Avoid mistakes with selection/maintenance by learning the language of motors.
The Line 6 drive motor failed again! You tell your supplier you're not sure if you need a Design B temperature-rated or Design D T-frame. After a pause, the supplier says, "We'll come out to see what you're doing with the motor." This is embarrassing for you, and costly for your company. But, knowing the following motor terms can prepare you for the next time.
Amperage. You need to understand amperage in three different contexts: full load, locked rotor, and service factor.
• Full load amps. Also known as "nameplate amps," it's the current you can expect under full load (torque).
• Locked rotor amps. Also known as starting inrush current, it's the current you can expect under starting conditions when you apply full voltage.
• Service factor amps. It's the current to expect when you subject the motor to a percentage of overload equal to the nameplate service factor. Many motors have a service factor of 1.15, meaning the motor can handle a 15% overload.
The Code letter indicates the inrush current or locked rotor current a motor requires when you start it.
The Design letter indicates the shape of the torque speed curve. The most common design letters are A, B, C, D, and E.
• Design A motors are rare, specialized motors for injection molding applications. The most important characteristic of Design A is the high pull out torque.
• Design B is the standard industrial duty motor. It has reasonable starting torque with moderate starting current and good overall performance for most industrial applications.
• Design C is for hard to start loads, due to its high starting torque.
• Design D, the "high slip" motor, has high starting torque. Its high slip rpm at full load torque means poor full-load efficiency. Design Ds are good for low-speed elevators, hoists, and punch presses.
• Design E arrived with great fanfare, because of superior efficiency. However, improvements in Design B motors have made the Design E a much less attractive choice than it was originally. Design B efficiencies often match those of Design E, while Design E has lower locked rotor torque.
Efficiency is the percentage of the input power the motor converts to work output. Most domestic motors carry an efficiency number on their nameplates.
Frame size. Motors come in various sizes to match the requirements of the application. Generally, the frame size gets larger with increasing horsepowers or decreasing speeds. To promote standardization in the motor industry, the National Electrical Manufacturers Association (NEMA) prescribes standard frame sizes for certain horsepower, speed, and enclosure combinations.
Full load indicates the speed the motor will run when putting out full-rated output torque or horsepower.
High inertia loads are those with a high flywheel effect. Examples include large centrifuges, punch presses, blowers, fans, and industrial washers.
Insulation class denotes the resistance of the insulating components to degradation from heat. The four major classifications, in order of increasing thermal capability are A, B, F, and H.
Load types. One of the most critical factors in motor longevity is matching motor to load type.
• Constant horsepower. This applies to certain types of loads where the torque requirement is less as the speed increases and vice versa.
• Constant torque defines a load characteristic where the torque required to drive the machine is constant, regardless of motor speed.
• Variable torque loads have characteristics requiring low torque at low speeds and increasing values of torque required as the speed increases.
Poles. This refers to magnetic poles in an energized motor. Poles come in sets of two (a north and south). In an AC motor, the pole quantity works in conjunction with the frequency to determine the synchronous speed of the motor.
Power factor (in percent) is a measure of a particular motor's requirements for magnetizing amperage.
Service factor. This multiplier indicates the amount of overload you can expect a motor to handle. For example, you can't expect a motor with a 1.0 service factor to handle more than its nameplate horsepower on a continuous basis. But, you can expect a motor with a 1.15 service factor to handle intermittent loads up to 15% beyond its nameplate horsepower.
Slip rpm is the difference between the motor's synchronous speed and the full load speed. When expressing this slip rpm as a percentage of the synchronous speed, it's "percent slip" or just "slip." Most standard motors run with a full load slip of 2% to 5%.
Synchronous speed is the speed at which the magnetic field within the motor is rotating. It's also approximately the speed the motor will run under a no load condition.
For example, a 4-pole motor running at 60 cycles would have a magnetic field speed of 1800 rpm. The no load speed of that motor shaft would be nearly 1800 rpm (probably 1798 rpm or 1799 rpm). The full load speed of the same motor might be 1745 rpm. The difference between the synchronous speed and the full load speed is the slip rpm.
Temperature. We all know heat is the number one cause of motor failure. In fact, 70% of motor failures are from overheating. When thinking of motor temperature, consider these two factors.
• Ambient temperature is the maximum safe room temperature if you are going to operate continuously at full load. In most cases, the ambient temperature rating is 40DegrC (104DegrF).
• Temperature rise is the temperature change you can expect, within the winding , from non-operating (cool condition) to full load continuous operating condition.
Time rating. The rating you find on most motors is for continuous duty, which means they can operate at full load torque continuously without overheating.
Torque is the twisting force exerted by the shaft of a motor. Units for torque include inch pounds, foot pounds, and inch ounces.
Full load torque is the rated continuous torque the motor can support without overheating within its time rating.
Peak torque. Many loads have cycling torques where the amount of torque required varies per the position of the machine. The maximum torque requirement at any point is the peak torque requirement.
Pull out (or breakdown) torque, is the maximum torque available when the motor operates at full voltage and full speed.
Pull up torque is the lowest point on the torque speed curve for a motor while accelerating a load to full speed. Some motor designs don't have a value of pull up torque because the lowest point may occur at the locked rotor point. In this case, pull up torque equals locked rotor torque.
Starting (or locked rotor) torque is the amount of torque available when you apply power to break the load away and accelerate up to speed.
This text is an adaptation of The Cowern Papers, courtesy Baldor Electric Co., Wallingford, Conn., edited by Mark Lamendola, EC&M Technical Editor. Cowern is an Application Engineer for Baldor.
