Induction Motor Energy Savings

A Variable Frequency Drive (VFD) can save power with an induction motor but that is not the normal reason I've found that a VFD was used on an induction motor. Normally I've found that a VFD was chosen to permit better speed control. The simplest method I've found for improving an induction motor is the addition of switched phase matching capacitors to the motors. The trouble of doing this though is that the capacitors will only match one mechanical load condition.

By 'save electrical energy by varying the frequency of input voltage,' did you mean save energy with the use of VFD (Variable Frequency Drive)?

If yes, then these links may help:

http://cr4.globalspec.com/thread/55271

http://ww1.microchip.com/downloads/en/AppNotes/00887a.pdf (page 17)

- MS

If you are adding a Variable Frequency Drive to supply an induction motor, You are adding a consumer of energy since the efficiency of the VFD is less than unity(or less than 100%), therefore, you are not saving electrical energy as such.

The idea of varying the frequency of the supply power is to change speed. Therefore eliminating (in some cases) the need to have a mechanical speed variator like Gearbox or pulleys etc. If properly designed, this method can save electrical energy.

BUT the most important: Use a VFD to have a smooth starting therefore reducing the peak demand on the grid (this peak usually is taxed by the utility using a Maximum Demand Metering), combined with the need to change speed away from the nominal speed of the motor ( reducing unnecessary high speeds when not needed... better process control). In these cases you will (proper design...) save energy by not using too much of it when not needed...

Depending on your use then it may be yes. If you use a vsd to control the start it may help to avoid running a motor into the demand peaks. (running peaks during resticted demand or peak periods of the day) This may save some energy and hence some costs. Whether the cost of a vfd will relate to a saving overall becomes debatable.

Yes you can save energy by using VFD,

But the quantum of saving differs based on the level of damper or valve opening without VFD.

For Example: Without VFD if the pump is operation with 50% valve opening at the outlet, the power consumption will be around 40 kW (for 100% valve opening power will be 45kW).

In this case if we provide a VFD & operate it at 50% of the RPM (ie 720) by keeping the valve at 100% opening.

Power consumption for the same load will be

power = 45 * (720/1440)^2 = 11.25 kW

Power consumption for the same load will drastically comes down from 40 kW to 11.25 kW after using the VFD.

So before using a VFD if you are well aware of the load you can estimate the potential energy saving.

Anbarasau,

Although the explanation illustrates the saving that might be possible, the work-out is not absolutely correct:

1) If the valve is open 100%, does not mean that the pump will be loaded at maximum power because all depends on the pressure at the outlet ( the water column or the hight at which you are pumping...).

2) For the same reason, closing the valve at 50% does not mean that the equivalent operation will be the half speed rpm of the pump.

3) The Power variation ratio is proportional to the cube of the rpm ratio (not the square ... giving you 5.625 kW for your example).

==> You need to use the pump curves for Flow against height (pressure) with the power curve to try and establish the exact quantity of flow you require at the height required, then use the curves at different speeds to find the rpm curve that matches your requirement of flow and H. This will then allow you to deduct the power required and see if you saved energy by keeping the valve open fully and adjust the rpm.

Use the pump Curves ... it is meant for that in general.

Well, that action would vary the speed of the motor. The motor is driving something. Can the process to which the something is connected tolerate a change in motor speed?

The simplest way to save energy in a motor, is to switch it off!