Full Load Amps
The amount of current the motor can be expected to draw under full load (torque) conditions is called Full Load Amps. It is also known as nameplate amps.

Locked Rotor Amps
Also known as starting inrush, this is the amount of current the motor can be expected to draw under starting conditions when full voltage is applied.

Service Factor Amps
This is the amount of current the motor will draw when it is subjected to a percentage of overload equal to the service factor on the nameplate of the motor. For example, many motors will have a service factor of 1.15, meaning that the motor can handle a 15% overload. The service factor amperage is the amount of current that the motor will draw under the service factor load condition.

The code letter is an indication of the amount of inrush or locked rotor current that is required by a motor when it is started. (See “Locked Rotor Code Letters” for more details.)

The design letter is an indication of the shape of the torque speed curve. Figure 1 shows the typical shape of the most commonly used design letters. They are A, B, C, and D. Design B is the standard industrial duty motor which has reasonable starting torque with moderate starting current and good overall performance for most industrial applications. Design C is used for hard to start loads and is specifically designed to have high starting torque. Design D is the so-called high slip motor which tends to have very high starting torque but has high slip RPM at full load torque. In some respects, this motor can be said to have a “spongy” characteristic when loads are changing. Design D motors are particularly suited for low speed, punch press applications and hoist and elevator applications. Generally, the efficiency of Design D motors at full load is rather poor and thus they are normally used on those applications where the torque characteristics are of primary importance. Design A motors are not commonly specified but specialized motors used on injection molding applications have characteristics similar to Design A. The most important characteristic of Design A is the high pullout torque.

Efficiency is the percentage of the input power that is actually converted to work output from the motor shaft. Efficiency is stamped on the nameplate of most domestically-produced electric motors.

Motors, like suits of clothes, shoes and hats, come in various sizes to match the requirements of the application. In general, the frame size gets larger with increasing horsepowers or with decreasing speeds. In order to promote standardization in the motor industry, NEMA (National Electrical Manufacturers Association) prescribes standard frame sizes for certain dimensions of standard motors. For example, a motor with a frame size of 56, will always have a shaft height above the base of 3-1/2 inches. (See “The Mystery of Motor Frame Size” for more details.)

This is the frequency for which the motor is designed. The most commonly occurring frequency in this country is 60 cycles but, on an international basis, other frequencies such as 40, and 50 cycles can be found.

An indication of the approximate speed that the motor will run when it is putting out full rated output torque or horsepower is called full load speed.

These are loads that have a relatively high flywheel effect. Large fans, blowers, punch presses, centrifuges, commercial washing machines, and other types of similar loads can be classified as high inertia loads.

The insulation class is a measure of the resistance of the insulating components of a motor to degradation from heat. Four major classifications of insulation are used in motors. They are, in order of increasing thermal capabilities, A, B, F, and H. (See “Motor Temperature Rating” for more details.)

Constant Horsepower
The term constant horsepower is used in certain types of loads where the torque requirement is reduced as the speed is increased and vice-versa. The constant horsepower load is usually associated with metal removal applications such as drill presses, lathes, milling machines, and other similar types of applications.

Constant Torque
Constant torque is a term used to define a load characteristic where the amount of torque required to drive the machine is constant regardless of the speed at which it is driven. For example, the torque requirement of most conveyors is constant.

Variable Torque
Variable torque is found in loads having characteristics requiring low torque at low speeds and increasing values of torque as the speed is increased. Typical examples of variable torque loads are centrifugal fans and centrifugal pumps.

Phase is the indication of the type of power supply for which the motor is designed. Two major categories exist; single phase and three phase. There are some very spotty areas where two phase power is available but this is very insignificant.

This is the number of magnetic poles that appear within the motor when power is applied. Poles always come in sets of two (a north and a south). Thus, the number of poles within a motor is always an even number such as 2, 4, 6, 8, 10, etc. In an AC motor, the number of poles work in conjunction with the frequency to determine the synchronous speed of the motor. At 50 and 60 cycles, the common arrangements are:

Poles Synchronous Speed
60 Cycles 50 Cycles
2 3600 3000
4 1800 1500
6 1200 1000
8 900 750
10 720 600

Per cent power factor is a measure of a particular motor’s requirements for magnetizing amperage.

The service factor is a multiplier that indicates the amount of overload a motor can be expected to handle. For example, a motor with a 1.0 service factor cannot be expected to handle more than its nameplate horsepower on a continuous basis. Similarly, a motor with a 1.15 service factor can be expected to safely handle intermittent loads amounting to 15% beyond its nameplate horsepower.

Slip is used in two forms. One is the slip RPM which is the difference between the synchronous speed and the full load speed. When this slip RPM is expressed as a percentage of the synchronous speed, then it is called percent slip or just “slip”. Most standard motors run with a full load slip of 2% to 5%.

This is the speed at which the magnetic field within the motor is rotating. It is also approximately the speed that the motor will run under no load conditions. For example, a 4 pole motor running on 60 cycles would have a magnetic field speed of 1800 RPM. The no load speed of that motor shaft would be very close to 1800, probably 1798 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 called the slip RPM of the motor.

Ambient Temperature
Ambient temperature is the maximum safe room temperature surrounding the motor if it is going to be operated continuously at full load. In most cases, the standardized ambient temperature rating is 40°C (104° F). This is a very warm room. Certain types of applications such as on board ships and boiler rooms, may require motors with a higher ambient temperature capability such as 50° C or 60° C.

Temperature Rise
Temperature rise is the amount of temperature change that can be expected within the winding of the motor from non-operating (cool condition) to its temperature at full load continuous operating condition. Temperature rise is normally expressed in degrees centigrade.

Most motors are rated for continuous duty which means that they can operate at full load torque continuously without overheating. Motors used on certain types of applications such as waste disposal, valve actuators, hoists, and other types of intermittent loads, will frequently be rated for short term duty such as 5 minutes, 15 minutes, 30 minutes, or 1 hour. Just like a human being, a motor can be asked to handle very strenuous work as long as it is not required on a continuous basis.

Torque is the twisting force exerted by the shaft of a motor. Torque is measured in pound inches, pound feet, and on small motors, in terms of ounce inches. (For more information see “Understanding Torque”.)

Full Load Torque
Full load torque is the rated continuous torque that the motor can support without overheating within its time rating.

Peak Torque
Many types of loads such as reciprocating compressors have cycling torques where the amount of torque required varies depending on the position of the machine. The actual maximum torque requirement at any point is called the peak torque requirement. Peak torques are involved in things such as punch presses and other types of loads where an oscillating torque requirement occurs.

Pull Out Torque
Also known as breakdown torque, this is the maximum amount of torque that is available from the motor shaft when the motor is operating at full voltage and is running at full speed. The load is then increased until the maximum point is reached.
Refer to figure 2.

Pull Up Torque
The lowest point on the torque speed curve for a motor that is accelerating a load up to full speed is called pull up torque. Some motor designs do not have a value of pull up torque because the lowest point may occur at the locked rotor point. In this case, pull up torque is the same as locked rotor torque.

Starting Torque
The amount of torque the motor produces when it is energized at full voltage and with the shaft locked in place is called starting torque. This value is also frequently expressed as “locked rotor torque”. It is the amount of torque available when power is applied to break the load away and start accelerating it up to speed.

This would be the voltage rating for which the motor is designed.

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