Can you explain the basic Motor Functions?

Basic Function To make it’s shaft rotate a motor must convert the available electrical supply into a rotary motion. Conveniently the origin of our electrical supplies are rotating generators. The commonest means of distribution, the Three Phase supply, is simply a tapping from three equally displaced poles around the periphery of the generator 120o apart. By connecting a 3 phase (3 ?) motor to this supply the motor will simply mimic the generator and rotate within the induced magnetic field. In order to understand the basic function of an electric motor acceptance of some basic principles is necessary …

• Any current flow along a conductor will produce a magnetic field around the conductor, proportional to the current flow. As the current flow increases so does the strength of the magnetic field.

• A magnetic field is measured by it’s magnetic or flux density, the ISO measurement unit for this is the Weber.

• If an alternating current is producing the magnetic field then the field will follow the pattern of the alternating voltage.

• If the Produced Magnetic field cuts across a conductor (including the conductor carrying the current) then a another current will be generated in that conductor. The induced current in the supply conductor will be in opposition to the applied current from the external supply, but much less than this value. Motor Design In an electric motor the magnetic effect is greatly increased by laying many smaller conductors alongside one another. These are the coils of the motor. The effect is further improved by laying the conductors inside a soft iron core, which provides an easy flow route for the magnetic field. This is the stator.

Finally in order to produce motion a rotor is utilised consisting of numerous soft iron plates. These plates are separated by a thin film of resin, with large connecting tie bars evenly spaced around the periphery. The moving magnetic field produced by the coils in the stator also cuts the rotor laminates, however due to the very low resistance between adjacent plates via the tie bars large circulating or eddy currents are formed within these plates. These currents in turn produce their own magnetic fields and a natural effect is that these are always in opposition to the magnetic force that first created them.

The net effect is a continuous opposition between the two magnetic fields around the periphery of the rotor, and these forces repel each other, the overall force of repulsion of the myriad smaller currents and eddy currents is to produce a rotation of the rotor. As the rotor turns the magnetic opposition force from the rotor causes further eddy current generation within the stator conductors, again because the fluctuating magnetic fields produced, this time by the rotor, are cutting the original conductors of the stator. These eddy currents are always in opposition to the main supply current. At rest these forces are minimal, however as the motor approaches full speed (see below) the forces will increase to a point where no current can flow from the supply.

Because this situation would cause the motor to stall the motor instead reaches an equilibrium point where just enough current is drawn by the motor conductors to maintain maximum rotation speed. This self governing effect is a major benefit of this type of induction motor construction. These motors are referred to as Squirrel Cage Induction motors from the construction method used for the Rotor assembly. Full Speed The maximum theoretical speed of these motors is a function of the supply frequency, in this Country (UK), 50 Hz, or 50 cycles per second. Thus the maximum speed attainable is 50 Hz x 60 seconds = 3,000 RPM. However, due to the required equilibrium situation outlined above, the maximum is practically rated at 2,900 RPM. Pole Pairs This speed is the running speed of a 2 pole motor, that is 1 North and 1 South magnetic pole for each phase group. Conveniently by increasing the pole pairs the speed can be reduced to fractions of this speed … 2 Pole 2,900 rpm 4 Pole 1,450 rpm 6 Pole 975 rpm 8 Pole 725 rpm etc. Load Current Because the supply current tends to diminish as maximum speed is attained, when a load is applied to the motor shaft it will tend to slow, this in turn reduces the eddy current level opposing the main supply current, hence the main current flow will increase. In turn this strengthens the driving force of the magnetic field until once more the maximum / equilibrium speed is attained, but now the motor will continue to draw a higher current to account for the increased loading.

The loading may be increased until a maximum safe level is reached for a given size of motor. Any further current input will not increase the magnetic strength because the iron will ‘Saturate’ or reach a maximum point. After this point additional current can only manifest as heat. All current flow through a conductor will produce some heat due to electrical resistance of the conductor. Although this is naturally dissipated by gas flow across the motor. However once past the saturation point and the heat build up becomes dramatic and can reach the point where insulation and even conductor breakdown and melting can occur. If this were left unattended then this would result in motor burn out. It is the job of the designer to ensure the motor size selected is adequate for all expected load conditions of a particular machine. It is due to the effect of heat build up that Thermistor protection is a common safety protection used for overload conditions.

Although another common method of overload control is current limiting overloads that will trip the motor if a preset level is exceeded. The above describes the basic operation of all 3 ??motors used to drive refrigeration compressors. Single Phase Motors - 1 ? A single phase motor uses a supply derived from 1 phase only of the 3 generated phases from the power station. The return path for the electrical current is referred to as Neutral, and is in fact the star point of the generator or sub-station transformer. Therefore the single phase supply has no natural rotation and the single phase motor cannot exploit the mimic effect. Instead the single phase motor creates a rotating field within the stator by using two separate coils. In order to obtain a rotation the supply current is made to flow along the two coils at different time intervals. This time interval or Phase Shift is achieved using a Capacitor. If a single coil alone were used this would produce an oscillating, but not rotating field.

A Rotor placed within the stator would simply sit and hum, as it jerked back and forth across the balance point of oscillation in tune with the supply frequency, no rotation will occur. However, by introducing a much smaller Start coil made to reach it’s peak at a different time to the main drive, or Run coil, by diverting it’s supply via a capacitor, a slight aberration is made to the otherwise oscillating field. Due to the relatively rapid oscillation the aberration effectively produces an overall field remarkably circular, and this produces the rotation required. Start Up The aberration required to maintain motion is fairly small, however to achieve initial rotation the stiction of the bearings and moving parts has to be overcome. This is normally done by using a far bigger capacitor to produce a larger and more powerful aberration. If the Start coil were exposed to this continuously it would need to be far stronger, however by using a switching arrangement to allow a temporary connection only of a bigger capacitor, a minimal size Start coil can be used for both Starting and normal running. Further details of connections and checks for 1?? motors is listed in Chapter 10. Capacitor Function Electrical supplies are subject to three major resistive effects in circulation around any circuit … Resistive Effect This is the natural effect of resistance to current flow due to impurities in the conductor, artificial loading etc. Lighting and direct heating circuits use resistive effect. Resistive effect has no effect on the relation of voltage and current flow. Inductive Effect Induction is caused when an AC supply generates eddy currents within a coil group which tends to oppose the normal current flow through the coil.

Inductive effect causes the voltage and current to move out of phase relative to the supply. An inductive supply is where the Current lags behind the Voltage. (see also Power Factor below). Capacitive Effect Capacitors have the opposite effect upon an electrical supply. They cause the phase difference in the supply that the current will lead the voltage. The overall effect of all three effects is known as impedance. Due to the major use of electrical generation to be in coil bearing induction motors, virtually all high power supplies tend to inductive. Power Factor This is a measurement of the amount of Phase or time difference between the Current and Voltage. The figure is the Cosine of the degrees of phase shift relative to 1 revolution of the generator. A side effect of poor Power Factor symptomatic of inductive supplies is that more current flows along the conductors than is actually used to drive the machinery and this is costly to the user. Large users often compensate for this by use of Power Factor Correction equipment. These are basically banks of large capacitors arranged to improve the power factor by exploiting the capacitive effect described above.

The foregoing explanations have been kept as brief as is feasible to enable a reasonable explanation of how the electrical supply typically drives motor compressors. For more information on Three Phase Power Supplies please contact us. Motor Starting 3 Phase Motors When a motor is first started the impedance of the circuit is minimal, hence the current drawn from the supply will rise to it’s maximum (Locked Rotor Amps). Typically this will between 5 & 7 times the normal full load current. This requires the local supply to be capable not only of supporting the normal maximum running load, but also that of the inrush. This requires heavier cabling, and a larger supply capacity. Additionally the user may be penalised for drawing higher peak loads, being charged for reaching a higher current threshold for perhaps a whole charging period of perhaps 1 hour, for only a few seconds during start up. In view of this most machines above 10 HP adopt a method of reduced in rush start up.

The four commonest methods are … i. Direct -on- Line This simply means the motor is connected in one movement to the mains supply. In rush will be maximum and equal, albeit briefly to the Locked Rotor Amps. Direct on line motors may be either Delta or Star connected. ii. Star/ Delta In this mode the motor will run normally in the Delta mode. However prior to switching directly to Delta the motor is briefly connected in Star pattern. This means there will be 2 fields in series upon any 2 phases of supply, instead of 1 when in Delta, thus the resistance of the motor is effectively doubled, and this halves the maximum in rush or LRA the motor can pull. Although current is considerably lowered enough impetus is provided to overcome the stiction of the static machine and commence rotation. An external automatic timer is used to control the switch over to Delta after about 2 seconds. iii. Part Wind In this mode the Motor designer splits the motor into 2 separate halves. By using 2 contactors the motor is started first on one part, and then the other. Again an external timer controls the 2nd contactor closing, normally with a 0.75 second delay. iv. Soft Start This is an external device best likened to an electronic dimmer switch. When the contactor closes a thyristor gradually winds up the voltage to the motor of a period of about 5 seconds. This mode may also operate in conjunction with Part-wind motors where the 2nd part-wind is brought on once the soft start has finished ‘Ramping In’ the 1st part. Single Phase Motors PSC Small single phase motors, below 1/3 HP, use a single Capacitor to provide the phase shift described earlier in this section. This is commonly referred to as Permanent Start Capacitor or PSC. CSR Larger 1?? motors require increased starting torque to the motor to overcome the stiction. This is achieved by connecting in parallel with the normal Run capacitor, a much higher value capacitor. By using a suitable relay the capacitor is simply disconnected once the machine runs up to speed. Control is achieved by measuring the voltage across the run capacitor.

The confusing point of both Start & Run capacitors is that they both operate upon the Start winding. The Run winding connects directly to the mains supply all the time the machine is running. 3 ? Motor Faults Single Phasing Over Voltage Under Voltage Mechanical Overload Mechanical Damage Contamination Damage.