By MARCONI
This think-piece addresses a possible cause for flickering incandescent lights when the supply has relatively high impedance, perhaps due to length, high resistance connections or resistivity of conductors – the situation existing I suspect for the OP’s home.
I am considering what would be the effect on any running loaded induction motors either in the OP’s home or without, not necessarily on the same phase, when there is a step change in energising voltage caused by a load being turned on or off – a surge/swell or a sag/dip. I know there are many other variations of the mains voltage which could be considered but this think-piece only covers steps up or down.
What puzzled me is how a step change in voltage/current downwards led to a brief oscillatory variation in mains voltage which caused the incandescent light to flicker at low frequency. Obviously, the lights change their intensity in such a way to flicker because the current through them changes because in turn the mains voltage changes in a decaying oscillatory way flowing the step change in load – the kettle being turned on for instance. The mains voltage changes because of increased or decreased current flowing through the supply conductors and some of the household wiring.
I reckon I can safely assume Darkwood and Mr Lunes are schooled in much of the theory of how a synchronous or asynchronous induction motor(IM) works. So, I can use terms they are familiar with.
In my first post I wondered where there are sub-mains frequency emfs which might cause decaying oscillatory low frequency currents to be impressed upon the wiring network (within and without the house) stimulated by a step-change in mains voltage following a step change in load. They exist in the spinning rotor of an induction motor. The frequency of the current in the rotor coils is slip x 50Hz. When an induction is run up and slip is small the rotor currents are of the order of low Hertz.
Let’s talk a bit more on slip. In a synchronous induction motor, which requires external excitation of the rotor and a starting only pony motor, the stator field and the rotor field rotate in step at the synchronous frequency – determined by the frequency of the ac and the number of poles, etcetera. There is no relative velocity between them. However there is a relative angular displacement between the stator’s rotating flux vector and the rotor flux vector – not much – but enough to create a rotating force interacting between the fields, and a rotating force is of course a torque.
So for a synchronous induction motor to generate a torque to drive a load connected to its shaft a displacement must exist and it gets larger as the torque increases.
As a thought experiment imagine two North-South bar magnets, each on their own spindle brought together but not quite touching with the spindles in line. The magnets will orientate so opposite poles are adjacent. Hold one spindle still and try turning the other spindle and one would feel the force of interaction. It’s the same when the spindles are rotating at the same speed. The physics is of course that forces moved through distance creates mechanical work. For the left spindle to do work on the right spindle it must have an angular displacement ahead in the direction of rotation or lead. For the right spindle to do work on the left spindle, the direction of rotation remaining unchanged, then the right spindle must lead on the left or the left lag on the right. When work is done energy is moved from on place or form to another. In the first case the energy transfer is left to right and the second case right to left.
Now to the asynchronous induction motor(AIM). Instead of a permanent magnet or dc electromagnet to create the rotor field it relies on the transformer effect of coupling between the changing magnetic field of the energised stator and the closed loop conductors of the rotor to excite the rotor magnetic field through induced currents in those conductors. Fleming’s left hand rule again and the rotating stator field exerts a torque on the rotor causing it to accelerate in angular speed until the resultant torque on becomes zero. A steady state condition.
In the steady state of an AIM, the relative angular velocity between the rotating stator and rotor fields, necessary to the induced currents in the rotor by Fleming’s Right Hand rule, means the mechanics of the rotor revolves at a lower speed than the stator field ; at a speed of (1-s) x Ns where Ns is the synchronous speed.
One needs to bear in mind too, that the changing displacement between the mechanical rotor and the stator field induces a current and thence a magnetic field whose flux vector is rotating around the rotor, with respect to the mechanical rotor at a speed of s x Ns. But the mechanical rotor is revolving at (1-s)Ns so the speed of rotation of the rotor field with respect to the windings of the stator field is
{(1-s) x Ns} + {s x Ns} = Ns. Lo and behold the two flux vectors rotate at the same speed and in step – no slippage. There is though, as explained before an angular displacement between them – which I will call D. So even though there is slippage in an AIM there is still synchronism between the flux vectors.
A little more physics I am afraid but stay with me please.
Real mechanical systems have friction, elasticity and inertia. In combination these determine the way a system responds to a driving force/torque. When all elasticity and inertia are present the system will oscillate in response to step change in force. If friction is present as well these oscillations will decay away. A motor connected driving a load through its shaft is such a system – the motor provides both the driving force/torque and contributes mechanically with the load to produce a harmonic oscillator. When there is a step change in torque the system will oscillate. How so if the stator and rotor fields are in synchronism?
The answer lies in the necessary angular displacement between the two fields, the flux vector phase angle D – remember the spindle bar magnet thought experiment earlier – to produce a force/torque between fields rotating in synchronism?
The final strait. When there is a step change in voltage let’s say down, the driving torque produced by the motor, which is proportional to Vsquared, decreases. Thus the twisting of the driving shaft and or stretching of any belts or chains lessens. What happens to D depends now on the relative inertia of the load and rotor. In the case when motor rotor inertia is higher than the loads, the angular deceleration of the rotor is slower than that of the load and D remains positive and the motor remains motoring. On the other hand, if the inertia of the load is higher than the motor – say a large fan – the motor attempts to decelerate faster than the load and the displacement angle D may approach zero and even become negative for a brief period. The interaction between the stator and rotor fluxes acts like a form of elasticity – magnetic elastic. When D is zero there is no torque applied to the load. When D is less than zero – the rotor field leads the stator field. In this situation the torque reverses in direction as the motor acts to decelerate the load. This is so-called regenerative braking. The rotor magnetic field is doing work on the stator field – the energy transfer is from the load to stator magnetic field. The motor is being driven by the load and acting as a generator.
In response then to the step change in voltage down, there is a consequent step change in shaft torque, which when subjected to an inertia, mass, friction system excites a decaying oscillatory accelerations one way and the other in the shaft. The displacement angle D also oscillates and decays to a new steady state where D is lagging once more and the motor is motoring.
The oscillation of the displacement of the flux vectors causes low frequency emfs and thus currents in the stator and rotor coils. Those emfs/currents in the stator windings are impressed on the supply network, and because they are of low frequency any inductive reactance in series with these currents present little impedance. Low frequency emfs/currents circulating in an already loaded and resistive network cause oscillatory variations in voltage drop and thence low frequency flickering of lights.
The oscillation of the angle D is sometimes called hunting or phase swinging and occurs in alternators too.
Actually of course the situation is even more complex because the Voltage- torque – speed characteristics of the motor interact with the kinematics of the load neither of which are linear.
For completeness – when slippage is less than zero the motor is acting as a regenerative brake. When slippage is between zero and one the motor is motoring. When slippage is greater than one the motor is generating.
This is the gist of what was going through my mind when I wrote my post.
I cannot write more now because my wife is giving me grief!
