Sparks Electrical News July 2019

CONTRACTORS’ CORNER

5

WORKING KNOWLEDGE WITH TERRY MACKENZIE HOY

AN ATTEMPT TO EXPLAIN THREE PHASE POWER AND MOTORS

A bout two years ago I rather grandly wrote that the power drawn by a three phase motor rated at 380 V/400 V was about 1,5 times the rating in kW. I was corrected by a smart electrician in Namibia who pointed out it was more like two times the rating in kW which it is, since I had forgotten to allow for the inefficiency of the motor. All electricians know that there is such a thing as single phase power and that there is such a thing as three phase power and, correspondingly, there are such things as three phase motors and single phase motors. I have never heard of a two phase motor in use although they were used in the USA some time back. Single phase motors have a two wire and earth connection and the two wires create an alternating magnetic field in the field winding, which is located in the static frame of themotor, know as the stator. The rotor of themotor consists of a set of copper bars set into the rotor and connected to a circular bar at each end on the rotor. If you were to remove the rotor

pole motor has three pole pairs, one for each phase. Since the stator induces current in the rotor (also consisting of a squirrel cage winding as above), the rotor always is at a speed slightly less than the rational speed of the magnetic field –if the field is rotating at say, 1 500 rpm, then the rotor will be rotating at about 1480 rpm at full load. The equation for the predicted motor speed is 2 x supply frequency/ number of magnetic poles. Then, for example, for a 50 Hz four pole machine this is 2 x 50 x 60/4 = 1 500 rpm. In general, fan suppliers like to specify two pole machines since they are higher speed and smaller and cheaper and the fan costs much less. Pump suppliers prefer four pole machines to limit surges in the pipe lines. I see I have not really explained three phase power very well but dwelt on motors… But to explain three phase power without diagrams is difficult. So… why not check the internet for further information? There’s a cop out… but oh, much better.

to increase the effect of this. Sometimes the capacitor is in circuit and then cuts out by means of a centrifugal switch. A handy tip is that if a single phase motor won’t start but just vibrates and no ‘electrical burn smell’ is present, it is probably the centrifugal switch. Give the motor a whack with a mallet and it may solve the trick. There is another type of single phase motor called a ‘shaded pole motor’ where the copper bars are sometimes set at an angle in the rotor with a copper ring creating an additional magnet field. This design has low starting torque and is not very efficient. Three phase motors are supplied with three phase power, normally supplied from three wires and sometimes four (three phases and a neutral). Between each of the wires and the neutral, the voltage peaks at different moments which creates a rotating magnetic field. The rotor has similarly a number of windings (called ‘poles’) in which themagnetic field is induced – all of which causes the rotor to spin. Typically a four

SPOT FINDER TEMPERATURE MEASUREMENT DEVICE It happens that if you connect single phase power to the stator then a magnetic field will be induced in the rotor but not much will happen since the field is an alternating field, not a rotating one. This is overcome by using two winding coils which then displace the magnetic field. Often a capacitor is used laminations the copper arrangement would look like a hamster exercise wheel. For some reason this type of motor is known as a ‘squirrel cage motor’ instead of a ‘hamster wheel motor’ (it is perhaps fortunate that this type of motor was not invented in South Africa – we’d have to have a ‘meerkat wiel’ which would be confusing, for sure).

OPTRIS’ two newly developed temperature measurement devices from Instrotech, Xi 80 and Xi 400, combine the benefits of infrared cameras and infrared thermometers. “With the Xi series, we are launching a semi-autonomous sensor which, in addition to traditional PC and software-based operation, also works as a smart pyrometer with analogue or digital outputs that seek the target on its own. This opens up new fields of application, for example, in mechanical engineering or multisensor applications,” explains Torsten Czech, Head of Product Management at Optris. The Xi series is a fusion between a robust, compact pyrometer andanadvanced IRcamera. The integrated spot finder function enables accurate temperature measurements in moving objects without having to readjust the sensor. Sensor adjustment takes place autonomously, i.e. without having to establish a connection with a PC. In addition to a direct 0/4- 20 mA analogue output, the devices also offer users digital interfaces such as Ethernet or RS485. An external processing interface permits the further processing of up to nine freely definable analogue outputs, making it ideal for OEM applications. The new microscope optics for the Xi 400 infrared camera allow reliable temperature measurement on tiny objects from 240 μm. In combination with a suit- able stand, this enables professional measurement of printed circuit boards and components in the elec- tronics industry. Circuit boards are a core part of electronic devices. They keep getting smaller while having to be more powerful at the same time. Temperatures of assembled circuit boards can easily be measured with the micro- scope optics of the Xi 400 thermal imager, thus identi- fying overheated areas quickly and preventing possible defects. The causes for excessive temperatures can be manifold: defective components, incorrectly dimen- sioned circuit paths or poorly soldered joints.

Enquiries: sales@instrotech.co.za

SPARKS ELECTRICAL NEWS

JULY 2019