The power factor represents the fraction of the total possible power that can be generated in a circuit. Since most of the loads in an HVAC are inductive, the current usually lags the voltage. Here are some examples:
When the voltage and current are in phase, they are both at their maximum values at the same time. For example: if we have a 120-volt circuit with 10 amps of current. The power is 1200 watts - 120 volts x 10 amps. This represents a power factor of 100%.
In the same circuit, if the current is out of phase, or lagging behind, the voltage by one eighth of a cycle, we would still measure 120 volts and 10 amps for a product of 1200. But, if we measured with a wattmeter, the power of the circuit would read 1000 watts for a power factor (PF) of 83.3%. The formula for calculating the PF is:
PF = wattmeter reading / (ammeter reading x voltmeter reading)
Using the above values PF = 1000 / (10 x 120) = .833
It is the counter emf that develops in the windings of a motor that opposes the applied emf or voltage which causes the lag in the current.
The counter emf of a motor depends on the speed of the rotor. In a no-load condition, the counter emf practically balances the applied emf. As the load increases the speed of the motor decreases, decreasing the counter emf. Because in the the drop in counter emf the applied voltage sends more current through the windings, this tends to keep the speed constant.
If the motor is overloaded, the current increases greatly due to the greatly reduced counter emf. This causes motors to overheat. A continuous overload on a motor is likely to burn out the windings.
If a rotor is locked in a motor so it cannot turn and power is applied, the current through the windings will be very high. Under these conditions, a motor will burnout very quickly. This is called a locked rotor condition.
As an HVAC technician, understanding power factor, emf, and counter emf is very important to help you properly diagnose and troubleshoot residential HVAC systems.
Want to learn more? Sign-up for our Free HVAC Training.
by Ron Walker
When the voltage and current are in phase, they are both at their maximum values at the same time. For example: if we have a 120-volt circuit with 10 amps of current. The power is 1200 watts - 120 volts x 10 amps. This represents a power factor of 100%.
In the same circuit, if the current is out of phase, or lagging behind, the voltage by one eighth of a cycle, we would still measure 120 volts and 10 amps for a product of 1200. But, if we measured with a wattmeter, the power of the circuit would read 1000 watts for a power factor (PF) of 83.3%. The formula for calculating the PF is:
PF = wattmeter reading / (ammeter reading x voltmeter reading)
Using the above values PF = 1000 / (10 x 120) = .833
It is the counter emf that develops in the windings of a motor that opposes the applied emf or voltage which causes the lag in the current.
The counter emf of a motor depends on the speed of the rotor. In a no-load condition, the counter emf practically balances the applied emf. As the load increases the speed of the motor decreases, decreasing the counter emf. Because in the the drop in counter emf the applied voltage sends more current through the windings, this tends to keep the speed constant.
If the motor is overloaded, the current increases greatly due to the greatly reduced counter emf. This causes motors to overheat. A continuous overload on a motor is likely to burn out the windings.
If a rotor is locked in a motor so it cannot turn and power is applied, the current through the windings will be very high. Under these conditions, a motor will burnout very quickly. This is called a locked rotor condition.
As an HVAC technician, understanding power factor, emf, and counter emf is very important to help you properly diagnose and troubleshoot residential HVAC systems.
Want to learn more? Sign-up for our Free HVAC Training.
by Ron Walker