3 phase motor speed control

Thread Starter

john mackay

Joined Oct 26, 2011
2
I came across a 3 phase fan today at work, the starter is marked slow and fast but only contains three interconnected contactors with no other internal or external controls. There are six wires out to the motor ie three windings. I thought this is a mistake which should read for / rev so I asked someone to watch the fan asI tried it both ways and it is indeed slow / fast. I`m baffled, how is the speed changed with this contactor arrangement?.
 

GetDeviceInfo

Joined Jun 7, 2009
1,734
have no idea what your contactor arrangement is, but you have a couple of options. Your motor may be wired with seperate windings to give you two independant speeds, or the single speed windings may be sectionally reversed giving you a 'consequent pole' configuration. Google is your friend.
 

gremlin

Joined Jul 22, 2010
19
it will i would imagen
switch the 6 wires from your 3 phase motor into
either star or delta configuratuion which will give you two speeds
this is quite comon on mixer agitators for the food industry

http://www.google.co.uk/url?sa=t&rct=j&q=star%20delta%202%20speed%20motor&source=web&cd=1&ved=0CEMQFjAA&url=http%3A%2F%2Fwww.mandc.co.za%2Fpdfs%2FStar_Delta_Starting_and_Dual_Voltage_Motors_Explained.pdf&ei=i4mpTu7pC8_d8QOpk_X5Cw&usg=AFQjCNEOHRbe7dfr8gLj31yPccLtYiWH5g

hope this helps

google star delta motor configuration
big three phase motors have to use bot star and delta one to start and one to run switched over after a few seconds of running
 

GetDeviceInfo

Joined Jun 7, 2009
1,734
it will i would imagen
switch the 6 wires from your 3 phase motor into
either star or delta configuratuion which will give you two speeds
this is quite comon on mixer agitators for the food industry

http://www.google.co.uk/url?sa=t&rct=j&q=star%20delta%202%20speed%20motor&source=web&cd=1&ved=0CEMQFjAA&url=http%3A%2F%2Fwww.mandc.co.za%2Fpdfs%2FStar_Delta_Starting_and_Dual_Voltage_Motors_Explained.pdf&ei=i4mpTu7pC8_d8QOpk_X5Cw&usg=AFQjCNEOHRbe7dfr8gLj31yPccLtYiWH5g

hope this helps

google star delta motor configuration
big three phase motors have to use bot star and delta one to start and one to run switched over after a few seconds of running
Let's flush out this misconception;

the only difference between star (wye) and delta, is the voltage across the windings. There is no change in speed. The poles are fixed and you use the same poles regardless of delta or wye. There is typically a loss in speed regulation at load with a wye connection compared to delta, however a low or no load, wye gives improved efficiencies.

The wye connection gives you 1/3 power output compared to it's delta configuration. This is what the 'soft start' concept is based upon.

Double check your linked doc, specifically the RPM ratings at the different connections.

Spending the last two days rebuilding moisture damaged contactors for a 37kw wye/delta starter.
 
Last edited:

Thread Starter

john mackay

Joined Oct 26, 2011
2
have no idea what your contactor arrangement is, but you have a couple of options. Your motor may be wired with seperate windings to give you two independant speeds, or the single speed windings may be sectionally reversed giving you a 'consequent pole' configuration. Google is your friend.
Thanks for the replies, GetDeviceinfo has the correct answer. On further investigation I found that the motor does indeed contain two separate windings, one of which has such a low resistance it does not register on my multi-meter.
 
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