NEMA MG1 Motor frequency variation during starting

[radug]

Hello,

NEMA MG1 is specifying a maximum 5% frequency variation during normal operation and also during starting:
"Medium motors shall start and accelerate to running speed a load which has a torque characteristic and
an inertia value not exceeding that listed in 12.54 with the voltage and frequency variations specified in
12.44.1.
The limiting values of voltage and frequency under which a motor will successfully start and accelerate to
running speed depend on the margin between the speed-torque curve of the motor at rated voltage and
frequency and the speed-torque curve of the load under starting conditions."

Is there defined anywhere the exact definition of what a variation is?

While trying to size a genset according to ISO 8528-5, usual manufacturer performance class G2 as in that standard, defines:
Transient frequency deviation from rated frequency during a sudden power increase <= -10%

I had a discussion with other colleagues whether the 5% frequency requirement during starting also applies for this requirement or not.

 

[waross]

You may get a lot more usable answer if you describe the intended application.
You may want to consider the interaction between a starting motor and a generator that is small enough that it is noticeably affected by motor starting.
Generally the first consideration of motor starting on a generator is not the effect on the motor, but the effect on other loads on the generator.
With the motor as the only load on a generator, a frequency drop when starting is beneficial to the motor.
As the starting load pulls down the generator speed, the UFRO feature will drop the voltage to maintain a safe V/Hz ratio.
This is similar to the action of a VFD when a low voltage and frequency is used to accelerate a motor with less heating than DOL starting.
A rue of thumb that has worked for me for numerous small generator applications starting residential air conditioners is a reserve of 3:1 times the motor size. (Usually based on the motor rated current.)
The voltage dip and frequency dip experienced by other loads has been acceptable.
For a generator starting one motor with no other loads, 2.5:1 has worked.
But, a large generator with a large base load will do better than a small generator and a small base load in comparison to the motor size.
Most generator manufacturers have software available online to size generators for motor starting loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[radug]

Thank you waross.

This is an industrial plant with a large genset that not only feeds motors with direct online starting but also UPS/DC system and other static loads.

There is not a concern related to motor starting itself. It is a discussion with our engineering subcontractors that stick strictly to NEMA MG1 requirement of 5% max frequency variation for motors regardless of diesel genset manufacturer recommendation or sizing software or above all, diesel genset standard ISO 8528-5, which defines 4 performance classes (3 usual and 1 special to be agreed with genset manufacturer) and even for the most demanding class G3 (except G4 that is the to be be agreed with manufacturer), allows 7% transient frequency droop. Our engineering subcontractor insists in defining a more demanding requirement than that in order to fulfil NEMA MG1 requirement.

 

[waross]

Sorry. Now I understand your question.
My take is that 12.44.1 applies to motor capability under stated conditions. It is not a requirement that those conditions be met.
Also I believe that 12.44.1 refers to sustained operation under the stated conditions, not transient conditions.
As I read NEMA 33.4.2.1.1. The performance of the prime mover, and by implication, the speed and frequency of a generator is outside the scope of NEMA standards.
As I read NEMA 33.4.2.5 under performance class G2 a generator is allowed a 24% voltage deviation during step loading.
I do not believe that NEMA 12.44.1 is intended to apply to transient conditions nor to generator capability.
That is one opinion. Wait for possible other opinions.

Bill
--------------------
"Why not the best?"
Jimmy Carter