Motor Starting Current

[mull982]

Hello

 

Can someone please explain to me what happens to motor starting current when trying to restart a motor that is already spinning. We had a motor that was trying to restart after it was stopped, but was still spinning. I have been told that there is a greater starting current associated with a greater starting torque when trying to start a motor that is already spinning due to the fact that the spinning motor and power supply are out of phase.

 

Can someone please explain to me what happens electrically in this instance or point me in the direction of where I can find some information. I have been trying to find information on this subject and have been unsucessfull.

 

Thanks for the help!

[marke]

Hello mull982

 

Welcome to the forum.

When the motor is connected to the supply, the supply voltage causes a rotating magnetic field in the stator. This field cuts through the rotor bars inducing a current to flow in them. The rotor current causes a rotor field and the interaction between the stator field and the rotor field causes the torque to drive the motor.

When the supply is disconnected from the motor, the rotor continues to spin for a period of time. The rotor retains a field which exponentially decays. While the rotor filed is significant, it will induce a voltage in the stator. The frequency of the stator voltage is dependent on the rotor speed and the voltage is dependent on the rotor flux.

If the supply is reconnected while the stator is generating voltage, there will be a current transient to re establish the stator field. If the voltage is out of phase. the transient can be very high.

Typically, the rotor flux will decay in less than a second, so reconnecting at greater than half a second will usually not result in a significant reclose transient.

The start current after the transient, will not be significantly higher on a rotating motor, but will equal the start current at that speed when started from zero. If the motor is spinning forward, the current will be less than the locked rotor current. If the motor is spinning in revers, the current will be a little higher than the current at zero speed.

 

If you are restarting a spinning motor, allow a delay of 0.5 - 1 second after disconnecting befor reconnecting the supply.

 

Best regards,

 

 

[mull982]

 

Marke

 

Thanks for the information!

 

What you are saying is that if the motor is for some reason restarted before the magnetic field has a chance to decay or collapse (less than .5s) then the motor will regenerate a voltage that is out of phase with the restarted incoming voltage leading to a transient current? What causes this current transient, is it due to the fact that there will be a difference in voltage potential between the regenerated voltage and incoming voltage or simply because they are out of phase? Do you know where I can read more about this condition?

 

Otherwise you are saying that if the motor is restarted after the field has collapsed, lets say after about 5s then this transient will not exist and the motor will start with the normal starting current. I wasn't sure from what you said that if the motor was spinning in the direction of rotation, would the restarting current be the same as starting from zero (LRC) or be less? If less, then why? Is this because the motor would require less starting/acceleration torque? I can see that a motor spinning opposite rotation would require more starting current but couldn't understand if you were saying a motor spinning in the direction of rotation would be the same or less than zero starting.

 

I have also read information regarding "Negative Coefficent of Temperature" relating to the temperature and resistance of the windings. From what I understand as the temperature of the windings increase their resistance decreases. When trying to restart a motor shortly after it has been running, could the motor windings still be hot giving them a lesser resistance, and therefore a greater starting current which could possibly trip a circuit breaker?

 

Thanks for all of the help.

[marke]

Hello mull982

 

if the motor is for some reason restarted before the magnetic field has a chance to decay or collapse (less than .5s) then the motor will regenerate a voltage that is out of phase with the restarted incoming voltage leading to a transient current?

Yes that is correct.

What causes this current transient,

When you initially apply an AC voltage to an iron cored reactor (motor, transformer, solenoid etc) there is no flux in the iron. There is an "inrush" current that flows for a very short time to establish the flux in the iron. The current then drops back to a steady state current, which in the case of a motor, is the start current of the motor.

When you "reclose" on a rotating motor with significant rotor flux, the inrush current is much higher as in addition to the establishment of the flux in the iron, the existing flux has to be dissipated. If the existing flux and the new flux are in phase, the inrush current is ery low. If the existing flux and the new flux are out of phase, the inrush current is very high. Associated with the high inrush current, there is a very high transient torque which can cause severe damage to the motor and to the driven load.

 

The start current is a function of the motor characteristics and the rotor speed. If full voltage is reapplied to a motor that spinning at say half speed, then the start current (after the inrush current) will be the same as the start current would be when the motor has accelerated to half speed from zero speed. The start current vers speed follows the same curve.

 

The windings of the motor, stator and rotor, are metallic and so have a positive temperature coefficient. As the temperature rises, the resistance increases by a very small amount.

The resistance of the stator has virtually zero influence on the start current of the motor. The resistance of the rotor however does influence the start current and start torque of the motor however it is very hard to see the effect.

 

Best regards,

[chris1373]

If you are restarting a spinning motor, allow a delay of 0.5 - 1 second after disconnecting befor reconnecting the supply.

 

Hello,

 

What happend with a star delta starter? Isnt it doing exactly wath you are telling not to do? Reconecting a spinning motor in less then a second, from star to delta?

 

Thanks for any help!

 

Chris.

[marke]

Hello Chris 1373

 

What happend with a star delta starter? Isnt it doing exactly wath you are telling not to do? Reconecting a spinning motor in less then a second, from star to delta?

Exactly

If you follow any of my discussions on star/delta starters, you will find that the open transition star/delta starter develops a very high current and torque transient at the transition from star to delta. It does more harm than a DOL starter!

 

Best regards,

[chris1373]

Hello Chris 1373

 

Exactly

If you follow any of my discussions on star/delta starters, you will find that the open transition star/delta starter develops a very high current and torque transient at the transition from star to delta. It does more harm than a DOL starter!

 

Best regards,

 

I agree with you in your observations about star/delta sarters, now i have it clear!

 

Marke, cant you help me with my other post? Please, i need it? The one that are about gens test!

 

Please!