Lesson 7 - Electricity SAQ

** a) Why do motors have a large initial current?

b) What is the difference between a soft starter and a variable speed drive?

c) For the following applications, recommend ways in which the maximum demand can be reduced.

i)     Five water pumps supplying water to the plant.

ii)    A large agitator that only runs at one speed.

iii)  A filter pump on a line that has a control valve on it.

Response:

a)    Motors have a large initial current because the equipment attached to it has inertia and this needs to be accelerated until it reaches operating speed.

b)    A soft starter is a device that reduces the voltage at start-up and shutdown. It operates in a fixed manner over a fixed time. The motor then runs at full voltage for the rest of the time. A variable speed drive adjusts the voltage and frequency output to a device or motor. It can be used to ramp a motor up or down slowly according to parameters. This allows for more control and flexibility. It can also be used to control the output to a device all the time.

c)    To reduce maximum demand:

i)     The pumps can be sequenced to start at intervals so that only one pump is started at a time. For very large systems, one or more pumps might be running (and pressurising the system) constantly, with others being brought on line sequentially as demand increases.

ii)     The agitator can be fitted with a soft starter.

iii)     The filter pump can be fitted with a variable speed drive and the control valve potentially even removed.