Three-phase motors, ECMs and variable-frequency drives

At the seniors' residence, Raj showed Theo three generations of motors in one mechanical room. The oldest was a 15-horsepower chilled water pump motor, connected straight to the 480-volt supply through a starter: it ran at full speed whenever it ran. Beside it, a newer pump of the same size was fed through a variable-frequency drive mounted on the wall, its display reading 47.2 hertz and 11.6 amperes. Upstairs, the corridor ventilation unit had been retrofitted with an electronically commutated fan motor the size of a dinner plate. "Same job, three ways of doing it," Raj said. "The pump on the drive uses maybe half the energy of the old one for the same water. But the drive and the ECM both have electronics that can hurt you after the power is off, and they fail in different ways."

This lesson covers the larger and newer motors technicians meet: three-phase induction motors, their nameplates, rotation and protection; electronically commutated motors (ECMs); and variable-frequency drives (VFDs), including the inverter drives in ductless heat pumps. It also covers the safety precautions for electronic drives.

Three-phase induction motors

A three-phase induction motor has three sets of windings in its stator, one for each phase. Because the three phase currents peak one after another, the magnetic field they create rotates around the stator smoothly, with no need for a start winding, capacitor or relay. The field induces currents in the rotor, usually a squirrel-cage rotor of aluminum or copper bars joined by end rings, and those currents create the rotor's own magnetic field, which follows the stator's rotating field. Three-phase motors are simple, rugged, efficient and self-starting, and they are used for compressors, pumps and fans above a few horsepower.

Like single-phase motors, their speed is set by frequency and the number of poles, with a few percent slip under load. Reversing any two of the three supply leads reverses the rotating field and the motor's direction. That is useful for fans and pumps, which must turn the right way, and critical for scroll compressors, as Lesson 4 described.

When a three-phase motor starts directly across the line, it draws a locked-rotor current typically about five to seven times its full-load current, for a second or two until it reaches speed. Large motors may use reduced-voltage starting, such as a soft starter that ramps up the voltage electronically, or part-winding starting on some chiller compressors, to limit the dip in building voltage.

Reading a motor nameplate

Dual-voltage motors have nine leads, or more, that are connected one way for the low voltage and another way for the high voltage; the connection diagram is on the nameplate or inside the terminal box cover. Connecting a motor for the wrong voltage either overheats it within minutes or leaves it unable to develop its torque.

Electrical tests on a three-phase motor compare the three phases. With the power locked out and the motor disconnected, the resistances of the three windings should be equal, within a few percent, and there should be no continuity to the frame; a megohmmeter checks insulation. With the motor running, the three line currents should be close to each other and at or below the nameplate FLA, and the three line voltages balanced within about 2 percent.

Single-phasing

If one supply phase is lost, for example because one fuse has blown or a contactor pole has burned open, a three-phase motor is said to be single-phasing. A stopped motor will not start: it hums, draws locked-rotor current and trips its overloads. A motor that was already running may keep turning on the two remaining phases, but its current in those two phases rises sharply, often to well above the nameplate, and it overheats quickly. A clamp reading that shows normal or high current in two lines and zero in the third is the sign. Overload relays sensing all three phases and phase monitors that open the control circuit on a lost or reversed phase protect against it.

When a three-phase motor fails with a burnt winding, the pattern of damage often shows whether single-phasing was the cause: two phases burnt and one untouched. Replacing the motor without finding the blown fuse, the bad contactor pole or the loose connection that caused it leads to a second failure.

Electronically commutated motors

An electronically commutated motor, ECM, is a brushless DC motor with a permanent-magnet rotor and a built-in electronic control module. The module converts the AC supply to DC and then switches current through the stator windings in sequence, sensing the rotor's position so that the field always pulls the rotor around. Because the magnets do not need induced current, ECMs are much more efficient than shaded-pole and PSC motors, especially at reduced speed, and their speed can be varied over a wide range. They are used in furnace and air handler blowers, condenser fans, and evaporator fans in walk-in coolers and display cases.

There are two common kinds. A constant-torque ECM has several speed taps, like a multi-speed PSC motor, energized with 24 volts or line voltage; each tap selects a programd torque. A constant-airflow ECM is programd by the equipment manufacturer and receives commands from the furnace or air handler control board; it adjusts its torque to keep a set airflow as filters load and duct pressure changes. Both have line voltage connected to the motor all the time, and the speed is selected by low-voltage signals. Switching the line voltage on and off to control an ECM, as is done with a PSC motor, can damage it.

Troubleshooting an ECM starts with the inputs: line voltage at the motor's power connector, a good ground, and the control signal at the low-voltage connector when the motor should run. If both are correct and the motor does not run, the module or motor has failed. Manufacturers' diagnostic tools can test some motors. The module holds charged capacitors, so after removing power, wait before unplugging connectors, as the manufacturer instructs; never unplug the power connector while the motor is energized, because the arc damages the pins. Replacement modules for constant-airflow motors must carry the correct program for the equipment model.

Variable-frequency drives

A variable-frequency drive, VFD, runs a standard induction motor at variable speed by changing the frequency of its supply. Inside, a rectifier converts the incoming AC to DC; a bank of large capacitors on the DC bus smooths it; and an inverter of fast transistor switches turns the DC back into AC by switching on and off thousands of times a second, a technique called pulse-width modulation. By changing the pattern of switching, the drive produces an output whose frequency, and voltage in proportion, can be set anywhere from a few hertz to above 60. The motor's speed follows the frequency.

The DC bus voltage is about 1.35 to 1.41 times the supply line voltage: roughly 325 to 340 volts DC on a 240-volt supply, and 650 to 680 volts DC on a 480-volt supply. Those capacitors can hold a lethal charge for minutes after the supply is opened, which is why drives carry a label stating the waiting time and many have a bus-charge light. Before working inside a drive or on its motor leads, lock out the supply, wait at least the stated time, then measure the DC bus at the drive's terminals with a meter rated for the voltage and confirm that it has fallen to a safe level. A dark charge light is not proof.

The output of a drive is not a smooth sine wave but a series of fast pulses, so ordinary meters may read it wrongly. Many true-RMS clamp meters have a low-pass filter or VFD mode for measuring drive output. Megohmmeter tests must never be done with the drive connected; disconnect the motor leads at the drive first.

Why drives save energy

Pumps and fans follow the affinity laws: flow is proportional to speed, pressure to the square of speed, and power to the cube of speed. A pump running at 80 percent speed delivers about 80 percent of the flow but needs only about half the power. Without a drive, flow is reduced by throttling a valve or a damper, which wastes the extra energy as pressure drop. That is why the residence's newer pump uses so much less energy than the old one for the same job.

# Synchronous speed, slip, affinity laws and DC bus voltage
f, poles, nameplate_rpm = 60, 4, 1760
ns = 120 * f / poles
print(f"synchronous speed {ns:.0f} rpm, slip {100 * (ns - nameplate_rpm) / ns:.1f} %")

full_kw = 11.2                        # pump input at 60 Hz (about 15 hp motor)
for hz in (60, 54, 47.2, 40):
    ratio = hz / 60
    print(f"{hz:5.1f} Hz: flow {ratio:4.0%}, head {ratio**2:4.0%}, power about {full_kw * ratio**3:4.1f} kW")

for line_v in (240, 480):
    print(f"DC bus on a {line_v} V supply: about {1.35 * line_v:.0f} to {1.414 * line_v:.0f} V DC")

synchronous speed 1800 rpm, slip 2.2 %
 60.0 Hz: flow 100%, head 100%, power about 11.2 kW
 54.0 Hz: flow  90%, head  81%, power about  8.2 kW
 47.2 Hz: flow  79%, head  62%, power about  5.5 kW
 40.0 Hz: flow  67%, head  44%, power about  3.3 kW
DC bus on a 240 V supply: about 324 to 339 V DC
DC bus on a 480 V supply: about 648 to 679 V DC

At 47.2 hertz, the frequency on the drive's display, the pump moves about 79 percent of its full flow while drawing a little under half its full-speed power. The real saving is somewhat smaller because of system pressure requirements and drive losses, but it is still large. The affinity laws are the reason drives are now standard on new pumps, cooling tower fans and air handler supply fans.

Inverter compressors

The attic ductless heat pump that Northfield installed last spring uses the same idea inside the outdoor unit. Its compressor is driven by an inverter board, which converts the 240-volt supply to DC and then to a variable-frequency supply for a permanent-magnet compressor motor, varying its speed to match the load instead of cycling on and off. Variable-speed compressors give steadier temperatures and better efficiency, especially at part load and in cold-climate heat pumps.

The inverter board stores energy in its DC bus capacitors just as a VFD does, often with a waiting time of several minutes printed on the board or cover. Service manuals give the test points and voltages for the DC bus and the compressor outputs, and most inverter units report faults with error codes on a display or a flashing LED. Inverter compressors are not tested like a single-phase compressor with C, S and R terminals: their three terminals are tested for equal winding resistance and insulation to ground, like a three-phase motor, with the board disconnected.