Drives & Power Products Group
New Berlin, Wis.
Selecting the right drive is essential to getting the best performance and efficiency from an electric motor. A motor drive controls the speed, torque, direction, and resulting horsepower of a motor. Dc drives typically control a shunt-wound dc motor, which has separate armature and field circuits. Ac drives control ac-induction motors and, like their dc counterparts, control speed, torque, and horsepower.
For instance, take a simple application of a fixed-speed motor driving a fan. Replacing a three-phase motor starter with variablefrequency drive (VFD) permits the fan to operate at variable speeds. One benefit is energy savings, because the system varies airflow by controlling motor speed instead of with an air outlet damper.
A drive can control two main outputs of a three-phase induction motor: speed and torque. To understand how a drive controls these two elements, let's take a look at acinduction motors. The two basic parts of the motor, the rotor and stator, work through magnetic interaction. A motor contains pole pairs — iron components in the stator, wound in a specific pattern to provide a north to south magnetic field.
With one pole pair isolated in a motor, the rotor (shaft) rotates at a specific speed, the base speed. The number of poles and the frequency applied determine this speed. Shaft speed, V, is found from
V = 120F⁄P – S
where F = frequency applied to the motor, P = number of motor poles, and S = slip.
Slip is the difference between rotor speed and the rotating magnetic field in the stator. When a magnetic field passes through the conductors of the rotor, the rotor takes on magnetic fields of its own. These rotor magnetic fields try to catch the rotating fields of the stator. However, they never do, and this difference is slip. Think of slip as the distance between the greyhounds and the hare they are chasing around the track. As long as they don ‘t catch up to the hare, they will continue to revolve around the track. Slip is what allows a motor to turn.
For example, slip for a NEMA-B motor is 3 to 5% of the base speed, which is 1,800 rpm at full load. Shaft speed in this instance would be V = 120(60)⁄4 – 54 = 1,746 rpm.
A convenient and economical method to adjust speed is to change the frequency applied to the motor. Changing the number of poles would also change motor speed, but this physical change would require rewinding the motor, and result in a step change to the speed.
The ratio of voltage to frequency (V/Hz) determines torque-developing characteristic of a motor. Changing this ratio changes motor torque. For instance, an induction motor connected to a 460-V, 60-Hz source has a ratio of 7.67. As long as this ratio stays constant, the motor develops rated torque. A drive provides many different frequency outputs and, therefore, many different torque curves.
Let's now look at how a drive provides the frequency and voltage output necessary to change the speed of a motor. All PWM drives contain an input converter, dc bus, and output inverter, with subtle differences in hardware and software from one product to another. In small VFDs, a single power pack unit may contain the converter and inverter.
Although some drives accept singlephase input power, we'll focus on the three-phase drive. But to simplify the accompanying illustration, the waveforms in the drive figures show only one phase of input and output.
The input section of the drive is the converter. It contains six diodes, arranged in an electrical bridge. The diodes convert ac power to dc power. The next section — the dc bus — sees a fixed dc voltage.
The dc-bus filters and smooths the waveform. The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460-V unit, average dcbus voltage measures about 650 to 680 V, calculated as line voltage times 1.414. The inductor (L) and the capacitor (C) work together to filter out any ac component of the dc waveform. The smoother the dc waveform, the cleaner the output waveform from the drive.
The dc bus feeds the inverter, the final section of the drive. As the name implies, this section inverts dc voltage back to ac. But, it does so in a variable voltage and frequency output. How this happens depends on what kind of power devices the drive uses.
Switching with IGBTs
Fairly involved control circuitry coordinates the switching of power devices, typically through a control board that dictates the firing of power components in proper sequence. A microprocessor or digital signal processor (DSP) meets all the internal logic and decision requirements.
Older drives were SCR-based. An SCR (originally referred to as a thyristor) contains a control element called a gate. The gate acts as the turn-on switch that allows the device to fully conduct voltage until polarity reverses — and then it automatically turns off. Special circuitry, usually requiring another circuit board and associated wiring, controls this switching.
Bipolar transistor technology began superceding SCRs in drives in the mid-1970s. In the early 1990s, those gave way to using insulated-gate bipolar-transistor (IGBT) technology. IGBTs switch the dc bus on and off at specific intervals. In doing so, the inverter actually creates a variable ac voltage and frequency output.
The output of the drive does not provide an exact replica of the ac input sine waveform, as shown in the "PWM output" figure. Instead, it provides constantmagnitude voltage pulses. The drive's control board signals the power device's control circuits to turn on the waveform positive half or negative half of the power device. This alternating of positive and negative switches recreates the three-phase output. The longer the power device remains on, the higher the output voltage. The less time the power device is on, the lower the output voltage. Conversely, the longer the power device is off, the lower the output frequency.
The speed at which power devices switch on and off is the carrier frequency, also known as the switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3 to 4 KHz). Older, SCR-based drives feature switch frequencies of 250 to 500 Hz. Obviously, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies decrease the drive efficiency because of increased heat in the power devices.
Drives vary in complexity, but each new generation tends to offer improved performance in smaller packages. The trend is similar to that of personal computers. Unlike PCs, however, drives' reliability and ease of use have dramatically improved. And also unlike computers, the typical drive of today doesn't spew gratuitous harmonics into the distribution system — nor does it affect the power factor. Drives are increasingly becoming "plug and play." As electronic power components get smaller and more reliable, the cost and size of VFDs will continue to decrease while performance and ease of use will only get better.
MAINTAINING A VFD
A variable-frequency drive is basically a computer and power supply. So apply to VFDs the same precautions as with these devices to ensure years of trouble-free performance. Maintenance requirements fall into three basic categories.
Keep it clean. Most VFDs fall into the NEMA-1 category (side vents for cooling airflow) or NEMA 12 (sealed, dust-tight enclosure). NEMA-1 drives are susceptible to dust contamination. Dust on hardware can restrict airflow, degrading performance from heat sinks and circulating fans.
Dust on electronic devices can cause malfunction or failure. Dust absorbs moisture, which also contributes to failure. Periodically spraying air through the heat-sink fan is a good preventive maintenance measure. Discharging compressed air into a VFD is a viable option in some environments, but typical plant air contains oil and water. To use compressed air for cooling, it must be oil-free and dry or it will likely do more harm than good. That requires a specialized, dedicated, and expensive air supply. This
practice still runs the risk of generating electrostatic charges and ESD. A nonstatic-generating spray or reverse-operated ESD vacuum will reduce static buildup. Common plastics are prime generators of static electricity. The material in ESD vacuum cases and fans is a special, nonstaticgenerating plastic. These vacuums, and cans of nonstaticgenerating compressed air, are available from static-control equipment specialists.
Keep it dry. Control boards subject to moist environments may eventually experience circuitboard corrosion, so keep obvious sources of moisture away from a VFD. Some manufacturers included a type of "condensation protection" in earlier VFDs. When the temperature fell below 32F, the software logic would not allow the drive to start. VFDs rarely offer this protection today. When operating a VFD all day, every day, normal radiant heat from the heat sink should prevent condensation. Unless the unit is in continuous operation, use a NEMA-12 enclosure and thermostatically controlled space heater if locating the unit where condensation is likely.
Keep connections tight. While this sounds basic, checking connections is a step many people miss or do incorrectly, and this requirement applies even to clean rooms. Heat cycles and mechanical vibration can lead to substandard connections, as can standard PM practices. Retorquing screws is not recommended because further tightening an already tight screw can ruin a good connection. If screws have merely worked loose, try retightening. Bad connections eventually lead to arcing. Arcing at the VFD input could result in nuisance overvoltage faults, clearing of input fuses, or damage to protective components. Arcing at the VFD output could result in overcurrent faults, or even damage to the power components. Loose control wiring can cause erratic operation. For example, a loose Start/Stop signal wire can cause uncontrollable VFD stops. A loose speed reference wire can cause drive speed to fluctuate, resulting in scrap, machine damage, or personnel injury.
Additional steps. Don't overlook internal VFD components as part of a mechanical inspection. Check circulating fans for signs of bearing failure or foreign objects, usually indicated by unusual noise or wobbly shafts. Inspect dc-bus capacitors for bulging and leakage. Either could be a sign of component stress or electrical misuse.
Measure voltage while the VFD is operating. Fluctuations in dc-bus voltage can indicate degradation of dc-bus capacitors. One function of the capacitor bank is to act as a filter section, smoothing any ac ripple voltage on the bus. Abnormal ac voltage on the dc bus indicates the capacitors are headed for trouble.
Most VFD manufacturers have a special terminal block for this type of measurement and also for connection to dynamic-braking resistors. More than 4 Vac may indicate a capacitor filtering problem or a possible problem with the diode bridge converter section (ahead of the bus). In such cases, consult with the VFD manufacturer before taking further action.
With the VFD in Start and at zero speed, output voltage should be 40 Vac phase to phase or less. Higher readings may indicate transistor leakage. At zero speed, the power components should not be operating. If readings exceed 60 Vac, expect a powercomponent failure.
Regularly monitor heat-sink temperatures. Most VFD manufacturers ease this task by including a direct temperature readout on the keypad or display. And finally, power up VFDs in storage every six months to keep dc-bus capacitors at peak performance capability. Otherwise, their charging ability will significantly diminish.
Some manufacturers advertise 200,000 hr — almost 23 years — of mean time between failures. Following these simple procedures makes it possible to obtain such impressive performance.