S

SCREENS

Are protection which can be placed over openings in the fan cover on a fan-cooled motor or ventilation openings of a protected motor to help keep out large particles and/or animals, but not block ventilation.

SECONDARY WINDING

Winding which is not connected to the power source, but which carries current induced in it through its magnetic linkage with the primary winding.

SERIES DC MOTORS

Where high starting torque are required for a DC motor, the series motor is used.  The load must be solidly connected to the motor and never decrease to zero to prevent excessive motor speeds.  The load must tolerate wide speed variations from full load to light load.  Typical areas of application are industrial trucks, hoists, cranes and traction duty.

SERVICE FACTOR (SF)

1.                  When used on a motor nameplate, a number which indicates how much above the nameplate rating a motor can be loaded without causing serious degradation (i.e., a 1.15 SF can produce 15% greater torque than the 1.0 SF rating of the same motor.

2.                  When used in applying motors or gearmotors, a figure of merit, which is used to “adjust”, measured loads in an attempt to compensate for conditions, which are difficult to measure or define.  Typically, measured loads are multiplied by service factors (experience factors) and the result in an “equivalent required torque” rating of a motor or gearmotor.

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.

SHAFT

The rotating member of the motor which protrudes past the bearings for attachment to the driven apparatus.

SHUNT WOUND DC MOTORS

Integral-horsepower shunt motors are used where the primary load requirements are for minimum speed variation from full-load to no-load and/or constant horsepower over an adjustable speed range at constant potential.  Shunt motors are suitable for average starting torque loads.

Typical applications include individual drives for machine tools such as drills and lathes, and centrifugal fans and blowers, which are regulated by means of the discharge opening.

SKEW

Arrangement of laminations on a rotor or armature to provide a slight angular pattern of their slots with respect to the shaft axis.  This pattern helps to eliminate low speed cogging effects in an armature and minimize induced vibration in a rotor as well as reduce associated noise.  It can also help to increase starting torque.

SLEEVE BEARINGS

A type of bearing with no rolling elements, where the motor shaft rides on a film of oil.

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.  It is expressed as a percentage of synchronous speed.  Slip generally increases with an increase in torque.

SPACE HEATER

Small resistance heater units mounted in a motor that is energized during motor shutdown to prevent condensation of moisture in the motor windings.

SPECIAL PURPOSE MOTOR

Motor with special operating characteristics or special mechanical construction, or both, designed for a particular application and not falling within the definition of a general purpose or definite purpose motor.

SPLASH-PROOF MOTOR

An open motor in which the ventilating openings are so constructed that drops of liquid or solid particles falling on it, or coming toward it in a straight line at any angle not greater than 100 degrees from the vertical, cannot enter either directly or by striking an running along a surface of the motor.

SPLIT PHASE START

Motors, which employ a main winding and an auxiliary winding, called the starting winding.  The windings are unlike and thereby “split” the single phase of the power supply by causing a phase displacement between the currents of the two windings thus producing a rotating field.

After the motor has attained approximately 75% of rated speed, the starting winding is automatically disconnected by means of a centrifugal switch or by a relay.  The motor then continues to run on a single oscillating field, which in conjunction with the rotation of the rotor, results in a rotating field effect.  Since there is no rotating field, after the starting winding is de-energized, the rotation cannot be changed until the motor has come to rest or at least slowed down to the speed at which the automatic switch closes.

Special starting switches are available as will as special reversing switches which have a means for shunting the open contracts of the automatic switch while the motor is running and thus permits the split phase motor to be reversed while rotating.  This type of starting is found typically on single-phase fractional motors.

SPEED

The speed of the motor refers to the RPMs (revolutions per minute) of the shaft.  For a three-phase AC motor the synchronous speed =

            120 x frequency (frequency is measured in Hertz or cycles per second)
            # of poles

The number of poles are a function of design.

STABILIZED SHUNT-WOUND MOTOR

A stabilized shunt-wound motor is a direct-current motor in which the shunt field circuit is connected either in parallel with the armature circuit or to a separate source of excitation voltage and which also has a light series winding added to prevent a rise in speed or to obtain a slight reduction in speed with increase in load.

STANDARDS ORGANIZATIONS

APIA            American Petroleum Institute

CE            Compliance to European Standards

CSA            Canadian Standards Association

EPACT            1997 U.S. Energy Policy Act

IEC            International Electrotechnical Commission

IEEE    Institute of Electrical and Electronics Engineers

ISO            International Standards Organization

NEMA            National Electrical Manufacturers Association

UL            Underwriter’s Laboratories

UR            Underwriter’s Laboratories Recognized

STARTING CURRENT

Amount of current drawn at the instant a motor is energized—in most cases much higher than the required for running.  Same as locked rotor current.

STARTING TORQUE

The torque or twisting force delivered by a motor at the instant it is energized.  Starting torque is often higher than rated running or full load torque.

STATOR

That part of an AC induction motor’s magnetic structure which does not rotate.  It usually contains the primary winding.  The stator is made up of laminations with a large hole in the center in which the rotor can turn; there are slots in the stator in which the windings for the coils are inserted.

STRESS CONES

A physical protection placed over the external connections point on medium and high voltage motor leads.  Stress cones are used to avoid di-electric breakdown of motor leads in the vicinity of the external connection.  Stress cones generally require an oversized conduit box on large motors.

SUFFIXES TO NEMA FRAMES

Letter suffixes sometimes follow the NEMA frame size designations.  Some of these suffixes, according to NEMA standards, have the following meanings:

            FRACTIONAL HORSEPOWER MOTORS

            INTEGRAL HORSEPOWER MOTORS

SURGE PROTECTION

A capacitor device usually mounted in the conduit box to flatten the voltage surges that may occur as a result of lighting or a power supply surge (short-period peak).  These surges can result in more than twice the rated voltage going to the windings and in turn cause winding damage.

SYNCHRONOUS MOTOR

A motor which operates at a constant speed up to full load.  The rotor speed is equal to the speed of the rotating magnetic field of the stator – there is no slip.  There are two major synchronous motor types: reluctance and permanent magnet.  A synchronous motor is often used where the exact speed of a motor must be maintained.

SYNCHRONOUS SPEED

The speed of the rotating magnetic field set up by the stator winding of an induction motor.  In a synchronous motor, the rotor locks into step with the rotating magnetic field and the motor is said to run at synchronous speed.  Approximately the speed of the motor with no load on it.