electrical academy · lesson 2 · about 12 minutes
Starting a motor: normal and heavy duty
A motor draws far more current for a few seconds when it starts. This lesson explains what makes a start heavy and why that changes the circuit.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.
The surge at start
When a motor is switched straight onto the supply, called direct-on-line starting, it can draw about six times its running current while it speeds up.
Normal protection is chosen so that this short surge does not trip it. The motor is up to speed in a few seconds and the current falls to its running value.
What makes a start heavy
- Three things describe a start: how long it takes, how high the current is, and how often it is repeated.
- A start is heavy when it lasts roughly 5 to 30 seconds, or when the current is more than six times running current, or both.
- Heavy loads usually have a lot of spinning mass: large fans, centrifuges and similar machines. Ask the owner of the machine, because the supplier of the motor knows its starting data.
Why it matters for the circuit
A long start heats cables and contacts much more than the running current does.
The overload setting must be raised or delayed so it does not trip during the start, and so the rest of the main circuit must be built stronger: contacts, wiring and protection chosen for the longer, hotter start.
Repeated starts
Every start adds heat. The number of starts per hour is limited, and the limit assumes a standard ambient temperature.
Between starts the cables and switching devices must cool. If the machine needs frequent starts, treat it as a separate design case.
check yourself
Three quick questions
Think of your answer first, then open the question.
Roughly how many times its running current can a motor draw at direct-on-line start?
About six times.
Name two machines with a heavy start.
For example a large fan and a centrifuge.
Why can the overload setting not simply stay as it is for a heavy start?
It would trip during the start, so protection and the main circuit must be re-chosen for it.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.