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Adjustable Drives

In today's environment with the increasing costs of energy, many electrical consumers are looking for ways to lower their monthly costs. One efficient and practical way to do this is the installation of variable frequency drives on electric motors that run pumps or fans. The following information will cover basic theory, application issues, and installation issues..

 

Basic Theory

 

The basic equation for a 3 phase electric motor is: Speed = (120 * F) / # of poles where:
120 = electrical constant, F = frequency and # of poles is determined at motor construction ie: a 2 pole, 4 pole or 6 pole machine. If we look at a 2 pole machine and 60 HZ supply, the speed calculates out to 3600 RPM. The only way to vary the speed is to change the F in the equation. We can accomplish this with a Variable Frequency Drive (VFD).

 

The basic construction of a VFD consists of 4 major components.
1. Rectifier: This converts our 3-phase AC voltage electrical supply into a constant DC voltage. For a 600 VAC supply, the DC voltage would approximately be 850 VDC, known as the DC Bus.
2. DC Bus: This is an inductive and capacitive circuit to maintain a constant and smooth DC Bus voltage that tries to resist changes from the main AC supply.
3. Inverter: Also known as IGBTs, this section converts the DC Bus voltage by pulsing it by a transistor network to form a variable voltage and variable frequency supply for a 3-phase electric motor.
4. Controller: Controls the pulses and calculates the magnitude of the voltage, current and frequency to obtain optimum motor performance under all conditions.

 

Application Issues

 

If we consider an electric motor running a water pump and the flow is manipulated via a control valve, we will look at the energy savings that can be achieved by installing a VFD to control the speed of the motor. If the pump is centrifugal, the physics governing its operation are known by the Affinity Laws. These laws have determined that the Horsepower, HP, required varies with the Speed cubed. When you have a system where the control valve is wide open, the speed of the motor is 100%, the flow is 100% and the HP required is 100%. As soon as you start to close the valve, the motor is still 100%, flow is 70% and HP required is approx. 90%. If the flow goes to 5 0%, motor is still 100% and the HP required is about 60%. If you install a VFD in this application and remove the control valve, and the flow required is 70%, the motor will be 70% speed, and the HP required will be 34%. If the flow required is 50%, the motor will be at 50% speed, and the HP required is 12.5%. As you can see, there is less HP required with the VFD installed: 34% vs. 90% and 12.5% vs. 60%. These HP differences translate directly into energy savings. This law also works for dampers and vanes in air flow applications. If you consider the increasing cost ofpower, and that most systems operate between 50% and 80%, the payback time of installing a VFD can soon be realized. TECO Westinghouse Motor Canada has software that can calculate energy savings.

 

Installation Issues

 

When installing a VFD, we must consider the ambient surroundings. The room must clean, dry, warm, and a maintained circulating air flow. Ideally an electrical room should be used. If there is not an ideal environment available, adjustments should be made. le: heaters, air conditioners, and NEMA 4 enclosures. This should be made aware up front to your VFD supplier.
The motor age must also be considered. If the motor is over 5 years old, the use of an output filter may be required. This will dampen the voltage spikes coming from the VFD and protect the motor insulation.
If the system voltage is 600 VAC, an output filter should also be used. Even with new motors, the insulation may be damaged from the VFD voltage spikes.
When talking with your VFD supplier, it is best to have as much information about your system and the application available. This way nothing should get
 overlooked and the proper equipment will be supplied.