Often while instructing a workshop, I receive the question about how design letters and code letters are used when designing and installing motor circuits. To begin with, we must understand that motor circuitry must be designed to provide protection for motor windings and components when motors are starting, running and driving loads.
Motor windings must be protected by overcurrent-protection devices (OCPDs), according to the type used and based on the amount of starting current required. OCPDs must be sized by percentages based on the type motor, starting method and design or code letter. If the OCPD is sized greater than 125 percent of the motor’s full-load current (FLC) in amperes, then a second stage of protection, such as an overload relay, is required.
Starting methods should be selected and based on the amount of starting current required to start and run the motor or the amount that is reduced by using a particular starting method, such as a solid state starter or an adjustable speed drive system.
Types of motors
The following are types of motors that must be considered when sizing OCPDs to allow motors to start and run. These motors are listed in Table 430.52 of the National Electrical Code (NEC):
• Single-phase AC squirrel cage
• Three-phase AC squirrel cage
• Wound rotor
• Direct current (DC)
Note, when Table 430.52 is used, the phases of the motor should be selected first, and the type of motor should be selected second.
Single-phase squirrel-cage motors
Induction motors are known as “squirrel-cage” motors in the electrical industry. An induction motor operates on the same principles as the primary and secondary windings of a transformer. When power energizes the field windings, they serve as the “transformer” primary by inducing voltage into the rotor, which serves as the secondary. Squirrel-cage motors have two windings on the stator (the stationary windings); one is the run winding, and the other is the starting winding. A motor with this additional starting winding on the stator is called a split-phase induction motor; it provides the ability to start and run. The starting winding has a higher resistance than the running winding, which creates phase displacement between the two windings that give split-phase motors the power to start and run.
The angular phase displacement is about 18–30 degrees, which provides enough starting torque (twist or force) to start the motor. The motor operates on the running winding after the rotor starts and has reached speed of about 75 percent of the motor’s synchronous speed. A centrifugal switch then disconnects the motor.
Three-phase squirrel-cage motors
Also listed in Table 430.52 are three-phase squirrel-cage motors. They are equipped with three separate windings per pole on the stator, which generates magnetic fields that are 120 degrees out-of-phase with each other. Three-phase motors do not require an additional starting winding. A three-phase induction motor will always have a peak phase of current. This is due to the alternating current (AC) reversing its direction of flow; then, a peak current will be developed, and as current reverses direction again, a smooth and continuous source of power is produced, once they are started and are driving the load.
Three-phase wound-rotor motors
Three-phase wound-rotor motors found in Table 430.52 are similar in design to squirrel-cage induction motors. They are three-phase motors having two sets of leads. One set consists of the main leads to the stator—the field poles—and the other set consists of the secondary leads to the rotor. The secondary leads are connected to the rotor through slip rings, while the other end of the leads are connected to the controller and the banks of the resistors. The speed of the rotor varies with the amount of resistance added in the motor circuit. The motor will turn slower when the resistance in the rotor circuit is greater and vice versa. The resistance may be incorporated in the controller or as a separate resistor bank.
Nonexcited and direct current (DC) are the two types of synchronous motors that are available. Synchronous motors are available in a wide range of sizes and types that are designed to run at fixed speeds. A DC source is needed to excite a DC-excited synchronous motor. The torque required to turn the rotor for a synchronous motor is produced when the DC of the rotor field locks in with the magnetic field of the stator’s AC.
Next month, this column will discuss DC and energy-efficient motors as well as starting techniques.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.