![]() ECMs have definite advantages over fractional and small horsepower single-phase motors without speed control however, they become cost-prohibitive in higher horsepowers. Now let’s look at some of those dependencies noted earlier (see Table 1). Figure 3: Electronically commutated motors use the same components as variable frequency drives to accomplish pulse width modulation however, the motor here is internal to the ECM. Remember, though, these ECM pulses happen rates in the tens of Hertz, where VFDs send pulses at switching frequencies in the tens of thousands of Hertz. The width of the pulses here varies the average voltage the motor sees, just like a VFD. The significant difference here is that the ECM has the motor and speed control built into a single unit. Most of the HVAC ECMs are three-phase permanent magnet motors. Note the similarities to the VFD in the ECM in Figure 3. Figure 2: Variable frequency drives use input rectifiers, direct current capacitors, and insulated gate bi-polar transistors to send pulse width modulation to the external alternating current motor. By varying the timing and duration of the pulses, the VFD can control the speed of the motor by varying the frequency of the pulses. In Figure 2, the output section draws from that DC power and sends pulses to the motor in a method called pulse width modulation. That DC power is then stored in capacitors. is 60 Hertz.Īll conventional VFDs are of the same basic design to use an input rectifier section to convert the AC to DC. The motor speed is determined by its number of poles and the frequency of its supply, which here in the U.S. Three-phase AC induction motors, which have been in use for more than 100 years, are fixed-speed devices when powered directly from the AC line. First, let’s look at a typical VFD powering an AC induction motor. Let’s take a look at how these things work. If not an exact duplicate, replacement requires at least the same type and possibly from the same manufacturer to assure correct operation. These aren’t the only types of ECMs in the world these are just the most common HVAC types.īecause the controls are integrated with the motor and because the controls are typically programmed by the original equipment manufacturer at the factory, the various types and sizes are not interchangeable. For HVAC applications, it’s the controls built into the ECM that determine its type. There are most commonly three types of ECMs: constant cubic feet per minute, constant revolutions per minute and constant torque. It’s important to note here that VFDs can drive permanent magnet motors, too. The use of permanent magnets contributes to two important areas that will be covered in more detail later: higher efficiency and higher cost. The intelligence built into the controls turns the ECMs into specific-purpose devices.īeyond that definition, ECMs are most often permanent magnet motors. In the HVAC world, ECMs are generally thought of as a motor and controls integrated into a single unit. ECMs use electronics, such as Hall Effect sensors, to sense the position of the rotor for commutation of the magnets. So now, 60 or 70 years later, we have ECMs that use electronic devices like power switching transistors to replace the mechanical commutator assembly of a DC motor. Then, things really got urgent when it was discovered that brushes lasted only minutes in the hard vacuum of space. The first patent for what was then called a “commutatorless DC motor” was claimed by Harrison Brailsford in 1955. Interestingly, the major influence for that better way came about during World War II when brushed DC motors in high-altitude aircraft failed when brushes rapidly deteriorate above 30,000 feet. They wear out and require regular replacement, so science found a better way of commutation. However, brushes have some inherent problems. ![]() These motors are simple to produce, are relatively inexpensive and are still in use. Figure 1: The brushes in a brushed direct current motor change the polarity of the motor’s magnets with each revolution. ![]() When the rotor gets to the proper position, the brushes, which connect the voltage to the windings that produce the magnetism, connect with the next section of the commutator to reverse the magnets’ polarity. Brushed DC motorsįigure 1 is a brushed DC motor. The trick is to change the polarity of the magnets at the proper time to keep the rotor spinning. The opposing and attracting forces between the stator and rotor poles cause the rotor to turn. Starting with the very basics, a motor requires magnets in the stator and magnets in the rotor. Both VFDs and ECMs each have their place. The trouble is, the answers are all dependent on a variety of the bounds that frame the answer, such as size, application, system components, personnel training and many other factors. Are ECM controls integrated with the motor?.
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