Understanding Three Phase AC Motors Part 2: NEMA Design Letters Example

Overview of Motor Specifications

The following are some common specification for three phase AC motors. These specifications are covered in greater detail elsewhere within the section.

FLA or Full Load Amps: As the name implies, this is the rated full load current of the motor.

RPM or Rotations Per Minute (Base Speed): This is the speed at which the motor will deliver its rated horsepower at rated voltage. This is the speed at which the shaft will rotate when driving a load. Note that the indicated RPM is typically less than the synchronous speed, due to slip.

Service Factor: A factor applied to the motor horsepower to determine the safe operating limits of a motor. For example, a 10 HP motor with a service factor of 1.0 can only be operated safely at 10 HP or below. However, a 10 HP motor with a service factor of 1.15 can be operated safely at 11.5 horsepower (for a short period of time, per manufacturer recommendations). A motor that is regularly operated above its rated horsepower (i.e. to the service factor limit) may have reduced reliability and service life.

Insulation Class: In order to determine appropriate ratings for a motor’s ability to handle heat, NEMA defines a motor insulation class. This includes classes A, B, F, and H.

Motor Design Letter: To outline motors with specific torque characteristics, NEMA defines letter codes A, B, C, and D. These letter codes express motor torque during operation, and are used to match motors with appropriate loads (with matching torque characteristics).

Nominal Efficiency: As motor efficiencies vary among motors of the same design, the NEMA nominal efficiency percentage represents the average efficiency typical of motors of the same type.

NEMA Motor Designs

Each motor exhibits specific torque characteristics during operation. NEMA outlines letter codes A, B, C, and D to help define common torque characteristics. The most common type of motor is the NEMA B motor, which are single speed motors often used for fans and conveyors. The graph below outlines the typical torque behaviour of a typical NEMA B motor.

Graph1

Starting torque (or locked rotor torque) is the torque the motor develops upon starting from rest with full rated voltage. When voltage is first applied to the motor, the motor develops a torque roughly equivalent to 150% of its rated full-load torque. For example, a NEMA B motor with a full load torque of 15lb-ft would exhibit about 22lb-ft of torque upon initial start.

As the motor picks up speed beyond the initial start, torque will dip slightly until it reaches the pull-up torque, where it begins to rise up to breakdown torque. For a NEMA B motor, this is about 200% of full load torque.

Just beyond the breakdown torque, the motor reaches full-load torque, which is the rated torque for the motor. Note that due to slip, the motor will never reach 100% synchronous speed.

This speed-torque curve useful in understanding motor performance and matching the appropriate motor to a load. Refer to the example below.

Graph6

In this example, we have a NEMA B type torque curve. For a single speed motor to operate properly, it must reach its rated speed to deliver its full load torque. Upon inspection of the graph, we see that variable torque load 2 and constant torque load 2 do not have curves which intersect with the synchronous speed/full-load torque point on the NEMA B torque curve. On the other hand, variable torque load 1 and constant torque load 1 both intersect with the synchronous speed/full-load torque point on the NEMA B torque curve. Thus, this motor is suitable for torque load 1 and constant torque load 1.

NEMA A motors are similar to NEMA B motors, with similar speed-torque curves. However, NEMA A motors have higher starting currents. NEMA A motors are not very common. If they are used, they are typically for applications similar to NEMA B motors.

NEMA C motors are used in applications with very high starting torque requirements (such as crushers or machinery that is heavily loaded). These motors are usually single speed. Below is the typical torque curve of a NEMA C motor.

Graph2

NEMA D motors are used for difficult start, high torque applications such as pumpjacks. This is due to their extermenly high starting torque of approximately 280%. NEMA D motors do not have a breakdown torque, unlike NEMA B motors. NEMA D motors can have significant slip, sometimes reaching double digit percentages. Below is the typical torque curve of a NEMA D motor.

Graph3

It is important to note the importance of in-rush current or locked rotor current. In order to start any  motor from rest, significant torque is required, and hence significant current is drawn. During a motor’s initial start upon application of full rated voltage, a motor can draw 600% to 650% of its full load current. This must be taken into account when specifying overcurrent protection devices.

Graph4

Pairing Motors and Loads

Manufacturers typically provide documentation outlining an equipment’s torque characteristics. This is useful in pairing an appropriate motor to a load. NEMA MG-1 is an additional source of typical torque characteristics.

In a previous example (shown below) we determined that the NEMA B motor is suitable for variable torque load 1 and constant torque load 1. Coincidentally, the curve for variable torque load 1 closely matches typical torque performance and torque requirements for centrifugal pumps. Because the starting torque of the motor exceeds the starting torque of variable torque load 1 and the full-load torque of variable torque load 1 is equal to the motor full-load torque, this NEMA B motor is a good match.

Graph6Now consider the torque curve for a pumpjack below, and the curve for a certain NEMA B motor.

Graph7

You’ll notice that the full-load torque of the motor and the pumpjack match at synchronous speed. However, this is not a good match since the starting torque requirements of the pumpjack are significantly higher than the starting torque of the motor. Thus, this motor will not be able to get the pumpjack started. A motor with a higher starting torque must be used.

Consider the same torque curve below, with the curve for a NEMA D motor. What can you see?

Graph5

Notice that the full-load torque of the motor and the pumpjack match at synchronous speed. The difference this time is that the starting torque of the motor significantly exceeds the starting torque of the pumpjack. Because there is sufficient starting torque. this motor will get the pumpjack started, and reach full-load torque. This is a good match.

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