Hawaii Building Engineers Association

3 Phase Motor Protection

Why Protect Your Three Phase Motors?

Historical data shows that 44% of motor failures are due to overloads and single-phasing.  (Cooper/Bussman’s Protection Handbook Bulletin http://www.bussman.com/library/docs/spd02/SPDSection11.pdf)

This can be serious, especially since a problem isn’t detected until the motor fails.  When that happens, which unfortunately may be on the weekend or night when it is difficult to get a repairman, the consequences may include inconvenience, loss of revenue, extensive repair costs, or irate customers. 

What Causes Overloads

Overloads are caused by excessive load on a motor.  This results in current that exceeds the

FLA

(full load amp) ampacity rating of the motor, resulting in overheating and damage to the motor.  Some causes are improperly sized motors or changes in the loads.

What Causes Single Phasing

Voltage imbalance means that the voltage between the phases isn’t equal.  Single phasing is the worst condition of voltage imbalance because one phase is lost (open) and the current in the other two phases increase to keep the motor running.  The result is overheating and a burnt out motor. 

Some causes are damaged motor starter contact, damaged switch or circuit breaker, open fuse in the circuit breaker, cable failure on the transformer primary or secondary side, or poor connections in terminals (Bussman, p5).

Air compressors

Air compressors Question

  I had been told that by replacing or changing all air compressor motors to variable speed drive (VSD), I will lower my electrical energy costs. I have three horsepower 100 rotary screw compressors and all three run all the time. Should I replace all these with variable speed drives?

This question can not be answered correctly without more data especially a load profile. Many times the conception of changing to a variable speed drive will save money and is very inaccurate. It is recommended that an assessment of your total system be done first before any equipment is changed. Variable speed compressors are great depending on how the overall system is in regards to the new drive, in some cases a drive may make it worse. Facts about variable speed drives include:

• A VSD is much more efficient, as a trim unit, than a similar size constant speed compressor version at loads of 25% to 75%.
• A variable speed is close equal to a constant speed compressor at 75% to 100% of load.
• A VSD, when applied at full load, will draw more power, for the same flow, than a similar size constant speed compressor. This can be 4 – 6% more in lubricated rotary screws and up to 13% more in oil free designs.

Surge Protection

Know the Benefits of Surge Protection

As energy demands increase for the Hawaiian Islands so does the likely hood of energy surges to our equipment. Power quality will diminish as use goes up and it could get worse before it gets better.

Why surge protection? Each year our buildings experience 300 potentially damaging electrical surges per year. Heating, Ventilation, Air Conditioning (HVAC) and elevator equipment are very sensitive to these surges. These units are a costly investment for the owners and typically there is no protection. A surge is a high amplitude short duration electrical fluctuation that can cause harm to electrical, electromechanical, and electronic equipment. Surges are caused by lighting, utility events, and internal events. Lighting is the most obvious and most sensational type of surge. Lighting rarely happens in Hawaii, what we see are utility events that consist of crossover of phases, capacitor switching, grid shifting, inductive loads, and open neutrals. Internal events in buildings are the more likely source of a surge. A general Electric (GE) and National Power Labs (NPL) studies have shown that 65 to 80 percent of transient surges are caused by internally from pumps (well or pool), A/C condenser motors, refrigeration motors and other motors around the building. These events can lead to 1) degradation 2) destruction 3) downtime.

Surge protectors provide protection against :

• Incoming surges
• Bounce surges from inrush current

A surge protector works by shunting the voltage to ground. A good ground is imperative the National Electrical Code (NEC) states maximum resistance at ground is to be 25 ohms.

Additional reading available from HBEA and HECO.         

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