start voltage current relationship 1

[pumpo]

Start voltage / amprage relation ship was given to me by one of our engineers as

Arated/Ameasured = Vrated/Vmeasured
so that if you see a V drop at the motor you will also see a decrease in Ampreres.
Is this correct????

 

[cbarn24050]

Hi, the short answer is no. The current is dependent on the motor speed which is dependent on the load.

 

[aolalde]

Pumpo:

During the starting condition yes, the voltage drop will reduce the inrush current proportional to the line voltage drop.

Under a steady HP load condition, the current will increase with a drop in voltage.

IL = HP*746/(1.73*VLL*EFF*PF)

Were
IL = Line current under load
HP= shaft load in Horse power.
VLL= Line to line voltage
EFF = motor efficiency ( regularly it will drop a while with lower voltage)
PF = motor power factor.

 

[pumpo]

could you help me with another query?
The software that we use calculates the start current from the Irms, Irms/Vrms X Vrated = Istart is that also correct if we have a correct voltage. The motor start I is estimated by the manufacturer because they do not manufacture or assemble the whole motor we do.

 

[Marke]

During start, a reduction in the starting voltage will reduce the starting current proportionally, however at full speed, this is not necessarily the case. A reduction in voltage will result in an increase in the work component of the current and at a moderate to high load, that will increase the running current.

The start current reduction is independent of load, but if the current / voltage is reduced too far, there will be insufficient torque to accellerate the load to full speed.
The start torque reduces with the squaare of the voltage or current reduction.

Best regards,

Mark Empson

http://www.lmphotonics.com