Motor Starting

Making life easier to live sometimes has the need for rather large motors. What comes to mind is those used in assisting the removal of all our man-made rubbish. Getting these motors to start turning is a little more complex than simply "throwing the switch". In some cases the use of motors actually becomes a matter of sustaining life, but this still doesn't remove the problem of getting them going in an orderly manner.

One such motor that comes to mind was a compressor on a mine that pumped 10m³ per second at a pressure of 7 atmosphere. This motor used quite a few kW and could not simply be energised without having dire consequences on the power as a result of start currents. The more interesting part, however, was the starter system.

This consisted of 3 large metal plates that would be slowly dipped into 3 awaiting tanks of water. What struck me, as the plates struck the water and sent sparks flying in all directions, was the amount and level of transients that must have made their way on to the power. Here we had some rather large inductors being subjected to intermittent power - the back-EMF must have been enormous!

Ok, so this motor never got switched off much (meaning this starting did not happen much either), but it was probably not cast a thought whenever there was a failure of some sensitive electronic equipment that it may have coincided with the starting of this compressor. Next time I investigate electronic equipment failure at a mine it will be one of the first questions I ask them is whether there are any water starters on site.

Agreed, things have become a little more sophisticated now. In modern motor starter systems the motor is now switched from star to delta with resistors to buffer the actual switching, but even this switching can still produce some fantastic back-EMF surges which will translate into transients on the power network.

Motor Starts and Flicker:

Maybe the starting is dealt with, and the transients kept under control. There is one other aspect that is often overlooked in some installations being inrush, and then further multiplied by starting frequency.

Of start-stop operations, the worst is start. With motors having possibly as high as 50:1 inrush it comes as no surprise that the lights dip when the contactor slams shut. Ok, agreed, the inrush quoted is for small motors. Bigger ones tend to be closer to 20:1 and even as small as 10:1. But, lest it be forgotten, we are dealing with much higher run currents. Even as low as 100A running could mean 2kA startup - and therefore even more ability to dip the lights!

On big motors the problem does not stop at just turning the motor on. There are two other issues that can protract the flicker caused while bringing the beast from rest to full run. These motors are often started with the windings wired in star formation and then, after a predetermined time or speed, switched to delta formation. So no longer is it one dip every time the motor is started, but two!

The second start-up issue with big motors is that while they are coming up to speed the motor goes through alternate phases of being a motor and a generator (finally being a motor when it is synchronised with the mains). This effect causes a heterodyne with the incoming mains and manifests as a flicker that is fast to start with becoming slower until the generator phase becomes insignificant.

One situation keeps cropping up with motors - pump stations and flicker. For some reason there is a serious reluctance to use motor speed controllers to control the flow of a pump. The desired method is to turn the pump on and off frequently to regulate the pump's throughput. If the feed to the pump is shared by another user, and is in any way weak, then the user is going to be plagued with changing lamp brilliance every time the pump is turned on or off.

But this is not just on big motors. It is on little ones too. One case involved a poor farmer who could not tolerate the flicker any longer and called in the complaints department. It was traced to an airconditioner unit used in a cellphone base station at the end of a rural line. Before the aircon was installed, the cellphone company approached the electricity supply company and asked if the extra load was allowed. Calculations were done and all appeared well.

Although someone may have taken into account that the aircon has an inrush-running current ratio of between 20 and 30, what was forgotten was the aircon would start and stop frequently as the space it was cooling was only a tiny shed.

As demonstrated, it is not just the fact that motors want a lot more from the supply to get going, it is the the fact that sometimes the user wants them to get going a little too often!


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© 25.08.03