S curve
(also sin, sin2, sin3 curve) An acceleration/deceleration circuit which reduces the rate of acceleration or deceleration at the beginning and/or end of the accel/decel period.

SCR
Silicon controlled rectifier, also known as a thyristor. Like a diode, this only allows electric current to flow through it in one direction. In addition, it can also be controlled so that the "turn-on" can be delayed indefinitely, so that it does not allow current to flow through it in either direction. In this way, it can control the output voltage of a circuit. Once turned on, it will continue to conduct after the control signal has been removed. SCRs are sometimes used in the phase-controlled rectifier section and inverter section of adjustable frequency drives.

secondary coil
In a transformer, the coil that is connected to a load in the electrical circuit.

serial communications
A method of transmitting computer information over a wire a single binary digit (bit) at a time. RS-232 and RS-485 are common serial communication standards.

service factor
A number that indicates the amount of continuous overload that a component can withstand. This is commonly applied to AC induction motors.

set point
or set speed The desired operating condition, as set by a control device.

set point controller
A device that continuously compares the actual operating condition with the set point and sends a correction signal to the controlled equipment if they are not the same.

sheave
A wheel with a grooved rim, often used in conjunction with a belt to connect a motor to a driven load. Also called a pulley.

shielded cable
Cable completely enclosed within a metal sheath. The sheath is to shield the sensitive, low level signals carried by the cable from external noise or to shield the area around the cable from noise-generating currents carried by the cable.

short circuit ratio
A measure of the impedance of a transformer, often the distribution transformer used to provide electrical power to a building. This is the ratio of the theoretical current that would flow in the case of a short circuit compared to the full load current rating of the transformer.

signal isolator
A signal isolator must be used when connecting a grounded signal source to a grounded signal follower. If the speed control input of an adjustable frequency drive is grounded, any input signal must be ungrounded. When a grounded input signal is connected to the signal isolator, it provides the isolation the drive requires.

sine wave
The ideal AC waveform for power distribution systems. While power utilities are careful to generate output voltage and current waveforms that are sine waves, electrical noise caused by connected loads can distort these waves.

six-step inverter
An output stage of an adjustable frequency drive that switches the inverter power components at a low frequency, just six times the output frequency of the drive. Six-step inverters generally must be connected to an adjustable-voltage DC bus so that the voltage to the motor can be controlled. see variable voltage inverter

skin effect
An electrical effect that causes high-frequency AC currents to travel mostly along the outside of a wire and not through its entire cross-section. As a result, wires often have a larger resistance to the flow of high frequency harmonic currents than they do to the fundamental frequency. When large amounts of harmonic distortion are present, this can cause additional heating in devices like transformers.

slip
The difference between synchronous speed (rotating magnetic field speed) and rotor speed of an AC induction motor. It is usually expressed as a percentage of synchronous speed.

slip compensation
A method of increasing the speed reference to the speed regulating circuit of a drive based on motor torque to maintain a constant motor speed as load changes.

speed range
The minimum and maximum speeds at which a motor must operate under specified conditions. It may also be expressed as a ratio. A 100:1 speed range for a motor speed control with a synchronous speed of 1800 rpm means it will operate down to 18 rpm under specified conditions.

stability
The ability of a drive to operate a motor at a constant speed without hunting.

star-delta
see wye-delta

starting torque
The torque produced by a motor during the start-up process, or the torque required by a load to get it moving initially.

step-over circuit
A step-over circuit will prevent operation of the drive at specified speeds. Typically the circuit allows two or more different speed ranges to be avoided.

stiffness
The ability of an electric distribution system to maintain constant voltage with load change. Also, the ability of a drive to maintain speed with load change. The greater the stiffness, the less the change.

surge
A transient wave of current, voltage or power in the electric circuit. A transient causes a high rate of change of current or voltage in the system, but lasts for only a short amount of time.

synchronous speed
The speed of an AC induction motorís rotating magnetic field. It is determined by the frequency applied to the stator and the number of poles in each phase of the stator windings.

system efficiency
The mechanical power supplied to the load compared to the total input power under specified conditions. It is usually expressed as a percentage of the total input power.