# Connecting 3 phase ac motor

As far as I understand in delta connection 3 lines and earth is connected to the ac motor. But if wye connection should one use the neutral or earth to the motor?

The centre-point of a star connection will be at zero volts when a load is perfectly balanced. With mixed loads (like a whole building), you cannot hope that the load will always be balanced, because there may be lots of single-phase loads which are being switched on and off at various times, and you care very much about your phase voltages being constant, so you have a neutral.

With a motor this is not the case - nothing[1] is going to cause the centre point to be at anything other than zero volts, which means that if you connected a neutral cable it would carry no current, and hence be a waste of time/effort/copper.

[1] Well, nothing probable.

The other answers already tell you what you need to know but I'll put it in plain English:

A 3-phase motor needs 3 connections to the phase windings of the motor PLUS the protective Earth connection to the motor frame. This Earth connection can either be a wire or can be the metallic conduit / flex that brings the power to the motor.

The Neutral conductor (if available) is not connected to the motor.

Note that for many 120 / 208 Vac motor controllers, a Neutral conductor is run from the panelboard (breaker panel) to the motor starter enclosure. The Neutral plus one phase conductor supplies power to the motor starter contactor and the run / stop controls.

This has two benefits:

1) the entire control circuit (including the contactor coil) is 120 Vac instead of 208 Vac. This allows:

2) Pilot indicators are 120 Vac instead of 208 Vac. It isn't as much of a problem these days but years ago, 120 Vac pilot lamps were much more common than the 208 variety.

FWIW - These pilot lamps were most commonly the same as used in old-fashioned parallel Christmas light strings: same size and same base. If I recall correctly, they were known as "S6" lamps.

In a WYE connection, the center point is not usually connected to anything outside the motor. If you see a fourth terminal on the motor, it may be the chassis/housing earth connection. This connection is required for safety purposes. You can verify this by measuring the resistance between the terminal and the motor housing.

• oh many thanks so neutral is never connected both in wye and delta? Commented Jul 22, 2014 at 18:23
• well... pure Generators come to mind.
– user16222
Commented Oct 20, 2014 at 20:39