Motor Starter Sizing Question 4

[Lovison]

We have a customer who has a 50HP motor installed already.
We have an application that calls for a 75HP motor will this require new starters?

Thought!

What about motor's that are purchased 50HP with a 1.35 SF
will the original motor starter be acceptable?

Thought!

If you install 75HP motor and only use 1/2 of the horsepower say 38 NBHP. Can you still use the 50HP starters?

Reason: We have a temperature rise issue involved. The motor amperage used on the 75HP will be far less than nameplated. We can't get the motor rewound for 125 Deg. F fluid temperatures. So if we derate the 75HP down to a 50HP with 1.35 SF = 67.5 NBHP can you still use the 50HP or Nema #3 starters and if so advise and if not advise.

I understand Code is code but if you took a 75HP and rerated it for 67.5 would the Code force you into a Nema 4 starter or could you still use the Nema 3?

What we are doing is trying to save the customer from having to put in new starters. But you guys are the pro's and I'm sure you know the service factors involved in the starter packages more than I.

Let me know - Thanks












wlovison@toyopumps.com

 

[jbartos]

Suggestion to Lovison (Mechanical) Aug 14, 2003 marked ///\\
We have a customer who has a 50HP motor installed already.
We have an application that calls for a 75HP motor will this require new starters?
///Yes, if the present one is not oversized, i.e. suitable for 75HP.\\Thought!

What about motor's that are purchased 50HP with a 1.35 SF
will the original motor starter be acceptable?
///No. The motors should not be purchased with 50HP 1.35 SF for continuous operation.
The motors with lower HP and high SF are usually applied to motor operated valves (MOVs) to unseat them. Then, they run under very low load.\\Thought!

If you install 75HP motor and only use 1/2 of the horsepower say 38 NBHP. Can you still use the 50HP starters?
///No. This is because of the 75HP motor inrush current that would damage the starter contacts and trip thermal overload relay furnished with 50HP starter.\\Reason: We have a temperature rise issue involved. The motor amperage used on the 75HP will be far less than nameplated. We can't get the motor rewound for 125 Deg. F fluid temperatures. So if we derate the 75HP down to a 50HP with 1.35 SF = 67.5 NBHP can you still use the 50HP or Nema #3 starters and if so advise and if not advise.
///No. The hitch is in the motor starting/inrush current that poses a problem to the 50HP starter.\\I understand Code is code but if you took a 75HP and rerated it for 67.5 would the Code force you into a Nema 4 starter or could you still use the Nema 3?
///Yes, since the motor inrush current would overload the NEMA 3 starter.\\What we are doing is trying to save the customer from having to put in new starters. But you guys are the pro's and I'm sure you know the service factors involved in the starter packages more than I.
///The right starter sizes are inevitable for the correct motor operation.\\\

 

[Lovison]

jbartos,

Clarification: As stated the motor would never really run much into the service factor other than maybe a short start up event.

The basic amp draw on the system is only 38 amps. The service is there only to protect a slightly higher fluid temperature.

We buy many motors with 1.35 standard. Do you size the motor starters for the FLA or the service factor conditions?



wlovison@toyopumps.com

 

[clarke]

Hi,
When sizing a motor starter the Full Load Amps (FLA) for the motor must be used as it will be 8X this load that the motor will pull on start up. I understand that you have derated the motor to 50kw from 75kw due to temperature but this seems to be a very inefficient way to run a motor because a motor is more efficient the closer to the full load it is ran and an average 75kw motor pulls 150amps you say you only pull 38amps, is there no way to cool the motor

 

[PUMPDESIGNER]

That temperature problem scares me.
All motors and overload protections are based on calculated heat rise, not actual.
If a motor shaft is conducting heat into the motor from a warm or hot process, you have an experiential consideration that is not part of the calculated heat rise used for motors. How much heat will be added to the total heat load that must be shed by the motor cooling system is a difficult problem to solve.

PUMPDESIGNER

 

[Marke]

During start, the shaft load is irrelevent. The motor impedance at a particular speed (slip) is the same independant of shaft load. If it is a 75HP motor, it starts like a 75hp motor whether it has full load or half load. If you use a reduced voltage starter, then the current may be reduced, but the impedance of the motor is still the same.

Select the starter for the motor rating, not the shaft load.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[boscorio]

Pumpdesigner's comments are something of which to take note. If heat being transferred through the shaft is a significant source of heat or if the motor is located in a very high ambient temperature environment but the starter overloads are not then overload heaters sized from a table are not worth much. Ambient compensated overloads of course only work if they're actually seeing the same ambient temperature that the motor sees. In such a case motor winding RTD'd or thermostats may likely be required to protect the motor reliably and without false trips.

 

[advidana]

Just replace the starter with one rated for 75hp. What if the load on the motor changes during operation and you use the wrong size starter and the motor causes problems. The client will not remember that you saved him some money-no he will come after you for damages. That is why the NEC code is followed.