plc noob
Member
I have to size a replacement motor starter with limited info . i have the hp 75 and voltage 240 . but nothing else . what is the formule for sizing overloads ? can it be done with htis limited info ?
sizing motor starters
- Thread starter plc noob
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seppoalanen
Member
75 hp => 75*0.75 = 56.25 kW
55 kW/380V/1500 RPM => In = 106 A => In240V is around 168A
So In is around 172 A for 75hp/1800 RPM motor.
55 kW/380V/1500 RPM => In = 106 A => In240V is around 168A
So In is around 172 A for 75hp/1800 RPM motor.
Is it a motor, or a gear-motor combination ?
I use SI units to convert like this:
I = P / (U x 1.73 x Kpf)
I is the line current (in the phase) in A.
U is the line voltage (between two phases) in V.
P is the rated shaft power in W (not kW or hp).
Kpf is a the power factor.
For small motors it can be as little as 0.55.
For bigger motors it can be up to 0.85
If there is a gear in between the motor and the outlet shaft, then you have to account for the efficiency of the gear as well. It can be as low as 0.60 for some gearmotors. So for smaller gearmotors it becomes somewhat of a gamble to only rely on some vague shaft power rating.
edit: For a 55kW/75hp motor my charts say approx 0.72 for the power factor. That gives I=184A.
I use SI units to convert like this:
I = P / (U x 1.73 x Kpf)
I is the line current (in the phase) in A.
U is the line voltage (between two phases) in V.
P is the rated shaft power in W (not kW or hp).
Kpf is a the power factor.
For small motors it can be as little as 0.55.
For bigger motors it can be up to 0.85
If there is a gear in between the motor and the outlet shaft, then you have to account for the efficiency of the gear as well. It can be as low as 0.60 for some gearmotors. So for smaller gearmotors it becomes somewhat of a gamble to only rely on some vague shaft power rating.
edit: For a 55kW/75hp motor my charts say approx 0.72 for the power factor. That gives I=184A.
Last edited:
Yes the number of poles do matter, but my formula only accounts for 4-pole motors. I dont have something for 2- or 6-pole motors right here and now.
An overload should be sized to exactly the rated motor current.
An overload should be sized to exactly the rated motor current.
Stationmaster
Member
I carry a sliding "pocket engineer" developed by Buss (the fuse company).
It says for 230v, 3ph, 75hp, 1800rpm, squirrel cage, induction motors, the FLA is 192. It often errs on the high side.
The best source of info is the nameplate, the next would be the manufacturer.
Do you have any historical "full load amp" measurements to refer to?
Solid state overloads are set to FLA. Try to select an overload block with your FLA somewhere near the center of the range.
"Heater element" type overloads are selected from the manufacturers chart.
(I admire your education/work ethic)
Stationmaster
It says for 230v, 3ph, 75hp, 1800rpm, squirrel cage, induction motors, the FLA is 192. It often errs on the high side.
The best source of info is the nameplate, the next would be the manufacturer.
Do you have any historical "full load amp" measurements to refer to?
Solid state overloads are set to FLA. Try to select an overload block with your FLA somewhere near the center of the range.
"Heater element" type overloads are selected from the manufacturers chart.
(I admire your education/work ethic)
Stationmaster
thanks for the info . the motor in question has no nameplate .apparently one of the maintenance guys removed it with a chisel to order a new one instead of writing down the info . i got the info i have from an old print in the machine . thank you for all the replys . very helpful
rsdoran
Lifetime Supporting Member
Since it is a replacement why not get the same or equivalent to what is there i.e. get the info off the starter?
rsdoran
Lifetime Supporting Member
You can pretty much call most distributors and you have enough information they can put one together for you.
Check this out: http://www.ab.com/en/epub/catalogs/12768/229240/229248/3005006/3178786/229361/index.html
http://www.elec-toolbox.com/usefulinfo/flamtrcharts.htm#AC%203%20Phase%20Induction%20Type%20Squirrel%20Cage%20and%20Wound%20Rotor
http://www.engineeringtoolbox.com/nema-starters-d_918.html
Check this out: http://www.ab.com/en/epub/catalogs/12768/229240/229248/3005006/3178786/229361/index.html
http://www.elec-toolbox.com/usefulinfo/flamtrcharts.htm#AC%203%20Phase%20Induction%20Type%20Squirrel%20Cage%20and%20Wound%20Rotor
http://www.engineeringtoolbox.com/nema-starters-d_918.html
Last edited:
Tom Jenkins
Lifetime Supporting Member
A rule of thumb is that FLA on a 460 VAC 3 Phase motor is 1.25 x hp. For 230 VAC it would, of course, be 2.5 x hp. Don't use this for sizing heaters, though.
If you can't get nameplate data, use the starter manufacturer's charts or the UL tables. Note, however, that old and new motors can have big differences in nameplate FLA. There can be quite a range, so make every effort to get the right motor data sheet. Also note that some motors have a 1.15 service factor, which can give you some margin for error, but that isn't universally true.
Number of poles affect efficiency and power factor,but this ins't reflected in many starter sizing charts.
If you can't get nameplate data, use the starter manufacturer's charts or the UL tables. Note, however, that old and new motors can have big differences in nameplate FLA. There can be quite a range, so make every effort to get the right motor data sheet. Also note that some motors have a 1.15 service factor, which can give you some margin for error, but that isn't universally true.
Number of poles affect efficiency and power factor,but this ins't reflected in many starter sizing charts.
If you still have the old starter/overload look at the heaters - they will have a number stamped on them such as FH28 or similar. Look up this number in the table here: http://www.southlandelectrical.com/WestinghouseHeaterChart.asp and it will give you the current and starter size.
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