IEC requirements on voltage drop when motor starting

[dnqhung]

hi everyone,

can anyone tell me the specific IEC requirement of voltage drop at the motor terminal when motor starting.
for a 4.5MW/6.6kv Motor, how much is the maximum permissble volt-drop when motor starting per IEC?

Please help me,

thank you

 

[EddyWirbelstrom]

I am not aware of any standard specifying minimum permissible motor terminal voltage during direct-on-line start.
Motor starting torque is proportional to the square of the applied voltage. If the volts are too low the motor starting torque may drop below load torque and the motor will stall.
There are standards for voltage drop at the switchobard for varous equipment, motors, contactor coils, electronic equipmient etc.

 

[dnqhung]

dear mnewman,

it took me alot time to find the standard.
I'm not familiar with IEC much. Some design books propose voltage drop during motor starting is 15%.
thank you for your advice.
By the way, can you tell me the code of IEC standard for voltage drop (in short time) at the switchobard for varous equipment, motors, contactor coils, electronic equipmient etc as you said.

one again, thank you.

 

[EddyWirbelstrom]

Hello Dnqhung
The Standards I was referring to were the following Australian Standards which are based on IEC Standards :

Permissible Voltage Drop at Equipment Terminals During Motor Start
Voltage drops in excess of 10% from nominal volts at switchboard should be investigated to ensure that equipment voltage tolerances are not exceeded.

Electronic Equipment and Sensitive Control Equipment
Permissible voltage tolerances should be obtained from specific equipment manufacturers.
AS 2834 Clause 3.2 specifies following voltage tolerances for computer equipment –
­ Steady-state voltages 415V +6% / -10%
­ Transient voltages 415V +7% / -7%

Contactors and Relay Coils
AS 60947.4.1 Clause 8.2.1.2 specifies the following limits of operation for contactors and alternating current relays –
­ Closing voltage tolerance 85% to 110% of control supply voltage Us.
­ Drop-out voltage 75% to 20% of control supply voltage Us.


3 Phase Induction Motors
AS 1359.101 Section 6.3 and Tables 2 and 12 specify following permissible voltage tolerances with respect to nominal voltage Un –
Zone A + / - 5% of Un Motor can continuously deliver rated torque, however temperature rise may be above rated value.
Zone B + / - 10% of Un Continuous operation not recommended.
Motor starting torque below rated value and temperature rise above rated value.

 

[7anoter4]

I agree with mnewman (Electrical) that the maximum permissible voltage drop on the
motor terminals at direct-on-line start [DOL] depends on a lot of other parameters as Driven Load Torque and motor +load moment of inertia.
As the start current value does not depend on Load [but depends on voltage] and it is relatively constant, the starting time it is what limits the voltage drop, actually. Less voltage, less motor torque [acceleration torque] then longer start time.
The start time is stated by manufacturer in the motor catalog usually, and also the start and maximum torque and motor and load maximum moment of inertia [J].
For an Induction Motor Squirrel Cage Rotor one may calculate the start time using a simple formula as follows [from ABB Motor Guide]:
Start time =J*2*pi*f/Tacc where: J =Jmotor+Jload moment of inertia
Pi=3.14156.. f=frequency [50 or 60 Hz]
Tacc= acceleration torque
Tacc=TM-KL*TL where TL is the average load torque. TM motor average torque KL is a constant depending on load type:
Lift motion Fan Piston pump
KL 1 1/3 0.5
TM motor average torque TM=0.45*(TStart+Tmax)
Now, as mnewman said, the motor torque is direct proportional with the motor terminal voltage [approximately] so:
New TM=Rated TM*(New Voltage/Rated Voltage)
The calculated start time has to be less than that shown on the manufacturer catalog.
Usually the maximum drop in DOL case for the motor manufactured as per IEC-34-12 will be 20%.
For low voltage motor [induction motor, squirrel cage rotor] the maximum voltage drop is stated in IEC 34-12.
As mnewman already mentioned the supply voltage and frequency must be in the limits stated by IEC-34-1for motor steady state –rated rpm, rated current, rated HP [kW].

 

[7anoter4]

Quote:
"Now, as mnewman said, the motor torque is direct proportional with the motor terminal voltage [approximately]"
First of all I apologize: mnewman did not say this. Secondly the torque is approximately proportional with SQUARE motor terminal voltage.Sorry.