Needham’s Motor Applications

Motor Applications and Control

Matt Needham’s lecture explains how to select, install, maintain, and protect electric motors used in air-conditioning and refrigeration systems.

1. Choose a Motor Suited to Its Application

• Don’t replace a motor based only on horsepower. A condenser fan motor, for example, may face more moisture, dirt, heat, or vibration than a motor in an indoor air handler.
• Check the motor’s horsepower, current rating, voltage, and whether it is single-phase or three-phase.
• Consult manufacturers or parts suppliers when confirming a replacement motor.

2. Check Voltage and Investigate Low Readings

• The lecture gives ±10% of the motor’s rated voltage as a general acceptable range, while emphasizing that it is a guideline rather than a precise failure threshold.
• Motors may be rated for ranges such as 208–230 V. Some are designed for 50 or 60 Hz to serve different markets.
• Needham says low voltage is more commonly encountered than excessively high voltage. Loose wires, poor contacts, or burned connections can cause a voltage drop.
• A motor operating at too-low a voltage may draw more current, generate extra heat, and overheat. A low reading should prompt inspection of connections and contacts.
• Solar power fed back into a local grid can sometimes contribute to high voltage.

3. Understand Single-Phase Loads on a Three-Phase Supply

• Single-phase loads can be supplied from a three-phase system, while an ordinary single-phase supply generally cannot directly provide three-phase power.
• In the example, three 230 V single-phase motors each draw 10 A and are connected across pairs of phases. Each line carries 20 A.
• If a fuse disconnects one motor, the example’s line currents become 10 A, 10 A, and 20 A.
• The key lesson is to trace which loads use each phase when interpreting clamp-meter readings.

4. Bearings and Lubrication

• Sleeve bearings are quieter and are often preferred where noise transmitted through ductwork is a concern. Ball bearings are more robust but typically noisier.
• Some bearings are sealed and require no routine lubrication; older serviceable bearings may need oil or grease. Needham’s example of a regularly oiled furnace fan illustrates that maintained bearings can last a long time.
• For bearings that require lubrication:
  • Oil-lubricated bearings generally need more frequent attention than grease-lubricated bearings.
  • Bearings that run longer hours or operate in warmer locations need more frequent lubrication.
  • A continuously running bearing in a hot location may need lubrication every few months, while an intermittently used bearing in a cool location may need it only every few years.
  • Remove a safety plug where necessary before greasing to avoid excessive pressure.

5. Mounting and Drive Arrangements

• Rigid mounts connect metal directly to metal. Rubber mounts or bushings isolate the motor, so a separate ground wire is important to provide a fault-current path if a winding contacts the motor housing.
• Mounting types mentioned include cradle mounts, clamp-style mounts, and end mounts.
• Direct drives connect a motor shaft to the driven equipment, often through a coupling. Black, powder-like residue beneath a coupling may indicate wear and a need for repair or replacement soon.
• Belt drives use pulleys and a belt:
  • A larger fan pulley makes the fan turn more slowly relative to the motor.
  • A larger motor pulley makes the fan turn faster.
  • The lecture mentions thinner Type A/4L-style belts and thicker Type B belts. Larger systems may use multiple belts.

6. Motor Current Ratings and Starting Current

• RLA (rated load amps) and FLA (full load amps) are treated as effectively equivalent operating-current references in the lecture.
• A brief startup or heavy-load current above the normal running value may be acceptable, but a motor should not run continuously above its rated operating current.
• LRA (locked-rotor amps) describes the high current associated with a stationary rotor at startup or when the motor is unable to turn.
• The lecture estimates compressor startup current at roughly five to seven times normal current and fan-motor startup current at roughly three to three-and-a-half times normal current, typically for a very short period.

7. Fuses, Overload Protection, and Restart Precautions

• Fuses and circuit breakers protect wiring and the overall circuit from excessive current.
• Overload protectors protect individual motors. Some small, inexpensive fan motors may lack separate overload protection.
• Thermal overload devices respond to heat and may take time to trip. Magnetic overload relays respond to excessive current more quickly, according to the lecture.
• If a motor has shut down on overload, allow it to cool before deciding it has failed; some protection is built into the motor or compressor.
• Needham recommends waiting at least two to three minutes before restarting an air-conditioning or refrigeration compressor. This allows it to cool somewhat and gives refrigerant pressures time to move toward equalization, making restart easier.

8. Switch Poles and Positions

The lecture offers a counting method for interpreting switch labels:

• Pole: The number of separate input circuits or contacts being switched.
• Position/throw: The number of output choices for each pole.

Examples discussed include single-pole, single-position; single-pole, double-position; double-pole, single-position; double-pole, double-position; and three-pole, single-position switches.

In a relay or contactor, energizing the coil changes the contacts’ state—for example, closing normally open contacts and opening normally closed ones.

9. Compressor Controls and Crankcase Heaters

• A thermostat can energize a contactor coil, closing contacts that power the compressor. When the thermostat is satisfied, the coil de-energizes and the contacts open.
• A crankcase heater warms an idle compressor. Its purpose is to help prevent refrigerant from migrating into the cold compressor and mixing with the oil, which could make startup more difficult or harmful.
• The lecture notes that crankcase heaters are particularly relevant where air-conditioning equipment sits unused through long, cold winters.

10. Contactors, Relays, and Motor Starters

• A contactor and a relay both use a coil to operate electrical contacts; a contactor is intended to handle higher current. The lecture gives 20 A as a rough dividing point, with smaller loads commonly using relays.
• A motor starter combines switching with overload protection. In the described thermal starter, excess motor current heats overload elements, which can open a control contact and de-energize the contactor coil.
• After a starter trips, the technician should let the motor cool, reset the device if appropriate, and investigate the cause—such as low voltage from a poor connection or a motor problem—before returning it to service.

Speakers and Sources

• Speaker: Matt Needham (identified in the subtitles as “Matt Nehm”), the lecturer.
• Sources or references mentioned: A refrigeration and air-conditioning textbook (Chapter 18); motor manufacturers and parts suppliers; and Lovejoy, cited as a coupling manufacturer.