View Full Version : Motor load & No load

03-03-2012, 01:46 AM

I've 4KW/480V/60HZ AHU fan motor with delta connection. motor name plate FLA is 7.1A. insulation F class.

this motor getting 60deg C within 10mins with load. and motor current is 6.5amps.

but without load condition motor current is only 0.9Amps only.

Does any one advice, without load condition motor supposed to be how much ? and is it motor temperature acceptable ?


03-03-2012, 03:56 AM
The name plate F class insulation will indicate an acceptable winding/ motor temperature, from memeory F class is high. possibly 80'C.
So where is the problem, the motor is doing its job. If motor is in the air stream all the better. No load equals low load low power consumption, and a greater potential to overheat without air flow cooling.

03-03-2012, 06:21 AM
Hi there Magoo. Think OP was just concerned at how quickly his motor heated up.
Had a sparkie change a 15 KW motor and the new one didn't last a day.
Doesn't help when the motor is not rated for the environment it has to work in, as in hot and dusty conditions.

03-03-2012, 10:49 PM
Class F is rated up to 155C

Is this a new installation or something that you are now checking?

04-03-2012, 12:31 AM
Have you carried out an insulation test and if so what reading did you record? Is there a cooling fan on the non drive end and is the air intake clean? Is the supply frequency in Singapore 50hz or 60 hz? as I was always led to beleive it was 50h, the load current and temperature are within the motors design. The motor spec is for North America and might not be compatible with the electricity supply characteristics of your country causing it to prematurely heat and eventually fail.

04-03-2012, 11:31 AM
yes. its correct F class insulation 155deg C. but motor getting 60deg C within 10min, is it acceptable?

megger infinity....
ambient temperature of 30oC
Motor 60HZ and supply frequency also 60Hz (its vessel)
there is a cooling fan on the non drive end.

main thing this is a Air handling unit's fan motor.

I am worried this is AHU system its already more cooling on the motor. then how it is getting 60Deg C within 10min... .

04-03-2012, 03:13 PM
You need thermographic picture to "see" the bearings.
Check that the belt is not too tight.
30C in 10 minutes is not ok.

04-03-2012, 09:24 PM
A totally enclosed motor with cooling fan and +30' C ambient, it will get hot. Do the sums, at 6.2 amps x 420volts x factor 1.73 x .90 effeciency x .095 power factor / 1000.
you have a 3.85 kw heater. With F class insulation at 150'C safety rating. An open type drip proof motor will run alot cooler , but in Singapore and high humidity would fail. So the design of totally enclosed and F class insulation is a good selection.

04-03-2012, 09:52 PM
Ooops Magoo, I think you made a small mistake.
The rated mechanical output of the motor is 4000 watts.
The efficiency for this motor will be something in the range of 85 % (or 0.85)
Using losses = output power X (1-efficiency) / efficiency) gives thermal losses of 4000 X (1-0.85) / 0.85 = 705 W.
Or, running idle gave the original poster 0.9 A, so 0.9 x 480 x 1.73 x 0.8 = 635 W.
So heat dissipation will be something around 700 W.

04-03-2012, 10:58 PM
If there is not enough resistance in air flow, centrifugal fan motor will overload and overheat.Example:If you open side panel on AHU at suction side and let air get in through side instead of duct, in some time thermal protection of fan motor will react.Check that no false air is geting in ductwork.Check voltage balance. That could be main suspect.Explain what you mean by "with load" and "without load".

04-03-2012, 11:14 PM
As Nike asks, are you checking the motor temperature with the air handling unit running or with the access doors open?

04-03-2012, 11:20 PM

05-03-2012, 01:57 AM
hi Nike,
When the AHU running access door fully closed.
balanced voltage.
With load = motor connected with fan using belt.
without load= motor not connected with fan ( belt removed)

for info... we are using 5 AHUs at the same vessel. but there is no problem and fan motor temperature is only 30~35deg C within 10mins. only this getting high temp...

05-03-2012, 03:21 AM
Hi Chemi.
amps * voltage * 1.73 * motor efficiency * power factor / 1000 = Kws drawn power.
Correct me where I have gone wrong. Motor rated FLA is 7.1 amps drawn power 6.2 amps. Motor rated 4.0 Kws.
6.2 amps / 7.1 * 100 = 87 %. 4000 watts * 87 % = 3480 watts
and I said 3850 watts, so an efficiency of .90 a modern HEM motor [ high efficiency motor ].
The unknown is power factor correction, if in this situation for example a high rise building the electrical design engineers will have carefully designed fluoro lighting on each floor between three phases to take advantage of fluoro ballast capacitors and get a reasonably close 0.95 to 0.98 power factor.

Goshen where are you, good time to comment now.

05-03-2012, 06:55 AM
If you have same AHU as this with trouble, and their fan motor is drawing same current, than this troublesome have mechanical issues.
Try to swap motors to see if problem is traveling with motor or it is fixed to particular fan.

What are the measured currents on other AHU-s?

install monkey
05-03-2012, 08:51 PM
good idea- its either bearings on the fan blade causing an increased resistance or volume control dampers/fire dampers closed causing a resistance

If you have same AHU as this with trouble, and their fan motor is drawing same current, than this troublesome have mechanical issues.
Try to swap motors to see if problem is traveling with motor or it is fixed to particular fan.

What are the measured currents on other AHU-s?

18-03-2012, 07:21 PM
A 3 phase motor will run fine with or without load. Check all 3 phase currents, and all 3 phase voltages. If the voltages all are within a few percent of each other, but the currents are not ( phase voltages are around 480V, but one phase current has more than a 5% or so difference to the others) then you have a shorted turn inside the motor. Rewind time or new motor time. Check the current on each motor winding wire ( you will need to do a bit of rewiring to split the 3 wires out so you can use a clamp meter on them) to make things simpler. Check winding resistances, with one end disconnected while you are doing this. If the resistance on the 3 windings is not the same within around 2 ohms then they definitely are shorted somewhere. not definitive, but a quick check that does not involve motor removal.

While the belt is off did you check the motor turns freely with no noise or stiffness in the bearings. As well check the fan on the end is actually turning and blowing air over the housing, I often come across fans that have cracked and do not flow air properly when the motor is running, even if they do turn with the shaft.