Don't go out on a limb

Every plant professional should be able to check the circuitry used within their plant. Remember to take these precautions when checking the reliability of three-phase motor branch circuits.

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By Stephen W. Garstang, P.E.

PlantServices.com

(Editor's Note: There are three figures that accompany this article that can be downloaded in PDF format via the "Download Now" button at the bottom of the page.)

The National Electrical Code (NEC) defines a branch circuit as the conductors between the final overcurrent device that protects the circuit and the loads the circuit is feeding. The branch circuit includes the final overcurrent device (disconnect switch and fuses or circuit breaker), the motor starter and associated control circuits, circuit conductors and the three-phase motor.

The plant professional ought to know how to test newly installed branch circuits and again during startup and while running. The Occupational Safety and Health Administration, in publication NFPA 70E, states that any such measurements are to be made only by qualified persons using voltage-rated tools and proper personal protection equipment suitable for testing live circuits.

Before energizing the load, you’ll need to check four things: a physical examination, the ambient temperature, insulation integrity and voltage.

During a physical check, look for problems related to shipping and installation. Vibration and jolting can loosen mounts and damage plastic parts. Tighten any screws and lugs. Failure to torque them correctly can result in high resistance, high connection temperatures at loose connections and damaged conductors on overtorqued connections. Many manufacturers provide connector torque specifications.

It’s generally accepted that standard three-phase motors have a life of 20,000 hours. Most motor nameplates show a maximum ambient operating temperature of 40°C (104°F). Also, most motors are wound with conductors that have a Class B insulation rating, which is designed for the same temperature. Operating in a temperature 20°F warmer can cut motor life in half. A motor operating in an ambient temperature of 140°F might have a life of only 5,000 hours.

The solution for many temperature problems is to use a motor with Class F insulation, which is rated for an additional 25°C above that of Class B. This means the surrounding ambient temperature could be raised and additional 25°C without decreasing insulation life (Figure 1). Don’t forget other items that may be affected by temperature, such as lubricants and motor control equipment. Several motor starter manufacturers offer starter coils having Class H insulation, which is rated for an additional 40°F rise above Class F.

Check the insulation before startup and before restoring power after a motor burnout. This requires a megger, a testing device providing a high-voltage, low-current DC output. It simulates the peak voltage of the AC supply, which is 1.4 times the voltage measured on a multimeter. Be sure to review the megger’s instruction manual before using it. If a variable-frequency drive is used as a controller, contact the manufacturer for guidance before making this measurement.

Disconnect the power and any electronic devices, including electronic overload protection, proximity switches and the like. Measure the conductor resistances of the short circuit protection device from the load side conductor to ground. This evaluates the wiring between the load side of the protective device and the line side of the motor starter. Close the starter contacts manually and retest. This measurement prevents damaging the insulation on the moveable contact carriers.

Check each leg of the motor starter overload protector load side conductors to ground to verify the integrity of the insulation between the load side of the motor starter and the motor itself. The readings are usually 100 megohms or more. A reading below 2 megohms suggests an incipient problem. Recheck periodically to monitor insulation degradation. A reading of 100,000 ohms or less signifies dangerous leakage to ground.

(Editor’s note: For related information, refer to “Testing the insulation resistance of rotating machinery,” Plant Services, April 2003, p. 69)

The NEC dictates that short-circuit protection should be able to clear a fault without extensive damage to the circuit’s components. This is interpreted as confining the damage to the branch circuit, which explains the need to check insulation integrity after each motor fault.

 Use a multimeter to check for high and low voltage on each phase at the line side of the disconnect switch or circuit breaker. The voltage should be within the motor’s nameplate tolerances, which is 10% for standard NEMA designs.

This test checks the highest branch circuit voltage, which is reduced by conductor resistance on its way to the motor. The lowest voltage won’t be found by checking at the control panel. It can only be measured at the motor when it is starting. Inadequate voltage at the disconnect may prevent the motor from starting (Figure 2). Possible problems related to low voltage include:

• An increase in full load current after starting.
• A decrease in starting current and dramatic decrease in starting torque.
• A large increase in running current and a large decrease in running torque.
• Reduced power factor, which is a measure of energy used for producing the magnetic fields that enable the motor to do work, but doesn’t directly contribute to horsepower. The power factor reduces system capacity and reduces voltage.