Direct Line Starting a 3-phase Motor Calcualtion Review/Comments

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73cj5

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I was asked to present a short 15 minute presentation on Direct Line Starting a 3-phase electric motor.

I have compiled the following and would ask that for any additional input and/or corrections.



I am using a 7.5 HP motor @ 208 VAC

General Information:

Direct line starting is also referred to as across the line or full voltage starting. In this configuration the motor current is the same as the line current. The voltage at the motor terminations is as the same as the line voltage.

Disadvantages to this configuration: The stator requires high current draw during start up and may damage the windings of three phase induction motors. Devices along the line may also be affected by voltage drop during start-up of the motor.

Common Uses:

Water pumps, compressors, fans, and conveyor belts

Calculations:

A. Disconnecting Means @ motor feeder:

Type-430.109
Single motor 115% FLC
24.2 amps x 115% = 27.83 amps
30 amp disconnect

B. Branch circuit over current protection:

Note: Short circuit & ground fault protection with ability to carry motor starting current(Complies with 430.52 (b) and (c))

430.52

FLC x the following:

Non time delay fuse 300% min / 400% max
Time delay fuse 175% min / 225% max
Inst trip breaker 800% min / 1300% max
Inverse time breaker 250% min / 300% max

C. Branch Circuit Conductors:

430.22

125% of motor FLC

430.150

7.5 HP @ 208 V 3 ph = 24.2 amps
24.2 x 125%=30.25

310.16

30.25 = 10 AWG @ 35 amps (75 degree C)

Apply temperature/deration factors as required)

D. Motor Controller

430.83

Types: HP rated Switch, Inverse Type CB, General Use Switch for 300 V or less/2 HP or less

7 1/2 HP motor = 7 1/2 HP rated controller

E. Overload Protection

430.32 (A)(1)

No service factor given
No temp rise given
All other motors 115%

No nameplate rated current available, use FLC from 430.250

24.2 x 115%=27.83 amps

F. Motor Ful Load Current

430.250

7.5 HP @ 208 V 3 ph = 24.2 amps


Thanks for taking the time to add any input, I appreciate it!
 
Who is your audience?

Who is your audience?

I am an electrical instructor and I spend several semesters and several classes getting the information out that you indicate you will be using. 15 minutes is not a lot of time to present "across-the-line" motor starting, let alone bringing the NEC requirements and calculations in. Therefore, I assume from your "General Information" the audience is versed in motor starting technigues... three phase power... as well as the NEC as it relates to motors?
Did you want input on the choice of material, your article references, or calculations?
 
A quick note...

A quick note...

73cj5 said:
I was asked to present a short 15 minute presentation on Direct Line Starting a 3-phase electric motor.

I have compiled the following and would ask that for any additional input and/or corrections.



I am using a 7.5 HP motor @ 208 VAC

General Information:

Direct line starting is also referred to as across the line or full voltage starting. In this configuration the motor current is the same as the line current. The voltage at the motor terminations is as the same as the line voltage.

Disadvantages to this configuration: The stator requires high current draw during start up and may damage the windings of three phase induction motors. Devices along the line may also be affected by voltage drop during start-up of the motor.

Common Uses:

Water pumps, compressors, fans, and conveyor belts

Calculations:

430.6 Ampacity and Motor Rating Determination.
[Gives instruction on when to use tables or name plate]

A. Disconnecting Means @ motor feeder:

Type-430.109(A)(1)
Must be HP rated
430.110(A)
Single motor 115% FLC
24.2 amps x 115% = 27.83 amps
30 amp disconnect

B. Branch circuit over current protection:

Note: Short circuit & ground fault protection with ability to carry motor starting current(Complies with 430.52 (b) and (c))

Table 430.52

FLC x the following:

Non time delay fuse 300% min / 400% max
Time delay fuse 175% min / 225% max
Inst trip breaker 800% min / 1300% max
Inverse time breaker 250% min / 300% max

C. Branch Circuit Conductors:

430.22

125% of motor FLC

430.150

7.5 HP @ 208 V 3 ph = 24.2 amps
24.2 x 125%=30.25

310.16

30.25 = 10 AWG @ 35 amps (75 degree C)

Apply temperature/deration factors as required)

D. Motor Controller

430.83

Types: HP rated Switch, Inverse Type CB [Rated in Amperes],General Use Switch for 300 V or less/2 HP or less [Does not apply in this situation]

7 1/2 HP motor = 7 1/2 HP rated controller

E. Overload Protection

430.32 (A)(1)

No service factor given
No temp rise given
All other motors 115%

No nameplate rated current available, use FLC from 430.250

24.2 x 115%=27.83 amps

F. Motor Ful Load Current

430.250

7.5 HP @ 208 V 3 ph = 24.2 amps


Thanks for taking the time to add any input, I appreciate it!
Click to expand...
I have indicated areas of concern in BOLD.
Hope this helps,
Wes
 
wgubitz said:
I am an electrical instructor and I spend several semesters and several classes getting the information out that you indicate you will be using. 15 minutes is not a lot of time to present "across-the-line" motor starting, let alone bringing the NEC requirements and calculations in. Therefore, I assume from your "General Information" the audience is versed in motor starting technigues... three phase power... as well as the NEC as it relates to motors?
Did you want input on the choice of material, your article references, or calculations?
Click to expand...

I am also an instructor and understand the complexity and general nature of the above question.

I will be presenting to a group of people with a general electrical background which is why I did not want to go too in depth.

I believe this request is more of a test of ability to instruct and general knowledge base of material.

I am looking to take on additional classes and this is what they would like me to present on.

Any input is appreciated and welcome. I use this forum frequently due to the excellent membership and knowledge.
 
Disadvantages to this configuration: The stator requires high current draw during start up and may damage the windings of three phase induction motors. Devices along the line may also be affected by voltage drop during start-up of the motor.
Click to expand...
I wouldn't say that part about damaging the windings, someone may challenge you on how that would happen, and it's only true under some very specific conditions, most of which are going to be tied to high inertia loads. In other words that is going to add a level of complexity that you don't want to have to defend in a 15 minute talk.

Better to say "... and high torque shock to the mechanical components, which may result in equipment damage." Other electrical equipment on the line may also be affected by voltage drop created by the high starting current of the motor.

That way you are being truthful but vague enough to not elicit a challenge from the audience. The other stuff in the 2nd sentence is just me being the brother of an English teacher and husband of a speech writer...
 
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