| | | Picture 5.15 As the motor spins, there are sparks at the commutator. These occur whenever the circuit is broken (i.e. the contact between the commutator and the brushes breaks). |
|  | Keeping the belt discharged | | | We have seen (page 24) that we can discharge a capacitor (like the belt) through a resistor. The time it takes to discharge is measured by the time constant, which is equal to the product of the resistance and capacitance. To reduce the chance of sparking, we need to keep the resistance of the belt to a minimum. So the belts are made of a special rubber that is impregnated with carbon. This means that the belts remain flexible but are quite good electrical conductors. So, even though they are rubbing against the mountings, they won’t charge up because any charge can be conducted safely away through the belts. | | | |
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| | 2. Inductive sparking | | | You may have noticed that when you switch off a light, there is a small spark in the switch. This is due to the inductance of the lighting circuit. When the circuit is broken, the inductance tries to keep the current flowing and produces a large voltage across the switch. The voltage is enough to cause a spark. The same effect will cause sparks at the brushes of a normal electric motor. As the motor is turning, the circuit is constantly broken (and reconnected) as the commutator and brushes swap the connections over. Every time it is broken, there will be a spark. The motors in a gassing room need to be made explosion proof. | | | |
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 | | Picture 5.16 The earth wire is bonded to the ground. |
| | | | Earthing | | You will often hear of electrical connections being connected to earth, earthed or grounded. This is, literally, making a good connection with the Earth. Often, at some point an earth connection is bonded to a metal pole that is driven into the ground. The reason that this is effective is that the Earth itself doesn’t really charge up. So its electrical potential is constant – often taken as zero. There are two reasons its potential doesn’t change: - it has such a big capacitance that any charge that builds up has no noticeable effect
- on average, there is as much charge flowing onto it as off it.
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| | | | Explosion proof motors | | | Motors (and other electrical equipment) are made explosion proof by sealing them in a special casing. The casing has to be strong enough to withstand any combustion of gases that get inside it. And it has to prevent the transmission of the explosion to the surrounding atmosphere. This would happen if hot gases escaped after they had been ignited, setting the surrounding gases on fire. | | | |
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