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Motor Starting Characteristics


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Those of us in the 50 Hz world have seen a change in motor designs in recent years, particularly the rotor design with a push for higher efficiency motors. The high efficiency is often achieved by the use of lower impedance rotor bars which results in lower slip losses. The down side of this has been that the high slip operating conditions have been severely compromised. This shows up as an increase in the Locked Rotor Current of the motor.


I have just completed a design project whereby two pumps are to be run of a diesel generator set. One pump is 75KW and the other is 37KW. The 75KW pump always will be started first. In order to optimize the generator set sizing, I went on the hunt for a 75KW 2 pole motor that had a high "starting efficiency". To my dismay, I found that although years ago, it was possible to find motors that would enable me to start this pump at around 300% start current, the modern motors needed start currents of around 400 - 450% The best I found was 350%. I do not know if there has been a similar trend in the Nema range of motors.


To me, this may have been a gain in the full speed running efficiency of motors, but it is a definite backward step when it comes to starting the motors. On weak supplies and standby generators, the starting current is very important. Pump stations with only one pump running from a dedicated generator have to have an engine sized for the start conditions. This results in an oversized engine which runs lightly loaded and quickly suffers glazed bores.

Perhaps as engineers, we need to bring some pressure to bear to ensure that the starting characteristics of induction motors are taken seriously by the regulatory bodies as well as the running efficiency.

I am interested in the comments and experiences of others.

Best regards,

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Also from the 50Hz part of the world and so I don't know that I'm going to add anything of value for you.


I remember a time when we could say with a high level of confidence that the LRC of a motor was in the order of 6 x FLC. With the advent of HF designs this has increased and typical LRC values now appear to be in the range of 7.5 - 8 x FLC.


As you have quite rightly stated this presents problems in areas of weak supply, but it also impacts on the selection of products that will be used to start these motors.


In the case of soft starters, manufacturers for years have concentrated their efforts on reducing the cost of product they release to market. The problem however is that this effort has been off-set by what is happening in the motor world. Five years ago, starting an unloaded screw compressor via a soft starter generaly required a unit capable of 3.2 to 3.5 x FLC for a period of between 8 and 10 seconds. These days the same compressor with a new generation motor requires between 4.2 and 4.5 times full load curremt and will take between 15 and 20 seconds to reach rated speed.


But it's not just the soft starter that is affected. The higher starting current and times impact on the selection of supply and motor cabling, the voltage drop that can be expected during motor starting, the selection of circuit breakers and/or fuses, and often a larger cabinet into which the equipment is to be installed will be required.


I don't know that there's all that much we can do to influence the design of motors, but it's certainly a subject worth raising for discussion.




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  • 2 years later...

Dear Sirs


Could you support me about starting current characteristics?

How can judg the starting current is Pass or Fail ?

EX: We had one motor with LRA = 28 Ams, We do Starting current test and it is 25 Ams. or 30 Ams so What case of consideration Pass , and Fail.


Thank you

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I am not aware of a pass and fail test on starting current as such. It really depends on what you aretrying to achieve.

If you are testing a motor against it's published specifications, then you can determine what tolerance you are prepared to accept.


Best regards,

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  • 1 month later...

My question is this I have a 200 hp 460 volt motor with FLA of 225 quincy screw drive compressor what would my starting amps be ?


right now it is hooked to a 400 amp breaker and it is tripping it instantly I had been figuring 1.75 * 225 to get starting amperage ??

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Hello jimmy


Welcome to the forum


The minimum starting current is a function of the load characteristics and the motor characteristics. - It is not possible to say what the start current will be without a lot more information.

Under full voltage start conditions, the motor will draw Locked Rotor Current which is typically 600% - 850% of the rated current of the motor.

As you reduce the start voltage, you will reduce the start current by the same ratio. The start torque will reduce by the current reduction squared.

The driven load will require a minimum start torque to spin it to full speed. This determines the minimum start current as a lower start current will not develop enough torque to start the load.

From my experience, I would expect that you will need in the order of 350% - 450% start current to start a screw compressor depending on the motor and compressor characteristics.


Best regards,

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