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Anonymous Poster

Motor Service Factor ( SF = 1 OR 1.15 ) ??

09/15/2010 7:12 AM

I am working in a water project , at which we are planning to buy new horizontal booster pumps , the technical specifications for the motors of the pump call out for motor with 1.15 service factor . After contacting one of the motor manufacturer they replied with the following answer :

" As motors are use with a VFD , motor will have service factor one (1.0) on inverter or 1.15 DOL ( Direct on line ) , the name plate of the motor will be 1.15 DOL"

I am not Electrical Engineer but I need to know if the manufacturer answer will provide motor with 1.15 service factor as per the project specifications ?? and why we should have this difference in the service factor value if the motor is running by VFD or DOL ????

any information about the above issue will be highly appreciated .. Thanks in advance

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#1

Re: Motor Service Factor ( SF = 1 OR 1.15 ) ??

09/15/2010 1:04 PM

Your motor nameplate value of 1.15 meets the specification. That additional capacity is included in the design.

Vfd's produce a power factor of near unity so the motor will operate more efficiently when run through a VFD.

Service factor is the amount of "headroom" built into the motor. It allows users to run the motor above the rated amp value. So, a motor rated at 10 AMPS at FLA can be safely run at 11.5 AMPS. (Without VFD control) Running at 11.5 AMPS will shorten the life of the motor, another problem negated by VFD's.

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#2

Re: Motor Service Factor ( SF = 1 OR 1.15 ) ??

09/15/2010 1:31 PM

What the supplier is stating is that even if the motor says 1.15 SF on the nameplate, it is only going to be 1.0SF when run off of a VFD. If your application does not have a VFD, this is irrelevant. If it does, TECHNICALLY the motor meets the stated specification, but the supplier is pointing out the erroneous nature of the specification to start with. You will not find a motor rated for operation on a VFD with anything other than a 1.0SF.

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#3
In reply to #2

Re: Motor Service Factor ( SF = 1 OR 1.15 ) ??

09/15/2010 6:43 PM
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#4

Re: Motor Service Factor ( SF = 1 OR 1.15 ) ??

06/04/2012 4:46 AM

Dear,

First I want to take your attention to the meaning of service factor. SF is nothing but safety margin on the motor to take care of the overload condition. This overload condition may damage the motor. There are two aspects of selecting SF;

1. Motor temperature-: Heating of the motor or temperature rise is function of I² x time (more appropriate it is I² x resistance of winding x time) in case of overload if current increases to 1.15 x rated then heating will go by 1.15² x time. This roughly means that if the temperature rise at full load is 80°C ('B' rise) the temperature rise at 1.15SF loading will be in the order of (1.15)² x 80°C = 105.8°C ('F' rise). That is why in many of the cases motor selected will have temp. class B with temp. rise limited to F.

2. Bearing life-: In general, when bearing run every 15°C hotter your grease life is halved. so when motor will run in it's service factor zone it will run hotter than normal, so do the bearing. This will reduce your bearing life.

This is all true when motor is supplied with the current of sinusoidal waveform. for VFD, motor will run hotter than motor running by sinusoidal current (as same will be supplied with discreet waveform i.e. pulsating one.) this will add to the heat say by 5% to 7% than normal. this will ultimately de-rate your motor & you will be available with motor having SF 1.0. The extra 15% will eat away by VFD heat. this is applicable when, motor running without VFD in SF zone will be provided with VFD & will run in SF zone.

Now, coming back to your application of pump. Please note that SF is applied to the equipment which may run in the SF zone continuously. for sake of example let's consider your pump. It may happen that normal operating & rated / design point may be different. & motor selected for the normal point may run in SF zone for design point. in such a scenario this SF will safeguard your motor.

if motor manufacturer is saying that motor will have 1.15SF when DOL i.e. supplied with sinusoidal waveform will have SF 1.0 when use with VFD. Above is the reason for same.

I hope this will help you.

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