By Roger Nichols
Spike, noise, surge, swell, transients, harmonics and sag are not the names of members of a new alternative rock group; they are characteristic problems encountered in power management.
AC power is often the most overlooked area in recording studio design. If you were a farmer and your horse was your livelihood, you would probably pay attention to how well he is doing. AC power is the main source of your income, and also the primary cause of all the hums and buzzes you must deal with on a day-to-day basis. They say that if you build a better mouse trap that they will beat a path to your door. Well, just wait until you have the quietest studio in town and see how fast everyone wants to work there.
Power quality can be measured by the Duration vs. Magnitude of a disturbance. Short fault durations, like transients, can damage sensitive electronic devices such as diodes, transistors and ICs. Lower level transients slowly eat away at internal semiconductor junctions within electronic equipment, eventually causing failures.
High-frequency noise can cause digital data errors in both digital audio and computer equipment, and can interfere with clock signals, causing timing errors and excessive jitter.
Voltage fluctuations effect motor operation and electronic equipment that require a steady power source.
Electric utilities must generate the service capacity to meet the peak demand, kVA (kilovolt amps), whether or not the customer is using that current efficiently. Utilities can only charge for the active power, or kW (kilowatts).
The ratio of kW (active Power) to kVA (apparent power) is called the power factor.
PF = kW / kVA
Induction loads, such as induction motors used to power fans in air handling systems, may operate at less than their full rated load because of poor power factor.
Total Power Factor also includes the effects of any phase difference between the fundamental voltage and current. In many cases, when the distribution system is serving only single phase receptacle loads, the phase difference at the fundamental is minimal. DPF is near 1.0 and PF represents the contribution of harmonics to the current. As the total harmonic distortion increases, the current waveform changes to a pulse with higher peak current. The RMS current increase produces a higher apparent power. The active power, or Watts, and displacement power factor do not change since they are based only on fundamental voltage and current. Total Power Factor decreases. More current must be carried by the system to deliver the same amount of active power. The different responses of DPF and PF can lead the way to the proper power factor correction methods.
Induction motors can suffer from harmonic current heating if the supply voltage is distorted. And the presence of negative sequence harmonic currents reduces motor torque. The combination of these effects causes motors to burn out. To test the proper operation of the motor and its power factor, use a true RMS tool (such as the Fluke 8060, Fluke 73, Protek 506, Wavetek 2030, Fluke 41B, Tektronix Wavemeter, and many others) to measure the three phases for proper voltage balance (Fig. 4). Look for obvious distortion in the waveform. Most motor manufacturers recommend less than 5% distortion for a fully loaded motor. Measure the three phases for proper current balance, then measure power and power factor at full or normal load. If total power factor and Displacement power factor are the same, you may need to add Power Factor correction capacitors. To minimize the effects of harmonics on induction motors, you can reduce the voltage distortion on the terminals by connecting the motor to a distribution center supplying only linear load. Or, you can consult with a power management expert and connect a harmonic filter at the source of harmonics.
To verify proper circuit operation, measure the three phases for proper voltage and balance. Then measure the three phase power and power factor. Notice the difference between the DF and DPF readings. If the Total PF reading is lower than the DPF reading, a portion of the load is probably non-linear. Examine the drive current for harmonics, typically 5th and 7th. Adjustable speed drives are a common source of 5th harmonic. The need for correction capacitors may be reduced when adjustable speed drives are installed on existing motors. Line reactors can be applied at drive inputs to reduce harmonic currents. Or to avoid harmonic frequencies from resonating with correction capacitors, filter networks can be designed to de-tune the resonant system.
RECEPTACLE LOAD CENTERS
At the load center, harmonic currents can cause circuit breakers to trip. Thermal magnetic breakers may trip prematurely from excess heat in the panel caused by harmonic currents. Breakers may also trip erratically when non-linear currents with high peak values are present. A peak sensing circuit breaker responds to the peak of the current waveform. Since the peak may be higher due to harmonics, this type of breaker may also trip prematurely at a lower RMS current.
To detect harmonics at a load center, check the phase voltage for flat- topping (a condition where instead of a sine wave, the waveform becomes flat on top resembling digital clipping).Then measure the current in the feeder conductors using a true RMS instrument. Remember that these instruments indicate the actual heating value of the current, including harmonics. Verify that you are operating within the load rating of the panel. Measure the feeder neutral current. If it reads high, triplen harmonics (see below) may be present. Then compare the current with the ratings for the conductors, lugs, and buss bars. Compare the individual branch circuit currents to the breaker ratings. Check the branch neutrals for overloads due to triplen harmonics. The same process can be repeated at other load centers fed from the same source.
Once you are sure that a receptacle panel is effected by harmonics, there are a few options to correct the situation:
- Balance load. Rearranging branch circuits and reduce the neutral current.
- Redistribute load to other existing or new panels.
- Zero Sequence harmonic Filters can be connected to the panel to reduce neutral current in the feeder. Filters should only be installed after consulting with a qualified engineer.
.If the neutral conductor is overloaded, a larger conductor will be required by code, or the existing receptacle panel can be replaced by a panel and main breaker that is rated for non-linear loads.
LIGHTING LOAD CENTERS
Excess heat caused by harmonics in lighting circuit conduit can cause conductor insulation to fail. In energy-saving electronic ballasts with solid-state power supplies, the phase and neutral currents can contain harmonics. Existing standards for the number of conductors in a conduit don’t always account for the heat caused by these harmonics. To find harmonic overloads in lighting circuits, you can make the same measurements you made at the receptacle load center. Measure the current in the feeder neutral. If the levels are high, compare the measured currents to the ratings of the conductor, lugs and buss bars. Feel the conduit for excess heat. To determine the overall level of harmonics, measure the total harmonic distortion in the phase currents. The THD generally refers to the RMS value of all the harmonic currents, divided by the fundamental (Fig. 5). The total harmonic distortion may be a problem if it exceeds 20%.
Harmonics on the AC line are usually caused by non-linear electrical loads. Some of these non-linear loads are: personal computers, certain types of lighting ballasts, electronic studio and office equipment, and adjustable speed motor drives. These devices draw non sinusoidal current in abrupt pulses when connected to a sinusoidal voltage source. These pulses form a distorted current waveshape which contains harmonics.
The harmonic current drawn by non-linear loads acts in an Ohm’s law relationship with the source impedance of the supplying transformer to produce voltage harmonics. The source impedance includes the supplying transformer and branch circuit components. For example, a 10 amp harmonic current being drawn from a source impedance of 0.1 Ohm will generate a harmonic voltage of 1.0 volt. Any load sharing this transformer or branch circuit can be affected by the voltage harmonics generated.
Computers used in console automation or hard-disk recording can crash or reset from excessive harmonic voltages in the supply power. Remember, the harmonics can come from devices anywhere on the same transformer or branch circuit.
Each harmonic has a name, frequency and sequence. The sequence refers to the phase or rotation with respect to the fundamental. In an induction motor, for instance, a positive sequence harmonic would generate a magnetic field that rotated in the same direction as the fundamental. A negative sequence harmonic would rotate in the reverse direction.
A simple way to determine the extent of harmonic distortion caused by a single-phase non-linear load would be to make two separate current measurements. Make the first measurement using an average responding current clamp or meter with clamp on probe. Make a second measurement of the same circuit using a true RMS current clamp meter. Divide the results of the first measurement by the second measurement. This will give you the A/R ratio. A ratio of 1.0 would indicate little or no harmonic distortion. A ratio of 0.5 would indicate substantial harmonic distortion. This test method works because an averaging meter will read a true sine wave correctly, as will the true RMS meter. If the waveform is distorted, the true RMS meter will read correctly, while the averaging meter will read up to 50% low, depending on the amount of distortion.
The above measurement method is not a substitute for a harmonic analyzer, but it is a simple way to determine if there is a need for more sophisticated equipment.
GROUNDING AND NOISE
As you can see by the material that has been covered, AC power contains more than just that pure mythical 60 Hz sine wave that you read about in text books. In most cases, filters added to the power line add noise of their own. The capacitors in the filter circuit leak current into the ground system. This noise is usually in the form of a reactive, non-linear leading current. The same type of noise on the ground is caused by switching power supplies found in most computers and digital audio gear. This ground noise usually shows up as hum in audio gear. Class A tube amps and balanced mic pre amps are particularly susceptible to this ground noise.
All of the power-consuming devices in a studio are connected to unbalanced power (Fig. 7). There are two wires supplying the 120 V power, with the ground for safety (and noise). If you measure between the two feed wires the results will be 120 V. If you measure between ground and one of them you will see 120 V. If you measure between ground and the other lead, you will see zero V. Well, you are supposed to see zero, but because of ground noise and currents, you will measure a couple of volts. Just remember, with unbalanced power, all of the power generated garbage ends up in the ground.
THE BALANCED POWER ANSWER
Balanced power consists of three wires (Fig. 8). The same three wires that are connected to most studio equipment. If you measure the voltage between the two feed wires, you get 120 V. If you measure between either one of the feed wires and ground you will see 60 V.
Quiet grounding schemes in studios sometimes border on the occult. I asked one studio why they had a water cooler in the control room with no water in it. The said that for some reason, when the water cooler was plugged into the same branch circuit as the guitar amps, that there was less hum in the amps. I unplugged it once. They were right.
Grounding circuits were never meant to carry current except during a short circuit. Objectionable ground currents are those that will provide you with a shock. Anything less than that is OK as far as Underwriters Laboratories is concerned.
We have all experienced ground loops in the studio. The really bad ones, with hum levels above the signal level, we try to cure. The ever present little hums, that make the DAT meters stick one segment up from the bottom, we try to ignore. We try breaking grounds in balanced cables at one end so that we do not have multiple ground paths for ground loops. We lift chassis grounds with special plugs and make sure that metal chassis do not touch each other. If we removed the currents from the ground, then we would have no current to loop.
With balanced power, you can use any type of grounding configuration you wish. Star, schmar. You can leave the grounds connected at both ends of your audio cables. You can throw away all of the ground lift adapters. You can finally plug everything in the way it was meant to be plugged in.