HVAC Electric Motors

Last Updated on Thursday, 05 November 2009 01:32 Written by Richard Saturday, 01 March 2008 21:01

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HVAC Electric Motors

Many Commercial HVAC Motors have disconnects nearby

The HVAC industry depends heavily on the electric motor. The electric motor is the basic component which powers blowers to move air, drives compressors to compress refrigerant, and powers pumps to move water for chilled water and hot water applications and fuel oil for oil burners. The electric motor is an integral part of all HVAC systems and many HVAC applications would be impossible to implement without the old electric motor. Depending on the application will depend on the type of motor needed for the job. It is important that the design engineer select the appropriate electric motor for the job otherwise big and continuous problems will occur throughout the life of the HVAC system.

Electric blower motors or propeller fans usually require electric motors with a low starting torque while compressors require electric motors with a high starting torque. These characteristics are important especially when selecting the appropriate electric motor for the application. A compressor electric motor with a low starting torque rating is doomed to failure while a blower motor or propeller blade electric motor with an unnecessarily high torque rating will end up costing more in energy costs over the life of the equipment and electric motor. This is why it is important that the design engineer who designs the equipment make the appropriate selection of electric motors for the equipment. It is also important that if an electric motor in an HVAC application fails for whatever reason that the electric motor be replaced with an exact replacement electric motor.

Three Phase Motors

In commercial and industrial HVAC applications the most common electric motor used is the three phase electric motor. The three phase electric motor is considered to be stronger and more reliable than the single phase electric motors used in smaller scale applications. Hermetically sealed and semi-hermetic sealed compressors are common in many commercial and industrial chillers, roof-top units, and even in air to air systems. Three phase electric motors do not require a capacitor that many other smaller single phase electric motors use and three phase electric motors have a good starting torque rating. This makes them ideal for use in refrigeration because compressors in air conditioning and refrigeration applications require a high starting torque from the electric motors. If the three phase electric motor is so good why is it not used in smaller applications like residential or light commercial? The answer is simple: Because typically only single phase power is available in residential and many light commercial buildings. The initial installation cost to install three phase power in many neighborhoods is high so three phase power is only usually installed for heavy commercial or industrial use.

Single Phase Motors

Split-phase motors are the workhorse of the HVAC industry. A split-phase electric motor has two windings - a start and and run winding. There two common split-phase motors used in HVAC applications.

One of the split-phased electric motors used in HVAC applications is the capacitor-start-induction-run motor and it is also a single phase electric motor. The capacitor start motor uses a start capacitor to boost its starting torque. Most capacitor start motors use a centrifugal switch to disengage the start capacitor from the start winding circuit when the motor reaches approximately 70 to 80 percent of its run speed.

The other common split-phase electric motor used for HVAC applications is the resistance-start-induction-run electric motor. This electric motor has a start winding and a run winding and single-phase current is introduced to both windings on start-up. The two windings are out of phase by 45 to 90 degrees which gives the motor a boost on start-up. This resistance-start-induction-run motor does not utilize a capacitor as the capacitor=start-induction-run electric motor does. The resistance-start-induction-run electric motor does use a centrifugal switch to drop the start winding out of the circuit just as the capacitor-start-induction-run motor does.

Another common electric motor used in many HVAC applications is the permanent split-capacitor (PSC) electric motor. Unlike the three phase electric motor, the PSC electric motor is a single phase electric motor. These electric motors are usually easily reversible from the clockwise direction to the counter-clockwise direction. The PSC electric motor uses a capacitor and has a good running efficiency. It only has a moderate torque rating on start up but it is used for some refrigeration and air conditioning compressor applications. Usually when a PSC electric motor is used in a compressor application is has two capacitors attached to its wiring. One is a start capacitor to boost the starting torque and the other is a run capacitor to increase its running efficiency. The start capacitor is usually only in the circuit for a second at start up and then it drops out of the compressor circuit by a special relay. It is important that the start capacitor drop out of the circuit quickly after start up. If the relay fails in a closed position and the start capacitor remains in the circuit the start winding in the PSC electric motor will burn up rendering the compressor useless.

The shaded-pole electric motor is used in HVAC applications but usually for only fractional horsepower applications where start and run torque requirements are minimal. The shaded-pole electric motor does not use a capacitor and not easily reversible.

electric motors, three phase electric motors, single phase electric motors