In a Permanent Magnet motor a coil of wire (called the armature) is arranged in the magnetic field of a permanent magnet in such a way that it rotates when a current is passed through it. Now, when a coil of wire is moving in a magnetic field a voltage is induced in the coil - so the current (which is caused by applying a voltage to the coil) causes the armature to rotate and so generate a voltage. It is the nature of cause and effect in physics that the effect tends to cancel the cause, so the induced voltage tends to cancel out the applied voltage (indeed were the effects to add, we should have a perpetual motion machine!).
Voltage is electrical pressure. Current is electrical flow. Pressure tends to cause movement, or flow so an electrical pressure is a force which moves electricity - or an 'electromotive force' (EMF). The induced voltage caused by the armature's movement is a 'back EMF' - 'back' because it tends to cancel out the applied voltage so that the actual voltage (pressure) across the armature is the difference between the applied voltage and the back EMF.
Voltage is electrical pressure. Current is electrical flow. Pressure tends to cause movement, or flow so an electrical pressure is a force which moves electricity - or an 'electromotive force' (EMF). The induced voltage caused by the armature's movement is a 'back EMF' - 'back' because it tends to cancel out the applied voltage so that the actual voltage (pressure) across the armature is the difference between the applied voltage and the back EMF.
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Last Updated: June 16, 2007
Last Updated: June 16, 2007
What are Permanent Magnet Motors?
The value of the back emf is determined by the speed of rotation and the strength of the magnet(s) such that if the magnet is strong the back emf increases and if the speed increases, so too does the back emf. It follows from this that if you use a weaker magnet to make a particular motor, you will get a higher speed motor!
If you apply now a load to the armature, it will slow down. The back emf will decrease so the difference between applied voltage and back emf will increase. It is this difference that causes the current in the armature to flow - so the current will increase as you increase the mechanical loading. It should be apparent therefore that an unloaded motor will take little current. It should also be clear that if you apply more voltage the motor will speed up, apply less and it will slow: this is what the motor speed controller does: it varies the voltage applied to the motor.
If you apply now a load to the armature, it will slow down. The back emf will decrease so the difference between applied voltage and back emf will increase. It is this difference that causes the current in the armature to flow - so the current will increase as you increase the mechanical loading. It should be apparent therefore that an unloaded motor will take little current. It should also be clear that if you apply more voltage the motor will speed up, apply less and it will slow: this is what the motor speed controller does: it varies the voltage applied to the motor.
