Electric motors, the machines that makes the world go around

The AC electric motor is the one device that can be classed as making the world go around in our modern society. Everything from generating the actual power, to running air conditioning, pressurising your mains water, pumping away sewerage, operating factories, mines and industrial plants and even now running your car. All of this is simply not possible without AC electric motors.

What is an electric motor?

The motor itself can vary in size from tiny 0.25kw single phase 240V up to high voltage (over 6000V) 3 phase units with power outputs of over 2000kw. It doesn’t matter the size or voltage, they all operate on the same basic concept of an electromagnet being created that spins its field around inside the motor housing and “drags” the rotor of the motor around to make the motor spin.  

Now this electromagnet we are describing here is in the windings in an electric motor. So an electric motor is a complex little device that operates using some very simple principles. For all the examples here we are talking about the most common motor that run most small pumps, this runs off AC 50Hz 240V single phase power.

AC power supply, how does that work?

What this means is the power supply:

• Alternates from positive to negative voltage 50 times a second.
• Single phase means the supply has one active wire and a neutral return wire.
• The voltage between the active and the neutral is 240V.

AC power is really great for making a motor rotate because it alternates its voltage from positive to negative constantly, unlike DC power which the positive and the negative stay in the same place constantly.

So an electric motor uses this power supply through its windings, a winding is literally a coil of copper wire usually 50-200 wraps which is fixed into the inside of the motor housing. In most motors there is actually a pair of these windings and they are mounted on opposite sides of the housing. These windings when energised make an electromagnetic field and as there are 2 of them and they are arranged to mirror each other they form 2 poles of a magnet. Therefore a single winding motor is called a 2 pole motor and this is the most common type of motor, you can also get 4, 6 and 8 pole motors but these are more uncommon and are usually in 3 phase specialised motors.

So these 2 coils when they are energised create an electromagnet. The magnetic field that is created will alternate from positive to negative 50 times per second. Now the motor windings are held in the outer frame of the motor which is usually steel or aluminium. As it is stationary this part is called the stator. There is a tubular shaped gap in the middle of the stator and inside this is another steel and copper/ aluminium cylindrical shaped device. This is the rotor because it is held in place with some bearings and it rotates. Set into the rotor are thick bars of a conducting metal (usually aluminium or copper) shaped like a little cage, the nickname for this contraption is a squirrel cage. When the main winding creates a magnetic field this inducts an electrical current into the cage bars in the rotor. These currents run through the cage bars and create a further electromagnetic field of their own. The electric motor turns by these 2 magnetic fields reacting with each other and either attracting or repelling at different points to push and pull the shaft around in a circle.

Now with 3 phase motors they start on their own as the 3 sets of windings can pull the rotor to start spinning. With a single phase motor, there is not enough force to start the spinning with 1 pair of poles. This is where the capacitor comes in to make the magic happen.

What a capacitor actually does in a motor.

So when a single phase motor needs to start it needs a little mechanism to provide a little kick start to get the shaft spinning. This is done by putting in a second winding called the start winding, which is not as heavy duty as the running winding, as it only needs to do work for a few seconds. The run winding is installed in the motor 90 degrees (1/4 turn) around from the run winding. Its purpose is to create a pull force to drag the rotor around 90 degrees so when the run winding reverses polarity as the AC current oscillates, the rotor can then be pulled initially to 180 degrees and then to 270 degrees and so forth. The momentum created by this and the oscillating magnetic field of the run winding is then able to sustain the motor rotating.

So now how does this start winding and the capacitor work? Well when power is applied to the windings they both create a magnetic field in the same way and at the same time. If they were both built the same this would mean that nothing really happens other than some buzzing when you turn the power on. The magic happens when you connect a large capacitor into the circuit with the starter winding. If you remember back to the start of the blog on capacitors, a capacitor has the effect on an electrical sine wave of advancing the current rise and fall slightly ahead of the voltage. This makes it a current leading power factor effect which is normal for a “capacitive” load.

As the run winding is wound from heavy low resistance wires and has lots of turns in its coil it creates a really strong inductor. When you power this circuit up, as it has a very strong “inductive” load, then you get the opposite effect where the current lags the voltage by a little bit of time.

Now the thing to remember with an electromagnet is that the magnetic field is created by the current flowing through. So when the start and run windings are now powered up you have a start winding that is creating a strong magnetic current very slightly ahead in time of the run winding. As the run winding has also started inducting current and magnetic field into the rotor on the previous oscillation of AC current, then the start winding is able to grab the rotor before the run winding and basically pull start the spinning. This is basically the electrical version of you using a pull start on a lawnmower or a kick start on a motorbike. It gets the shaft spinning enough for the main part of the motor to be able to then sustain the shaft spinning.

Now the motor is spinning up to speed, then a switch sometimes a mechanical centrifugal switch or more commonly these day an electrical relay cuts out the start winding (or sometimes just the large capacitor) at a shaft speed of about 70-80% of maximum. This is because when the large capacitor is activated in the start winding and the motor is at maximum speed, this capacitive effect of advancing the current can actually cause the magnetic fields to “push” against the shaft and slow it down. This makes the motor produce less power, run very hot and a lot use a lot more amps and amps are what you pay for when the meter box is spinning.

Now all single winding (2 pole ) motors have a maximum speed. This speed is created by the frequency of the AC power, that is if the power is 50Hz or 50  oscillations a second then 50 x 60 seconds is 3000 so the motor can run at 3000rpm. No motors actually run at 3000rpm as there are things like friction and power factor that cause inefficiencies, so most motors run at between 2800rpm and 2900rpm.

When a motor accelerates up to this maximum speed, it gets to what is called the “synchronous speed” that is the rotating magnetic field created by the stator has pulled the rotor magnetic fields up to speed so they are matching. At this point there is very little torque created by the motor as it is gliding. An electric motor only creates the torque (the moving force) when the rotor slows down slightly by having a load placed on it and then the magnetic fields once again start being “pulled” around and this creates the torque to move the load.

It is from this strange effect that all pumps, air conditioners, cordless tools, even electric cars are able to do the work they do today. From some accounts, electric motors use upwards of 40% of all worldwide electricity demand. So the motor and the humble capacitor literally do make the world go around. 

While we don’t sell motors in our webshop, we do sell capacitors and bearings to help repair your pump motor when it breaks down. If there is a part you need (including electric motors) then send us a message or give us a call and we can locate it for you.