Star / Delta Overload Placement

[frothblower]

What criteria is used to determine whether the overload is placed in the Star or Delta windings? I am in refrigeration and usually place the overload in the Delta Windings set to Full load Current recently received some serious verbal abuse from a "knowlegeble Client" who stated that I should know that all cold rooms should be set to trip on startup. Should I ?

 

[Marke]

Hello frothblower
If you put the overload protection in series with the delta contactor, that means that you will not have any protection during start. If the timer fails and the motor is held in star, (it does happen) you have no protection. The correct place to put the protection is in series with the motor windings such that it is in circuit all the time that the windings are energised.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GGOSS]

Hello frothblower,

just to add a few comments to those already made by Marke, in a star/delta starter the overload relay is fitted directly below the 'line' contactor and is therefore connected within the delta loop of the motor. This provides protection both in star and in delta however it is important to note that the current setting is made on the basis of motor FLC/sqrt3.

Some caution needs to be taken however as some standard bimetalic overload relays provide a dual FLC setting scale. One for standard 3 wire motor connection, the other for star/delta connection scaled at FLC/sqrt3, thus eliminating the need for the user to do the calculation him/her self.
Hope this helps.

Regards,
GGOSS

 

[GusD]

Hi frothblower

The only way to protect your motor is to use a dual function O/L relay, so that you can protect your motor during start (or failed start) and during Delta running.
As per the info below ,one has to be careful when setting the O/Ls for a Delta connected motor.
As a rule ,we set motor overloads as per Table 44 on the CECode or possibly any other code .This rule says that for motors with a Sfactor of 1.15 OLoad is set at 115% of motor nameplate FLAmps or Table 44 and 125% for motors with a 1.25 Sf.This is true for most motors unless ,they have a Star/Delta starter.
The following example indicates how to determine the OLoad setting of 75 HP ,575 volts,3phase(SCIM) controlled with a Wye/Delta starter.The motor is rated for continuous Duty and has a Sfactor of 1.2.

Sorry about the Table info.Is in the CECode.Maybe not much different in the USA or VDE rules in Europe.
Step1= Use Table 44 to determine the FLA of the motor. Table 44?--75 hp,575 v,3Ph SCIM draws 77 Amps.

Step 2==Determine the Phase value of the current.
77Amps/Sroot3(1.73)=44.45 Amps

Step 3==Use rule 28-306(1)to determine the overload setting.44.45x1.25=55.56 Amps

Step 4 == Rule says to comply with the marking requirements outlined in Ru28-306 (2)

If I wasn't thinking about a Star/Delta motor and I set my
OLoads at FLA of 77x 1.25 =96.25 Amps.it would not be hard to burn the motor out,if one wasn't paying attention.




GusD

 

[GGOSS]

Hello GusD,

Great post!

My only concern is the information contained in your Step 1, as the actual motor FLC rating will vary between manufacturers.

For example recently we were asked to provide a quotation on a start package at 355kW, 415VAC 3-phase. Similar tables to that described in your Step 1 suggested and FLC of 560 amps, however when we reviewed actual data for motors available throughout Australia found that this could be anywhere between 530 amps and 630 amps. In addition if you analyse data for submesible motors, you will find that the FLC is almost always somewhere between 10% and 25% higher than that of a standard cage motor.

Due to the above-mentioned experiences and many other similar ones, where possible I like to refer to the motor 'nameplate' FLC rating and service factor, for the selection of switchgear, cables etc, and for setting up of the overload relay.

Regards,
GGOSS

 

[jbartos]

Suggestion to frothblower (Mechanical) Jul 6, 2003 marked ///\\What criteria is used to determine whether the overload is placed in the Star or Delta windings?
///Normally, the overload relay with its thermal elements is located one set of leads from the main contactor to the motor terminals and connected directly to the motor terminals with their outputs. The rating of the thermal elements is based on the motor full load amps FLA divided by sqrt3, since thermal elements will be inside the delta connection and the star connection will draw FLA/sqrt3 current.\\ I am in refrigeration and usually place the overload in the Delta Windings set to Full load Current recently received some serious verbal abuse from a "knowlegeble Client" who stated that I should know that all cold rooms should be set to trip on startup. Should I ?
///Yes, the thermal elements placed in inside delta connection have the same rating, FLA/sqrt3, for star connection start and delta connection run.\\\

 

[frothblower]

Thanks for the feedback. I already knew how to do the calculations. I was hoping for an answer on the lines of in application (A) place the overeload here, in application (B) put it there. I will continue to put the overload in series with the main contactor. I will also keep my eyes open for the dual protector.