How To Repair A Larg Dc Brush Motor

Chuck Yung
EASA Senior Technical Support Specialist

There are times when a DC motor or generator experiences a catastrophic failure and the customer wants to know why it happened. One type of failure that seems to stimulate lively conversation is when the failure involves dramatic damage to the brushholders and commutator. The term "flashover" describes the appearance of the failure; the very proper noun conveys an accurate mental image of the failure.  See Figure 1.

The questions that arise next are predictable: "What caused this?" and "What can be done to prevent a recurrence?" Or, if the motor was recently repaired: "What did you lot practice to my motor to cause this?!" The purpose of this commodity is to assistance you reply those questions.

The causes of a flashover tin can exist partially explained by the insulating backdrop of air, and Ohm'due south Law. Air is an electrical insulator, although the dielectric breakdown voltage of air is depression compared to the insulating materials we use in electrical motors. Within an operating DC motor, nosotros discover rut, carbon dust and other contaminants, and perhaps even humidity. Each of these will reduce the dielectric strength of air.

Equally for Ohm's Law, E/R = I; winders utilise this frequently to evaluate shunt fields and to extrapolate the temperature rise of those fields. Merely it also applies to the armature circuit.
At the moment a DC motor is energized, before the armature starts to rotate, the armature current is express merely by the available kVA of the power supply.

Consider the instance of a 500 hp motor, with a 500V armature circuit. Static resistance of the armature-interpole circuit measured but 0.02 ohms, so the short excursion armature current could reach 25,000 amps if the drive has sufficient kVA: 500/0.02  = 25,000 amps.

Furnishings on armature
Fortunately, drives ramp up the armature voltage, rather than applying information technology instantly. Every bit soon equally the armature begins to rotate, the inductance provided by the armature becomes a cistron in suppressing the armature current. Paraphrasing the now-defunct IEEE Standard 66: When voltage E is practical across a circuit consisting of a resistance and inductance L in series, the maximum rate of rise is given past the equation di/dt = Due east/Fifty amperes per 2nd; where E equals volts, and L equals henrys. In other words, the armature current decreases chop-chop equally the armature speed increases.

Every DC motor tin can exist used as a generator, past driving information technology mechanically and applying electric current to the fields. When operating every bit a motor, there are times where the motor might be driven past an overhauling load (eastward.chiliad., a loaded conveyor running downhill; or a hoist lowering a heavy load). When that happens, the counter-emf (electro-motive strength) produced overcomes the practical emf, and flashover is likely. In layman'south terms, operating conditions crusade the armature electric current to increase apace, and generated voltage/current trigger the flashover.
A list of operating events that tin can cause a flashover is included in Tabular array 1.

If the interpoles are not correctly adapted to maintain castor neutral throughout the operating load range, the shifting neutral results in arcing as the load increases exterior the black ring region. That tin, in and of itself, trigger a flashover. (The black band region can be described as this: Weakening / strengthening the interpoles, independent of all else, until the brushes brainstorm to spark produces a band within which no sparking occurs. That band is referred to as the "black band." For more information, see the Associates and Last Exam section of Fundamentals of DC Operation and Repair Tips.)

Preventive measures
Working to aid your customer understand the basics of how a DC motor operates tin can get a long way towards helping them avert problems. 1 of my most vivid "triggers" of a flashover is the customer who installs a newly rebuilt compound motor with more than than l% compounding. (The percentage compounding describes the percentage of total field flux contributed past the serial fields, at full load.) They check rotation and discover that the motor needs to be reversed. We all know that the right way to do this is to bandy the A1 and A2 leads (the large wires that are thoroughly taped). But, says the client, information technology is and so much easier to swap the shunt field leads (they are smaller, and probably held in a concluding strip by screws) instead. That shortcut has worked in the past — on directly shunt motors.

With a compound-wound motorcar, this time-saving shortcut inverse the motor from a cumulative connection to differential. The motor runs fine unloaded, and even with a moderate load. But when the load is increased to the point that the serial overpowers the shunt fields, ending occurs. Since this is a newly rebuilt motor, there is a very good take chances that your customer will arraign you. Later all, you simply rebuilt the motor. So information technology is of import to educate the customer to avert only such a state of affairs. (And yes, I have had many, many calls where a newly installed motor failed exactly as just described.)

If someone blames a flashover on "drive settings," that implies that the drive is accelerating or decelerating the motor likewise quickly. If so, a competent drive technician should be able to accommodate that to reduce the chance of flashover. Blaming the drive may instead mean that the motor is in an application calling for a regenerative drive, just the client replaced the drive with a less expensive model that cannot handle the regenerative mode. (And the client might not acknowledge having done so until yous printing the upshot.) One example would be a chemical compound wound motor driving a roller coaster. When the cars are coasting downhill, the regenerative mode is used to foreclose dangerous over-acceleration.

A compound wound motor, in such an awarding, requires a drive that has connection points for the shunt, armature and separate series field leads. This is to allow the motor to operate with a cumulative connection in both directions of rotation. If a compound wound motor is operated from a bulldoze with simply shunt- and armature circuit leads, in a reversing application, it will be cumulative in 1 management but differentially compounded in the reverse direction. The college the percentage compounding, the greater the risk of speed instability and/or flashover. Run across Table 2.

Specific to any DC motor, there are several preventive measures to reduce the take a chance of a flashover. The outset of these is to simply chamfer the end of the commutator confined. Voltage stress varies exponentially to the inverse of the radius. Chamfering the customary square corner at the end of the commutator to a 1/16" (1.6 mm) radius reduces the voltage stress to approximately 15%, significantly reducing the opportunity for flashover to occur. See Effigy two.

Add together flashover protection
If a client has chronic bug with flashover, have a lesson from the traction motor industry and add flashover protection. Install four equally spaced short lengths of angle iron in line with the terminate of the string ring expanse. The bolted connectedness must be electrically audio and the edge closest to the commutator must be bare metallic (no paint or other coating). The bare metal provides a reliable path to ground, if an arc is to occur, thus minimizing damage to the costly brush boxes and commutator. See Figure 3.

Flashover detection is commercially bachelor and reliable. It has long been known that, at the moment a flashover begins, the field polarity reverses. Automated instrumentation, past monitoring the polarity of the field current, can shut the motor downwards before the fault electric current causes impairment.

If the application is a fan, blower or downhill conveyor, where the motor might start while the load is free-wheeling in opposite, the solution could be a brake – either mechanical or otherwise, interlocked with the drive to release the restriction when the motor starts. One option the terminate user might consider is to use the shunt fields as the dynamic brake. If they practise and so, the field current should not exceed 1/3 of the rated shunt field current. Otherwise, the shunt fields might overheat and neglect prematurely.

The manufacturer has more breadth than we practise as repairers, and then it is common to see larger machines designed with a compensating winding (a.k.a. "pole confront confined"), imbedded in the face of each field pole to finer extend the influence of the interpoles. Those compensating windings, but like interpoles, must exist connected correctly so as to yield the correct interpole force. Misconnected interpoles or compensating windings (i.e., the wrong number of circuits) radically change functioning and are much more than likely to spark and/or flashover.

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How To Repair A Larg Dc Brush Motor,

Source: https://easa.com/resources/resource-library/flashover-causes-and-cures-for-damage-to-brushholders-commutators-1

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