Connecting a 3-phase 480 V Delta Motor w. a 3-Phase 480Y/277V Xfmr 2

[joozu6]

I submitted this post in "Electric Motors & Controlls" but wanted to make sure I submitted it in the right catagory.

I work for a utility company and a customer of ours requested a 3-phase padmount 480V delta transformer to connect a 3-phase 480V pump motor. These are rare and it is unfeasible for us to stock, so we decided to provide them with a 3-phase 480Y/277V connection.

Has anyone experienced any problems with connecting a 3-phase 480 volt delta motor w. a 3-phase 480Y/277V xfmr? The motor has a soft start and a breaker to protect from motor faults (phase imbalance). The motor is getting 480 volts per phase. The equipment is grounded for safety. It doesn't seem to me to be a problem.

Our customer brought up the idea that a phase imbalance on the primary transformer side will cause the motor to burn up, where as with a delta transformer this wouldn't happen. It seems to me that a phase imbalance would affect them whether it is a delta or a wye connection, i.e., the delta voltage is equal to the load times the difference of two phase of the transformer primary (Vdelta = Z x (Ia - Ib)), so that if one primary phase has low voltage it will still cause an imbalance on the motor. The breaker and motor protection will protect them and us either way.

Am I thinking correctly? Suggestions? Thank you much.

 

[twosockets]

joozu6,

I agree with you. A 480 Volt pad mount transformer is now considered obsolete in much of the U.S. and has been replaced with 480/277 volt grounded wye type.

Some have been tempted to leave the grounded wye phase back at the transformer and supply to the customer only the three hot phases. In addition to being against code in most places in the U.S., it can create some serious safety problems and should not be done.

 

[dpc]

The motor will work fine with either type of transformer connection. It doesn't care how the transformer is connected as long as it gets the voltage it is looking for at its terminals.

Voltage imbalance is a concern, regardless of delta or wye connection. Motors don't like voltage imbalance. Having a delta winding on the tranformer doesn't guarantee balanced voltages.

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 142-1991 (Green Book) IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems.
Par. 1.4.2 Ungrounded Systems (No Intentional Grounding).
These systems are grounded through the system capacitance to ground. In most systems, this is extremely high impedance, and the resulting system relationships to ground are weak and easily distorted.
Two principal advantages are:
1. The first ground on a system causes only a small ground current to flow, so the system may be operated with a ground fault present, improving the system continuity (especially convenient for more expensive processes).
2. Economic aspect. No expenditures are required for grounding equipment or grounded system conductors.
However, the grounded systems have numerous advantages, e.g. greater safety, freedom from excessive system over voltages that can occur on ungrounded systems during arcing, etc.

The ungrounded system, i.e. 480V transformer, would be suitable for a dedicated load, only.

 

[jbartos]

Suggestions to joozu6 (Electrical) Dec 2, 2003 marked ///\\I submitted this post in "Electric Motors & Controlls" but wanted to make sure I submitted it in the right catagory.

I work for a utility company and a customer of ours requested a 3-phase padmount 480V delta transformer to connect a 3-phase 480V pump motor. These are rare and it is unfeasible for us to stock, so we decided to provide them with a 3-phase 480Y/277V connection.

Has anyone experienced any problems with connecting a 3-phase 480 volt delta motor w. a 3-phase 480Y/277V xfmr? The motor has a soft start and a breaker to protect from motor faults (phase imbalance). The motor is getting 480 volts per phase. The equipment is grounded for safety. It doesn't seem to me to be a problem.
///This is a preferred design since it has more advantages; however, not always the best, depending on a specific application.\\Our customer brought up the idea that a phase imbalance on the primary transformer side will cause the motor to burn up, where as with a delta transformer this wouldn't happen.
///Yes, the motor will burn if the voltage becomes imbalanced and if the motor is improperly protected. However, if the motor is sensitively protected it is not supposed to burn.\\ It seems to me that a phase imbalance would affect them whether it is a delta or a wye connection, i.e.,
///True.\\ the delta voltage is equal to the load times the difference of two phase of the transformer primary (Vdelta = Z x (Ia - Ib)), so that if one primary phase has low voltage it will still cause an imbalance on the motor. The breaker and motor protection will protect them and us either way.
///True. The 480V/277V solidly grounded will allow the ground current, including harmonics, to circulate since there is the soft starter causing wave distortion thus harmonics, etc.\\\

Am I thinking correctly?
///Yes.\\ Suggestions?
///Some.\\

 

[rbulsara]

What dpc said is correct and concise.

In fact, I would (and most do) prefer a Y secondary transformer to a delta transfomer, as it provides a convenient means of system grounding and a balance voltage system. This is how most installations are today.

A delta secondary service on the other hand will have to be conrner groudned, a much less preferred system. Having ungrounded system will have even more problems than any help.

 

[busbar]

At 480V in the last 20 years, utility-provided grounded-wye service to a customer’s delta- or wye-connected load such as a motor has become nearly universal. The likelihood of stator damage from transient overvoltage occurring with an ungrounded source is far greater than with problems using any variety of transformer-winding configuration.

It’s unlikely that the requester could support his misguided opinion with any published ANSI standard or peer-reviewed text. There is the rare case of overcurrent on one secondary phase with hi-side fuse operation for certain transformer configurations, but that’s why 3-pole motor-overload protection has been Code boilerplate for many editions.

 

[jbartos]

Suggestion: The ungrounded system may support an expensive process or safety where the cost of the transformer and motor are unimportant unless of course there is a transformer or motor prices more than a human being life, for example.
The ungrounded systems have ground fault monitoring and alarm systems. This means that a ground fault or leakage does not go unnoticed.

 

[joozu6]

You guys are a great help. Thank you for your comments.

 

[twosockets]

rbulsara,

I must disgree with your assertion that an ungrounded or corner ground delta system is all that is available. Have you thought about grounding a center tap of one of the transformers, creating 480/240 volt 4-wire delta? Many people prefer this to either an ungrounded system or a corner ground.

As always, a 4-wire system, regardless of whether it is wye or delta, needs to be metered as a 4-wire system. A common error is to meter the anticipated load and not the source.

A preferred alternative to 480-volt 3-wire ungrounded delta for those applications where a phase fault to ground would cause a costly shutdown is 480/277 volt 4-wire wye with a high resistance grounded neutral.