Saddle Torque: A torque developed, usually well below rated speed, that is much less than normal torque expected at that point. On the torque curve it creates a depression or ‘saddle’ and thus the name. Also called the ‘cusp’ of the curve.
Salient Pole: A motor has salient poles when its stator or field poles are concentrated in to confined arcs and the winding is wrapped around them (as opposed to distributing them in series of slots).
Schematic Diagram (Elementary Diagram): A schematic or elementary diagram is one that shows all circuits devices of a controller. The diagram does not show the physical arrangement of the devices or the actual wiring to the devices.
Screens: 1/4-inch mesh that covers the openings on open enclosure motors to keep rodents and other vermin from entering the motor cavity.
Seals - Double Lip: A rubber shaft seal to keep contaminants such as oil, water and dust from entering the bearing cavity.
Seals - Double Sealed Bearings: Bearings which have seals on both sides to keep the lubricant inside the bearing housing and keep out contaminants.
Seals - Labyrinth: A non-friction seal with a close fit of many turns which protects the bearing cavity from contaminants which can cause bearing failure.
Seals - Taconite: Seals which provide a seal so tight as to filter out iron ore (one of the finest dusts) from entering the bearing cavity.
Secondary Winding: The secondary winding of a motor is a winding which is not connected to the power source, but which carries current induced in it through its magnetic linkage with the primary winding.
Self-Release Coupling: A special feature of vertical hollow shaft motors which permits free spinning of the shaft while the pump water is receding without the pump shaft disengaging from the motor.
Semiconductor: A material, usually silicon or germanium, which permits limited current flow.
Service Deviation: See operating/Service Deviation.
Service Factor: A multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading at rated voltage and frequency.
Service Factor - Gear: A method of classifying loads and sizing gear reducers based on severity of load. The service factor for gear applications is determined by AGMA and is contingent on the duty of the gear box.
Service Factor - Motor: A multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading at the rated voltage and frequency. The service factor advises how much extra horsepower the motor is capable of handling. For example, a 10 hp motor with 1.15 service factor can operate to 11.50 horsepower.
Service of a Controller: The service of a controller is the specific application in which the controller is to be used; for example:
- General purpose.
- Definite purpose.
1.Crane and hoist.
2. Elevator.
3. Machine tool (Drill).
Set Speed: The desired operating speed.
Shaft Runout: Term used to advise how must shaft play there is at the end of the shaft extension in relation to the flange of the motor.
Shock Load: The load seen by a clutch, brake, or motor in a system which transmits high peak loads. This type of load is present in crushers, separators, grinders, conveyors, winches, and cranes.
Short-Circuit: A defect in a winding which causes part of the normal electrical circuit to be bypassed. This frequently results in reducing the resistance of impedance to such an extent as to cause overheating of the winding, and subsequent burnout.
Short Shaft Motor: NEMA standard MG-1 defines shaft length as the dimensions AH, or the distance from the face, flange or base of the machine to the end of the shaft. In a NEMA type TS frame a shorter than standard shaft extension is defined. This is usually used when the motor is direct connected to the load via couplings and no overhung load is encountered.
Silicon Controlled Rectifier (SCR): A solid state switch, sometimes referred to as a thyristor. The SCR has an anode, cathode and control rectification since it can be turned on at will. The SCR can rapidly switch large currents at high voltages. It is small sizes and low in weight.
Six Step Inverter: An old inverter design in which the outgoing produced wave takes the form of six steps, three up and three down.
Skew: Arrangement of laminations on a rotor or armature to provide a slight diagonal pattern of their slots with respect to the shaft axis. This pattern helps to eliminate low speed cogging effects in an armature, minimizes induced vibration in a rotor, and minimizes harmonic stray currents.
Skewing: Refers to time delay or offset between any two signals in relation to each other.
Slaving: A method of connecting controllers in cascade (series) or parallel. A number of slave units can be utilized, each running a drive at a different speed. When the manually operated master controller calls for a speed change, the slave units will respond in proportion, maintaining the speed ratios between them.
Sleeve Bearing: A bearing that is made of a sleeve bushing, not a ball or roller bearing.
Slewing: An incremental motion of the motor shaft or machine table from one position to another at maximum speed without losing position control.
Slinger: A device on the shaft or rubbing on it that prevents entrance of abrasive material into the bearing. Also the washer-like attachment to a shaft or part of the shaft that prevents oil from leaking into the motor or out of the bearing.
Slip: The difference between the speed of the rotating magnetic field (which is always synchronous) and the rotor in a non-synchronous induction motor is known as slip and is expressed as a percentage of a synchronous speed. Slip generally increases with an increase in load.
Slip Compensation: Method of increasing the speed reference to the speed regulator circuit, based on the value of motor torque, to maintain motor speed as the load on the motor changes.
Slip Ring: A conductor band mounted on an armature and insulated from it. A brush slides on the band as the armature rotates. The function of the slip ring system is essentially the same as a commutator and brushes. Slip rings are also used to transmit current from the armature in a generator application.
Slip Speed: The speed difference between speed at any load and the synchronous speed.
Snowmaking Motor: A specialized motor design for use in snowmaking applications.
Space Heater: Motor accessory used to prevent moisture condensation in the motor during periods of rest. When the motor is not operational, the space heater is energized to keep the motor temperature 10 degrees above ambient.
Special Purpose Motor: A motor with special operating characteristics, special mechanical construction, or both, designed for a particular application and not falling within the definition of a general purpose or definite purpose motor as defined by NEMA.
Speed Range: The minimum and maximum speeds at which a motor must operate under constant or variable torque load condition. A 50:1 speed range for a motor with top speed of 1800 RPM means the motor must operate as low as 36 RPM and still maintain regulation within specifications.
Speed Regulation: The numerical measure in percent of how accurately the motor speed can be maintained. It is the percentage of change in speed between no load and full load.
Spherical Roller Bearing: A special bearing design used for extended life or higher thrust when designs merit. This bearing will take some radial load, but only if thrust is applied at all times. Spherical roller bearings, provided by USEM, employs sprig loading to ensure the bearing will not be damaged during starting and momentary upthrust conditions. These springs push up against the lower the lower race so the lower race is kept in contact. Since the spring pressure may be several thousand pounds, a considerable load is imposed on the guide bearing during start-up. Care must be taken not to specify life factors that would cause bearing failures due to insufficient load during normal operation.
Stability: The ability of a drive to operate a motor at constant speed (under varying load) without "hunting (alternately speeding up and slowing down). It is related to the characteristics of the load being driven and the electrical time constants of the drive's regulator circuits.
Stable Torque: The torque of a motor is stable if the motor torque required for a load never exceeds 75-80% of the maximum motor torque allowed.
Stacked Bearing: A ball or roller bearing that is preloaded, with two bearings in opposition, or just two bearings (thrust) together on one shaft. Used where weight or thrust exceed the capacity of a single bearing or where there is a heavy thrust in both directions parallel to the shaft.
Stalling Torque: The torque at which the load causes the motor to stall (or stop). The maximum torque a motor can deliver while running at rated voltage and frequency. No more than 80% of this value should be used for stable operation.
Starting - Across The Line (Full Voltage): Standard starting method used on motors. In this starting method, the motor terminal voltage equals the line voltage, the motor current equals the line current, and the starting torque equals the rated starting torque. This type starting is used where system capacity and stiffness are sufficient to stand the high starting current without excessive voltage drop.
Starting - Autotransformer Reduced Voltage: In this starting method, an autotransformer is placed in series with the motor during starting. The transformer action reduces the voltage applied to the motor terminals. Because of the transformer action the line current is less than the motor current. For a given starting current on the line side, the motor terminal voltage can be higher than for other reduced voltage start methods. Thus, this method gives the highest motor torque per line ampere.
Starting - Capacitor: In this method, large capacitors are connected with the motor so that the capacitors supply much of the current during the start cycle. Careful sizing and switching of the starting capacitors is required to ensure that the capacitor current is not all placed on the line at once, and that the capacitors are not left connected with the motor after acceleration is accomplished.
Starting - Double Delta: This method accomplishes the equivalent of reduced voltage starting by changing a delta connected winding from parallel groups to series groups during the start. Frequently called "double delta part winding".
Starting - Part Winding Start: This starting method uses only a portion (usually 1/2, but sometimes 2/3) of the motor winding, increasing the impedance seen by the power system. It is to be used only for voltage recovery, and must not be left on the start connection for more than 2-3 seconds. The motor is not expected to accelerate on the start connection, and may not even turn.
Starting - Series Reactance Reduced Voltage: In this method, a voltage-dropping reactance is placed in series with the motor during starting. The impedance seen by the power system then is that of the reactance plus that of the motor.
Starting - Series Resistance Reduced Voltage: This starting method has a voltage-dropping resistance placed in series with the motor during starting. The impedance seen by the power system then is that of the resistance plus that of the motor.
Starting - Solid State Reduced Voltage: In this method, a solid-state starter, consisting of power SCR’s controlled by logic circuits, is used to chop the sine-wave power so that only a portion of the wave is applied to the motor.
Starting - Solid State Variable Frequency: In this method, a solid state variable frequency inverter is used to start the motor. With the variable frequency power source, the motor can supply full torque at full load current for the duration of acceleration. The initial applied frequency is very low, and the frequency is gradually increased to the desired running speed.
Starting Torque: The torque exerted by the motor during the starting period (a function of speed or slip).
Starting Variable Speed Drive: In this method, a variable speed drive (VFD) is installed between the motor and the driven load. This drive may be an eddy current clutch or fluid clutch (sometimes called fluid coupling). Use of such a drive allows the motor to accelerate without accelerating the driven load. After the motor is ramped up to speed, then the load is brought up to operating speed.
Starting - Wye Start/Delta Run: This method is actually reduced voltage, but is accomplished by changing the motor phase connections such that a winding that is designed to run with phase voltage equal to line voltage on delta connection is wye connected for starting to put less than line voltage on each phase.
Stator: That part of an induction motor’s magnetic structure which does not rotate. It usually contains the primary winding.
Steady Bushing: An option to vertical hollowshaft motors which enable the motor to give impression of operating as a solid shaft motor.
Stiffness: The ability of a device to resist deviation due to load change.
Stopping - Counter Torque Braking for Reverse-Running Loads: A form of reversing used in wound rotor motors. (USEM does not produce wound rotor motors.)
Stopping - DC Dynamic Braking: A form of braking and stopping the motor which does not cause rotation is reverse is to circulate direct current in the stator windings. This sets up a magnetic field that is stationary and tends to oppose the motion of the squirrel-cage through the field. The speed-torque curve for this situation is like that for normal running, except that the curve starts at synchronous speed and goes toward a peak torque near zero speed.
Stopping - Eddy-Current Braking: An electrical method of slowing a machine or load. Similar in principle to the DC dynamic braking of a squirrel cage motor. Stationary magnetic coils set up a controllable magnetic field throughout which a conducting disc or cylinder can turn. This disc or cylinder is coupled mechanically to the shaft that is to be brakes. When it turns through the magnetic field, eddy currents are generated and these cause a drag on the rotating shaft, just as a torque is set up in a squirrel-cage when currents flow in its short-circuited conductors.
Stopping - Mechanical Braking: Mechanical brakes are either disc type or shoe (or drum) type. Commonly, the shoes are raised from the wheel by an electromagnet and are set by a spring when the magnet is de-engerized.
Stopping - Plugging: A type of motor braking provided by reversing either voltage polarity or phase sequence so that the motor develops a counter-torque which exerts a retarding force to brake the motor.
Submersible Motor: A motor whose housing and terminal box is designed so that the motor can run under water (or another allowable liquid) -- completely submerged at a temperature of water not above 25° C.
Surge: A transient wave of current, voltage, or power in the electric circuit. Note: A transient has a high rate of change of current or voltage in the system.
Surge Protection: The process of absorbing and clipping voltage transients on an incoming AC line or control circuit. MOVs (Metal Oxide Varistors) and specially designed RC (Resistor-capacitor) networks are usually used to accomplish this.
Switch: A switch is a device for opening and closing or for changing the connections of a circuit. Note: A switch is understood to be manually operated unless otherwise stated.
Synchronous Speed: The speed of an AC induction motor's rotating magnetic field. It is determined by the frequency applied to the stator and the number of magnetic poles present in each phase of the stator windings. Mathematically it is expressed as Speed (RPM) = 120 x Applied Frequency (Hz) / Number of Poles Per Phase.
System Efficiency: The ratio of the mechanical power supplied to load to the total input power under specified operating conditions. The input power includes requirements for auxiliary functions, such as motor field, phase control, switching equipment, overload protection, and fans.
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