force & velocity constitute the term power.
Similarly, In an Electrical motor, There are two quantities like its rotational speed; rpm(rotation per minute) & torque. Motor’s power is directly proportional to these two quantities.
Electrical power
Electrical Power is a flow rate of electrical energy with respect to time !
P=E/t ( E is energy & t is time)
When a voltage is get applied across a load. It takes current according to its capacity. Load produces output parallelly while consuming electrical power.
P=E/t
P=E/Q*Q/t (Q/t=I, E/Q=V | Q is charge, V is voltage & I is current )
P=V*I
Electrical power across a load depends on the factors like voltage & current. The units of power is ‘wattage‘. In short, It is represented as ‘watt‘ or W or kW.
A name plate induction heater is shown below. Which shows its power rating of 1.5 kW & having the single phase voltage rating is 230 V AC.
A name plate of air compressor made by Ingersoll rand is shown below. Machine’s motor rating is 22 kW. Machine’s voltage rating is 400 Volts 3 phase/50 Hz.
Types of Electrical power
In Electrical, There are 2 types of power, AC & DC.
- DC stands for direct current. It remains constant at its magnitude through out the time.
- AC stands for Alternating current. It continuously changes its magnitude with respect to time. It also changes its direction at fixed intervals.
Both the powers are described through a GIF below:
Frequency in AC power
Frequency term exist in AC because it is making cycles form positive half to negative half. While DC do not have frequency due to its fixed magnitude.
Frequency means number of cycles with respect to time. In a waveform, It is counted as number of cycles per seconds. In a GIF of AC cycle above, Wave is moving from positive to negative & than negative to positive. Both; 1 positive half & 1 negative half signifies one cycle.
Hz is the unit of Frequency . 50 Hz Frequency shows 50 cycles per second.
Term Power factor in AC power
DC power flow remains stable through out the time. Its voltage & current both remains stable. That’ why DC power only depend on voltage & current. P(dc)=V*I . Voltage & current both moves together without any angle in between them.
DC power phenomenon’s is described through a GIF next. In which an electrical load is connected across a battery source through a switch. When switch get closed. Voltage & ;at the same time; current starts flowing. There is no angle between voltage & current.
AC power contains a term frequency. In AC, voltage & current are not stable. Due to connected load type, an angle get created in between voltage & current. That angular term is known as power factor(cosΘ).
That’s why AC power; in along with voltage & current; also depend upon power factor(cosΘ). P(ac)=V*I*cosΘ
Power factor varies from 0 to 1. At 1 power factor, Maximum power is get delivered while power get zero at 0 power factor.
In a GIF below, In which a load is connected across an AC supply through a switch. When the switch is connected, voltage appears across the load. Current starts flowing in a parallel to voltage.
In an AC power GIF above. Voltage & current both moves together at same angle . Angular difference becomes 0. Power factor cosΘ; cos0 becomes 1. Power again becomes P=V*I.
But What will happen when an angle get exist in between voltage & current ?
For this, We have taken an example of Electric motor connected to an AC supply. A gif of waveform of voltage & current is shown below :
According to GIF, When voltage is applied across the motor. Current wave is not moving in parallel with voltage waveform. There is angular difference in between voltage & current. That angular term makes the power term entirely different.
Let suppose motor’s power rating is 1 kW , Rated voltage is 230 V AC. Than ampere rating becomes at different power factor ratings :
P(ac)=V*I*cosΘ ; I=P/(V*cosΘ)
- At power factor 0.9, I=1000/(230*0.9)=4.83 Amps
- At power factor 0.8, I=1000/(230*0.8)= 5.43 Amps
- At power factor 0.7, I= 1000/(230*0.7)=6.21 Amps
An angular term; power factor; produces an unthinkable impact on power. As power factor decreases current in line increases. That’s why power factor term is included in AC power. P(ac)=V*I*cosΘ
Torque & Speed relation for Power
Electrical motors produces rotational motion with rated torque with rated speed.
Since motor produces rotational motion that’s why its speed is categorized as rotation per minute (RPM). While Torque is a rotational force having a rating of ‘newton meter‘ or ‘kilogram force meter‘.
Torque & RPM, Both defines the power rating of electrical motor. Imagine, there are two electrical motors with power rating of 1 kW & 10 kW.
Both the motors have same rated speed of 1440 rpm But different torque rating. 10 kW produces much higher torque than 1kW. That’s why motor power is product multiple of torque & speed.
Mathematically, Power(Watt)= (torque*Speed*2π)/60
A name pale of Induction motor is shown above Which is having the specifications :
- Power- 0.75 kW ; 750 W
- Speed- 1440 rpm
- Torque- 0.51 K.g
Torque is specified in Kilogram force meter
Next finding the torque in newton-meter !
- Power (formula)= (Torque*speed*2π)/60
- Torque= (Power*60)/speed*2π
- Torque= (750*60)/1440*2π
- Torque= 4.9 N-m
Electrical power & horse power
In an ancient times, Horses are the mode of transportation before the innovation of Electrical motors & mechanical engines. When engines becomes reality than these are compared with horse power(hp).
Even these days, Electrical motor & engine’s ratings are defined with horse power(hp). 0.75 kW rated electrical motor is equivalent to 1 Horse power (hp).
An image of submersible pump set by KSB is shown below. Which is having kW rating of 5.5 kW & HP rating is 7.5.
BASIC ELECTRICAL ELEMENTS
There are three basic electrical elements :
- Resistor
- Inductor
- Capacitor
- Resistor : Resistor provides opposition to the flow of electrical current. That opposition property of resistor is known as resistance Which is measured in numeric value. Resistance’s units of measurement are ‘ohm‘ & represented through a symbol Ω.
- Inductor : Inductor stores energy in magnetic form. Apart from resistance it produces reactance which is known as ‘Inductive reactance’. Reactance is also a type of resistance Which generates when an voltage get applied across the inductive load. It is also having a units ‘ohm‘ & represented through a symbol Ω.
- Capacitor : Capacitor stores energy in a form of electric charge. In along with resistance, It also generates reactance when an AC voltage get applied. This reactance is known as ‘Capacitive reactance’. Similarly, It is also having the units ohm & represented through a symbol Ω.
Different load’s impact on power factor
- Loads which majorly carries resistive element is known as Resistive load. Resistive loads are having very minor impact on power factor. Because both voltage & current are in phase when an AC voltage is applied across resistive load.
- Loads which majorly carries Inductive element is known as Inductive load. Inductive loads leads to power factor lagging. In which current lags the voltage.
- Loads which majorly carries Capacitive element is known as Capacitive load. Capacitive laod leads to power factor leading, In which current leads the voltage.
Both Capacitive & Inductive loads are counter to each other. Both are in opposite directions. The power factor graph is shown below. The unity power factor is marked at the center. It is located between the leading and lagging power factor.
SUM UP
- Power is defined as the rate of work done or the flow of energy over time. It is expressed as a product of force and velocity. In electrical systems, it is the product of voltage and current.
- In traction and motor systems, power is determined by drag and velocity or torque and rotational speed.
- Electrical power is calculated as P=V*I. It has types AC and DC.
- Both the AC & DC type has different characteristics, including the impact of frequency and power factor.
- DC remains stable while AC varies, influencing power delivery. Basic electrical elements include resistors, inductors, and capacitors, influencing power factors based on their characteristics.