9inch grinder on domestic power supply

[Mikefromlondon]

Let me also butt in, a 2Kw kettle and a 2Kw grinder are two different things, all appliances that run on electric motors possess so called inductance as opposed to resistance, we call it inductance because its effective resistance changes with the speed of the motor, when not powered up the kettle's resistance remains the same or almost same, as Bernard said, may be change very slightly by insignificant amount as the kettle heats up, an electric motor on the other hand possesses a very low resistance when not powered up, so initially when you first apply power to it, the motor is not running so all that power rushes in many times more in magnitude than when it picks up the speed and starts running at normal designed speed, when it does, it also becomes a generator and starts churning out reverse energy, or power but of opposite polarity, which then opposes the incoming power, but incoming power is always stronger, so it still requires energy to spin yet it will only need a little, so at starting point in time a motor may draw as much as 3 times the normal current it requires but when it has gained acceleration and stabilised it will draw its normal designed current or in other words a 2Kw grinder may suck as much as 6kw for a very brief period, and then settle down to 2 Kw normal running, in fact it will draw much less than its designed rating when it is not under any load, so in fact it may actually draw only 200 watts as there is no loading on it but once you put load on it, that is when it would start to draw more and more current up to its designed rating, and it would continue to exceed its designed power rating if you then subject it to a further loading, so if a grinder was designed to cut a concrete slab 2" thick and it is then using 2kw, but if you now start cutting a 4" slab it will be now subjected to a lot more loading so its speed will drop and its back EMF will reduce so it will not oppose the incoming current as much as it did before hence the grinder may start to draw 3Kw, and since it is designed to tolerate (its internal windings are of a certain size and tolerate a certain amount of heat) so that grinder will eventually cook itself with heat and burn out.

A kettle will draw 2kw irrespective of how much water you put inside, it will just take a little longer when full, and when totally dry it will obviously overheat and burn the element as the element can get red hot and melt.

Bulbs also behave like Inductive loads, but they are not in true sense inductive, because they do not have back emf, but the filament of conventional Tungsten bulbs can draw 10 times as much current on first switching on for a split second , this is the characteristic of tungsten when cold its resistance is very low, or tenth of its normal operating temperature, so once the bulb fully lights up, its resistance will increase, so imagine you have 10 x 100watt bulbs, that is a total load of 1Kw, and on first switch on, they can all start drawing 10 times more, so a load of 10Kw for a split second, but in practice that would not happen since 1.5mm wire would limit inrush current, so no fear of tripping any MCBs.

 

[^woody^]

Surely the initial draw of power from the off state to the on state is the greatest draw for either a kettle or a grinder?

A grinder can't go faster as load is applied and so it's power input can't be greater than it's stated power consumption.

The OP's question relates to the initial turning on of the grinder tripping the power. ie the initial rush of power from the off to the on state - same as a kettle. That contradicts your assertion of power draw increasing under load for a grinder.

 

[bernardgreen]

Usually an induction motor draws 5 to 7 times its rated current during starting before the speed builds up and the current is modified by the back EMF. In wound rotor motors the starting current can be limited by increasing the resistance in series with the rotor windings.

In squirrel cage designs, electronic control systems are used to control the current to prevent damage to the motor or to its power supply.

Even with current control the motor can still overheat because, although the current can be limited, the speed build up is slower and the inrush current, though reduced, is maintained for a longer period.

 

[Mikefromlondon]

Surely the initial draw of power from the off state to the on state is the greatest draw for either a kettle or a grinder?

I don't mind explaining it further if you can absorb it.

No, it is greatest ONLY for a GRINDER as it uses an ELECTRIC MOTOR, (so the key thing to remember is that any electric motor on any appliance can draws anything between 2 to 4 times as much current on starting up - for just under a second ( time and inrush current depends on motor design, Edit, as Bernard Green stated 5 to 7 times, all depends if it has to run a load as well where it will gain speed slowly) that is because when a motor is in a stationary state or running very slow, it is not generating an opposite current to oppose an inrushing current, this opposite current called back EMF, it will only be present when the motor has started to run, and if this Back EMF was not there initially to oppose or push back some of the inrushing current, then the motor would keep drawing too much in rush current from the mains and get overloaded with too much power and burn up or trip fuses. All motors when running will by law of physics generate back EMF when their windings are perfect, and not shorted out.

As an example, to show you how Back EMF can reduce an inrushing current, I will use a simple maths to demonstrate the effect of Back EMF but i will use just a Resistor for explaining,

Suppose you have a big resistor of 23 Ohms, and you apply 230 volts across it, then using Ohms Law, it will draw 230/23 = 10 amps, and this will give you 230 x 10 = 2300 watts of power being used up by that resistor, and depending on its size, it will start getting very hot, can even become red hot and burn up if its size could not handle all that heat or if it could not dissipate that power,

(Voltage divided by Resistance = current) and Voltage multiplied by current is watts , as well as Voltage squared and divided by Resistance can also give you watts as well.

But suppose you also apply another voltage of opposite polarity to your resistor but only in series (Definitely not in parallel) so if you were to apply 130 volts of opposite polarity, then the net voltage now being applied to your resistor is 230-130 = 100v, so now your resistor will only see 100 volts across it thus it will now be using a lot less power i.e. 100 x 100 /23= 434 watts!

But this opposite voltage can only occur in a electric motor, and not in a kettle, so a kettle will have a resistive element that does not change and Does not produce back EMF, will always use the same current or power at switch on or even after it has been switched on for some time, its power will be constant as long as the input mains voltage remains steady at same level.

The thing to now remember is that why a kettle element cannot produce back EMF is because it is purely a resistive in nature, and is not based on Electromagnetics in nature, only things based on Electromagnetic in nature can produce Back EMF, such as Motors and Solenoid coils, relays and Transformers,

Therefore In a Motor this opposing voltage becomes automatically applied in series in the opposing direction, no matter which way is the motor turning, but its magnitude (voltage) will depend on how fast that motor is turning, the faster it is turning the greater is the Back EMF and if greater is the opposing voltage , lesser will it draw the current






[quote = A grinder can't go faster as load is applied and so it's power input can't be greater than it's stated power consumption.[quote/]

When you apply a load to a free running grinder wheel, it will tend to bind and slow down, causing back EMF to reduce, thus allow more mains current to rush in, which in turn will start to use more current and provide more power to oppose it from binding, or coming to a halt, under all load conditions motors starts to draw more energy from mains automatically, to try and maintain a speed, but there is a limit when it can no longer oppose the binding action if that force is greater than the power motor can generate.

The OP's question relates to the initial turning on of the grinder tripping the power. ie the initial rush of power from the off to the on state - same as a kettle. That contradicts your assertion of power draw increasing under load for a grinder.

Again, see my answer above, when you apply a voltage to an electric motor, it will start reacting to applied power (voltage & current), drawing much more current initially ( 5 - 7 times for a few milliseconds to a few seconds depending on motor and loading on it) as it starts to spin from scratch, until it has reached full load free speed, so say on a 2Kw rated grinder it may start revving up to say 3000rpm with no load, having drawn 7 times its rated current for just under a second, and after that since there isn't yet any mechanical loading on that grinder and it is running load free, it will just use enough energy from the mains to maintain that load free speed, so the energy a 2kw grinder is using will be a few hundred watts only , just to keep it running, but when you start cutting something it will start slowing down and cause the back EMF to drop, which in turn will allow more mains current to flow in to try and maintain that speed, thereby increasing its power to continue to run, and finally if a 2kw grinder is designed to run at 2000rpm at full load, that is when it would be using its ideal or designed maximum rated power of 2kw, and if you now put more pressure on it and by doing this you reduce its speed to say 1000 rpm, now the Back EMF would drop down even further, this will cause it to start drawing more and more current from the mains to flow into it, whereby it may suddenly start to draw 3Kw, or even more, and a motor designed for a 2kw load could easily get overloaded by a 3kw power being pushed into it, and burn out.

But anyways, all electric motors draw far more current at initial switch on point, to the extent that OP's MCB trips up, one way to avoid this is to use a different class of MCB, or what might help is a long extension lead, as the lead can act as a current limit resistor thus limiting peak in rushing currents to well below the MCB tripping point, even an MCB rated at say 16A may take a fair bit of overload for a few seconds before it trips.[/quote]

 

[^woody^]

Ok, points awarded for effort

 

[Mikefromlondon]

Thanks much obliged,