Please Help me Understand Motor Contribution in SCCR Calcs......

ron

Senior Member
This has been the subject of debate, but yes, that is the general accepted concept at the moment, ASSuming the drive is not line regenerative. Technically, because there is a generic statement in IEEE standard 141-1993 (Redbook), section 4.2.5 that says "adjustable speed drives can contribute current from the motors to a short circuit...", most calculation software packages such as SKM Power Tools and ETAP will assume the worst (that he drive MIGHT be regenerative) and go ahead and leave the motor contribution in. But in reality if it is a simple VSI drive with a diode front-end rectifier, it cannot contribute fault current backward from the motor unless it completely melts down and becomes it's own bolted fault, which at that point becomes its own new problem.

As to the earlier concept of "just put a diode to block it" (sic), remember it's AC, so you would need TWO diodes to block anything, and then if it is blocked in both directions, nothing works! The VFD does because the diodes are configured as a RECTIFIER to DC, effectively blocking anything from going the other way. So technically it COULD be done if VERY motor were on a VFD (non-regen of course), something we VFD manufacturers would highly promote...:thumbsup::thumbsup::thumbsup::thumbsup:
Of the generally available pump and HVAC type VFDs sold from major manufacturers, generally 6 pulse, would you suggest that an overwhelming percentage of them are non-regenerative? That would be my guess.
 

GoldDigger

Moderator
Staff member
Regenerative VFDs are specified mostly where there is a high lifting or inertial load on the motor and the energy lost in stopping or slowing and then restarting is a significant cost. Or where the heat dissipation in a braking resistor is a problem on its own.

Unless one of those conditions (or similar) is present in the application, the extra cost of a regen front end is not justified.
 

dionysius

Senior Member
Location
WA
Regenerative VFDs are specified mostly where there is a high lifting or inertial load on the motor and the energy lost in stopping or slowing and then restarting is a significant cost. Or where the heat dissipation in a braking resistor is a problem on its own.

Unless one of those conditions (or similar) is present in the application, the extra cost of a regen front end is not justified.
So that I do not have to spend a night on the web help me here. A VFD is a Variable Frequency Drive so 60 Hz is turned into DC then sent to an Inverter to make AC with desired frequency which will be a speed governor on an induction motor. Regen means that as we accelerate or inject energy the energy (or some of it at least) is recaptured when we slow back down. Now that means a storage device (capacitor??? battery???) is required??? Now someone spoke of a bypass.....what is that.

Please sanity check if all I just said is correct.....
 

GoldDigger

Moderator
Staff member
So that I do not have to spend a night on the web help me here. A VFD is a Variable Frequency Drive so 60 Hz is turned into DC then sent to an Inverter to make AC with desired frequency which will be a speed governor on an induction motor. Regen means that as we accelerate or inject energy the energy (or some of it at least) is recaptured when we slow back down. Now that means a storage device (capacitor??? battery???) is required??? Now someone spoke of a bypass.....what is that.

Please sanity check if all I just said is correct.....
Basically the regen-capable VFD contains a DC to AC grid interactive inverter which can take extra energy in the DC bus capacitor and generate AC back into the AC supply circuit. It does this by switching the DC to force current back onto the AC line in the opposite direction of normal current flow through the input rectifier.
For this to happen the DC bus voltage has to be higher than the normal rectifier supplied bus voltage. This happens as a result of the AC back EMF caused by slowing of the motor being allowed to push current back into the DC bus by the switching elements. This circuitry is part of the normal braking and speed control operation of a VFD. The input side AC generation circuitry is not standard and adds to the cost.
 

Sahib

Senior Member
Location
India
This happens as a result of the AC back EMF caused by slowing of the motor
The energy back flow from an induction motor takes place when the speed of the rotor exceeds that of the rotating magnetic field i.e. when the motor tends to speed up above its synchronous speed.
 

kwired

Electron manager
Location
NE Nebraska
So that I do not have to spend a night on the web help me here. A VFD is a Variable Frequency Drive so 60 Hz is turned into DC then sent to an Inverter to make AC with desired frequency which will be a speed governor on an induction motor. Regen means that as we accelerate or inject energy the energy (or some of it at least) is recaptured when we slow back down. Now that means a storage device (capacitor??? battery???) is required??? Now someone spoke of a bypass.....what is that.

Please sanity check if all I just said is correct.....
The storage is what you are trying to refuse to accept>> the inertia of the driven load. That energy doesn't just magically appear, it was consumed by the motor to drive the load, but when we put a slower speed command into the drive we must decelerate to attain that speed. One way is to let it coast and let that energy be given up to friction and other losses, the other is for the drive to do the braking, by doing so it takes the energy stored in the inertia of the load and gives it up as heat, in some cases with additional help of a braking resistor, or if designed to do so dump that energy back into the supply side of the drive. But if the drive is what is the main cause of deceleration it is because the motor is feeding electrical energy back to the drive - acting like a generator.

Ever try to take a resistance reading across motor windings when the motor was not "energized" but was turning? Your ohmmeter doesn't hold a steady display because of generator effects of the turning rotor. Stop that rotor and you will be able to get a stable reading on your meter.
 

iwire

Moderator
Staff member
Location
Massachusetts
Now someone spoke of a bypass.....what is that.
VFD sometimes die, and if that stops a necessary motor that can be an issue so often a standard across the line motor starter is also wired in so that if the VFD dies you can use this 'bypass' motor starter to run the motor.
 

kingpb

Senior Member
To make any sense here the motors would have to be so great as to be able to compete with the POCO generators. This is just never close to the case.
Possibly in your limited experience; but there are many situations where motor load (sc contribution) is significant. One instance would be two 21,000 Hp boiler feed pump motors let alone the rest of the plant load.

A couple 50Hp motors arn't going to be a game changer, but 20 motor control centers filled with a bunch of motors on each could be, it's all relative.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Of the generally available pump and HVAC type VFDs sold from major manufacturers, generally 6 pulse, would you suggest that an overwhelming percentage of them are non-regenerative? That would be my guess.
Yes, I'd estimate it at 99.99999%, which is why it's kind of ridiculous that the IEEE Red Book makes that statement that way, instead of adding a footnote that says "In rare occurrences, VFDs may be line regenerative and possibly contribute to Fault Current".

Also because Bypass Contactors are actually somewhat common in HVAC drives, they could just reference that a Bypass Starter in the VFD package as another way the SYSTEM could end up contributing. But technically it's not the VFD itself, it would ONLY allow contribution if in bypass at the moment of the fault.
 

Ingenieur

Senior Member
Location
Earth
This may help to understand the concept of regen
motor lowering a load like an elevator

motor torque 100
load torque 90 basically wt x g /gearing

if no power to motor the load will accelerate or freewheel
with power we can use it to brake

the rotor is turning in the load direction
armature is slowing the rotor its f < rotor positive slip = generating
if the load got lighter motor direction would reverse and be lifted
it produces a negative torque of say -75 to retard the load
the load still produces 90
90 + (-75) = 15 excess
some is used to power the motor
the excess is wasted on a resistor grid or manipuated and synched to the grid
 
Last edited:
Top