Motor temperature rise and ambient temperature
Motor temperature rise and ambient temperature
(OP)
Hello,
Our Sag mill motor is rated 1500HP with 4000V, 212A and 0.8 lead power factor, insulation class F and permissible temperature rise 80K. Our motor is actually used at 112% of nominal charge (1320kW, 0.943 lead P.F., 190A, 4220V). We want to know if we can still increase the load without damaging the motor or if the motor capacity is already reached.
In my mind, as it is a synchronous motor, I can go up to 125% of nominal load by adjusting P.F. to 1.0 instead of 0.8 lead. So 4000*212*1.73*1 = 1468kW.
Rated Input power is 4000*212*1.73*0.8 = 1175 kW
1468/1175 = 125% of rated load.
We could also continue overexciting the motor and maybe reach a little more if the temperature limits are respected…
But I want to be sure that we respect the allowable maximal temperature of the motor.
So if I look at the RTDs, what values should I setup my alarms and trip to avoid damaging the motor ? Is it based on actual ambient temperature + permissible temperature rise (80K), or is it based on motor class insulation maximal temperature (155°C for class F)
Ambient temperature of the mill is 20°C and not 40°C. Insulation class is F (155°C) and permissible temperature rise is 80K.
Does the critical temperature to avoid is 155°C – 10°C (hot spot) = 145°C since it is a class F insulation motor? Or does the critical temperature to avoid is 20°C + 80K = 100°C. So that over 100°C temperature rise within the motor will exceed 80K..
There is a huge difference between both so I really need to understand the way it works to adequatly protect the motor.
Do I have to prevent exceeding the 80K temperature rise within the motor whatever ambient temperature is (below 40°C..because I know there are factors to consider if I'm over 40°C ambient) or do I have to prevent exceeding the maximal allowable temperature based on motor class insulation, or both .. ??
Thanks !
Dave
Our Sag mill motor is rated 1500HP with 4000V, 212A and 0.8 lead power factor, insulation class F and permissible temperature rise 80K. Our motor is actually used at 112% of nominal charge (1320kW, 0.943 lead P.F., 190A, 4220V). We want to know if we can still increase the load without damaging the motor or if the motor capacity is already reached.
In my mind, as it is a synchronous motor, I can go up to 125% of nominal load by adjusting P.F. to 1.0 instead of 0.8 lead. So 4000*212*1.73*1 = 1468kW.
Rated Input power is 4000*212*1.73*0.8 = 1175 kW
1468/1175 = 125% of rated load.
We could also continue overexciting the motor and maybe reach a little more if the temperature limits are respected…
But I want to be sure that we respect the allowable maximal temperature of the motor.
So if I look at the RTDs, what values should I setup my alarms and trip to avoid damaging the motor ? Is it based on actual ambient temperature + permissible temperature rise (80K), or is it based on motor class insulation maximal temperature (155°C for class F)
Ambient temperature of the mill is 20°C and not 40°C. Insulation class is F (155°C) and permissible temperature rise is 80K.
Does the critical temperature to avoid is 155°C – 10°C (hot spot) = 145°C since it is a class F insulation motor? Or does the critical temperature to avoid is 20°C + 80K = 100°C. So that over 100°C temperature rise within the motor will exceed 80K..
There is a huge difference between both so I really need to understand the way it works to adequatly protect the motor.
Do I have to prevent exceeding the 80K temperature rise within the motor whatever ambient temperature is (below 40°C..because I know there are factors to consider if I'm over 40°C ambient) or do I have to prevent exceeding the maximal allowable temperature based on motor class insulation, or both .. ??
Thanks !
Dave





RE: Motor temperature rise and ambient temperature
RE: Motor temperature rise and ambient temperature
120 deg C trip is the norm for class F machines.
Never a good idea to overexcite a motor. More losses and harmonics.
Muthu
www.edison.co.in
RE: Motor temperature rise and ambient temperature
The final word is the temperature and the ambient plus rise should not be exceeded. Otherwise I see no reason not to mess with any of the drive values - as long as you realize you're dramatically minimizing motor life by all this brinkmanship.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor temperature rise and ambient temperature
My permissible temperature rise is 80K. The standard ambiant tempersture is 40degC. So 80+40=120 degC. So i should avoid exceeding 120degC based on 40degC ambient temperature. But if my ambient temperature is 20degC, does my maximal temperature remain 120degC or does it change to 80+20=100degC ?
Also, class F insulation àllow 155degC. So what happen if I exceed the 80K permissible temperature rise but stay within the 155-10(hot spot)= 145degC? As I understand the 25degC beteeen 120 and 145 is a safety margin but can the motor works in that safety margin whitout damaging itself?
RE: Motor temperature rise and ambient temperature
All the temp limits on the motor are absolute based not relative based. The 120C is the absolute limit for safe operation. I would expect the motor to be damaged at anything above 120C and that operation there would be technically irresponsible. I would remove any staff that put my $150K motor in that situation. It shouldn't be your position to bail out management's undersized motor.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor temperature rise and ambient temperature
Do you have a standard or a norms reference for that conclusion? Because I made some searches and I did'nt see it nowhere.. I've looked in Nema MG-1 and some others and stil can't find this written clearly
RE: Motor temperature rise and ambient temperature
RE: Motor temperature rise and ambient temperature
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor temperature rise and ambient temperature
RE: Motor temperature rise and ambient temperature
synchronous motor has a lowest stator current with power factor or cos φ 1 so you should try to come to this value.
It all depends on the load and the value of rotor current , which should not exceed a maximum value from nameplate .
With these settings heat of stator winding should be the most economical and all the current from the grid should be active.
Your motor now with leading power factor make reactive power and power factor correction of the grid .
Good luck !