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, installation issues, and TECO Westinghouse Motors Canada products offered to the HVAC industry.

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.
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.

TECO Westinghouse Motors Canada Products

Fluxmaster 50:

¼ to 1 HP, 230 VAC 1 phase input and 230 VAC 3phase output.
This VFD is ideal for chemical injection pumps, which are usually in this range. If we look at an injection pump for boiler feed water, and the feed water is at 100%, the chemical pump is at 100%. If the boiler feed water goes down to 50%, the chemical is still 100%. If you install a flow transmitter on the feed water line, and hook that to the Fluxmaster 50 as its speed reference, the VFD will adjust the speed of the chemical injection pump to follow the feed water flow rate. It will not take long to recover the cost of a $500 VFD in saved chemical.

Fluxmaster 500

¾ to 40 HP, 230/480/600 VAC 3 phase input, 3 phase output.
This VFD is ideal for small fans and pumps. Very easy to program and set-up. Comes in NEMA I and NEMA 4 enclosures. Will control temperature or pressure or flow rate with an optional built in PID controller. Also,the cost is very economical.

Fluxmaster 1000

20 to 250 HP, 230/480/600 VAC 3 phase input, 3 phase output.

This VFD is ideal for larger fans and pumps. The display is a touch screen so set up is very easy and in English text. Comes standard in a NEMA 12 enclosure, which is dust tight and drip proof Will control temperature orpressure or flow rate with a built in PID controller. Also, the cost is very economical.

HVAC PAC Options

TECO Westinghouse Motors Canada has a full production facility in Edmonton AB. At this facility we do option packages with line reactors, fused disconnects and bypass systems along with any other customizable options the installation requires. This also is the Canadian head office with over 10 million dollars of motor and drive inventory.

Summary

In summary, the installation of variable frequency drives can save you considerable amounts in your energy costs. Other benefits include better control of your system, less damage to mechanical equipment, and better control of your process.

Above presentation was made the membership of the Building Operators Association by Mr. Troy Metz, Control Product Specialist Western Canada and Mr. Brian Smolik, Technical Sales Representative of Teco - Westinghouse.



First published March 2001