Cyndi Nyberg
Former EASA Technical Support Specialist
There are a number of different types of DC generators: shunt, series and compound, each of which can be separately or self-excited. A DC generator is built and designed exactly the same as a DC motor, and can be run as such. Regardless of the type, there are a number of reasons why a generator won’t produce the correct voltage, or any voltage at all.
Let’s start with the basics of how a DC generator works. When the armature is rotated, the magnetism from the fields produces a voltage in the armature. If the generator is self-excited, then the small voltage produced in the armature in turn is supplied back to the fields, which induces current in the fields.
As the current in the fields increases, the strength of the field increases, which, in turn, increases the voltage produced in the armature. This process continues until the fields reach full strength and the armature produces its rated voltage. If it is a separately excited generator, when the fields are supplied with the rated strength before the prime mover begins to rotate the armature.
If the generator requires a small amount of magnetism in the fields to start to produce a voltage in the armature, then how is that possible in a self-excited generator that has no external source of power to the fields? When the fields are deenergized, the pole pieces retain a small amount of residual magnetism. The residual output voltage can be measured with a voltmeter, although it may be very low – in some cases just a fraction of a volt. It only takes a very small amount of residual magnetism to produce a small amount of voltage in the armature, which then supplies the fields.
Effects of incorrect data
Now what happens if you have repaired a generator, and it won’t produce any voltage when it is being driven? The most common explanation is that the machine was idle for a long enough period that the residual magnetism faded away.
If the fields or armature have been rewound, it is possible that the winding data or connections are incorrect. If the data is incorrect, there will likely be an output voltage, but it will not be the correct voltage.
If there is a short or open connection in either the fields or the armature then there may be reduced (if shorted) or no output (if open) voltage when the generator is driven. If it is shunt wound, and the field or armature leads are reversed, then the field polarity will oppose the residual magnetism of the field poles, and the output voltage will not build.
If it is compound wound, a similar situation can occur. If the series field is reversed, but the field and armature polarities are correct, the correct voltage will be produced at no load, but the performance of the generator will be affected when it is loaded. Figure 1 shows the standard connections for a shunt and compound wound generator. These connections are for counterclockwise rotation facing the end opposite the drive. For clockwise rotation, interchange A1 and A2.
When the generator is self-excited, connections should be made as shown by the dashed lines. When the shunt field is separately excited, it is usually isolated from the other windings of the machine, but the polarity or the voltage applied to the shunt field should be as shown for the particular rotation and armature polarity.
DC generator design
An advantageous thing about a DC generator is that since it is designed the same as a DC motor, it is often possible to run it as a motor. If you can run it in the service center, many potential issues can be assessed before it is sent out into service. If the generator can be test run as a motor, any problems with the field or armature windings, or the internal connections, can be addressed.
The armature can be tested for shorts and opens with a growler, barto-bar or surge test. The fields can be AC voltage drop tested, and (using DC power) checked for correct polarity on each field. If it is test run, with rated current on the fields and rated voltage on the armature, the speed should be approximately 10% above the rated speed.
If not, then there may be a problem with the windings or internal connections.
If the windings check to be correct, have not been rewound and are not shorted or open, what happens if the generator doesn’t produce any output voltage?
Generator produces no output voltage
If the generator is separately excited, check the field excitation current supply. If there is no field voltage, the armature won’t produce any voltage. If there is a small amount of residual magnetism on the fields, the armature may produce a very small voltage.
If the incorrect polarity voltage is supplied to the fields, then the generator will not produce the correct output.
If the correct excitation voltage is delivered to the generator, check the fields for open circuits.
Next check the lead markings and make sure the connections are correct. If the shunt fields or armature leads are reversed, then the output voltage
cannot build.
Keep in mind that the armature leads markings may be correct, but the brush holder leads may have been swapped. This reverses the armature polarity relative to the inter-poles.
Flashing the fields
In a self-excited generator, it may be necessary to restore residual magnetism to the fields. The residual may be lost due to a number of factors, including being idle for an extended period of time. Without residual magnetism in the shunt fields, the armature will not produce voltage when driven at the rated speed. You can measure the residual by disconnecting the shunt field leads.
Measure the voltage at the field leads it may be a very low voltage, but that may be all that is necessary. If no voltage is present, you will need to flash the fields. Use jumper leads to connect a battery to the field leads and operate the unit. There should now be output voltage at the armature leads. If not, there is an open or misconnection in the armature or field circuit. See Figure 2.
Additionally, if you are measuring negative voltage on your voltmeter, but the F1 is on the positive and the F2 lead on the negative of the voltmeter, then that will tell you that even though there is residual on the fields, it is the wrong polarity. If it is a shunt-wound machine, simply switch the numbers on the shunt field leads.
Use fuse for safety
Another way to restore the residual is to use a piece of lead wire or welding cable to directly short the armature leads together. You should use a fuse for safety (a 30A fuse will be sufficient), and wear the appropriate personal protective equipment (PPE), since there will be an arc when you remove the cable. Short the armature leads for just a fraction of a second and then open the shorted connection. This provides a stronger source of excitation than a 12v battery would.
Use the DC inductive kick method to verify the brush neutral position. (The AC method, which requires that AC voltage be applied to the shunt field, will destroy the residual magnetism required for self-exciting generators.) If the generator produces voltage, let it run for a few minutes to restore residual magnetism to the field poles. Reconnect the fields for self-excitation, and run the unit. Voltage should build normally, until the rated voltage is produced.
Note: If the fields were recently rewound and the generator requires frequent field flashing, the field poles might not have been installed in the correct positions. Residual magnetism should be of alternate polarity, just like field excitation. If the field poles were randomly reinstalled, or the shunt field leads reversed, the building self-excitation voltage may cancel out the residual magnetism.
As the generator output voltage starts to build, it cancels the residual magnetism, and the output voltage suddenly drops to zero. In that case, it may even be necessary to use a compass to determine the residual of each pole, and move any that are out of sequence (i.e. alternating polarity) to the correct position.
Brushholder leads
Most of the time, when a DC motor or generator is disassembled, the brush holder leads must be disconnected. If those leads are inadvertently swapped when it is reassembled, the polarity of the armature (relative to the interpoles) will be incorrect. When the armature is driven in the correct direction, the output voltage is now opposing the field, so the voltage will start to build, then immediately collapse to zero. This happens because the reversed polarity cancels out the residual. Once the net field flux equals zero, there will be no armature output.
Generator doesn’t produce correct voltage
If the generator produces voltage, but it is too low or too high, there are a number of things to check. Check that all lead markings and connections are correct. If the output voltage is half or twice rated voltage, verify the field connections. It is likely that the shunt fields are connected for either double, or half, the rated voltage. Many generator applications include a rheostat for regulation of the generator output voltage. Check the condition of the rheostat for shorts or opens.
A simple procedure for doing so is to connect an ohmmeter to the rheostat “start” and “finish” terminals and then observe the resistance while adjusting the rheostat through its full range. The resistance should change smoothly, with no sudden change in resistance. If the rheostat is open or shorted, it must be replaced. Be sure the replacement rheostat has the same resistance range and is rated for the field current.
If the generator is compound wound, determine whether it should be connected cumulative or differential. Certain applications, such as the swing generator of a dragline, must be connected differentially. If a generator application calls for cumulative connection, a differential connection will cause a dangerous loss of output voltage should the generator suddenly cease to produce voltage when the load is critical. Imagine the result if a crane motor suddenly lost power.
If the generator is compound wound, determine whether it should be connected cumulative or differential. Certain applications, such as the swing generator of a dragline, must be connected differentially. If a generator application calls for cumulative connection, a differential connection will cause a dangerous loss of output voltage should the generator suddenly cease to produce voltage when the load is critical. Imagine the result if a crane motor suddenly lost power.
If the armature was rewound, the connection may be incorrect. For example, an armature that was connected duplex but should be simplex would produce only half rated voltage.
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