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Some of the terms you are likely to come across when servicing motors.
Force- an action that produces motion, expressed in Newtons (N) or pounds (lb).
Thrust- is a force that causes an object to rotate. The applied force through the distance it acts, expressed in meters (M) or feet (ft). T = F x D or the torque of a motor when converted to machine motion. Torque is therefore expressed in Newton meters (N-m) or pound feet (lb-ft).
Work- a force acting on an object causing it to move through a distance, expressed in joules (J) or foot pounds (ft-lb) (*not lb-ft).
Inertia- is the opposition of a body to a change in velocity when acted on by an external force. The motor’s force must overcome the inertia of the load, coupling components, and motor rotor.
Power- is the rate of work expressed in Watts (W) or horsepower (hp). P = I (current) x V (voltage). 1hp = 550ft-lb/sec or 746 watts. For a motor P (in Watts) = .105 Na (rpm) x T (torque in N-m) or P (in hp) = Na x T (lb-ft)/5252.
Magnetism- is the ability to attract iron and certain other materials.
Electromagnetism- is the magnetism created by the motion of an electrical charge.
Magnetic Flux- are the lines of force between the north and south poles of a magnet, expressed in Webers (Wb). Flux density is the flux per unit area expressed in Wb/m*2.
Lorentz Force – is the force exerted on a current carrying conductor in a magnetic field. It is measured in Newtons (N).
Faraday’s Law – If the flux linking a loop of wire varies as a function of time, a voltage is induced which is proportional to the rate of change of the flux.
Electromotive Force- is the voltage produced if we move a conductor through the lines of flux of a magnetic field. E = Wb/m*2 x l (length of conductor) x N (speed in RPM) all divided by 10*8 expressed as Emf.
CEMF- or counter electromotive force is the electromotive force that is generated by the armature (rotor) of a motor that opposes the input voltage supplied to it when the motor is driven in the forward direction. It reduces or limits the armature (rotor) current.
Poles – Are the elements that develop the magnetic flux fields in a motor. The number of pole pairs in a motor must be the same number or a multiple of the number of phases in the applied voltage. Ex: A 3 phase motor would require 3, 6, 9, 12 etc. pole pairs. The stator poles must also be physically displaced from each other by an angle that is equal to the phase angle between the individual phases of the applied voltage. Example: in a 3 phase input voltage the individual phases are 120 degrees apart (3 x 120 = 360 degrees), therefore the stator poles are also 120 degrees apart in a 3 phase AC motor.
Salient Poles- are poles, N and S that are salient, or “sticking out”. Motors which rotate as a result of the creation of torque due to the reaction between magnetic fields normally have salient poles, poles which stick out in order to concentrate the magnetic flux into angular sectors to concentrate and focus the alignment force between the fields.
Rotating Field – refers to the rotating (not physically rotating) magnetic field in the stator of an AC motor.
Slip – refers to the difference between synchronous speed and the rotor speed. Slip(S) = (Ns-Nr) / Ns and 0 < S < 1. A locked rotor = 1 and a rotor at synchronous speed = 0.
Breakdown Torque – is the point at which an excessive load on the motor will cause it to stop.
Power Factor- is the ratio of real power to apparent power: (watts/volt-amps) x 100 (0 > p.f. < 1). A power factor of 1 (unity) is ideal. Power factor can exceed unity when rotor current is greater than stator current.
Power factor correction (PFC) - is a technique of counteracting the undesirable effects of electric loads that create a power factor that is less than 1, (or unity). In an electric circuit, if the load is resistive, the voltage and current waveforms are in phase. This is called unity power factor. If the load is inductive, current lags behind voltage and if the load is capacitive then current leads voltage. Motors, fluorescent lighting fixtures, etc. are normally inductive loads; hence power factor is always less than unity in most factories.
To bring the power factor to unity, we add capacitors across the load so that the leading and lagging currents cancel each other out. This is what is meant by power factor correction.
Compensating Windings – are windings which are placed in the pole face of the DC field windings which are parallel to and the opposite the polarity of the armature winding. They are often used on DC motors designed for heavy overloads or rapidly changing loads to minimize the possibility of magnetic flashover between the field and armature windings.
Commutating Poles or Interpoles – are small windings located between the main field windings to assist in the reversal of the voltage applied to the commutator.
Locked Rotor – is the condition in which the armature/rotor is mechanically constrained by an excessive load (or by a pole screwed into the armature/rotor for limit testing by the manufacturer). This condition will result in maximum torque by the motor and maximum current draw.
Thermister – refers to a temperature sensing device attached to the motor windings to alert the motor drive to an extreme increase in temperature within the motor.
- PTC (positive temp. coefficient) Thermister is a non-linear device which increases in resistance with a corresponding increase in temperature. These are widely used in motor construction. Once the motor winding temperature reaches the device’s trip temperature its resistance increases several orders of magnitude for a small increase in temperature.
- NTC (negative temp. coefficient) Thermister like the PTC is a non-linear device which decreases in resistance with a corresponding increase in temperature.
- RTD (Resistance Temperature Detector) is a device which experiences a linear positive change in resistance with respect to temperature. Because of their linearity, stability, and accuracy over a wide temperature range RTD’s are the gold standard in temperature measurement devices. However due to their construction they are also the most expensive sensing devices and rarely used in motor construction.
Induction – refers to the current “induced” by constantly moving a permanent magnet in and out of a coil of wire, or by constantly moving a conductor near a stationary permanent magnet.
Wye Motor Connection – the 3-phase winding connections in an AC motor stator are connected so that the similar ends of the coils are connected together and the other ends are connected to the 3 incoming supply service lines, also called a "star connection". The internally-connected point is brought out as the neutral connection. The line-to-line voltage (between any 2 lines) is 1.732 times the voltage between any line and neutral. However the line currents are the same. Wye systems are used wherever you have single phase loads. By connecting supply lines from line to line or line to neutral you get two different voltages for distribution within a building.
Delta Connection – the 3-phase winding connections in an AC motor stator are connected in such a way that the dissimilar ends of each winding are connected together to form a triangle and the 3 incoming supply service lines are then connected to the junctions. There are only 3 wires and all three of the windings will have the same line voltage value while the line currents are 1.732 times the supply currents. Delta connections are used wherever you don’t need a neutral, especially in electric motors but the biggest use of delta is in power transmission. It’s way too expensive to run a fourth (neutral) wire all those miles, especially since a 3-wire delta transmits the same amount of power as a Wye connection. At the destination, (a distribution transformer outside the home or business), the primary of the transformer is wired delta and the secondary is wired wye. This creates the neutral that can be used to derive single-phase power where needed.
Key – is the component placed between the motor shaft and the hub of the power transmitting element such as a gear, sprocket etc.
Keyseat – is the corresponding groove in the shaft of the motor. Note: this is commonly confused with the keyway.
Keyway – is the corresponding groove in power transmitting equipment.
Plugging – reversing the stator direction of rotation to bring the motor to a rapid stop. Unfortunately it results in the generation of a lot of heat.
Dynamic Braking – is the result of applying DC voltage to the stator to make it stationary. The result is that the rotor slows and eventually aligns with the stationary applied voltage.
Rectification – is the act of converting an AC voltage source into DC voltage.
Inverting – refers to the conversion of a DC voltage source into AC voltage.
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