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Maximum Statring Voltage Drop for Motors 2
- Thread starter JatTiw
- Start date
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1
- #2
Voltage drop during a motor starting is the function of motor characteristics (LRA, subtransient reactance), starting method and source impedance. Starting method may have to do with what starting torque is required. So it varies by actaul field conditions.
The motor itself does not care so much about the voltage drop at its terminals during the starting, it can even recover from a 35% voltage drop if the source hold on, but not many other equipment on the same bus like it. Its own contactor may drop out.
In practice you may want to keep the voltage drop during starting to 15 to 20%.
If you be more specific as to what is your actual concern and what you are trying to resolve, you may get a better and helpful response.
The motor itself does not care so much about the voltage drop at its terminals during the starting, it can even recover from a 35% voltage drop if the source hold on, but not many other equipment on the same bus like it. Its own contactor may drop out.
In practice you may want to keep the voltage drop during starting to 15 to 20%.
If you be more specific as to what is your actual concern and what you are trying to resolve, you may get a better and helpful response.
- Thread starter
- #3
I cant imagine how can they regulate something they do not control.
However NEMA does have Code letter desginations A,G,H etc..for motor desings which relates to maximum kVA (LRA) under 'full voltage' starting which helps determine probable voltage drop if you know your system parameters. Besides the torque-speed characteristics.
However NEMA does have Code letter desginations A,G,H etc..for motor desings which relates to maximum kVA (LRA) under 'full voltage' starting which helps determine probable voltage drop if you know your system parameters. Besides the torque-speed characteristics.
In some areas, there are regulations that say that the voltage must be held within 5% - 15% of the nominal voltage. Authorities then regulate the maximum motor size that can be connected to the supply based on supply impedance, starting method etc. In otherwords, it is often a regional thing with a limit on the maximum disturbance that can be tolerated.
Best regards,
Mark Empson
Best regards,
Mark Empson
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1
- #7
howardbarr
Electrical
There are no specific industry standards related to the maximum voltage drop that a motor is allowed to cause during starting. NEMA standards do say that the motor must be capable of operating successfully at up to 10% deviation from rated voltage, implying that the motor must successfully accelerate at 90% voltage applied at the motor terminals.
Electrical utilities will often have restrictions on motor starting that are based on their specific situations. These restrictions may require specific reduced voltage starting methods under certain conditions. Automatic system voltage regulation may be used to bring the voltage back to nominal when starting a large motor, but there will be a dip before the regulation can function.
As already pointed out, severe voltage drop can cause problems throughout a facility. Low voltage may allow magnetic contactors to drop out, de-energizing other equipment. Vapor lamps may go dark and require start-up time before functioning again. Electronic equipment may malfunction.
It is recommended that a thorough study of system load and stiffness be made before adding a large motor starting current. The result of the study may be requiring reduced voltage starting for the motor or increasing the system stiffness feeding the motor.
Electrical utilities will often have restrictions on motor starting that are based on their specific situations. These restrictions may require specific reduced voltage starting methods under certain conditions. Automatic system voltage regulation may be used to bring the voltage back to nominal when starting a large motor, but there will be a dip before the regulation can function.
As already pointed out, severe voltage drop can cause problems throughout a facility. Low voltage may allow magnetic contactors to drop out, de-energizing other equipment. Vapor lamps may go dark and require start-up time before functioning again. Electronic equipment may malfunction.
It is recommended that a thorough study of system load and stiffness be made before adding a large motor starting current. The result of the study may be requiring reduced voltage starting for the motor or increasing the system stiffness feeding the motor.
- Thread starter
- #8
Hi Howardbarr
I think you are referring to part 12(12.45.1 & 12.45.2) of MG-1-1998. According to it motor shall start successfully with +/- 10% variation in voltage.
At instant when motor is started the voltage drop for big motors are of range of 15-20% for 3-4 sec.
Now my question is if motor causes voltage drop of 15-20% for 3-4 sec & fails to start then it should be considered as problem with power system or it should be problem with motor. (Assuming all torques (such as Lock rotor, pull-in, load torques) are in accordance with NEMA MG-1 specifications)
I think you are referring to part 12(12.45.1 & 12.45.2) of MG-1-1998. According to it motor shall start successfully with +/- 10% variation in voltage.
At instant when motor is started the voltage drop for big motors are of range of 15-20% for 3-4 sec.
Now my question is if motor causes voltage drop of 15-20% for 3-4 sec & fails to start then it should be considered as problem with power system or it should be problem with motor. (Assuming all torques (such as Lock rotor, pull-in, load torques) are in accordance with NEMA MG-1 specifications)
If your motor is built to MG-1, I believe the standard is that it will start successfully at 90% voltage, provided load inertia meet values complying with those given in MG-1. MG-1 also states that for lower starting voltages, the speed-torque curves must be evaluated to determine if starting will be successful.
Starting at much lower voltages is obviously possible, since reduced-voltage starting is commonly used. It just depends on your driven equipment.
So, to answer your last question, if motor load meets MG-1, and the motor failed to successfully start at something less than 90% voltage, then the motor manufacturer would probably tell you the problem was with your system.
Please bear in mind that the 90% voltage is based on the motor nameplate voltage which is generally less than the system nominal voltage to start with. So the situation may not be as bad as you think.
For large motors, it is a good idea to provide a minimum starting voltage in the specifications. If you purchase the motor with the driven equipment, you can place some of the responsibility on the supplier to provide a system that will successfully start at whatever voltage you specify.
Hope that helps.
Starting at much lower voltages is obviously possible, since reduced-voltage starting is commonly used. It just depends on your driven equipment.
So, to answer your last question, if motor load meets MG-1, and the motor failed to successfully start at something less than 90% voltage, then the motor manufacturer would probably tell you the problem was with your system.
Please bear in mind that the 90% voltage is based on the motor nameplate voltage which is generally less than the system nominal voltage to start with. So the situation may not be as bad as you think.
For large motors, it is a good idea to provide a minimum starting voltage in the specifications. If you purchase the motor with the driven equipment, you can place some of the responsibility on the supplier to provide a system that will successfully start at whatever voltage you specify.
Hope that helps.
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion to JatTiw (Electrical) Oct 8, 2003 marked ///\\Hi everybody,
I am interested to know any NEMA or IEEE standards governing the maximum voltage drop caused by starting of induction motor.
///The motor nameplate rated voltage minus minimum transient rated starting motor voltage would be the limit for the sought maximum voltage drop.
Visit
for discussion on specified transient voltage during motor starting.\\
I am interested to know any NEMA or IEEE standards governing the maximum voltage drop caused by starting of induction motor.
///The motor nameplate rated voltage minus minimum transient rated starting motor voltage would be the limit for the sought maximum voltage drop.
Visit
for discussion on specified transient voltage during motor starting.\\
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion: Visit
for: 75% minimum starting voltage (of motor nameplate (rated) voltage) of a fan motor
etc. for more info
http://www.energy.ca.gov/sitingcases/magnolia/documents/applicants_files/afc_cd-rom/14_AppendixE.pdf
for: Minimum starting voltage specificationfor: 75% minimum starting voltage (of motor nameplate (rated) voltage) of a fan motor
etc. for more info
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