Line Current vs. Phase Current and what they mean to me (on the exam)

[1-3-2-4]

Hi all,

I'm currently registered for the April exam for Mechanical with the Machine Design concentration. I've been reviewing things and have a question on some electrical stuff (eek!). I worked a problem in the 2001 NCEES exam (# 108 in the AM section), and I'm a bit perplexed. The problem asked for the full load current, and I'm confused how to tell if said motor is wye or delta connected. Am I supposed to assume?

I understand the basic premise between line and phase voltages, but I'm unsure why this problem assumed the motor was Delta connected. Lindeburg's MERM supplies all the right equations, and the sample problems therein either specifically ask for phase current or they give the motor's wiring designation.

However, the same type of question pops up in Lindeburg's Practice Problem book. Problem 67-2, © asks for line voltage, but never describes the wiring.

Am I wrapping myself around the axle on this? Do I just assume delta wiring when asked for anything beyond 'phase' voltage? Sorry if this is coming across as rambling, but I'm starting to talk myself in circles.

Thanks!

 

[Guillaume]

You might need to give a little bit more info on how each problem was stated. Basically, if the line voltage and the electric 3-phase power supplied to the motor are given, you don't have to worry of your motor connection (the equation will still be S = sqrt(3) * VL * IL)

Please put both problems so I can try to explain it a little bit better.

 

[1-3-2-4]

You might need to give a little bit more info on how each problem was stated. Basically, if the line voltage and the electric 3-phase power supplied to the motor are given, you don't have to worry of your motor connection (the equation will still be S = sqrt(3) * VL * IL)Please put both problems so I can try to explain it a little bit better.

Sure thing.

NCEES 2001, Problem 108:

A fan motor at full load draws 24.2 kW. It operates on a balanced 460 V/60Hz/3-phase electrical power supply at a power factor of 0.90 and has a mechanical efficiency of 93%. The full load current (amperes) is most nearly: A. 30, B. 34.0, C. 51.6, D. 58.9

Lindeburg PPI Practice Problems, Problem 67-2

A 200-hp, three-phase, four-pole, 60 Hz, 440 V (rms) squirrel cage induction motor operates at full load with an efficiency of 85%, power factor of 91%, and 3% slip. A. Find the speed in RPM, B. Find the torque developed, C. Find the line current.

A similar problem exists in the MERM. Example 67.6:

A pump is driven by a three-phase induction motor running at its rated values. The motor's nameplate lists the following rated values: 50 hp, 440 V, 92% lagging power factor, 90% efficiency, 60 Hz, and 4 poles. The motor's windings are delta connected. The pump efficiency is 80%. When running under the pump load, the slip is 4%. What are the a. total torque developed, b. torque developed per phase, and c. line current?

All of these problems were solved by the same method of Il=P/[(efficiency)(Vl)(pf)(sqrt(3))]. However, only the last problem stated a delta connection. Since phase current = line current (wye), and phase current = sqrt(3)/line current, I'm unsure how I can assume to use the delta formula. I understand if they ask for the phase current, but the line (or load current) eludes me.

Does this clear up my confusion (does that even make sense? lol)?

Thanks!

 

[sam314159]

High horsepower motors are usually run in delta. To avoid large starting currents, the motor can be started in wye and switched to delta.

• This is straight out of the Power Reference Manual.
• If a delta motor is started in a wye configuration instead of a delta then the starting current is divided by 3.

Edit: Fixed typo

 

[1-3-2-4]

High horsepower motors are usually run in delta. To avoid large starting currents, the motor can be started in wye and switched to delta.

• This is straight out of the Power Reference Manual.
• If a delta motor is started in a wye configuration instead of a delta then the starting current is divided by 3.

Edit: Fixed typo

So from a non-motor guy, what is considered 'high hp'? Anything over 10 hp? 50? I didn't know about switching circuits like that...good to know in case I ever need it.

 

[sam314159]

Right on the same page, it says:

Types of small AC motors include split-phase, universal, capacitor, and series motors. Large AC motors are almost always three-phase.

I am unaware of hard cut-off HP limit beyond which a motor is considered "large" but it sounds like 3-phase motors are considered large motors. I even asked a couple of people here at work and no one can think of a hard limit.

 

[1-3-2-4]

Right on the same page, it says:

Types of small AC motors include split-phase, universal, capacitor, and series motors. Large AC motors are almost always three-phase.

I am unaware of hard cut-off HP limit beyond which a motor is considered "large" but it sounds like 3-phase motors are considered large motors. I even asked a couple of people here at work and no one can think of a hard limit.

Thanks for the follow up. Based on this, the nature of the problems (the size of the motors), I'm going to assume delta connections unless told otherwise or asked for phase current. Thanks again.

 

[Dolphin P.E.]

Right on the same page, it says:

Types of small AC motors include split-phase, universal, capacitor, and series motors. Large AC motors are almost always three-phase.

I am unaware of hard cut-off HP limit beyond which a motor is considered "large" but it sounds like 3-phase motors are considered large motors. I even asked a couple of people here at work and no one can think of a hard limit.

Thanks for the follow up. Based on this, the nature of the problems (the size of the motors), I'm going to assume delta connections unless told otherwise or asked for phase current. Thanks again.

I wouldnt assume anything. If you are looking for the Line current, then it is irrelevant to know if the motor is delta or wye connection. Consider the motor as a blackbox. (as in 1st and 2nd problem).

If they go phase specific, they would tell you what type of connection. (as in 3rd problem).

 

[1-3-2-4]

Right on the same page, it says:

Types of small AC motors include split-phase, universal, capacitor, and series motors. Large AC motors are almost always three-phase.

I am unaware of hard cut-off HP limit beyond which a motor is considered "large" but it sounds like 3-phase motors are considered large motors. I even asked a couple of people here at work and no one can think of a hard limit.

Thanks for the follow up. Based on this, the nature of the problems (the size of the motors), I'm going to assume delta connections unless told otherwise or asked for phase current. Thanks again.

I wouldnt assume anything. If you are looking for the Line current, then it is irrelevant to know if the motor is delta or wye connection. Consider the motor as a blackbox. (as in 1st and 2nd problem).

If they go phase specific, they would tell you what type of connection. (as in 3rd problem).

Got it, thanks.