7.15.1 - Rotating machines

The vast majority of motors used in industry are of the three-phase squirrel-cage induction type. Smaller motors are usually single-phase induction machines. Induction motors have important advantages, such as robustness, minimal maintenance needs, and self starting characteristics, but all draw very high starting currents from their supplies {Fig 7.23}. This starting current is a short-lived transient, and may usually be ignored when calculating cable sizes.

Fig 7.23 - Starting current of an induction motor

Although the starting current may be several times the running current, the value depending on the machine characteristics and the connected mechanical load, its short duration will not lead to overheating in usual circumstances. If frequent starting is a requirement, larger supply cables may be necessary to avoid damage to insulation. A problem could arise when fast-acting fuses or circuit breakers are used for short-circuit protection; the high starting current may result in operation of the protective device. A common, but unsatisfactory, remedy for this difficulty is to increase the rating of the protective device, leading to a loss of proper overload protection. A possible solution is to use dual rated fuses (gM types). For example, a 25M40 fuse has a continuous rating of 25 A and the operating characteristics of a 40 A fuse.

A word is necessary concerning motor ratings. Many years ago it was decided to replace the horsepower as the unit of output power with the kilowatt. Unfortunately, the old horsepower is a very long time in dying. Many machines still have rating plates giving output power in horsepower. The conversion is straightforward. Since one horsepower is the same as 746 W, horsepower is converted to kilowatts by multiplying by 0.746.

It is sometimes practice to stop a motor very quickly by feeding it with a reverse current. When this method is provided it is important that the machine does not begin to move in the reverse direction if this would cause danger.

Where other types of motor, such as wound rotor and commutator induction or thyristor fed dc types are used, the cables must be suitable for carrying running currents on full load, which will usually mean that they are large enough to carry the short duration starting currents.

Every motor rated at 0.37 kW (0.5 horsepower) or more must be fed from a starter which includes overload protection. Such devices have time-delay features so that they will not trip as a result of high starting current, but will do so in the event of a small but prolonged overload. They have the advantages over fuses and single-pole circuit breakers that all three lines of a three-phase system are tripped by an overload in any one of them. If only one line were broken, the resulting 'single-phasing' operation of the motor could cause it to overheat.

It is often necessary to provide a means to prevent automatic restarting after failure of the supply. For example, if the supply to a machine shop fails, the machine operators are likely to use the enforced break in production to clean and service their machines. If so, when the supply is restored, the presence of hands, brushes, tools, etc. in the machines when they automatically restart would cause serious danger. The necessary 'no-volt protection' is obtained by using a starter of the type whose circuit is shown in {Fig 7.24}. The coil is fed through the 'hold-in' contacts, which open when the supply fails; the motor can then be operated only by pressing the 'start' button. This requirement does not apply to protected motors which are required to restart automatically after mains failure. Examples are motors supplying refrigeration and pumping plants. It is important that lock-off stop buttons are not used as a means of isolation.

Fig 7.24 - Direct-on-line starter