During the design process of mating a motor to a machine, is it better to concoct a costly machine part or order a motor with one or more modifications? For many applications, it is more economical to select a modified motor. Not only can modifications increase the dependability of motors in certain applications, they can also lengthen motor life in harsh environments and reduce total machine cost.
Motor manufacturers typically offer — and list in their catalogs — predesigned modifications that are available in a few days or a couple of weeks. For example, if you need a model number motor with an oversized conduit box, most manufacturers can supply it in a few weeks.
The basic motor is already designed; the oversized conduit box is probably in stock. The motor engineers just combine the two drawings and produce the motor. Other motor requirements dictate designing a motor with the features never ordered before, such as a 5-hp, 1,750-rpm motor with neon pink paint, a special shaft configuration, 16 thermistors located at specific points in the windings and a few more seemingly weird features. One such unit is a special, or custom-designed, motor. However, if you offer a firm purchase order for 100 per month for 2 years and you are willing to pay in advance, such a special order becomes a customer model or specification number motor. If this design catches on, it will be a model number unit. Until that time, it may be best to stick to the common motor modifications. By knowing these, you may be able to reduce the total machine costs and still obtain short motor delivery cycles.
One important point: Many motor manufacturers have more standard modifications than are listed in their catalogs, as the list is constantly expanding. So don’t hesitate to ask if the needed modification is a standard offering, or if the manufacturer has a modification close to what you need. If you can use one of the standard offerings, you may save money and shorten delivery time over that required for a special modification.
One of the most common changes is modifying the motor shaft, Figure 1. A little longer or shorter, or a shaft diameter change might be just what’s needed. Special steps on the shaft or different key ways may also meet your requirements.
An example is a special pump that mounts above plating tanks. It circulates the solution in the tank and directs the flow to a specific location. For years, engineers added a shaft extension of stainless steel. It slipped over the shaft of a standard motor and locked in place with set screws in the motor keyway. However, the stresses on this coupling along with slight misalignment and runout occurrences led to motor shaft breakage and out of balance problems.
The solution was a custom motor with a correct length shaft made of stainless steel.
Other common modifications to a shaft include an extension out the opposite end from the drive end of the motor, creating a double shaft motor, and reducing the motor shaft length to give greater clearance in the belt cover.
Faces and flanges
Adding a C-face and eliminating the mounting feet are common changes. When you combine a second C-face along with the double shaft, you create a motor that is ready for a brake. In some automotive applications, engineers specify one type of brake for all their brake motor applications in a plant. Thus, these motors must also come with brake mounting provisions. The brakes are added during motor installation.
You can also request special flange endplates with gussets for stiffness, Figure 2.
Another group of popular changes involves protecting motors from extreme environments. It is easy to add shaft seals, weather proof coatings and even stainless steel shafts. Complete stainless motor housings are also available, up to about 10 hp.
Chemical and water vapors shorten motor housing life and cause premature bearing failure. In one application, the motors were protected from this environment in several ways. An epoxy coating, Figure 3, protected the housing. An air purge fitting provided continuous air pressurization of 1 to 3 psi from the plant’s air system. These modifications keep moisture and chemical vapor from entering the motor and bearings. The result is a pump motor that will last many times longer in such environments than standard motors.
Motors used in high and low temperature applications require non-standard lubricants. For some high-temperature applications, you can also request higher temperature windings and insulation as motor modifications.
In very cold applications where motors are subject to freezing, space heaters can be installed into the motor. These heaters, connected to a separate power circuit, activate when the motor is turned off to keep condensation from forming inside the motor. You can also install condensation drains to let moisture drain harmlessly.
Motor internals, as well as externals, can be coated with a special varnish to inhibit mold.
Reversing the air flow through a motor is used in some pump or blower applications. Pump seals, for example, can have leakage spraying into the air as water vapor spins off the pump shaft. This airborne water can be pulled into the motor by its cooling fan. Typically open dripproof motors are used in these applications. These motors generally have air flowing from the shaft end to the opposite end of the motor. With reverse air flow, dry air is drawn in the opposite end from the shaft and blown out the shaft end, preventing moisture from getting into the motor.
For another application, Figure 4 shows a totally enclosed motor with the fan on the shaft end, opposite from conventional designs.
Another option is to design motors without cooling fans (TENV). This usually requires going up one motor frame size.
Conduit boxes and capacitors
Often motor manufacturers will relocate the conduit box, and capacitors on single-phase motors, to another location on the motor, Figure 5. Changing the conduit hole location or size in the conduit box is also popular and can make an otherwise costly motor installation easy. Some applications require motors without a conduit box.
Switches and wires
You can request On/Off switches on the motor, or a cord or cord and plug, Figure 5. Many saw applications and hydraulic pump applications must be ready to plug in to power, right out of the equipment box. For a customer, the cost of taking an off-the-shelf stock motor, installing the On/Off switch and plug plus six or eight feet of power cord into the conduit box of the motor takes much time. This method usually costs more than the additional cost charged by the motor manufacturer.
Most motor manufacturers can easily meet your custom paint requirements to match a machine or company logo color, eliminating costly masking and repainting by the customer, Figure 1, Figure 2, Figure 3, Figure 4, and Figure 5.
Hydraulic pump mounting
Hydraulic pumps usually require a separate bracket mounted on the surface of the hydraulic reservoir, Figure 5. The motor must be carefully aligned with the pump. Special hydraulic pump endplates are available for most commonly used hydraulic pumps. These endplates can save alignment and mounting time for the pump assembler.
Thermal protection is available in several ways. Manual as well as automatic reset overloads in blowers and fans are common methods to protect the motor from overheating. Other available thermal protection includes pilot circuit devices installed in series with the motor starter holding coil and thermostats placed on the windings to monitor winding temperature. They will shut the motor off at the control if one or more phases of the motor overheat.
Typically a normally closed thermostat protects the motor. In manually loaded equipment, like saws, rather than turn the motor off, a normally open thermostat can turn on a red warning light. An operator can then let the motor cool without shutting it off during a cut.
You can choose from a number of temperature sensing devices that monitor winding and bearing temperatures. Their leads are brought into the motor conduit box or into a separate conduit box, ready to connect to an external temperature monitor.
The mechanical changes available are countless. You can select from precision balancing and higher grades of bearings to oversized bearings and larger than standard shaft diameters. Custom nameplates using your company’s logo and identification are also available for the motor.
In some applications, developing a special endplate casting, typically out of cast iron or aluminum, can solve an unusual need for a different, rather than standard, mounting. Many unusual mounting requirements are some type of face mount. Motor manufacturers can even provide special lifting devices to set the motor in place.
Other mechanical modifications include special running or starting needs requiring high torque or low torque, Figure 2. Just about any voltage available is also available in a custom motor design. You can even get custom motors that handle frequent starts and stops.
Either a motor manufacturer or a service shop can make these modifications. The service shop offers the advantage of obtaining a motor with special features quickly, for example when you must replace a failed motor. The local service shop can also obtain a standard motor and modify it. Motor manufacturers will often give you the name of shops capable of handling your needs.
If time is not critical, it is generally more economical to have the motor manufacturer produce the desired unit.
Ted Atkins is vice president, Baldor Electric Co., Fort Smith, Ark.