Chuck Yung
EASA Technical Support Specialist
If you have ever tried to figure out the field resistance from the information on a DC motor nameplate, you probably wonder what in the heck the manufacturer was thinking! You know Ohms Law, but the nameplate information just doesn’t seem to follow it.
Ohm’s Law: R = E/I
I = E/R
RI = E
Where R = resistance E= voltage I= current
For those accustomed to the format of the AC motor nameplate, DC nameplate information can seem confusing – even misleading. Misinterpret the field voltage / field current / rpm information, and a technician is liable to conclude that something is wrong with the motor. A customer may even question whether a repair was done correctly. The most confusing case is the DC motor with dual voltage fields and field-weakening capabilities. An example will help to illustrate the problem:
Field voltage: 150 / 300
Field amps: 3.2 / 1.2
RPM: 1750 / 2500
DC nameplate
With an AC nameplate, it would be natural to assume that the 150 volt, 3.2 amp and 1750 RPM ratings go together, with the 300 volts, 1.2 amp and 2500 rpm ratings also related. That assumption is wrong, unfortunately. On the DC nameplate, the first field current rating and the first rpm correspond to the high field voltage rating.
What’s the logic?
DC coil strength is reported in ampere-turns: Field current x turns per coil. Field weakening, by reducing the voltage applied to the fields, also reduces the current carried by the field conductors (I = E/R). Since the turns remain constant, a change in field current causes a corresponding change in field strength. As the shunt fields are weakened, the motor speed increases.
The first rpm listed on the nameplate is the base speed at full load. The second number, if there are 2 speeds given, is the field-weakened speed or the maximum safe speed. This may be limited by the construction of the armature — or it could represent the limit for stable speed control. It provides a warning to the end user to limit the speed.
Attempting to run a DC motor beyond that speed limit could result in the rapid acceleration associated with field loss; the motor “runs away” or accelerates so quickly that catastrophic failure is almost certain. Those who have tried to dodge flying commutator bars know how important it is not to exceed the maximum safe speed.
In the example above, 3.2 field amps should produce the rated base speed (1750 rpm) at full load. The second field current rating (1.2 amps) is the lowest safe current when field-weakening is used, and should result in a speed increase to 2500 rpm. That 2500 rpm (in this example) is the maximum safe speed – the highest rpm at which the motor should be run.
Field current (amps)
The field current on the nameplate is based on the field being at full operating temperature, typically under full load conditions. In other words the fields are “hot.” Conversely, the nameplate field ohms are typically given for ambient conditions with the machine not yet operating, i.e., “cold.” The temperature used is typically 25 degrees C (68F).
Since the nameplate field current refers to “hot amps,” and the resistance we are after is “cold” resistance, it’s necessary to adjust the resistance in order to apply Ohm’s Law. A “rule of thumb” multiplier of roughly 1.2 to 1.3 permits a quick estimate; the table below gives accurate multipliers for each insulation class if the fields operate at the listed temperature. Multiply the nameplate current times 1.2 to get a ballpark value for cold amps, then use that value to estimate field resistance.
Use these multipliers to correct field resistance to ambient.
Note: It is important to know the operating temperature of the shunt fields, so as to select the correct multiplier. Assuming a 1.53 multiplier, our same example would indicate a field resistance of 68 Ohms. If the nameplate also lists the “hot amps” we can work backwards to determine the approximate field temperature expected under normal operating conditions.
Current for low field voltage
What about the field current for the low voltage rating? Treat the field coils as basic circuits. The dual voltage field circuit is comprised of 2 circuits that can be connected in series or parallel.
Using Ohm’s Law: 300 volts / 3.2 amps = 93.75 ohms. Since the high voltage connection is series, 93.75 ohms divided by 2 = 46.88 ohms per circuit. Connecting those two resistors of 46.88 ohms in parallel, the resulting resistance = 23.44 ohms. If 150 volts (the low voltage rating) is applied, the expected current is 6.4 amps.
This information is also helpful when troubleshooting dual-voltage field connections: Not everyone marks their 4-lead shunt fields in the same way. A DC motor accidentally connected so that only half the fields are energized will operate, but half the fields will have a short thermal life. The clue, of course, is that 2 fields look brand new while the other 2 fields exhibit severe thermal damage. Confirming the customers drive parameters, especially the current, may prove the misconnection. If the electrician compares field current to the expected value, this error can be avoided.
Summing up
For DC machines, the nameplate format actually is very logical when we know what we are looking at. Understanding the reason behind the labeling will help enable the repairer to avoid costly mistakes, and provide an additional level of quality control.
Caution: With modern DC drive parameters set by field current, the manufacturers recognize the need to report hot amps so that installers don’t make a similar mistake in applying Ohm’s Law.
When a nameplate is missing or defaced, the installer may calculate the field current from what they know: the applied voltage (of the drive) and the resistance (which can be measured). Trouble is, the controller will hold the field current constant, and that may saturate the fields. So the table of multipliers for various temperature classes is critically important. Share that with your customers to avoid perceived problems with DC field current.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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