Motor starting while coasting

[Guest]

Gentlemen,

 

I just want to ask if it is advisable to start a motor (Fan application, directly coupled with the motor) while it is just stopped and the fan is still rotating or coasting. The motor is 100 HP with soft starter. Is there also an effect with the bearing life?

 

Regards,

 

Glenmar

[marke]

Hello Glenmar

 

In most situations, there is no problem soft starting a rotating machine (unlike inverters). You must allow sufficient time for the rotor flux to decay to a very low value, - allow a minimum OFF time of say 1 - 2 seconds.

Some starters have soft start schemes that need to motor to be stationary for the model to work correctly, for example a torque control start. This may not perform correctly when used on a rotating machine.

 

I would suggest that current control and voltage control systems should not be a problem, anything else check up first.

 

I do not expect bearing life should be an issue.

 

Best regards,

[jraef]

Marke,

Why would the torque control starting scheme have trouble starting into a rotating motor?

[marke]

Hi Jeff

 

Some of the torque control systems rely on the motor being at close to standstill t start and make measurements for setting up the "model" based on these measurements. A little bit like the initial start up on vector drives. A rolling start could preload the model with the wrong parameters and affect the results.

 

Best regards,

[Murph095]

Starting a rotating motor can be detrimental to the overall health of the motor, but more importantly whatever its drive load is. A rotating induction motor with no input voltage becomes a generator and devlops a back EMF. If incoming voltage(AC) is applied, the incoming phase and back emf will be out of phase and a fairly large current inrush and negative torque aspect will be developed. As Mark said, if the motor is given sufficient time to degrade the back emf, it should be no problem. A motor fan should be no problem whatsoever.

[marke]

Hello Murph095

 

Welcome to the forum.

Yes you are correct, the motor can act as a generator while there is a rotor field. The rotor field is caused by current flowing in the rotor which in turn requires a stator field. When the stator field is removed, the rotor field will decay at a rate determined by the resistance and inductance of the rotor circuit.

This filed will generally collapse to a very small value within a second or so, so provided that the power has been removed for more than a few seconds, the motor generator effect is not an issue.

 

Best regards,

[Murph095]

Mark,

depending on what that application is it can be severe. My company has a product that provides ride through capability for any 3 phase ac induction motors. Voltage sags and momentary interuptions are typically only 3-30 cycles. When these happen many motor contactors will de-energize and shut the motor down. Our product will monitor back emf and when supply power is restored, it will re-engage the contacts when the 2 phases will line up. We have worked with several large electrical utilities and power quality groups to develop and verify this. Many motors have long wind down times and while the phase indifference may not be an issues due to degradation of the signal to restart the motor as fast as we do it is a significant factor as most companies want to alleviate waiting for the motor to wind down or having to restart the motors manually.

 

thanks for the welcome

 

Murph

[marke]

Hi Murph

 

Yes, temporary interruption of the supply is a major problem and on this forum and others, I have been pretty vocal about open transition switching (as in star delta starters) for this very reason.

Generally, if there is a temporary interruption, I recommend a two second delay before reclosing the contactor. This is adequate time to overcome the problems. The big issue is that if the interruption is well upstream, the voltage generated by the motor can hold the contactor closed and also fool any monitoring equipment.

My recommendation to anyone using a standby generator set is not to use chaser contactors to bring the main supply back on, but to either synchronous shunt switch, or to use a two second delay. Many use two contactors with the auxiliary contacts form one contactor controlling the second giving a 1 cycle or so interruption.

 

Have a good day,