In easy to understand terms why do motors have capacitors and how do they work to make that motor turn>?
capacitors
- Thread starter markymark2406
- Start date
All single phase motors have to be started and have whats called a start winding.
A capacitor is used to start the motor, it is inline with the start winding inside the motor, dependant on size of the motor there may be a means of switching the capacitor out of circuit once the motor is up and running this can be achieved by means of a centrifugal switch.
How the Capacitor makes the motor turn and therefore start is achieved because the Capacitor causes the current to lead in the starter winding compared to the current through the main winding, this generates a rotating magnetic field that causes the rotor to turn.( you need to understand the relationship between the phases and how capacitance and inductance affect AC circuits)
This has to be done with single phase motors as they cant achieve the rotating field without additional components, three phase motors have their individual phase windings wired at different angles in order to achieve a rotating magnetic field and therefore do not need a start winding.
I don't suggest you do this but you can start a single phase motor that doesn't have any starting components( ie Capacitor removed) by hand by rotating the rotor shaft(in the right direction)and once moving it will then start spinning.
A capacitor is used to start the motor, it is inline with the start winding inside the motor, dependant on size of the motor there may be a means of switching the capacitor out of circuit once the motor is up and running this can be achieved by means of a centrifugal switch.
How the Capacitor makes the motor turn and therefore start is achieved because the Capacitor causes the current to lead in the starter winding compared to the current through the main winding, this generates a rotating magnetic field that causes the rotor to turn.( you need to understand the relationship between the phases and how capacitance and inductance affect AC circuits)
This has to be done with single phase motors as they cant achieve the rotating field without additional components, three phase motors have their individual phase windings wired at different angles in order to achieve a rotating magnetic field and therefore do not need a start winding.
I don't suggest you do this but you can start a single phase motor that doesn't have any starting components( ie Capacitor removed) by hand by rotating the rotor shaft(in the right direction)and once moving it will then start spinning.
A single phase supply can only generate an oscillating magnetic field, not a rotating one. Imagine it like a weight on a piece of string. You can hold the end of the string and swing it from side to side like a pendulum, but it won't whirl round.
However, if you can give the string a bit of a prod away from you as the pendulum reaches the end of it's swing, it will start to go round in a circle. Once it has started going round in a circle, a side to side movement is all that is necessary to keep it whirling round.
The capacitor and second field winding are what give the rotor in a single phase motor the 'prod' to set it spinning initially. Once the rotor is spinning it is no longer needed, and may be switched off by a centrifugal switch on the rotor.
If the connections to the start winding are reversed, then the motor will run in the opposite direction.
However, if you can give the string a bit of a prod away from you as the pendulum reaches the end of it's swing, it will start to go round in a circle. Once it has started going round in a circle, a side to side movement is all that is necessary to keep it whirling round.
The capacitor and second field winding are what give the rotor in a single phase motor the 'prod' to set it spinning initially. Once the rotor is spinning it is no longer needed, and may be switched off by a centrifugal switch on the rotor.
If the connections to the start winding are reversed, then the motor will run in the opposite direction.
There are three main ways of making a motor that will run on single phase.
One is to use a capacitor as tim suggests. This tends to be more common on larger motors.
Another is to do some magnetic tricks to make the two sides of eash pole magnetise at different speeds resulting in what is known as the shaded pole motor. Theese are often used for fans.
The final way is to use what is known of a universal motor which is basically an adaptation of DC motor principles to AC systems. Theese are generally used where high speeds are desired (mainly power tools)
One is to use a capacitor as tim suggests. This tends to be more common on larger motors.
Another is to do some magnetic tricks to make the two sides of eash pole magnetise at different speeds resulting in what is known as the shaded pole motor. Theese are often used for fans.
The final way is to use what is known of a universal motor which is basically an adaptation of DC motor principles to AC systems. Theese are generally used where high speeds are desired (mainly power tools)
As Plugwash says:
Look carefully at one of these motors and you'll see that the pole faces are split and there is a thick copper ring around the smaller part. That small part is the shaded pole. The ring is like a shorted turn on a transformer and its effect is to make the magnetic field passing through it lag behind the rest of the pole.
Look carefully at one of these motors and you'll see that the pole faces are split and there is a thick copper ring around the smaller part. That small part is the shaded pole. The ring is like a shorted turn on a transformer and its effect is to make the magnetic field passing through it lag behind the rest of the pole.
You guys know motors, wow!
Does that mean with 3-phase motors that its inductive starting and how would this work? 180 degree separation. Is the "pushing" taking place from one phase to next phase etc?
Capacitors. They still hold their charge for a short while after power is turned off right, do you just earth it get rid of the charge?
They also interfere with circuit testing, is that right?
Does that mean with 3-phase motors that its inductive starting and how would this work? 180 degree separation. Is the "pushing" taking place from one phase to next phase etc?
Capacitors. They still hold their charge for a short while after power is turned off right, do you just earth it get rid of the charge?
They also interfere with circuit testing, is that right?
Mike2007 said:
The phases are normally 120 degrees apart.If you can imagine that the phases are arranged around the stator (stationary) in windings and are reaching their peak voltage and current values (ignoring power factor) at different times, one after the other, then this sets up a rotating magnetic field (as current in a wire sets up a magnetic field around it)
As these magnetic fields sweep past the rotor they induce a voltage in the rotor windings.These windings are shorted out at the ends of the rotor so a current can flow.This current 'reacts' with the stators magnetic fields and repels, so the rotor is pushed around.
Depending on which order the stator windings voltage rises and falls determines the direction of rotation, which is why direction can be changed by swapping any two supply lines.
Speed is determined by how fast the voltages rise and fall (frequency), and how the windings are wound (no. of poles).This is the speed of the stator fields and is known as the synchronous speed.The rotor needs to travel slightly slower otherwise if it travelled at the same speed as the stator fields then the fields would not 'cut' the rotor windings.The difference between synchronous speed and rotor rpm is known as slip.
When a load is applied to the rotor shaft it has the effect of slowing the rotor so then the slip is bigger so the stator fields cut the rotor windings faster, so more current is induced and more torque is produced to cope with the load.
This is seen as an increase in current on the supply to the motor.That is why a motor draws a larger current on start up as the stator fields are cutting the rotor windings at max speed thus inducing a big current.
The correct way to discharge a capacitor is to use a high ohm resistor (100k-200k) which is permanently connected across the terminals, but there is the old fashioned way with the screw driver
Hope this helps, I got carried away
From Mike2007:
They can hold their charge for a LONG while after power is turned off or, more precisely, after they've been disconnected. That's why high voltage capacitors should always be stored with a shorting link across their terminals. If you ever see one of these brutes lying around with one or more terminals floating free I suggest you give it a wide berth.
As for discharging them, ricicle is right about the resistor (known as a bleed resistor) but the screwdriver trick can be dodgy. I once accidentally shorted a 68mF capacitor with a scope probe. It destroyed the probe and blew six inches of track clean off a PCB - and it was only 25 volts!
Yes, capacitors can interfere with circuit testing, mostly because so many of them don't appear on circuit diagrams. Here's a post from someone who was fooled by a hidden capacitor:
http://www.diynot.com/forums/viewtopic.php?t=101411
A search through old posts will undoubtedly turn up many more.
They can hold their charge for a LONG while after power is turned off or, more precisely, after they've been disconnected. That's why high voltage capacitors should always be stored with a shorting link across their terminals. If you ever see one of these brutes lying around with one or more terminals floating free I suggest you give it a wide berth.
As for discharging them, ricicle is right about the resistor (known as a bleed resistor) but the screwdriver trick can be dodgy. I once accidentally shorted a 68mF capacitor with a scope probe. It destroyed the probe and blew six inches of track clean off a PCB - and it was only 25 volts!
Yes, capacitors can interfere with circuit testing, mostly because so many of them don't appear on circuit diagrams. Here's a post from someone who was fooled by a hidden capacitor:
http://www.diynot.com/forums/viewtopic.php?t=101411
A search through old posts will undoubtedly turn up many more.
Space-cat
I read this bit in your reply to the previous OP but did not see notes on an actual "Capacitor". When I have tested for a dead circuit with my voltage/continuity tester I sometime get poor illumination for voltage (which startled me a great deal). Is this like you say "capacitive coupling" showing power if only a tiny amount, which still stops me touching the cores directly!
As far as capacitors on single phase motors are concerned, they will discharge through the start winding and the field winding quite quickly when the motor is switched off.
Some motor faults can leave the capacitor charged when the supply is isolated.
I had a faulty vacuum cleaner. With the plug removed from the socket, I got a shock from the plug pins when I operated the on/off switch. This provided enough information to correctly identify the fault without further investigation.
What was the fault?
Some motor faults can leave the capacitor charged when the supply is isolated.
I had a faulty vacuum cleaner. With the plug removed from the socket, I got a shock from the plug pins when I operated the on/off switch. This provided enough information to correctly identify the fault without further investigation.
What was the fault?
TicklyT, the main winding is open circuit.
Mike2007, 'capacitors' are everywhere and they don't all look like the ones you find on motors. Lay two pieces of wire on your bench and you have a very small capacitor, so small you will have great difficulty trying to measure it. Lay them side by side along their length and you will have something big enough to measure on a decent meter. Swap your wires for two large plates with a very thin spacer between them and it will be bigger again. You now have something that looks like the symbol for a capacitor. Put another spacer on top and roll them up into a cylinder and you have something that looks like a motor capacitor.
All of these capacitors work in exactly the same way; it's just a matter of geometry. The larger the area and the smaller the gap, the bigger the capacitor. So how do they apparently conduct electricity when there is no connection from one side to the other? The answer is that they don't; you cannot get DC through a capacitor. What they do is store charge. When you put some volts across a capacitor you force electrons into the negative side and suck an equal number of them out of the positive side. The larger the capacitor, the more charge it will accept for a given voltage.
Reverse the voltage and the charge changes sides. Now a movement of charge is what we call current so if you keep reversing the voltage you will see current apparently flowing through an open circuit.
Back to that 'dead' circuit of yours. Somewhere in there you have two wires lying side by side, one live and the other isolated. There's your capacitor. It's very small so even if you short the isolated one to ground you will get very little current. That hypothetical 1nF I mentioned would pass a mere 75 micro amps but this is enough to light a neon or move an old fashioned moving coil meter. It will certainly register on a modern digital multimeter with its 10 megohm input impedance. Now connect a standard light bulb from your 'dead' wire to earth and see what your meter says.
Mike2007, 'capacitors' are everywhere and they don't all look like the ones you find on motors. Lay two pieces of wire on your bench and you have a very small capacitor, so small you will have great difficulty trying to measure it. Lay them side by side along their length and you will have something big enough to measure on a decent meter. Swap your wires for two large plates with a very thin spacer between them and it will be bigger again. You now have something that looks like the symbol for a capacitor. Put another spacer on top and roll them up into a cylinder and you have something that looks like a motor capacitor.
All of these capacitors work in exactly the same way; it's just a matter of geometry. The larger the area and the smaller the gap, the bigger the capacitor. So how do they apparently conduct electricity when there is no connection from one side to the other? The answer is that they don't; you cannot get DC through a capacitor. What they do is store charge. When you put some volts across a capacitor you force electrons into the negative side and suck an equal number of them out of the positive side. The larger the capacitor, the more charge it will accept for a given voltage.
Reverse the voltage and the charge changes sides. Now a movement of charge is what we call current so if you keep reversing the voltage you will see current apparently flowing through an open circuit.
Back to that 'dead' circuit of yours. Somewhere in there you have two wires lying side by side, one live and the other isolated. There's your capacitor. It's very small so even if you short the isolated one to ground you will get very little current. That hypothetical 1nF I mentioned would pass a mere 75 micro amps but this is enough to light a neon or move an old fashioned moving coil meter. It will certainly register on a modern digital multimeter with its 10 megohm input impedance. Now connect a standard light bulb from your 'dead' wire to earth and see what your meter says.
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