Open a motor catalogue in the USA and you will see the words Design A, Design B, Design C and Design D. These are NEMA motor types, and the letter tells you how the motor starts and how it behaves under load. Pick the wrong design and a motor can stall, trip its breaker or overheat. This post explains what each letter means.
What Is NEMA?
NEMA is the National Electrical Manufacturers Association, the US trade group that writes standards for electrical equipment. Its motor standard is NEMA MG-1. For three-phase squirrel-cage induction motors, MG-1 defines designs A, B, C, D and the newer design E, each with its own mix of starting torque, starting current, breakdown torque and slip.
Three Terms You Must Know First
Locked-rotor torque is the torque the motor produces at standstill, when the rotor is not yet turning. It decides whether the motor can start its load at all.
Breakdown torque is the highest torque the motor can produce before it stalls. If the load torque rises above it, the motor stops.
Slip is the difference between the synchronous speed of the magnetic field and the actual rotor speed, shown as a percentage. A 4-pole motor on 60 Hz power has a synchronous speed of 1800 rpm. If it runs at 1750 rpm at full load, its slip is about 2.8 percent. The synchronous speed comes from the formula 120 x frequency / number of poles, so a 2-pole motor on 60 Hz runs at 3600 rpm.
Design A
Design A has a maximum slip of 5 percent, a high to medium starting current, normal locked-rotor torque and normal breakdown torque. It is a general-purpose design for fans, pumps and machine tools, and its breakdown torque is higher than that of Design B.
Design B: The Most Common Motor
Design B is the most common three-phase induction motor in the USA. It also has a maximum slip of 5 percent, but its starting current is lower than Design A, and it has a high locked-rotor torque with normal breakdown torque. That combination suits the everyday loads found in HVAC systems: fans, blowers and pumps.
The curve below is the torque-speed curve of a NEMA Design B motor. Read it from left to right: the horizontal axis is speed, from 0 at standstill to 1.0, which is synchronous speed, and the vertical axis is torque in per-unit values. Torque starts at about 1.0 at standstill, rises to a peak of about 2.3 near 85 percent speed, and then falls sharply to zero at synchronous speed. At synchronous speed the torque is zero, because the rotor turns at the same speed as the magnetic field, so no voltage is induced in the rotor.
Diagram: Torque-speed curve of a NEMA Design B motor. Horizontal axis: speed from 0 (standstill) to 1.0 (synchronous speed). Vertical axis: torque in per-unit values. Torque peaks at about 2.3 before falling to zero at synchronous speed.
Design C
Design C is built for high starting torque. It has a maximum slip of 5 percent, low starting current, high locked-rotor torque and normal breakdown torque. Use it where the load is hard to start: conveyors, positive-displacement pumps, compressors and crushers. A Design B motor might stall on such a load, while a Design C motor breaks it away and gets it moving.
Design D
Design D is the high-slip motor. Its slip at full load is 5 to 13 percent, much higher than the other designs. It has a low starting current and a very high locked-rotor torque, and its full-load speed is lower. That makes it right for loads with very high inertia and sudden changes, such as cranes, hoists and punch presses.
Design E, the newest design, is a high-efficiency motor with a high starting current, normal locked-rotor torque and a maximum slip of 5 percent. It is built to meet the energy-efficiency rules for premium-efficiency motors.
Diagram: A squirrel-cage rotor. The bars and end rings carry the rotor current, and the rotor design shapes the torque-speed curve of the motor.
Design Letter or Code Letter? Do Not Mix Them Up
Students often confuse two different letters on a motor nameplate. The design letter (A, B, C or D) describes the torque and speed behaviour of the motor. The locked-rotor code letter, which runs from A to V, is different: it tells you the locked-rotor kVA per horsepower, used to estimate the starting current. For example, code letter G means 5.6 to 6.29 kVA per horsepower at locked rotor.
Difference Between the Designs at a Glance
Design A: high to medium starting current, normal locked-rotor torque, higher breakdown torque than B, slip up to 5 percent. General purpose: fans, pumps, machine tools.
Design B: lower starting current, high locked-rotor torque, normal breakdown torque, slip up to 5 percent. The everyday workhorse: HVAC fans, blowers and pumps.
Design C: low starting current, high locked-rotor torque, normal breakdown torque, slip up to 5 percent. Hard-to-start loads: conveyors, compressors, crushers.
Design D: low starting current, very high locked-rotor torque, slip 5 to 13 percent. High-inertia, shock loads: cranes, hoists, punch presses.
Key Takeaways
NEMA Designs A, B, C and D describe the torque-speed character of a three-phase induction motor. Design B is the general-purpose workhorse for fans, blowers and pumps. Design C gives high starting torque for conveyors and compressors, and Design D gives very high starting torque with 5 to 13 percent slip for cranes and hoists.
Objective Questions
1. Which NEMA standard defines motor Designs A, B, C, D and E?
a) NEMA MG-1
b) NEMA ICS-2
c) NEMA WC-3
d) NEMA 250
Answer: (a) NEMA MG-1
Explanation: NEMA MG-1 is the motor standard. For three-phase squirrel-cage induction motors it defines Designs A, B, C, D and the newer Design E.
2. What is the synchronous speed of a 4-pole motor on 60 Hz power?
a) 900 rpm
b) 1200 rpm
c) 1800 rpm
d) 3600 rpm
Answer: (c) 1800 rpm
Explanation: Synchronous speed = 120 x frequency / poles = 120 x 60 / 4 = 1800 rpm.
3. Which NEMA design is the most common three-phase induction motor in the USA?
a) Design A
b) Design B
c) Design C
d) Design D
Answer: (b) Design B
Explanation: Design B is the most common three-phase induction motor in the USA, the general-purpose workhorse for fans, blowers and pumps.
4. Which NEMA design has a full-load slip of 5 to 13 percent, with very high locked-rotor torque?
a) Design A
b) Design B
c) Design C
d) Design D
Answer: (d) Design D
Explanation: Design D is the high-slip motor. Its slip at full load is 5 to 13 percent, much higher than the other designs, with low starting current and very high locked-rotor torque.
5. On a motor nameplate, what is the difference between the design letter and the locked-rotor code letter?
a) They mean the same thing
b) The design letter (A, B, C or D) describes the torque and speed behaviour, while the code letter (A to V) gives the locked-rotor kVA per horsepower
c) The design letter gives the kVA per horsepower and the code letter gives the torque behaviour
d) The design letter is for single-phase motors only
Answer: (b) The design letter (A, B, C or D) describes the torque and speed behaviour, while the code letter (A to V) gives the locked-rotor kVA per horsepower
Explanation: Students often mix these up. The design letter describes how the motor starts and behaves under load. The locked-rotor code letter, from A to V, tells you the locked-rotor kVA per horsepower, used to estimate the starting current.
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