Ultrasonic Detectors:
Another Tool for the Power Quality Engineer

Bruce Lonie, Power CET, Santa Clara, California

We use infrared to detect light the eye can not see and ultrasonic detectors for detecting sounds the ear cannot hear.   The human ear's hearing range is limited to 20kHz.  Ultrasonic detectors provide engineers with a method of detecting sounds greater than 20kHz.  Power quality and facility maintenance engineers should consider the use of ultrasonic detection equipment for evaluating equipment and facilities.

Loose electrical connections emit characteristic sounds that are beyond the range of the human ear.  The ultrasonic detector (or translator) provides a method of converting inaudible sounds to sounds and tones that match our hearing capabilities. Electricity flashing over an insulator makes a crackling sound that can be detected even when there are no visible signs of arcing.

The basic operation of the ultrasonic detector is relatively simple, as illustrated by the block diagram in Figure 1.  A transducer, generally equipped with some type of mechanical sound focusing aid, collects the sound energy and passes it to the amplifier.  The amplifier feeds the modulator section where the collected sound is mixed with a signal from the oscillator section.   The mixing of the two signals in the modulator produces a sum frequency, f1 + f2, and a difference frequency, f1-f2.  The difference frequency is passed on to the output amplifier that provides the audible signal.   Arcing and corona discharge will be heard as a frying or buzzing sound on the speaker/headset.   The closer the transducer is to the source, the more intense the signal.

Ultrasonic energy resulting from a loose electrical connection should be in the range of 405OkHz.  Mixing a 40kHz signal from the oscillator section of the detector unit produces a difference signal in a range up to lOkHz that can be heard from the speaker (or headphone) on the detector unit.

Portable ultrasonic detection equipment is available from several suppliers and the cost varies with the capability of the equipment.  More expensive equipment has the ability to tune the oscillator over a broad range (20kHz to 1OOkHz) that can be used by the experienced operator for more precise problem determination. Less expensive equipment employs a fixed frequency oscillator, which dictates an operating range.  For example, a 40kHz oscillator would provide detection in the range from 20kHz to 60kHz.

Accessories for ultrasonic equipment will increase the cost of the system, but may be required, depending on the specific application.  Systems can run from $1,000 to $6.000, including all accessories.

A typical use for the device might be to detect loose or arcing connections inside an enclosed cabinet or panelboard.  It will determine if the situation is safe or if the equipment should be de-energized before opening.  All high-voltage switchgear and panelboards should be checked with ultrasonic equipment prior to opening or starting a project. Ultrasonic detection equipment can identify loose connections that thermography (infrared) could miss. Connections often do not pro-duce enough heat to be detected, or the surrounding heat source masks the problem.  Figure 2 shows the Ultraprobe being used to check the tap connections on a transformer.

The use of ultrasonic detection technology is not limited to electrical applications, but can be used in motor bearing analysis, gas and steam leaks, vacuum leaks, and liquid leak amplification, etc.  With the ultrasonic transmitter (source), usually an accessory, this technology can be used to detect leaks in containers, hatches, automobiles and aircraft, to name just a few.