Clarification on Example D8 Motor Circuits

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E2BRUCE

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I am preparing for the state of Michigan Masters Exam. While reviewing Example 8 I ran accross a few questions. The example given has 2-30 H.P. ,460V, 3-phase wound-rotor motors with a primary nameplate current of 38 A and secondary nameplate current of 65 A, with a temperature rise of 40 deg. C. I understand that the motor overload would be 125% of the nameplate rating, the example shows that as the 38 A for primary. 430.32(E) says that secondary circuits can be protected by the motor overload device. in this example would one always use the nameplate rating for the primary? With this type of motor likely to appear on my test, how would I select branch circuit short circuit protective device for these motors, is this based on the the primary's FLC or the larger of the two? :confused: Any help would be much appreciated.
 
Nium said:
Wouldn't that be the secondary current rating?
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The example given has a nameplate current of 65 A on the secondary. It also says that it is a 30 HP motor but this current rating is for a 50 HP motor. I am not sure how the secondary windings effects the branch circuit overcurrent protection?
 
Thanks Laszlo. Have to admit I'm a little weak on anything but basic motors. I was sorta thinking dual voltage at first myself.
 
Ok, now we're there. Being a wound rotor type motor, the seconary or rotor current is still considerably higher. In the example from Annex D in 2008 NEC they show two separate branch circuit ampacities but just one overload ampacity for these motors, why is this?
 
E2BRUCE said:
Ok, now we're there. Being a wound rotor type motor, the seconary or rotor current is still considerably higher. In the example from Annex D in 2008 NEC they show two separate branch circuit ampacities but just one overload ampacity for these motors, why is this?
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I don't know what is in the NEC on for wound rotor induction motors.
My involvement with wound rotor machines has mostly been on variable speed drives for fairly high powers. Typically these have a limited speed range from about 60% to 100% and include facilities for bypass operation - that is with the slip rings shorted so that it runs much like any other induction motor.
Usually we have standard motor protection on the stator.
For the rotor, we usually fit a winding insulation resistance monitor and high speed fuses in series with the rotor winding. We build the power electronics to accommodate short circuit current but we have found that the slip rings are less tolerant to high overloads.
 
Isn't the current on the secondary induced by the primary and used for controlling the speed? Resistors are placed in the secondary wiring for control; maybe overloads are unnecessary since they are included in the primary?
 
430.32E Wound-Rotor Secondaries

The secondary circuits for wound-rotor ac motors, including conductors, controllers, resistors, and so forth, shall be permitted to be protected against overload by the motor-overload device.
 
Thanks Stopmoving, I did see 430.32 E for the overloads while looking further in to this so now that makes sense to me :) , but in Example 8 in Annex D they show the secondary ampacity for the conductor ampacity. Why is this part of that example? I will probably have to draw an actual diagram for my masters exam and with a wound-rotor type motor I am not sure how this would effect the branch circuit conductors? FYI: this diagram will have to include feeder overcurrent protection, feeder conductor size, branch circuit overcurrent protection size and overload protection size for a feeder suppling multiple motors. It is my guess that it will be a 120/208 V feeder having both single phase and three phase motors. I also susspect that one or more of these motors will be of the wound-rotor type, probably the largest, so i am concerned that if I don't learn as much as possible this could change a few different portions of the question. This is worth 25% of my exam.
 
Stopmoving said:
Isn't the current on the secondary induced by the primary and used for controlling the speed? Resistors are placed in the secondary wiring for control; maybe overloads are unnecessary since they are included in the primary?
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Think of at as a transformer, which it is. If you overload the secondary, you can burn up the secondary winding even with the OL protection on the primary.
 
weressl said:
Think of at as a transformer, which it is. If you overload the secondary, you can burn up the secondary winding even with the OL protection on the primary.
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One of the difficulties in monitoring the rotor current is varying frequency if it is speed controlled and the very low frequency if it is run normally at full speed, the situation that would pertain if rotor resistors are used specifically for starting.
You could use Hall-effect transducers I suppose and there are probably other techniques. It's just not something I have done or seen done.
 
rebuilt a OLD 3 speed Crane, that had wound rotor motors, overloads and contractors were sized for the primary. used secondary current to size the wire to the resistors without protection. per manufactures drawing.
 
Besoeker said:
One of the difficulties in monitoring the rotor current is varying frequency if it is speed controlled and the very low frequency if it is run normally at full speed, the situation that would pertain if rotor resistors are used specifically for starting.
You could use Hall-effect transducers I suppose and there are probably other techniques. It's just not something I have done or seen done.
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Good ole' thermal overload protection will perform fine, since all we're talking about is OL protection.
 
RETRAINDAILY said:
rebuilt a OLD 3 speed Crane, that had wound rotor motors, overloads and contractors were sized for the primary. used secondary current to size the wire to the resistors without protection. per manufactures drawing.
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There you go, the secondary amperage is used to size the wire to the control resistors.

Use the primary amperage for the 430.xx calculations.
 
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