Troubleshooting Less Common Motors

Troubleshooting Logic Using Voltage Rules

Many electrical problems can be explained using the logic in these three rules:

1. When a conductor cuts or is cut by magnetic lines of force, a voltage is created in the conductor.

2. The voltage value increases as the speed of cutting lines of force increases. An example is the speed of a generator’s armature.

3. The voltage value increases with an increase in the number of lines of force.

Example: Increasing the excitation amperes in the shunt field of a DC generator, or the rotor windings of an AC alternator, will increase the voltage value.

The speed at which lines of force cut conductors can be very high when AC or DC is switched off. The arc that forms when the load-carrying contacts open is destructive to the contacts, but the effect of the arc is to slow the shutdown of power. When the shunt field of a DC machine is shut off, the magnetic field from thousands of turns of wire will collapse. As the magnetic field collapses, its lines of force cut the shunt field’s turns, producing a very high voltage. If this shutdown is instantaneous (as done with electronic switching), the result is an extremely high voltage spike.

Any sudden voltage change from full voltage to zero or to another voltage will cause the amperes and the resulting magnetic field around conductors to change at the same rate. Any conductor within this magnetic field will have voltage transformed into it. The result can be a very destructive spike that arcs through insulation, and can also destroy electronic circuitry. The voltage spike value increases as the length of the conductors increases. If the distance between an electronic speed controller and the motor it controls is over 50 feet, spike damage to the motor’s winding is common. Spike-caused insulation breakdown will occur within the first few turns of a motor’s line lead. Spike voltage can be dampened with reactors—a coil of heavily insulated wire wound around laminated iron. A reactor bucks sudden power changes.

Power lines in Canada that feed the eastern United States have problems with magnetic storms from the sun. Although the magnetic field is barely detectable, the voltage becomes very high and destructive when accumulating over hundreds of miles. These storms have moved magnetic North so far that survey crews have had to adjust for it.

Steel conduit that gets hot is another problem caused by magnetism. A conduit containing multiple, unevenly loaded conductors gets hot from eddy current. Eddy current circulates in steel when magnetic lines of force cut this ferrous metal. (This heat is a direct power loss.) If the amperes are the same in each conductor, their magnetic fields cancel each other and reduce the heating effect.

A neutral wire or a bare equipment ground wire can become dangerously energized by being located close to the magnetic fields of other wires. An equipment ground wire (that doesn’t go back to the transformer) can be ineffective if connected to a ground rod that doesn’t carry enough amperes. In the case of dry sand conditions, it would be safer to use an ungrounded power supply. (Poorly designed electrical systems in desert locations can be deadly where plumbing is involved.)

Troubleshooting Less Common Motors

Troubleshooting Identified Motors

The troubleshooting procedures under “Tÿpical Winding Problems” in Chapter 6 can be used on all types of three-phase motors.

The motor’s connection first must be identified. The schematic of the motor then is used to test equal circuits and, if necessary, to locate the problem. Comparison testing is the most accurate and dependable test procedure.

All circuits should be given the proper winding-to-frame test (with the usual precautions), starting with an ohmmeter.

Power supply problems (unbalanced voltage, low or high voltage, spikes, etc.) are all in-plant problems that cause motor failure. These problems don’t cause immediate motor failure so they are often overlooked.

Bearing breakdown is high on the motor failure list. Information is found under “Bearing Maintenance, ” later in this chapter.

Keeping good records on special motors can shorten troubleshooting time. Some motors and controls need more attention than others. Odd characteristics of a motor, such as high amperes of the multispeed consequent-pole motor’s low-speed connection, should be noted in the motor’s maintenance record. Control contacts that deteriorate sooner than normal should also be noted, as well as solutions to past problems. Having easy access to this type of information can shorten (or prevent) downtime.

Special motors such as multispeed or special-frame motors are expensive and hard to replace. (They usually have to be rewound or repaired.)

Special-duty motors often have copper or other alloys (other than aluminum) in their rotors. Broken rotor bars occur much more often with these alloys than with cast aluminum. Symptoms of broken rotor bars are found in Chapter 3, “Broken Rotor Bars.”

Troubleshooting the Synchronous Motor

The brush-type synchronous motor components most likely to break down are

The DC exciter fields and discharge resistor

The amortisseur (squirrel cage) winding

The stator winding

Bearings

The brushes, brush holders, and slip rings can have extremely high voltage while the motor is operating. Use caution with these components.

The DC Exciter Field and Discharge Resistor

The DC exciter field and its discharge resistor are a closed circuit (Fig. 7.1) until the motor control energizes the DC field and then disconnects the discharge resistor

When the motor starts, the DC field will have high voltage transformed into it. The transformed voltage is controlled by the overloading effect of the discharge resistor. If the resistor develops a faulty connection, it won’t load the field circuit enough and voltage will become too high. The DC field may not fail immediately, but its insulation has been stressed and will eventually break down. Most DC field failure is related to high-voltage stress.

A shorted DC field coil may keep the machine from reaching a speed high enough (95 to 98 percent of synchronous speed) to apply the DC voltage. If too many turns are shorted out, a circulating current develops and forms a pole in the coil’s iron. This pole will buck the pole formed by the amortisseur (squirrel cage) winding, causing loss of starting torque, and the motor won’t be able to get up to speed.

FIGURE 7.1 The function sequence of the contacts 41 ⁰ . Normally closed contact 41 ⁰ opens after normally open contacts (also 41 ⁰) close.

For the first test, visually check each coil for signs of shorted coils. (The coils are usually wrapped and varnished, making it hard to see signs of a short.)

The DC exciter field can be tested with the same procedures that are used on the shunt field of a DC motor. (The resistance of the field coils controls the current.)

The coils can be comparison-tested using the motor’s DC source and a voltmeter (voltage drop test) or by using an ohmmeter. The current should be limited with resistors to about one-half of the field’s ampere rating.

For the DC voltage drop test, divide the applied voltage by the number of coils—to get the approximate voltage drop across each coil. The voltage read across the coils should be +5 percent of each other.

An ohmmeter or microhmmeter can also be used to compare the resistance of the coils. The allowable difference is +2 percent.

The discharge resistor limits the transformed voltage in the DC field each time the motor starts. Its purpose is to overload the circuit and cause a voltage drop, the same as an overload affects the secondary of a transformer.

The resistors are usually made of ni-chrome wire, which expands and contracts each time the motor starts. If it is adjustable, the movable contact (clamped to it) may loosen. The discharge resistor, its control, and all connections in the circuit should be checked at least once a year.

Connections can be checked with an infrared gun (as the motor starts).

If a coil is shorted, the rest of the coils in its (series) circuit may be damaged. They should be individually surge-tested for turn-to-turn insulation damage.

The DC field circuit’s resistance to ground should be tested yearly with an ohmmeter and a megohmmeter. Early detection of insulation leakage from brush dust or other conducting contaminants will avoid a catastrophic breakdown.