Current Affairs: Motor Starter Basics - Part 2 (November 1999)
Disconnecting means usually consist of an enclosed safety switch or circuit breaker with an external operating handle. Like motor contactors, switches and circuit breakers used in motor circuits must be capable of making and breaking motor locked-rotor current in addition to continuously carrying full-load current. Consequently, these devices carry horsepower ratings as well as continuous current ratings, and both must equal or exceed the corresponding motor ratings for a safe application.
Motor Disconnecting MeansIn general, each motor requires a disconnecting means that is located within sight of the motor, although an exception is provided if the disconnecting means that is part of a remotely located controller is capable of being locked in the open position. Installing a local disconnect switch at the equipment where it is conveniently accessible to maintenance personnel is the most effective way to meet this requirement. The exception, however, is widely used, and for this reason the disconnects provided with all standard starters and MCCs are designed to accept padlocks.
In many facilities, operation and maintenance procedures are based on the presence of a local disconnect. The client should always be consulted before relying on the NEC exception to delete this feature from your design.
Controller Disconnecting MeansDisconnecting means for the motor controller are also required to be located within sight of the controller, but no exception is provided for lockable devices. It is common for manufacturers to include only contactors and overload protection in starters furnished with packaged equipment, assuming that the electrician will install a disconnect switch in the supply circuit to the equipment. While this is usually indicated on the shop drawings, if it has not been anticipated by the electrical designer, there may be a claim for additional cost by the contractor.
The most effective means of ensuring that proper controller disconnecting means are provided is to specify a combination motor starter, which includes disconnecting means and short circuit protection in addition to the basic components.
Don't forget that variable-frequency-drives (vfd's), dc drives, "soft starters," and other electronic devices that supply power to motors are considered motor controllers under the NEC and must have a disconnect within sight. Modern vfd's are installed close to the motor to avoid over-voltage problems, making it difficult to keep all the vfd's in a mechanical room within sight of a single distribution panel or MCC.
As with starters, to reduce cost and size, most vfd manufacturers do not provide a disconnect switch as part of their standard drive package. If the optional input disconnect is not specified, a separate switch or circuit breaker must be installed.
I have most frequently seen the "within sight" requirement overlooked in retrofit applications, where it has been common practice to install a vfd near the motor, reconnect the motor to the output of the vfd, and reuse the conduit and wire from the existing MCC or starter to supply the vfd. If the starter or MCC is not within sight of the new vfd, a local disconnecting means must be installed to comply with the NEC.
The NEC also requires a controller disconnecting means to interrupt the control power supply to the starter. In most cases, the control power supply is derived from line voltage via a control power transformer located within the controller enclosure, and interrupting the main power supply circuit also disconnects control power. If there is a separate external control power supply circuit, additional contacts must be provided in the disconnect to interrupt it.
Wrapping It UpIf only the motor itself is included in your hvacr equipment package, a local disconnect may not be required. However, if you are specifying a starter, vfd, or other motor control equipment, a disconnect is required within sight of the equipment. Careful coordination between mechanical specifications and the electrical design will ensure that requirements for disconnecting means are met, but not duplicated.
Next month we'll wrap up our discussion of motor starter basics with another critical coordination item: short circuit protection.