Do RPM reductions affect start limitations?

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VinceS

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MG1 gives general guidance on the number of starts per hour for a motor of various sizes for across the line starts. Does reduction gearing have any affect of this limitation?

My point, I'm trying to make a case, using MG 10-2001, against load cycling motors to save money. At my first presentation I was told MG1 need not apply because we use RPM reduction.

Anyone with pros or cons in this area feel free to advise...
 
Reference are both NEMA STD's

Reference are both NEMA STD's

NEMA Standards Publication MG 1-2006 Revision 1-2007 and NEMA MG 10-2001
 
I believe that the number of starts per hour is based on the fact that the inrush current required in starting a motor increases the heat in the motor and impacts the life of the motor winding insulation. Having a gear reduction drive on the output of the motor shaft won't have much impact on the heat generated from starting.
 
math example from NEMA MG 10

math example from NEMA MG 10

I am assuming, that the ratio of blower RPM to motor RPM equates to some sort of gearing ratio. Therefor I am correct to use gearbox output RPM / motor RPM when determining the number of starts.

? Copyright 2001 by the National Electrical Manufacturers Association.

Example 2:
25 HP, 2-Pole, 3550 rpm, Design B motor belt connected to a 5000 rpm blower with a Wk2 of 3.7 lb?ft.2

Load Wk2 referred to motor shaft = (5000/3550)^2 x 3.7=7.34ft(lb)^2

From Table 7:
A = 4.4
B/Load Wk2 = 26/7.34 = 3.5
Minimum off time = C = 115 seconds

The value of B/Load Wk2 is less than the maximum number of starts per hour and therefore the motor must
be limited to 3.5 starts per hour with a minimum off time between starts of 115 seconds.
 
So, how is Wk2 calculated?

So, how is Wk2 calculated?

The major issue I have is that Wk2 cant be calculated when load weight is unpredictable. Furthermore, if load is such that it can trip O/L's the stress on the motor will lead to early failure.
 
O/L tripping and early failure

O/L tripping and early failure

VinceS said:
The major issue I have is that Wk2 cant be calculated when load weight is unpredictable. Furthermore, if load is such that it can trip O/L's the stress on the motor will lead to early failure.
Click to expand...

I think you missed on the last phrase:
Furthermore, if load is such that it can trip O/L's the stress on the motor will lead to early failure.
Click to expand...
The O/L protects the motor damaging internal heat by "simulating" the heating effect of motor current in the O/L heaters, hence tripping to cut off high currents in the windings! Honestly, you do away with extensive motor failure by having those O/L's functional. In fact, some O/L's have "AUTO" reset functions - i.e. you can't reset the O/L trip when the heaters inside are still hot.

BTW, the effect of having a reduction gear is clearly seen in the formula for calculating the no. of starts per hour. The "reduction ratio squared" increases the value of the divisor (load WK2) of "B" cautioning you to lower the number of starts per hour, up to a value set by the "A" value. (From your example: w/o reduction gear; no. of starts (calc) = 26/3.7 = 7- this value greater than A =4.4, so 4.4 is the limit. With reduction gear = 26/7.34 = 3.5 starts - this value is less than A value therefore this limits no. of starts)
 
VinceS said:
MG1 gives general guidance on the number of starts per hour for a motor of various sizes for across the line starts. Does reduction gearing have any affect of this limitation?

My point, I'm trying to make a case, using MG 10-2001, against load cycling motors to save money. At my first presentation I was told MG1 need not apply because we use RPM reduction.

Anyone with pros or cons in this area feel free to advise...
Click to expand...

As it was pointed out mechanical RPM reduction will have no effect on the number of restarts as limited by the manufacturer. The MG-1 is not necessarily strictly adhered to as far as limitations. The cumulative heat damage is also affected by the driven inertia, eg. how long will the run-up continues before the current levels off.

The following would be considerations on this application:
Contact the motor manufacturer and give them the driven inertia to calculate the optimum thermal performance.
RTD's in MV motor windings are often used to calculate the thermal performance of the motors in combination of the I2T and it often allows the motor to be used at optimum thermal performance. It will actually calculate - based on previous data and 'remembered' - of how much time will you have until the next safe restart. It calcualtes the predicted cool-down time.
In case of SSRV's and ASD's the inrush current is reduced, so the thermal damage is far less than FVS. The manufacturer may be able to calculate the resultant safe restarts.
Remember that the cycle is heat up/cool down. The motor is hotter if you start it again after a short time of a previous run as it may not have cooled down to the run temperature after the accumulated extra heat due to startup.
 
Thank you all..

Thank you all..

Mr Weressl, your insight is on the mark, I will incorporate RDT's wiring into my design. More than likely it won't be obtained until the first motor fails, but the RTD at high temp will bypass the interval timer allowing the motor run cool.
 
As it was pointed out mechanical RPM reduction will have no effect on the number of restarts as limited by the manufacturer.
Click to expand...
I think above conflicts with the next statement:
The cumulative heat damage is also affected by the driven inertia, eg. how long will the run-up continues before the current levels off.
Click to expand...
Reasons:
  • NEMA governs designs related to electrical equipment. This is an association trying to standardize every electrical equipment design to facilitate interchangeability and issues guidelines for safe and proper operation of electrical equipment.
  • The formula : Allowable no. of starts per hour (for design A and B motors) = "B"/load WK2 means that the load inertia determines no. of starts, the value of "B" being fixed depending on the motor HP and speed.
It is clear that the mechanical RPM reduction affects the allowable number of starts. In the case of the OP's example, the speed change reduces the allowable number of starts by virtue of a larger load inertia the drive motor is subjected to.

MG 10-2001, Table 7 has been prepared as a guide to:
  • a) the minimum off time required to allow the motor to cool sufficiently to permit another start;
  • b) the maximum number of starts per hour (irrespective of load Wk2) to minimize the effect of winding stress imposed by repeated starts; and
  • c) a means of adjusting the number of starts per hour as a function of the load inertia. It should be recognized that each start is one factor in the life expectancy and reliability of the motor, and, as a result, some reduction in life expectancy and reliability must be accepted when a motor is applied at the upper range of the starting duty determined by Table 7.
Click to expand...
The problem, as I understand here is to prevent motor overheating and failure. Embedded RTD's can solve the limits of the number of restarts as you can't run motors when the temp interlocks on (putting off motor control circuit)
 
VinceS said:
Mr Weressl, your insight is on the mark, I will incorporate RDT's wiring into my design. More than likely it won't be obtained until the first motor fails, but the RTD at high temp will bypass the interval timer allowing the motor run cool.
Click to expand...

Make sure that the RTD's - 2 per phase for a total of 6 - are connected to an appropriate protective relay. The RTD will not necessarily 'bypass' the interval timer, but actually measures the temperature and gives a more accurate thermal modeling than calculated from current alone. Also in case the inertia can not be calculated it will determine the optimum cooling period, the safe restart time. The RTD will only protect against thermal damage but not the repeated mechanical stress of the restart.

Some of the relay models:
Eaton MP3000, GE/Multilin 369R or 469, etc.
 
topgone said:
I think above conflicts with the next statement:

Reasons:
  • NEMA governs designs related to electrical equipment. This is an association trying to standardize every electrical equipment design to facilitate interchangeability and issues guidelines for safe and proper operation of electrical equipment.
  • The formula : Allowable no. of starts per hour (for design A and B motors) = "B"/load WK2 means that the load inertia determines no. of starts, the value of "B" being fixed depending on the motor HP and speed.
It is clear that the mechanical RPM reduction affects the allowable number of starts. In the case of the OP's example, the speed change reduces the allowable number of starts by virtue of a larger load inertia the drive motor is subjected to.


The problem, as I understand here is to prevent motor overheating and failure. Embedded RTD's can solve the limits of the number of restarts as you can't run motors when the temp interlocks on (putting off motor control circuit)
Click to expand...

You are correct and I misstated my case. Given the same equipment the addition of a mechanical speed reducer will result in a greater inertia on the motor shaft.

NEMA establishes a requirement, but the manufacturer is free to produce anything they can sell. Should a motor have a higher than NEMA rating the interchangebility may not necessarily work. One of the biggest differences between motors that otherwise meets the NEMA MG1 is the breakaway torque. Found that out the hard way......:roll:
 
weressl said:
You are correct and I misstated my case. Given the same equipment the addition of a mechanical speed reducer will result in a greater inertia on the motor shaft.
Click to expand...
For the same driven load, the inertia referred to the motor shaft reduces as the square the reduction.
 
weressl said:
Make sure that the RTD's - 2 per phase for a total of 6 - are connected to an appropriate protective relay. The RTD will not necessarily 'bypass' the interval timer, but actually measures the temperature and gives a more accurate thermal modeling than calculated from current alone.
Click to expand...
Totally agree.
Actually measuring the temperature takes away the need for estimations, calculations, and/or guesswork.
 
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