Acceptable voltage drops when starting mv motors

[RMurdoch]

I am typing to decide which type of starter I need to start some medium voltage motors and am wondering what is a acceptable voltage drop due to inrush current. For instance I have run a cold start test on one of the medium voltage motors in the system and it drops the bus voltage at the MCC to 88% nominal voltage. At what point do I need to use a soft start of VFD

 

[davidbeach]

88% of nominal voltage for how long? You need both magnitude and duration to determine impact.

 

[RMurdoch]

88% is the instantaneous voltage drop since this project is still in the design phase and dynamic characteristics are still not know the total duration of the voltage drop is unknown

 

[Electic]

There are two aspects to your question.


One consideration is what voltage drop can the motor withstand, for how long, while still accelerating the load and not overheating. This is a mechanical problem at some level, that can become complicated if the load is high inertia. Some relays will model residual heat in a motor etc.


The other consideration is what can the buss withstand without bothering other loads. A soft start can help reduce bus effects. Is your 88% calculated at the motor terminals or at the bus?

In general dipping voltage to 88% would be too low at a bus, but you must determine what is acceptable based on other connected loads and how often the motor is started.

If this is a refinery blower that gets started once every 3 years 88% might be okay.

 

[dpc]

1. Impact of low voltage on starting motor and its load. Can motor accelerate the load at this voltage without stalling or overheating. This depends much on the nature of the load.

2. Impact on other loads on the bus. NEMA contactors have to stay pulled in down to 85% voltage. Below that, you're on your own. Reduced voltage to running motors means their current will tend to increase when the voltage dips.

3. Utility requirements.