Chuck Yung
EASA Senior Technical Support Specialist

Once upon a time, DC motors oper­ated from batteries or motor-generator sets. For the past 50+ years, most DC motors have operated from solid state power supplies – rectifying AC power to DC power. When motors began operating from rectified power, one of the problems experienced was the presence of AC “ripple” in the leads that were supposed to deliver DC power to the machine. Absent a spe­cific standard, a recurring question is: “How much is too much?” Before I try to suggest an answer to that question, let’s talk about what AC ripple is and explain why it is not desirable.

One source describes AC ripple as “the small unwanted residual periodic variation of the direct current (DC) output of a power supply … from an alternating current (AC) source.” We express ripple factor as the ratio of the root mean square (rms) AC voltage over the DC voltage. That is usually expressed as a percentage. Since it is easier to measure the peak-peak volt­age when using an oscilloscope, the ripple is often reported as the peak-peak AC to DC voltage ratio.

Full-wave rectification
Those of you with experience using (or troubleshooting) rectified power supplies, such as the typical service center test panel, know that full-wave rectification is “better” than the output from a half-wave rectifier. The formu­las below illustrate this:
Full wave:  Vpp = I/2fC 

Half wave:  Vpp = I/fC 

where:
Vpp is peak-peak voltage ripple
I is current
f is frequency of the AC power
C is capacitance.

The full-wave rectifier can produce 130% of the input AC voltage, whereas the half-wave rectified supply only produces about 90% of the AC input voltage. 

Aside from all the issues that AC ripple causes for radios and TVs, there is a more immediate concern for those of us involved with electric motors. First, both the AC and the DC current contribute to heating of the windings. While DC current is carried uniformly across the conductor, AC current is carried on the surface of the conductor. The higher the frequency of the AC current, the closer to the surface. So the heat generated by the additional AC current should be cause for concern.

Evaluating the rms current, we would use:

Second, the greater the AC ripple, the more commutation is affected. One symptom of excessive AC ripple is a bluish coloration to the commutator instead of the normal chocolate patina.

Another is chronic sparking, despite our best efforts to adjust brush neutral. When brush neutral, spring pressure, brush spacing and brush grade all check out as correct, sparking at the brush-to-commutator interface could be a symptom of voltage ripple.

Bigger not always better
Modern drives use a capacitor to smooth out the AC ripple. In a well-designed drive, the capacitor is sized for normal load conditions. As the load decreases, the voltage ripple increases. An overly robust power supply could have more voltage ripple than a drive that is operating near capacity. With that in mind, here are some classic indications of an AC ripple problem:
• A field supply that seems to produce sparking under field-weakening but not at base speed.
• A motor operating from a grossly oversized drive. (“Gee, we only need a 150 hp motor, but let’s use this 400 hp drive we have in stock.”)
• A DC motor that sparks when running light product, more than when running with a heavier load. An example might be an extruder or mixer that is used for different consistencies of product.

Ideally, a technician can use an oscilloscope to evaluate the power supply. In the real world, that is not al­ways possible. A low-tech preliminary check is to use an AC ammeter and a DC ammeter to measure the current in the field leads and then in the armature leads. If there is AC current observed, document how much, at what volt­age, and under what load conditions. If field weakening results in a greater ratio of AC current to DC current, that suggests that the output capacitor size could be changed to better reflect the load. The same comparison should be made for the armature circuit – over the normal load range encountered.

Using true-rms ammeter
Document the AC and DC volt­ages, as well as currents, for the field supply as well as the armature sup­ply. Using a true-rms ammeter and a conventional peak-reading meter is common practice when checking for harmonics on an AC power source. If the true-rms meter yields a higher reading, that is a strong indication that there are harmonics (i.e., fre­quencies besides the line frequency) present. The literature suggests that anything over 5% harmonic content should be dealt with. IEEE 519 sets a limit of 5% for harmonic distortion of voltage and a limit of 3% for a single harmonic. The impedance of the sys­tem makes such a finite number for current harmonics difficult to assign. (See tables 10.2 and 10.3 of IEEE 519 for THD limits for voltage and current, respectively.) IEEE 113: Test Procedures for Direct-Current Machines, which was allowed to lapse, suggested a limit of 2% for voltage ripple in DC voltage supplies. It further stated that, “If a capacitor is used to block the DC component of voltage, it should be of sufficient size so that the AC voltage drop across the capacitor is less than 2% of the AC component of the volt­age measured.”

With adjustable speed drives (ASD), which most of us call VFDs (variable frequency drives), filters or line reactors can be used to improve the waveform.

Limit of 5% to 10%
Asfor how much AC ripple is too much in a DC supply, a limit of 5% to 10% at full load has been suggested by various engineers. The 5% limit is based on engineering texts and con­sistent with harmonics standards (see “Coil Pitch and Search for the Perfect Sine Wave,” Currents, January 2004). 

Should you find a larger AC ripple, the drive manufacturer should be con­sulted. Knowing the actual operating conditions, a drive engineer should be able to improve the capacitor size to smooth out the ripple. 

A low-tech method to reduce the AC ripple is to place a full-wave bridge rec­tifier between the power supply and the motor to further rectify the DC power. Be careful, though, when considering this method. The rectifier will “fix” the output polarity. In other words, regard­less of the polarity of the DC into the rectifier, the polarity of the output will remain constant. There are other ap­plications where that knowledge can solve other problems, but make sure your customer is not switching the field polarity to reverse a small motor. 

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Categories: DC motors, Design/theory/application

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