Reversing Motor Starters

Question 1

What does any basic reversing motor starter consist of?

Answer 1

Two contactors and an overload device.

Question 2

How would someone reverse the direction of a three phase squirrel cage induction motor?

Answer 2

Interchange any two of the three phases.

Nema standards interchange lines 1 and 3.

Question 3

In a reversing motor starter, what are the holding contact terminal numbers:
For a NEMA starter?
For an IEC starter?

Answer 3

2 and 3 for forward, 4 and 5 for reverse.

13 and 14 for both.

Question 4

How would one prevent a dangerous short between line 1 and line 3 on a reversing motor starter?

Answer 4

By the use of a mechanical interlock, allowing only one armature to seal in at a time.

Question 5

Why do reversing motor starters use electrical interlocks? How is this achieved?

Answer 5

To prevent coil energization, in the event that the opposite coil is already energized. It is achieved by placing a normally closed contact of the opposite coil, in series with the original coil.

Question 6

There are seven circuit sections in which to place interlock contacts in a reversing motor starter, what are they?

Answer 6

General stop.
Forward stop.
Reverse stop.
Forward start.
Forward holding.
Reverse start.
Reverse holding.

Question 7

What is the purpose of pushbutton interlocking?

Answer 7

To allow the change in rotation of the motor with the use of one button, and to not require a general stop first.

Question 8

How many physical pushbuttons are present in a general reversing motor starter circuit, utilizing push button interlock? How are these labelled?

Answer 8

There are three only. The “Stop” button, “Fwd start” button, and the “Rev start” button.