Three phases are better than one
May 1, 2006 12:00 PM, By John Battison, P.E., technical editor, RF
Most engineers come to radio having been brought up on single-phase 240V primary power supplies with a small pole transformer hanging outside the house. Many small radio stations exist efficiently with a similar type of power supply. But when transmitter power increases are authorized it becomes necessary to consider a three-phase power supply and its attendant additional wiring.
On the surface, three-phase operation may appear to be a lot more complicated than single-phase work, and for that reason is not preferred by some engineers. In his early work, Nikola Tesla, the father of three-phase operation, showed that it was actually more efficient than the more popular single-phase system.
Figure 1. A typical delta connection.
However, in larger stations three-phase power is generally the choice for several reasons. When the load on a three-phase system is properly balanced, a 25 percent to 30 percent saving may be made on the cost of the conductors for a single-phase system with the same kVA rating.
The power delivered by a three-phase system does not pulse in the same way as a single-phase supply in which the power actually falls to zero three times each cycle. A three-phase system power never falls to zero. This creates smoother and better operating characteristics in the three-phase systems.
Transformers can be smaller in a three-phase system because the kVA rating will be about 150 percent higher than for similar size single-phase transformer.
The load in a three-phase system is generally connected in one of two ways: wye or a delta connection. The wye connection is sometimes called a star connection.
Carry the load
With a delta connection, each section of the three loads receives the full wire-to-wire voltage. In the case of the star connection shown in Figure 2 the voltage across each leg will be 1/v3 times the line voltage. Sometimes the center point of the star connection is grounded but not always. Some transmitters obtain reduced power operation by switching the power transformer from a delta to a star connection.
Figure 2. A star or wye connection with the optional ground connection.
The number of phases used in a primary power supply is not limited to three. Six, nine or even 12 can be used. In some applications multiphase operation is frequently more efficient. Multiphase operation is often used in high-power transmitters because with a given type of rectifier it is possible to obtain more power and a higher dc voltage from a multiphase rectifier system.
Three-phase operation is flexible. It's possible to exchange the number of phases as desired by varying the manner in which the secondaries are connected and by using additional transformers.
There is, however, a caveat in connection with the use of three-phase motors � especially fan motors. Always connect a phase monitor on such a circuit. If a phase fails, motors will usually continue to run but will be too slow, with resultant tube or equipment damage.
There is an interesting special circuit, which is mentioned in case a reader comes across one in an older, high-power transmitter. The primary connections may be puzzling. The circuit uses two transformers to convert three-phase power to two-phase for hum-free operation of the filaments of the final pair of tubes operating in phase quadrature. This is called the Scott connection (named after its inventor), and it is shown in Figure 3. The two secondary outputs go to the final tube filaments.
One of the problems that broadcast engineers experience is that of harmonics in the power supply. Harmonics in a three-phase system are frequently produced by variable speed drives for ac motors and electronic drives for dc motors. Power supplies for ac/dc that use pulse width modulation are a good example of electronic devices producing harmonics.
Figure 3. A Scott connection has similarities to a wye.
Circuit breakers that trip when not expected, as well as overheated wiring and transformers are frequently the result of harmonics on the power line. Harmonic generation can have a positive or a negative sequence. Positive sequence harmonics rotate in the same direction as the fundamental and, as might be expected, negative harmonics rotate in the opposite direction.
Positive harmonic sequences cause circuit breaker tripping, and transformer and wiring overheating. Similar problems are caused by negative sequence harmonics as well as slow speeds in induction motors resulting in overheating and excessive power consumption. When overheating problems are encountered, and especially when motor speeds are slow, it is worth checking for harmonic invasion.
Apart from some solid-state power supplies, electronic ballasts can also produce harmonics. Equipment using power in square pulses is usually more prone to developing harmonics. In some cases harmonic problems have been found to occur with four-wire wye connected systems. If harmonic interference is suspected a scope will usually trace the problem.
Just as in the case of a single-phase power supply, power factor correction has to be applied to each phase in a three-phase system. The need for power factor correction is quite simple to understand when we examine a typical motor circuit that consists of inductance (the motor windings) and resistance.
Under no load conditions a motor looks like a circuit with a lot of inductance and low resistance. Most of the power drawn is used to energize a magnetic system. The current used is 90� out of phase with the voltage.
Now consider true or real power. In electrical terms, this occurs when electricity is changed into some other form of energy. In the case of a motor torque is produced as well as friction losses; heat is also another transformation of electricity into energy. The only real power produced under no load conditions is that used to overcome friction and other losses. So the circuit looks like a small resistance in series with a large inductance.
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