To understand how electricity causes an electric motor to turn, and to understand the difference between single- and three-phase power, let us relate electric motors to a water wheel. Picture a water wheel next to a waterfall. This particular wheel has just one cup on it, which fills with water whenever it is positioned under the waterfall. The weight of the water in the cup causes the wheel to turn. The wheel has just enough momentum to complete one revolution, but is kept turning by the re-filling of the cup.
A single-cup water wheel is like a single-phase electric motor. They work fine for small jobs, which don’t take much inertia to get them spinning, or which aren’t pulling too much of a load, but getting all your energy input only once per revolution isn’t the most efficient way to transfer energy.
Three-phase power, on the other hand, is like a water wheel with three cups spaced equally around the wheel. Let’s call the cups L1, L2 and L3. As the wheel makes its way around completing one revolution, it gets boosted three times, instead of just once. It doesn’t spin any faster, but it is easier to get it spinning in the first place (more starting torque), and it is more efficient because more of the water is being used to make the wheel turn.
In an electric motor, electricity is the water, the cups are the stator, and the wheel is the armature. A single-phase motor gets one “cup” of electricity per revolution, and a three-phase motor gets three. Three cups per revolution is a much more efficient way to get an armature spinning, and to keep it spinning. That is why large motors are three-phase.
Frequency of the Power
One complete revolution is called a cycle in electricity. As I’ve said, a single-phase motor gets one hit per cycle, and a three-phase motor gets three. The number of cycles per second is known as the frequency of the power, and is described in terms of hertz – one hertz is one cycle per second. The speed of rotation of an electric motor, rated in terms of revolutions per minute (RPM), is determined by the frequency of the power. (A motor’s RPM also is determined by the basic design of the motor, which will be covered later.) In North America, the utilities deliver 60-hertz power, but in many other parts of the world, the standard is 50 hertz. Motors will run on either frequency, but will turn more slowly on 50-herz power, and, thus, develop less horsepower. A motor that develops 5 HP at 60 hertz only will put out a little more than 4 HP at 50 hertz. There is less work going into keeping the motor spinning, so it will turn slower, and produce less power.Transformers
Understanding how the utilities deliver three-phase power to end-users helps us understand some of the electric motor failure modes. Whether the power is generated by nuclear energy, hydro, steam, natural gas, oil, coal or wind, the utilities use three-phase generators. They deliver the three-phase power to the grid at very high voltages for transmission efficiencies. To bring it down to a usable voltage like 240 volts or 480 volts, they use transformers, usually a separate one on each phase.Figure 1 shows a three-transformer, three-phase 240-volt system. L1, L2 and L3 represent the three legs of the three-phase power. Single-phase 240-volt power is available between any two of the legs. This is an important concept to understand, because it explains how three-phase power can become unbalanced – that is, the voltage in all three legs is not the same.
Voltage unbalance is the leading power-quality problem that results in motor overheating and premature motor failure. Voltage unbalance degrades the performance, and shortens the life of a three-phase motor. Voltage unbalance at the motor stator terminals causes phase current unbalance far out of proportion to the voltage unbalance. Unbalanced currents lead to torque pulsations, increased vibrations and mechanical stresses, increased losses, and motor overheating, which result in a shorter winding insulation life.
Unbalance can occur when the single-phase loads on the three legs downstream from the transformers are not equal. In Figure 2 (p. 56), Farmer Jim on the right has a three-phase pump on his farm. His neighbor Rob has a machine shop, but all of his machines run on single-phase motors hooked up to L2 and L3 of the three-phase power on the pole. Next to Rob is Bob’s Dairy, and all of Bob’s pumps and equipment also are running on L2/L3 single-phase power. Since the L2/L3 pair is carrying a heavy single-phase load from Jim’s neighbors, the voltage of that pair will be reduced by the time it gets to Jim’s farm, which will cause an unbalance condition in Jim’s three-phase motor.
Winding damage likely will occur, because the windings served by that pair will carry more of the load and overheat. A relatively small unbalance in voltage will cause a considerable increase in temperature rise. In the phase with the highest current, the percentage increase in temperature rise will be approximately two times the square of the percentage voltage unbalance. To illustrate the severity of this condition, an approximate 3.5-percent voltage unbalance will cause an approximate 25 percent increase in temperature
rise.
Submersible motor manufacturers recommend not exceeding 10-percent current unbalance at the rated input motor load, and 5 percent at service factor load for their motors. If your power is more out of balance than this, you still may be able to operate a three-phase motor by oversizing the motor such that the overloaded winding is not carrying more load than it was designed to carry. For instance, a 10-HP pump operating on three-phase power that is 15 percent out of balance may run just fine with a 15-HP motor. Consult your motor manufacturer for its recommendation for your particular situation.
To calculate the percentage of current unbalance:
1. Add the three line amps values together.
2. Divide the sum by three, giving you the average current.
3. Pick the amp value that is furthest from the average current (either high or low).
4. Determine the difference between this amp value and the average.
5. Divide the difference by the average. Multiply the result by 100 to determine the percent of unbalance.
There is another three-phase configuration called open delta that uses two transformers instead of three. Open delta power is harder to balance than conventional three-phase power, so extra care must be taken to ensure that the motor will not be damaged. One source for a more complete description of open delta is Franklin Electric’s Submersible Motor Application Manual. If you do not have a copy, call Franklin at 800-348-2420. Another good source for information on three-phase power is the Electrical Engineering Pocket Handbook published by the Electrical Apparatus Service Association; phone 314-993-2220 to request a copy.
Next month, we look at devices that can be used to convert single-phase power to three-phase. ’Til then….
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