Electric Motors: Operation and Troubleshooting
February 1, 2015 by Dave Demma
Imagine a world without the electric motor: No elevators, no DVD players, no computer hard drive, no electric clocks, no Toyota Prius’, no washing machines or dryers, no conveyor belts at the supermarket checkout stand, no manufacturing plants of any kind, no power tools, no electric windows in your car and back to hand cranks to start the engine, no hair driers-the list goes on and on.
And of course: no HVAC/R systems whatsoever. Three of the four major HVAC/R components rely on electric motors; two for air movement, and the third to drive a compressor. Yes, I suppose it is within the realm of possibility to use large convection condensers and evaporators, and even a gas driven compressor. But the idea is beyond impractical. We need electric motors to provide comfort cooling and refrigeration. Without them we would be taking a huge step back to pre-industrial revolution times.
We are all familiar with the sixth grade science experiment where a steel nail is wrapped with several turns of copper wire. When the wire is connected to an electric power source (typically a battery) the steel nail becomes a magnet. Since the nail only operates as a magnet when the coil is connected to an electric power source, it is given the name “electro-magnet”.
A motor is nothing more than a series of electro-magnets, aligned such that the physical movement between the magnets resulting from the normal attraction between the north and south poles, and the normal repelling between like poles, is transmitted into movement via a rotating shaft.
The two major parts of any motor are the stator and the rotor.
Stator – It has been given this name because it is the “stationary” electro-magnet in the motor. It consists of one or more individual coils located on the stator, which when supplied with power become electro-magnets. Each coil (electro-magnet) is configured such that its north and south pole are spaced 180º apart from each other on the interior of the stator. For example, in a three-phase motor there would be three electro-magnets (or poles), each with its own north and south, you would have six individual poles situated inside the motor housing. They would be spaced 60º apart from each other, with the north/south orientation of the magnets consistent (see Figure 1).
Rotor – The rotor is the “rotating” electro-magnet of the motor, with the motor shaft being an integral part of the rotor. As the rotor rotates, the shaft rotates with it. The rotor of an induction motor consists of an iron core having longitudinal slots around its circumference in which heavy copper or aluminum bars are embedded. These bars are welded to a heavy ring of high conductivity on either end. The magnetic field generated when the stator is powered produces an induced current, which runs through the copper/aluminum bars in the rotor, resulting in the rotor becoming an electro-magnet. The composite structure is sometimes called a squirrel cage, and motors containing such a rotor are called squirrel cage induction motors (see Figure 2).
Once the motor has started, the natural attraction/repelling between the stator electro-magnet and the rotor electro-magnet will produce a rotating movement.
The three windings in a three-phase motor are powered with a three phase voltage supply. The phases are 120º out of phase with each other (see Figure 3). With a 60 Hz power supply, power is supplied to coil one 60 times per second. Likewise, power is supplied 60 times per second for coil two and coil three. However, due to the 120º phase shift, the power supplied to coil two is delayed with respect to coil one, and power supplied to coil three is similarly delayed with respect to coil two. The three phase power feeding the stationary coils in the stator produces a rotating magnet field. Picture a small playground wheel that children play on. Strategically placed 120º apart around the circumference of the wheel are three parents. In a timed fashion, parent one pulls/pushes the wheel in a clockwise direction. One second later parent two does likewise, and parent three follows one second later than parent two. This is similar to the effect generated by the rotating magnetic field in a three-phase motor.
Unlike a three-phase stator, single phase motors have an inherent problem starting in that the magnetic field they produce is not a rotating magnet field. It is a pulsing magnetic field, alternately getting larger, then smaller, but always pulsing in the same direction. This pulsing magnetic field produces no starting torque.
There are several methods used to produce starting torque in single phase motors:
Split Phase: This arrangement utilizes an auxiliary winding in the stator, typically fewer turns of smaller wire, and electrically placed 90º apart from the main winding. This winding has higher resistance (lower inductance), and produces an approximate 30º-phase difference from the main winding. The auxiliary winding is disconnected from the motor circuit by means of a centrifugal switch, which opens at approximately 75 per cent of the motor’s full speed. This method will provide moderate starting torque in smaller motors (1/3 HP or less) without the need for a capacitor (see Figure 4).
Permanent Split Capacitor: One method in solving the lack of starting torque problem inherent with single-phase motors is to add a second motor winding (two phase motor). The two windings would be spaced electrically 90º apart. The permanent split capacitor (PSC) motor offers low starting torque, and is typically used in two HP or smaller applications. The main advantage of the permanent capacitor is that it offers the flexibility of operating at different speeds such as blower motors (see Figure 5).
Capacitor Start: This method also uses an auxiliary winding, but typically larger wire and more turns than the split phase winding motor. In addition to the physical separation between the main and auxiliary windings, the addition of the capacitor in the auxiliary winding circuit will produce a 90º-phase shift. This provides excellent starting torque. Again, the auxiliary winding will be disconnected from the motor circuit by either a centrifugal switch or current/voltage relay (see Figure 6).
Capacitor Start Capacitor Run: This is a variation of the capacitor start motor described above. The auxiliary winding is wired with a start capacitor and a run capacitor in parallel. The large start capacitor provides excellent starting torque. It is removed from the circuit using either a current or voltage relay as the motor reaches approximately 75 per cent of rated motor speed. A smaller run capacitor remains in the circuit, providing improved running conditions without drawing excessive current (see Figure 7).
This provides a basic overview of the common motors used in the HVAC/R industry. In an upcoming issue, Part II will cover motor failure troubleshooting and methods for preventing motor failures. <>