Winter here in the Northwoods usually means subzero temperatures, feet of snow, and people walking around in clothing layered so thick they walk like mummies. So far though, this winter has had a couple of really cold snaps, but nothing too bad. They’re not layered in clothing, but the combination of technology and more equipment is resulting in drives continued success in extreme weather.
Eaton’s Matt Callahan, writing at Pumps & Systems explains how modern drives are operating in conditions that would stop their predecessors.
Because of advanced control capability and potential energy savings, the use of adjustable frequency drives (AFDs) for motor control is expanding rapidly throughout many industries and environments. Users expect AFDs (also commonly known as variable frequency drives, or VFDs) to be reliable and perform consistently, even in harsh-temperature environments. High- and low-temperature extremes always provide challenges for electrical equipment, but modern solutions allow AFDs to deliver the same performance that is provided under normal operating conditions.
Before going into too much depth, Callahan describes drives 101:
In general, motors are sized for the maximum load of an application, but this output level is not always required. To achieve energy savings when lower output is needed, AFDs control the frequency of the alternating-current (AC) power being delivered to the motor. The power module of the AFD contains the components that are responsible for delivering this AC power.
The three basic parts of the power module are the converter, direct-current (DC) bus and inverter sections. The incoming AC power passes through diodes, which split the power into the positive and negative components forming DC power.
This DC power is stored on the DC bus in capacitors. The DC power then passes through the inverter section, which uses insulated-gate bipolar transistors (IGBTs) to reconstruct an AC sine wave at the desired frequency that is required to run the motor.
These components and the circuit boards for the control modules can be affected by high or low temperatures, which can lead to drive failure. AFDs are rated for an ambient temperature range designed to protect these components. However, users have options for operating AFDs at or beyond those temperature limits.
Callahan writes that one known factor in electrical failure is heat. The combination of external temperatures and the heat generated from the device can lead to some extreme temperatures. He said that most drives are rated to work in temperatures up to 122 °F (50 °C). However, when a specific application needs to operate such high temperatures, there aren’t many options to choose from. Some drives are liquid-cooled, whereby coolant is pumped through the drive to remove heat. This option has a couple of expensive disadvantages: a higher initial cost and additional maintenance.
Ventilation may be a better option.
Another option for AFDs that are in an enclosure is to add increased ventilation or air conditioning. This solution may be practical for a large control cabinet that has other temperature sensitive components. However, it includes additional cost, increases space requirements and adds another potential point of failure.
Another solution is de-rating the drive for the higher-than-rated ambient temperature. The drive’s internal cooling system includes the heat sink and cooling fan, which are designed to properly cool the drive when operating at its full-load rating.
However, if a drive is not run at its full-load rating, the maximum heat generated does not reach the design limit, and excess cooling capacity exists. This excess cooling capacity can be used to run a drive at higher than normal ambient temperatures and is called de-rating.
On the opposite end of the temperature spectrum are drives operating in extremely cold environments. Callahan writes that many drive applications are designed to operate to temperatures as low as -14 F. Oil and gas pumping applications deal with these chilly temperatures during the winter.
There are some solutions to consider:
The first solution for these applications involves adding a heater to the enclosure. This heater will warm the AFD and other components to maintain an acceptable operating temperature. However, the amount of heat provided depends on the heater size. To provide sufficient heating, a larger heater and enclosure may be needed, which also increases energy costs. In addition, this solution is not feasible if the AFD is not enclosed.
Some AFDs have an alternative mode of operation that allows them to function in temperatures as low as minus 22 °F (minus 30 °C). One program for alternative operation decreases the cold temperature fault from minus 10 C to minus 30 C with an alarm trip point at minus 20 °C.