It’s inspiring to realize that working DC motors have been around for over 160 years and, for the past century, DC machines over 30 kW 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, the default method of cooling a DC machine is with that darned blower (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 aftermarket consideration. Service centres often can help end users select the best cooling method and correctly size it for a particular application.
The rule of thumb for cooling electrical equipment is 100 cfm (2.8 m3/min) per kW of losses. When 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 by as much as 25% is prudent to ensure there is adequate air flow to cool a machine. Recognize, too, that at slower operating speeds, less air flow is available from the factory-integral fan(s), and the winding temperature is likely to be higher (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, and the recommended air flow can be determined in the same way as for other electrical equipment. Although DC machine efficiency is not part of the typical nameplate, unlike an AC machine, it can be determined by calculating the losses as a percent of input power.
To obtain the input power, convert the field and armature ratings to kW (watts = volts x amps) and add them together. If the output power is given in hp, convert it to kW (recall that hp x 0.746 = kW).
For example, consider a 400-hp DC motor (0.746 x 400 = 300 kW), with a 500V armature circuit rated at 645A (500V x 645A = 322.5 kW), and shunt fields rated for 240V and 3.5A (240V x 3.5A = 0.84 kW). (Note that the armature circuit includes the armature, interpoles and—when present—series fields.)
Input power = 322.5 kW + 0.84 kW = 323.34 kW
Out power = 300 kW
323.34 kW input - 300 kW output = 23.34 kW losses, for about 93% efficiency
100 cfm x 23.5 kW losses = 2350, and would require a 2350 cfm blower at 2 in. or 3 in. (5 cm to 8 cm) of water column static pressure.
Effect of carbon dust
Still, the drawback remains: the carbon dust generated by brush 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 found that this by itself was not enough.
To optimize cooling, they also needed to increase the static pressure inside the motor while simultaneously forcing more cooling air through the armature. This was accomplished by installing a flat baffle on the commutator end (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 increased the velocity of the air passing between the baffle and risers.
This yielded two additional benefits: first, the faster air flow 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°C to 100°C] range.)
Manufacturers of any equipment keep up with what the competition is doing, especially when it works. Not surprisingly, several other manufacturers have (almost) copied the revised cooling scheme, and now install the blower on the drive end. As of this writing, though, none have apparently realized the need for the additional baffle on the commutator end.
Improving motor performance
The good news is that if there is a blower installed on the drive end rather than the commutator end, a service centre can improve motor performance by fabricating a baffle for the commutator end.
For AC motors operating from a variable frequency drive (VFD), some manufacturers offer an already-engineered aftermarket blower. Some mount 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 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 centres can fabricate robust framework to support and properly position the auxiliary blower. For TEFC (IP54) or WP (IP 23 or 24) enclosures, adding an aftermarket 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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Chuck Yung is a senior technical support specialist with EASA, the Electrical Apparatus Service Association (easa.com).
This article originally appeared in the March 2018 issue of Electrical Business Magazine.