Chuck Yung
EASA Senior Technical Support Specialist
Although the earliest practical DC motor was built by Moritz Jacobi in 1834, it was over the next 40 years that men like Thomas Davenport, Emil Stohrer and George Westinghouse brought DC machines into industrial use.
It’s inspiring to realize that working DC motors have been around for over 160 years. For the past century, DC machines over 30 or 40 kW have been cooled in the same manner – by mounting a squirrel cage blower directly over the commutator.
End users and repairers alike quickly realized that the air that cooled the motor also forced highly conductive carbon dust directly into the very windings that were the lifeblood of the machine. Doing so still seems contrary to the desire to maintain a high insulation resistance-to-ground to preserve or extend winding life rather than end it.
Even so, DC machines remain popular largely because of the ability to control speed and torque. A DC motor can be expected to operate over a large speed range, and a shaft-mounted fan can only provide cooling in direct proportion to the speed at which it is turning; thus you have the the default method of cooling a DC machine with that darned blower. See Figure 1.
Finding best cooling option
Today, we also have inverter-fed AC machines operating at wide speed ranges, so auxiliary cooling has become an after-market consideration. We as repairers must be able to select the best option and correctly size whatever method of cooling seems best suited to a particular application.
The rule of thumb for cooling electrical equipment is 100 CFM (2.8 cubic meter / minute) per kW of losses. If the efficiency of the motor is known, the simplified calculation is:
hp x 0.746 = kW
(1 – efficiency) x kW = losses
Losses x 100 = CFM recommendation
Rounding up as much as 25% is prudent to ensure that there is enough airflow to adequately cool a machine. Recognize, too, that the slower the operating speed, the less airflow available from the factory-integral fan(s), and the higher the winding temperature is likely to be. See Figure 2.
Most popular cooling method
For a DC machine, an auxiliary squirrel cage blower mounted on the commutator end remains the most popular cooling method. Although DC machine efficiency is not part of the typical nameplate, unlike an AC machine, it is very straight-forward to calculate:
The output power is given in hp or kW, recall that (hp x 0.746 = kW)
Watts = volts x amps, so use the field and armature ratings, then add them together to obtain the input power.
For example, a 400 hp DC motor (0.746 x 400 = 300 kW), with a 500V armature circuit (note that the armature circuit includes the armature, interpoles and – if present – series fields) rated at 645 amps, and shunt fields rated for 240V and 3.5 amps:
500V x 645a = 322,500 watts or 322.5 kW
240V x 3.5a = 840 watts or 0.84 kW
The input power = 323.34 kW (322.5 + 0.84 = 323.34)
323.34 kW input – 300 kW output = 23.34 kW losses, for about 93% efficiency.
100 CFM times 23.5 kW losses = 2350, and would require a 2350 CFM blower at 2 or 3 inches (5 to 8 cm) of water column static pressure.
Effect of carbon dust
Still, the drawback remains: The carbon dust, generated as the brushes wear, is blown directly into the very windings we want to preserve. Not too many years ago, one manufacturer addressed this by installing the blower on the drive end (opposite the commutator), but they found that by itself was not enough.
To optimize cooling, they also needed to increase the static pressure inside the motor, while at the same time forcing more cooling air through the armature. This was accomplished by installing a flat baffle on the commutator end (see Figure 3), positioned just above the risers. This increased the static pressure (denser air cools better), forced more air to pass through the axial vent ducts in the armature backiron, and it increased the velocity of the air passing between the baffle and risers. That yielded two additional benefits: First, the faster air did a better job of expelling the carbon dust from the motor. Second, it increased cooling at the risers and the brush holders. Ideally, the brush and commutator temperature should be in the 140°F to 210°F (60° – 100°C) range.
Manufacturers of any equipment keep up with what the competition is doing, especially when it works. So it should not be a surprise that several other manufacturers have (almost) copied the revised cooling scheme. Several manufacturers now install the blower on the drive end. But – as of this writing – none have apparently realized the need for the additional baffle on the commutator end.
Improving motor performance
For repairers, that is good news. As long as we realize that a blower is installed on the drive end, rather than the commutator end, fabricating a baffle for the commutator end is an excellent way for us to improve motor performance for our customers.
For AC motors operating from a variable frequency drive (VFD), some manufacturers offer an already-engineered aftermarket blower. Some are mounted directly to the fan cover with a shaft sized to accept the original external fan. Other kits require removal of the shaft-mounted fan; a squirrel cage blower mounts directly to the fan cover. Either way, a suitable constant volume of air is provided to eliminate the problem of reduced cooling at reduced speed operation.
For those instances where the original equipment manufacturer (OEM) does not offer enhanced cooling options, revert to the 100 CFM per kW of losses guideline. Competent blower manufacturers offer help in sizing blowers for specific applications and most service centers can fabricate robust framework to support and properly position the auxiliary blower. For TEFC (IP54) or WP (IP 23 or 24) enclosures, adding an after-market blower is relatively simple. For ODP enclosures, the same rule for volume and static pressure applies, but the complexity of execution is much greater.
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