In the 2005 National Electrical Code, Article 430 was rearranged to make requirements on adjustable-speed motor drives more detailed and easier to locate in the new Part X. These requirements were previously scattered throughout Article 430. The most common method of varying the speed of motors involves varying the input frequency to the motor. Varying the voltage to the motor is possible, but it is more difficult because too much or too little voltage can cause destructive heating to the motor insulation. For this reason, variable-frequency motors—not variable-voltage units—are the usual choice of most electrical designers and contractors.
An adjustable-speed drive is defined in Article 100 as power-conversion equipment that provides a means of adjusting the speed of an electric motor. The informational note that accompanies the definition states, “a variable frequency drive is one type of electronic adjustable speed drive that controls the rotational speed of an AC electric motor by controlling the frequency and voltage of the electrical power supplied to the motor.”
Varying frequency not voltage
Most of the variable speed is done by varying the frequency, not the voltage. Section 430.120 in Part X of Article 430 states that the provisions in parts I through IX are applicable to motors unless modified or supplemented by Part X.
Since most motors used in the United States are 60 Hertz, the motor may heat up with higher or lower frequencies. There is a very lengthy informational note in 430.126 covering motor overtemperature protection that is worth reading, since overheating certainly can occur with variable-frequency driven motors.
It states: “The relationship between motor current and motor temperature changes when the motor is operated by an adjustable-speed drive. In certain applications, overheating of motors can occur when operated at reduced speed, even at current levels less than a motor’s rated full-load current. The overheating can be the result of reduced motor cooling when its shaft-mounted fan is operating (at) less than rated nameplate RPM (Revolutions Per Minute). As part of the analysis to determine whether overheating will occur, it is necessary to consider the continuous torque capability curves for the motor given the application requirements. This will assist in determining whether the motor overload protection will be able, on its own, to provide protection against overheating. For motors that utilize external forced air or liquid cooling systems, overtemperature can occur if the cooling system is not operating. Although this issue is not unique to adjustable speed applications, externally cooled motors are most often encountered with such applications. In these instances, overtemperature protection using direct temperature sensing is recommended [i.e., 430.126(A)(1), (A)(3), or (A)(4)], or additional means should be provided to ensure that the cooling system is operating.”
Motors with built-in internal overload protective devices can be a problem with variable-speed drive (VFD) units, since there may not be a way to adjust the devices for the motor’s added internal heat. Many VFD systems have load- and speed-sensitive overload protection and thermal memory retention built into the system in case of shutdown or power loss.
Since these units are solid state, the overload sensors embedded in the motors may be designed to communicate with the variable-frequency controller, thus providing appropriate overload protection. Many VFD units are designed with bypass circuits that permit the VFD to be bypassed to allow motor operation at rated full load speed. Where a bypass is employed, 430.124(B) requires that additional motor overload protection be provided in the bypass circuit since the built-in overload protection in the VFD may also be bypassed.
The requirements for the minimum size and ampacity for branch circuit or feeder circuit conductors are located in 430.122(A) through (D). Circuit conductors supplying power conversion equipment, included as part of variable-frequency drive units, must have an ampacity of not less than 125% of the rated current to the power conversion equipment, usually the nameplate ampacity on the conversion unit.
A new informational note in the 2020 NEC states that circuit conductors on the output of the VFD are susceptible to breakdown under certain conditions due to the output wave form of the drive. Factors could be due to the output voltage, frequency and current, as well as the length, spacing between and dielectric strength of the conductors.
Insulation degradation can occur due to these conditions. Harmonics in the electrical system may also be a concern and should be isolated if possible. Care must be taken with these systems to ensure these VFDs are sized properly, the conductor insulation issues are considered and proper overload protection of the motors is provided.