A Motor Controller and Disconnect in One?

Nov 1, 2001 12:00 PM, By Bertram R. Newman, Altech Corp.

A recent development in UL standards can save you space, time, and money.

Like most things, manual motor controllers and disconnect switches have their advantages and disadvantages. Unlike disconnect switches, motor controllers make it possible to start and stop a motor without opening both legs of power to the motor. On the other hand, disconnects can be installed anywhere in the circuit—a function motor controllers lack. Not confined to the load side, disconnects can operate on the supply side, making it possible to interrupt power before the motor. What if you could install the controller and the disconnect switch on the same load side of the branch circuit, or combine both functions into one component?

After recently undergoing UL-508 testing, a line of miniature, rail-mountable devices with ratings as high as 60A now qualify as manual motor controllers suitable for use as motor disconnects. The testing, which lasted nearly a year, covered all the tests for temperature, overload, endurance, dielectric voltage withstand, and short circuit response. The components are now panel-ready and they conform to newly modified NEC installation procedures. Available in 1-, 2-, or 3-pole configurations, these miniature controllers perform a single or dual job by interrupting all ungrounded conductors in a single or 3-phase circuit.

While these units perform as combination controller/disconnect devices, they are also suitable as combination controller/overload protection components. Other applications include using the components in AC resistance applications, in AC discharge lamp circuits (ballast) or incandescent circuits (tungsten). In fact, you can use these new 508-listed units to replace a long-standing series of components called “supplementary protectors,” which are only UL recognized, not listed.

Why combine? For one thing, the combination saves the cost of the second component. This cost can become substantial, especially in larger installations. Because one component takes the place of two, you save room in any panel application. You save labor by wiring one, rather than two, components. You do not need to submit the components to UL for investigation as part of a system, so you can use them in field installations directly. This same advantage applies to customer acceptance testing and local inspections. And with easier wiring, maintenance costs are going to be less over the life of the installation.

Fig. 1 (right) shows the elements of a typical motor load circuit. You can use one of the new devices as a:

Standalone controller
Standalone disconnect switch
Combination controller and disconnect switch
Combination controller and overload protection

UL 508 also permits manufacturers to combine three features—the disconnect switch, the motor controller, and the overload protection functions—into the device. While the NEC changes permit the 2-way combinations mentioned above, it is silent on the 3-way approach. You should verify this point with your local NEC inspector.

Listing vs. recognized. What is the difference between UL-listed and UL-recognized? Motor controllers and disconnect switches often look alike and perform similar functions in many common applications, so a difference in listing could create confusion.

A UL-Listed device is UL tested and approved for most common industrial applications. You can field-install a listed component, and it needs no additional UL investigation. A recognized component meets the requirements for a limited specific use. UL doesn’t intend for you to use it for separate installation in the field.

How do you tell one from the other? Look for the mark. The listing mark (the familiar UL in a circle) includes the listing identifier that gives you full information as to what the product class and device characteristics are. The recognized mark is a mirror image reversed UR. The Photo (above) shows examples of both marks—one from a manual motor controller and one from a supplementary protector.

Component selection. How do you pick the device for your application? You must consider these basic questions:

What is the supply voltage?
How many poles must be interrupted?
What is the full load amperage for the circuit?
What is the ambient temperature for the equipment?
Is DIN rail mounting OK?

If you are going to use the device as a dual component—operating as a motor controller and a disconnect switch—remember it must interrupt all ungrounded legs of conductors to the motor. Figure this in selecting the number of poles for your device.

You must size the switch/controller so it is no lower than the horsepower rating for the motor in the circuit. This is typically the same as the motor nameplate horsepower. You can usually determine the size by using the motor nameplate rated-load current and locked-rotor current of the motor, and reading the horsepower from one of the tables in Art. 430, Part N—for most industrial applications that will be Table 430.151(B). If you don’t find the exact value of the current you have, use the next higher horsepower value in the table.

When a device operates as a controller, overload protector, or disconnect switch (singly or in combination) and provides additional short circuit protection, you can select trip characteristics. For most motor branch circuits, you determine short circuit/inrush current and then select an appropriate trip characteristic. This characteristic essentially ensures tripping of the component in less than one hour at 145% of rated current, and no tripping in more than one hour at 113% of rated current. Fig. 2 (right) shows a typical trip characteristic curve for one of the miniature motor controllers in this category.

Now you can safely and legally use a dual-, or even triple-function component for motor installations. Applied properly, such components will save you space, time, and money.

Newman is a technical consultant with Altech Corp., Flemington, N.J.