Example D8 in the NEC

[Dennis Alwon]

Can someone explain this? I don't understand the secondary on a motor and why it is used as it is in the calculation

(b) Two 30-hp, 460-V, 3-phase, wound-rotor motors, nameplate primary
full-load current 38 A, nameplate secondary full-load current 65 A,
40°C rise.

Conductor Ampacity
The full-load current value used to determine the minimum required conductor
ampacity is obtained from Table 430.250 [see 430.6(A)] for the
squirrel-cage motor and the primary of the wound-rotor motors. To obtain
the minimum required conductor ampacity, the full-load current is multiplied
by 1.25 [see 430.22 and 430.23(A)].
For the 25-hp motor,
34 A × 1.25 = 43 A
For the 30-horsepower motors,
40 A × 1.25 = 50 A
65 A × 1.25 = 81 A

 

[Smart $]

Simply put...

Squirrel-cage induction motor rotors create their magnetic field using permanent magnets.

Wound-rotor motors use energized coils to create the magnetic fields of the rotor. These coils are called the motor secondary (I think, as I'm not speaking from experience).

You can look up a lot more info on the 'net.

 

[Dennis Alwon]

Simply put...

Squirrel-cage induction motor rotors create their magnetic field using permanent magnets.

Wound-rotor motors use energized coils to create the magnetic fields of the rotor. These coils are called the motor secondary (I think, as I'm not speaking from experience).

You can look up a lot more info on the 'net.

Thank you. If there is only one branch circuit to these motors why do they calculate the conductor amps for the motor and it's secondary

 

[Dennis Alwon]

It sounds like we have to wire the secondary and primary side of the motor??? 430.24 talks of wiring the secondary. I can't get an image of this... any help. I did check the web but I only see primary wiring that is field wiring

 

[Smart $]

Thank you. If there is only one branch circuit to these motors why do they calculate the conductor amps for the motor and it's secondary

I do not know for certain. I surmise the branch conductor size is based on total FLA. However, if the overload protection is divided into separate primary and secondary sensing, I believe smaller conductors can be used to branch out yet again, as in one set to the primary windings and another set to the secondary.

 

[Smart $]

It sounds like we have to wire the secondary and primary side of the motor??? 430.24 talks of wiring the secondary. I can't get an image of this... any help. I did check the web but I only see primary wiring that is field wiring

Google Images "wound-rotor wiring" and see if that helps.

A lot of the diagrams show a control circuit which has no direct connection to the supply...

 

[iwire]

Not an expert here but when I worked at an amusement park some of the motors where wound rotor motors. The primary was the incoming power from the utility.

The secondary came out of the motor, went through either contactors or rotary drum switches and into resistor banks. This provided speed control of the motors. Short the secondaries together and you had full speed, introduce resistance into the secondaries and you reduced the motor speed.

The primary connected to the stationary motor windings, the secondaries connected to the rotating armature windings via brushes.

 

[Dennis Alwon]

Not an expert here but when I worked at an amusement park some of the motors where wound rotor motors. The primary was the incoming power from the utility.

The secondary came out of the motor, went through either contactors or rotary drum switches and into resistor banks. This provided speed control of the motors. Short the secondaries together and you had full speed, introduce resistance into the secondaries and you reduced the motor speed.

The primary connected to the stationary motor windings, the secondaries connected to the rotating armature windings via brushes.

Thank you-- That's is what it appeared to be however it would appears to be but I am confused still. If the secondary ampacity is larger than the primary then how does the primary protect both? I can't imagine you need to pull 2 circuits.

 

[iwire]

Thank you-- That's is what it appeared to be however it would appears to be but I am confused still. If the secondary ampacity is larger than the primary then how does the primary protect both? I can't imagine you need to pull 2 circuits.

In my cases it was just one three phase supply to the motor. The secondaries never physically connect to the utility at all. The only source of power for the secondaries is a magnetic connection between the stationary and rotating parts.


I learned just enough to service the resistor banks, controls and how to bypass the secondaries so we could drive the motors with modern VFDs.

 

[Dennis Alwon]

Thanks Bob & Smart$... I appreciate the help. I had heard of these motors but never ran across them. I suspect we are responsible for the secondary side and that side may see 65 amps but it obviously doesn't get back to the overcurrent protective device at the panel.

 

[GoldDigger]

Thanks Bob & Smart$... I appreciate the help. I had heard of these motors but never ran across them. I suspect we are responsible for the secondary side and that side may see 65 amps but it obviously doesn't get back to the overcurrent protective device at the panel.

The secondary is current limited by the induction from the primary. If the motor is designed to run at full speed with the wound rotor secondary shorted then there does not seem to be any need for overcurrent protection of the secondary windings.
Just like there is no need for overcurrent protection for a PV panel, just for more than two panels in parallel.

If the secondary acts like a transformer secondary when the rotor is locked or during startup, and secondary overcurrent will correspond to primary overcurrent (LRA) and the motor overload protection would kick in.

 

[Dennis Alwon]

The secondary is current limited by the induction from the primary. If the motor is designed to run at full speed with the wound rotor secondary shorted then there does not seem to be any need for overcurrent protection of the secondary windings.
Just like there is no need for overcurrent protection for a PV panel, just for more than two panels in parallel.

If the secondary acts like a transformer secondary when the rotor is locked or during startup, and secondary overcurrent will correspond to primary overcurrent (LRA) and the motor overload protection would kick in.

That makes sense. Thanks. It does seem to act like a transformer since the amperage increase. Wouldn't that mean the voltage increases also since I assume wattage would stay the same???

 

[JoeStillman]

That makes sense. Thanks. It does seem to act like a transformer since the amperage increase. Wouldn't that mean the voltage increases also since I assume wattage would stay the same???

I would expect to have a pretty low voltage on that secondary circuit. The secondary current is proportional to the speed - as you vary the current, you vary the speed. I don't think you can calculate the voltage unless you have a lot more info - especially the power transferred to the shaft.

 

[hmspe]

Simply put...

Squirrel-cage induction motor rotors create their magnetic field using permanent magnets.

I respectfully disagree. A squirrel cage motor does not use permanent magnets, but has rotor bars that are shorted together with a ring at each end of the rotor. See https://en.wikipedia.org/wiki/Squirrel-cage_rotor.

 

[ggunn]

I respectfully disagree. A squirrel cage motor does not use permanent magnets, but has rotor bars that are shorted together with a ring at each end of the rotor. See https://en.wikipedia.org/wiki/Squirrel-cage_rotor.

A squirrel cage is just a type of fan. Any kind of motor can spin it.

 

[Dennis Alwon]

A squirrel cage is just a type of fan. Any kind of motor can spin it.

I hear you but Table 430-248 thru 430-250 have ampacities for induction-type squirrel cage. Here is the heading from the table

Induction-Type Squirrel Cage and Wound Rotor (Amperes)

 

[JoeStillman]

I don't think the OP is interested in Squirrel Cage motors, since the problem involved sizing wire for the secondary current. This implies a Wound-Rotor motor, which is not the same thing. See 430.23.