Replacing old low-horsepower, electro-mechanical motor starters with new variable speed drives (VSDs) may seem like an attractive solution. After all, drives offer many advantages over motor starters: They don't create inrush current or generate switching transients, they have high displacement power factors, and they're less expensive. In fact, just as programmable logic controllers have replaced mechanical relay-based control devices, solid-state drives may someday replace many, if not most, mechanical starters (Sidebar below).

But beware. Drives — both big and small — can negatively affect the distribution system and the motors themselves by introducing harmonics and reducing motor-drive capability. That's not necessarily a problem for big drives that serve high-horsepower motors. The fact that they represent a larger investment means more care in preventing those problems is bound to be taken. It's the smaller drives, which don't garner as much attention, that create more potential for problems. If you're planning on replacing low-horsepower mechanical motor starters with VSDs, take the time to consider potential problems and plan accordingly.

Harmonics and smaller drives. The larger the VSD, the more harmonics you can expect. However, a 250-hp drive is almost sure to attract much more attention from engineers to mitigate its harmonics than a 5-hp or smaller drive, which frequently merits little if any attention. Given the economics and effect on operations, this often makes perfect sense. But there are other times when a careful analysis of smaller horsepower drives is warranted.

On single-phase systems, drives will generate 3rd and, to a lesser degree, 5th harmonics. In commercial buildings, on four-wire systems (3-phase conductors and a neutral conductor), they'll contribute to the 3rd harmonic current that adds up in a shared neutral. This is a good reason for installing neutral conductors twice the size of phase conductors. So as small drives add their contribution to the total harmonic load, any line measurements — particularly current measurements — require a true rms meter for accuracy. In the presence of harmonics, average responding current meters can provide readings that are off by as much as 40%.

On 3-phase systems, the 5th will be the predominant drive-generated harmonic. The 5th harmonic is a negative sequence harmonic: it creates reverse torque that will tend to make motors turn backward. The 5th harmonic won't affect the drive's own motor, or other motors with drive control, but it will affect motors with across-the-line mechanical starters. The across-the-line motor, driven by the much larger fundamental current, will still turn forward, but the 5th will cause additional heating and, over time, can be extremely damaging to the stator insulation. If a drive shares a bus with an across-the-line motor, such as in a motor control center, it could damage the motor.

Note that this 5th harmonic will probably have virtually no effect on the upstream distribution system (i.e. cause minimal voltage distortion upstream), because its harmonic current is such a negligible portion of the total. But at the local level, where source impedance is at its highest, a bank of low-horsepower drives can cause enough voltage distortion at the local point of common coupling (PCC) to affect the motor loads that share that PCC.

The first line of defense against harmonics should be in the drive itself. A reactor coil, sometimes called a link inductor, is integrated into the DC link of many drives and tends to reduce current distortion on the line side of the drive. It will also protect the drive from transient overvoltages (notably capacitor switching transients) that can travel onto the DC link and cause DC overvoltage trips.

In some lower-cost drives, manufacturers have cut costs by eliminating the reactor coil, making the drive, in effect, a “harmonics generator.” This is especially critical when you install the drive on a bus with an across-the-line motor. In this instance, you can correct the situation by installing input line reactors or isolation transformers.

But buying a drive with a link inductor is no guarantee against problems. A similar harmonics problem occurs when you install a large number of small drives on a bus where they can cumulatively create enough harmonic distortion to trigger problems.

In this instance, it may not make sense to install a passive filter tuned to trap the 5th and 7th harmonics, as you would typically do with a high-horsepower drive, because the small loads can be highly dynamic, with constantly varying loads that a single passive filter won't mitigate. In such cases, you can use active harmonic filters, which track the harmonic currents and generate an out-of-phase counter current of the same harmonic and same amplitude to cancel the original harmonic. These filters are especially effective for dynamic loads with constantly varying harmonic currents.

Motor compatibility and smaller drives. VSDs can also create motor compatibility problems, especially when retrofitting drives to older motors. The high-speed switching of the IGBT in conjunction with long cable runs can cause overvoltage reflections (also known as standing-wave voltages or peak-to-peak or corona voltages) with peak voltages two to three times the DC link voltage.

Many drive manufacturers will specify that cable not exceed 100 feet, but sometimes even this can be too long. These overvoltage reflections tend to puncture insulation on the first few windings of the motor, causing premature failure of stator insulation. This is a problem common to high- and low-horsepower drives with PWM outputs. But lower-cost, low-horsepower motors are especially vulnerable. Their stator windings are often random-wound, a less costly manufacturing process, but one that could create a high potential between adjacent wires, making them that much more vulnerable to overvoltage reflections.

At one time, low-pass filters were commonly placed at the drive output to reduce the overvoltages. In recent years, however, manufacturers have designed inverter duty motors rated at 1,500V specifically to withstand overvoltage reflections. (Inverter duty motors are specified in NEMA MG-1, Section IV, Part 31.) Many drive manufacturers now require you to use inverter duty motors with their drives.

If your VSDs are generating harmonics, the first thing to do is take measurements at key points in the distribution system to determine the present level of harmonics. You can take measurements of harmonic distortion of waveforms as well as of individual harmonics with handheld power quality analyzers. You can also view overvoltage waveforms with handheld oscilloscopes.

Smith is a product specialist with Fluke Corp., Everett, Wash.

Sidebar: The Advantages of VSDs

Drives just keep getting smaller. Somehow engineers have been able to cram more horsepower into smaller, more compact packages, to the point that low-horsepower drives now have more or less the same footprint as mechanical starters.

Historically, heat dissipation needs made it difficult to build smaller drives. A drive first converts the AC sine wave into DC, then stores the DC power in capacitor banks in a section referred to as the DC link. The inverter then switches the DC to create a pulse width modulated (PWM) signal — a kind of synthetic AC. That process was a prime generator of heat.

In newer drives, however, a semiconductor device called an insulated gate bipolar transistor (IGBT) does the switching. Recent developments in IGBT design have resulted in greater current carrying capacity. Of more relevance in this context, however, is that IGBT switching speeds have become faster and faster, on the order of 100 to 200 nanoseconds.

The faster a switch, the more efficient it is. Why? With the exception of a very small voltage drop across any semiconductor, switches don't consume energy when they're off or on. They only waste energy by generating heat in the transition stage from off to on and vice versa.

So higher switching speeds result in less heat loss and increased efficiency. The result is smaller heat sinks and fans and more compact drives.

In addition to energy savings, drives offer many other benefits that contribute to the stability and robustness of the electrical distribution system:

  • Drives don't have inrush currents; they're typically limited to 110% of rated current. The inrush currents associated with starting motors across the line can cause nuisance tripping of the motor. They can also cause voltage sags that disturb other loads. Drives will instead “soft-start” motors, typically ramping a motor and load up to speed in about 20 to 30 seconds.

  • Drives have high displacement power factor, eliminating the need for power factor correction capacitors.

  • Drives isolate the switching transients (spikes) caused when a motor is turned off. They will typically ramp down a motor over 20 to 30 seconds. When the drive finally turns off, the motor is at low-speed and low-current, producing a relatively small spike that's easily absorbed within the DC link section of the drive itself.

  • Drives have programmable motor control, protection, and even communication functions that are far beyond what's provided by the contactor, trip unit, and auxiliary contacts of the mechanical starter. For example, you can program drives to reverse motor rotation, which eliminates the need for an additional contactor.

  • Using a drive in a single-phase system makes it possible to replace single-phase motors with more rugged 3-phase motors. This is because the VSD can accept a single-phase line-side voltage and output a 3-phase signal on the load (motor) side. In other words, drives “transform” single-phase to 3-phase voltage.