Constant current vs Constant impedance vs Constant power loads

[shortcircuit1]

Hello,

What are Constant current,constant impedance and constant power loads?

I am assuming for constant current loads the current will be constant but the voltage or impedance will be varied to attain constant current and for constant power loads both the current and voltage will be varied to meet the constant power across the load?

How do they work and what are the different applications for these?

 

[luckylerado]

Constant current is used for airfield lighting so that all lamps burn with equal intensity and as you stated, the voltage is regulated as the impedance of the load changes in order to keep the current the same across all of the lamps. I have never had any experience with the other types that you mention.

 

[GoldDigger]

There are two simple examples of constant power loads.

One is a motor which is mechanically loaded at anything up to its full load power.
If you reduce the voltage the motor's speed will decrease slightly (more slip) and the current being drawn will increase to provide the same torque and power which was being produced at the higher voltage.
You can look at it this way:
If the motor is to continue to produce the same power (turn at roughly the same speed), the small decrease in speed resulting from the voltage drop will reduce the back EMF of the motor, which will in turn cause more current to be drawn.
With the exception of a several percent factor related to slip, the motor will continue to drive its full mechanical load and consume nearly constant power (as measured by a wattmeter) from the AC source.

A second common constant power load is the regulated driver to an LED lamp. The driver tries to deliver the same DC power to the LED array independent of the incoming voltage. Whether the driver tries to maintain constant power or tries to sense the voltage and/or duty cycle of the AC source determines whether or not that driver allows the lamp to be dimmed by control of the incoming line voltage.
The constant power driver will draw more current from the source to keep the internal capacitor charged as the input voltage decreases.

Constant current loads include the airport and some street lighting described in an earlier post. But keep in mind that it is the system of transformer and lamps that is constant current, not the lamps themselves. The lamps are designed to operate at a constant current, but do nothing by themselves to regulate that current.

A simple constant voltage load is a zener diode. It will conduct as the applied voltage exceeds the zener voltage and as the voltage rises slightly it will consume much more current so that the internal resistance times the current makes up the difference between the applied voltage and the zener voltage. If the source voltage is stiff, this will at some point destroy the diode.

An electric resistance heater is a constant impedance load (the typical nichrome wire has a very low temperature coefficient of resistance, unlike an incandescent light bulb.)

 

[shortcircuit1]

There are two simple examples of constant power loads.

One is a motor which is mechanically loaded at anything up to its full load power.
If you reduce the voltage the motor's speed will decrease slightly (more slip) and the current being drawn will increase to provide the same torque and power which was being produced at the higher voltage.
You can look at it this way:
If the motor is to continue to produce the same power (turn at roughly the same speed), the small decrease in speed resulting from the voltage drop will reduce the back EMF of the motor, which will in turn cause more current to be drawn.
With the exception of a several percent factor related to slip, the motor will continue to drive its full mechanical load and consume nearly constant power (as measured by a wattmeter) from the AC source.

A second common constant power load is the regulated driver to an LED lamp. The driver tries to deliver the same DC power to the LED array independent of the incoming voltage. Whether the driver tries to maintain constant power or tries to sense the voltage and/or duty cycle of the AC source determines whether or not that driver allows the lamp to be dimmed by control of the incoming line voltage.
The constant power driver will draw more current from the source to keep the internal capacitor charged as the input voltage decreases.

Constant current loads include the airport and some street lighting described in an earlier post. But keep in mind that it is the system of transformer and lamps that is constant current, not the lamps themselves. The lamps are designed to operate at a constant current, but do nothing by themselves to regulate that current.

A simple constant voltage load is a zener diode. It will conduct as the applied voltage exceeds the zener voltage and as the voltage rises slightly it will consume much more current so that the internal resistance times the current makes up the difference between the applied voltage and the zener voltage. If the source voltage is stiff, this will at some point destroy the diode.

An electric resistance heater is a constant impedance load (the typical nichrome wire has a very low temperature coefficient of resistance, unlike an incandescent light bulb.)

If i get the summary right,basically if current is to be kept constant then other two parameters will be varied to keep the constant current delivered to the load.And if the voltage is to be kept constant power and impedance will be varied to keep the constant power to the load.

 

[GoldDigger]

If i get the summary right,basically if current is to be kept constant then other two parameters will be varied to keep the constant current delivered to the load.And if the voltage is to be kept constant power and impedance will be varied to keep the constant power to the load.

Try stating that again. The second half of your sentence does not seem to make sense.

In any situation you start out with the assumption that one or more parameters can vary uncontrollably and you vary the other(s) to keep something constant.

Be very careful to distinguish between a load which is inherently one of the three types (like a zener diode for constant voltage or a motor for constant current) and one where it is the combination of the underlying load and a control system that does the regulation (like a driver and LED combination that draws constant power.)

 

[shortcircuit1]

Try stating that again. The second half of your sentence does not seem to make sense.

In any situation you start out with the assumption that one or more parameters can vary uncontrollably and you vary the other(s) to keep something constant.

Be very careful to distinguish between a load which is inherently one of the three types (like a zener diode for constant voltage or a motor for constant current) and one where it is the combination of the underlying load and a control system that does the regulation (like a driver and LED combination that draws constant power.)

Yes just looked at it...Doesn't make sense...Just got the point...Thanks for the elaborate explanation with examples...

 

[rian0201]

motor during starting behave like constant impedance and changes to being constant power loads..


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[GoldDigger]

motor during starting behave like constant impedance and changes to being constant power loads..


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Good point. The single cycle inrush portion, unlike the Locked Rotor Amps, can actually vary from start to start depending on phase and residual magnetism.

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