Electronic motor drives seem to be everywhere, but do you really know what they are or how they work?
Electronic motor drives fall into one of two categories: AC and DC. AC motor drives control AC induction motors, DC motor drives typically control shunt-wound DC motors (which have separate armature and field circuits), and they both control the speed, torque, direction, and resulting horsepower of a motor.
A drive can control two elements of a 3-phase AC induction motor: speed and torque. To understand how a drive controls these two elements, let's quickly review AC induction motors. The two basic parts of the motor, the rotor and stator, work through magnetic interaction. The stator contains pole pairs, or iron pieces, that are 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, or base speed, which is determined by the number of poles and the frequency applied, as shown in this formula:
RPM = (120×Frequency/No. of Poles) - Slip
In addition to frequency and number of poles, the formula includes an effect called “slip,” which is the difference between the 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 fields try to catch up to the rotating fields of the stator, but they never do this difference is referred to as slip. Think of slip as the distance between greyhounds and the hare they chase around the track. As long as they don't catch up to the hare, the dogs will continue to run around the track. Slip is what allows a motor to turn.
You can change the speed of a motor by adjusting the frequency applied to it. You could change motor speed by altering the number of poles, but this requires rewinding, and it could result in a step change to the speed. So, for convenience, cost-efficiency, and precision, it's best to change the frequency. The torque-developing characteristic of every motor is the volts-per-hertz ratio (V/Hz). Change this ratio to change motor torque. An AC induction motor connected to a 460V, 60 Hz source has a ratio of 7.67 V/Hz. As long as this ratio stays in proportion, the motor will develop rated torque. Now where does the drive come into play? A drive provides several different frequency outputs. At any given frequency output of the drive, you get a new torque curve.
Just how does a drive provide the frequency and voltage output necessary to change the speed of a motor? That's what we'll look at next.
Fig. 1 (right)schematically shows a basic pulse width modulated (PWM) drive. All PWM drives contain the parts shown, with subtle differences in hardware and software components. Although some drives accept single-phase input power, we'll focus on the 3-phase drive.
The converter is the input section of the drive. It contains six diodes, arranged in an electrical bridge, that convert AC to DC power.
The DC bus section filters and smoothes out the waveform. The diodes reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you will find an average DC bus voltage of about 650V to 680V. Multiply the line voltage by 1.414 to get this value. 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, which is the final section of the drive. As the name implies, this section inverts the DC voltage back to AC. But, it does so in a variable voltage and frequency output. How? That depends on what kind of power devices your drive uses. Bipolar transistor technology began superceding silicon-controlled rectifiers (SCRs) in drives in the mid-1970s. In the early 1990s, those gave way to insulated gate bipolar transistor (IGBT) technology, which will form the basis for our discussion. If you have several SCR-based drives in your facility, you should understand how they operate. (See Sidebar below.)
Today's inverters use IGBTs to 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 doesn't provide an exact replica of the AC input sine waveform (Fig. 2, right). Instead, it provides voltage pulses of a constant magnitude.
The drive's control board signals the power device's control circuits to turn on the positive or negative half of the waveform. This switching of positive and negative waveforms re-creates the 3-phase output. The longer the power device remains on, the higher the output voltage. Conversely, the less time the power device is on, the lower the output voltage and output frequency.
The speed at which power devices switch on and off is the carrier frequency, or switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3000 to 4000 times per sec (3 KHz to 4 KHz). (With an older, SCR-based drive, switch frequencies are 250 to 500 times per sec). As you can imagine, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies increase heat in the power devices and decrease the efficiency of the drive.
Shrinking cost and size
Although the complexity of drive designs is increasing, quality is rising, as well. Drives are also coming in smaller packages with each generation. The trend is similar to that of the personal computer: more features, better performance, and lower cost with successive generations. Unlike computers, however, the reliability and ease of use of drives has dramatically improved. And also unlike computers, today's typical drive doesn't spew gratuitous harmonics into your distribution system nor does it affect your power factor. Drives are increasingly becoming plug-and-play devices. As electronic power components improve in reliability and decrease in size, the cost and size of drives will continue to decrease, and their performance and ease of use will only get better.
Dave Polka is the manager of ABB's Training Center in New Berlin, Wis.
Sidebar: What if You Have SCRs?
SCRs (originally referred to as thyristors) contain a control element called a gate. The gate acts as the “on switch” that allows the device to fully conduct voltage. The device conducts voltage until the polarity of the device reverses and then it automatically turns off. Special circuitry, usually requiring another circuit board and associated wiring, controls this switching. The SCR's output depends on how soon in the control cycle the gate turns on. Thus, you would approach troubleshooting differently if you had an SCR-based drive.