Operating Induction Motor at Reduced Speed 1

[gcaudill]

Anyone care to guess, or have some tool, to estimate the temperature rise that will result from operating a non-inverter rated induction motor at less than base speed from an inverter supply?

I have:
5HP motor, 1750RPM, 184T frame, 230V/3/60, 87.5% Eff (at full load), NEMA design B, open drip proof enclosure.
Operating at:
.3HP, 175RPM, 100F. ambient, sensorless vector mode inverter supply.

Please include any assumptions in your guess/estimation.

Thanks in advance.

 

[DanDel]

I would expect that the main problem with operating a non-inverter rated motor from an inverter will be insulation failure due to voltage spikes caused by the inverter, not overheating. Contact the motor manufacturer to be sure.

 

[gcaudill]

I'm not interested in an inverter vs. non-inverter rated motor conversation (with all due respect to DanDel's appropriate recommendation). How about temperatures?

 

[electricpete]

Some very general discussion from NEMA VFD application guide:

"5.2.1.3 Reduced Speed
There are two general methods of motor cooling or ventilation: 1) speed dependent, 2) speed
independent. These methods may be affected by operation on variable frequency.
In the case of speed dependent ventilation (totally-enclosed fan cooled or open drip-proof motors) where
the rotation of the cooling fan is supplied by the motor, cooling depends on motor speed. Therefore,
cooling will decrease as motor speed decreases. The magnitude of the decrease depends on the speed
range. The rate of decrease depends on motor construction. Motors in this group may have only 20 to 50
percent of base speed cooling at very low speeds. Care should be taken to be certain that unidirectional
motors are operated only in the intended direction of rotation.
In the case of speed independent ventilated motors (totally-enclosed non-ventilated, totally-enclosed airover,
blower-cooled, etc.), cooling variation with speed is minimal, effectively staying constant. As a result,
motors of this type are better suited for operation at very low speeds or for wide constant horsepower
speed ranges."

It is my understanding that for inverter fed motors, the rating shall include identification of ratio between base speed and the lower speed at which constant torque operation is no longer possible due to reduced cooling.

 

[RadarRay]

This link doesn't fully answer your question but you may find it interesting reading. If they can do the curves, there must be an equation that does what you want.

http://www.patchn.com/mtrwhtpaper.htm

 

[GusD]

hllo gcaudill

A lot of motors used with Inverters are not necessarily Inverter rated.Often times, people needed a variable speed
application, but they already had an expensive motor that they wanted to use.Not all the non inverter type motors used in these situations burned up or failed .Some operate for many years if the VFD is properly sized and protected .
If the application is not a CTorque motor should operate down to 30/40% speed without overheating.If load requires a CTorque you would have to provide external cooling for motor to operate at low speed and full load.
Some motor HP derating,if at all possible would help the system.

GusD

 

[GusD]

hllo gcaudill

A lot of motors used with Inverters are not necessarily Inverter rated.Often times, people needed a variable speed
application but they already had an expensive motor that they wanted to use.Not all the non inverter type motors used in these situations burn up.Some operate for many years if the VFD is properly sized and protected .
If the application is not a CTorque motor should operate down to 30/40% speed without overheating.If load requires a CTorque you would have to provide external cooling for motor to operate at low speed and full load.
Some motor HP derating,if possible would facilitate this system.

GusD

 

[moturcu]

Hello,
Due to the low back emf at low speed, current regulator in the vector controller makes regulation more aggresively. You can see this from the frequency spectrum of the motor current for low and base speed operations. If current regulator is hysteresis type you will see more components at high frequencies for low speed operation. If the current regulator is constant frequency PWM or space vector PWM you will start to see higher magnitude of currents at lower frequencies. Regardless of the current regulation scheme, this will essentially increases copper losses and temperature. Depending on the magnitude of the high frequency components, you will see proportional increase in core losses as well.

It is hard to estimate from the core loss point of view. But comparing frequency spectrums at low and base speed can give you rough idea about percentage increase on losses.

 

[electricpete]

Here are some excerpts from NEMA MG-1 1998, Rev 2:

"30.2 GENERAL PURPOSE MOTORS USED WITH ADJUSTABLE-VOLTAGE OR
ADJUSTABLE-FREQUENCY CONTROLS OR BOTH

30.2.2 Application Considerations

30.2.2.2 Torque

30.2.2.2.2 Torque Derating Based on Reduction in Cooling
Induction motors to be operated in adjustable-speed drive applications should be derated due to the reduction in cooling resulting from any reduction in operating speed. This derating should be in accordance with Figure 30-2. This derating may be accomplished by or inherent in the load speed-torque characteristics, or may require selection of an oversized motor. The curves are applicable only to the NEMA frame sizes and Design types as indicated, and as noted, additional derating for harmonics may be required. For larger NEMA frames or other Design types consult the motor manufacturer. The curves in Figure 30-2 represent the thermal capability of Design A, B and E motors under the conditions noted, and are based on non-injurious heating which may exceed the rated temperature rise for 1.0 service factor motors (see 12.44) for the class of insulation. This is analogous to operation of a 1 .I 5 service factor motor at service factor load (with rated voltage and frequency applied) as evidenced by the 115 percent point at 60 hertz for a 1 .I 5 service factor motor.

30.2.2.2.3 Torque Derating During Control Operation
Induction motors to be operated in adjustable-speed drive applications should also be derated as a result of the effect of additional losses introduced by harmonics generated by the control. The torque available from the motor for continuous operation is usually lower than on a sinusoidal voltage source. The reduction results from the additional temperature rise due to harmonic losses and also from the voltage-frequency characteristics of some controls. The temperature rise at any load-speed point depends on the individual motor design, the type of cooling, the effect of the reduction in speed on the cooling, the voltage applied to the motor, and the characteristics of the control. When determining the derating factor, the thermal reserve of the particular motor is important. Taking all of these matters into account, the derating factor at rated frequency ranges from O to 20 percent. Figure 30-3 shows examples of a derating curve for a typical motor for which the thermal reserve of the
motor at rated frequency is less than the additional temperature rise resulting from operation on a control and one for which the thermal reserve is greater. It is not possible to produce a curve which applies to all cases. Other motors with different thermal reserve, different methods of cooling (self-circulation cooling or independent cooling), and used with other types of controls will have different derating curves. There is no established calculation method for determining the derating curve for a particular motor used with a particular control that can be used by anyone not familiar with all of the details of the motor and control
characteristics. The preferred method for determining the derating curve for a class of motors is to test representative samples of the motor design under load while operating from a representative sample of the control design and measure the temperature rise of the winding."

My take on the meaning is that 30.2.2.2.2 tells us the effect of the reduced cooling (neglecting heating from harmonics) can totally be accounted for by the derating curves of Fig 30.2. (Do you have access to that curve? if not I can try to describe it or post it somehow).

My take on meaning of 30.2.2.2.3 is that when we also factor in the added heating from harmonics, the result can no longer be calculated or given by precise derating factor and must be given by testing ... the manufacturer will generate a curve like Figure 30-3. It is the familiar curve torque vs speed which looks like a mountain with top chopped off. Level part is constant torque, the slope on the right is constant hp, the slope on the left is limit from reduced cooling. The point where the slope on the left ends and the level part begins is where the effect of reduced cooling becomes limiting IF we are driving a constant torque load. In the example curves given it appears that beginning of that region is around 67% speed.

As Gus mentioned for pump/fan loads you could go lower without concern... 30-40% sounds reasonable to me.

Just passing along what I have read from the standard. I certainly may have misunderstood it.

 

[GusD]

Hello Gcaudill

No doubt the information you looking for is available in Websites,trade books and numerous papers.The answer is also available at most of the places we all work for.
When I look at my work place, I don't see a single pump system powered from an Inverter that has an externally powered cooling fan for the motor.Reason why we limit the minimum speed to 30 hz.Below 30Hz, motors may Overheat in excess of their allowable Temp Rise.A Variable Torque Application like a pump,blower or similar load does not draw High Amps at the low speeds.There is the reason why we limit this minimum to 30Hz.I also walk around the Crusher Plant where we have many Apron Feeders.These are considered Constant Torque loads.Every one of these motors is equipped with an External powered fan, which maintains constant air flow regardless of the motor speed.Note that these motors are required to provide 100% Torque at very low speeds.They too have a minimum speed of 10Hz.Unless I missed your question,if your motor is driving a Variable Torque Load such as a pump or fan,you should have no problems by reducing speed to 30 or 40Hz ,depending on your motor.
GCAUDILL-One consideration to be taken into account is the fact that most of the motors we have are TEFC type.Your motor is ODP,you might have to monitor temperature rise for awhile.This type of motor may not cool down as efficiently as a TEFC motor.
Good motoring


GusD

 

[GusD]

Hi Gcaudill
Me again.
I have to mention that the pumps I spoke about on the previous answer are Centrifugal pumps not PD pumps.Also the Fans or blowers, can be very high Inertia loads ,which would call for proper accelerating Torques.

I'll sleep better now.

GusD

 

[mdonner]

I started many water pump applications, with pump idling at 10% rated speed, and none had temp rise problem (100F environment).
I believe the reason is the low power consumption at reduced voltage (v/Hz=constant), so the reduced cooling (1:10 squared = 1:100 cfm) has no impact.
Your figures show similar 1:10 control range, and may be variable torque load. If your application is CT load, the above does not apply.

 

[UKpete]

Taking the problem step-by-step, the temperature rise of the motor will depend on the losses and the cooling.

Looking at the losses first, and putting aside the effect of the inverter harmonics for a moment, a reasonably well balanced design of induction motor may have rated condition losses in the very approximate proportions 25% stator copper loss, 15% rotor copper loss, 40% stator core loss, 20% friction and windage loss (and I've neglected stray loss). These figures will vary according to the motor rating, and I don't have access to design calculations covering 5hp, but they will do for this. Your machine will have a total loss of about 470W at the rated condition, based on stated 87.5% efficiency.

Running at 1/10th speed will dramatically reduce the iron loss, which is proportional to something between speed and speed squared. The stator copper loss will also reduce; at .3hp torque will be about 60% of full load torque and assuming current reduces in roughly the same proportion, the stator copper loss will be less that 40% of the full load value. This is approximate, the equivalent circuit of the induction motor has the magnetizing current branch across the motor terminals but as the voltage is reduced (in proportion with the speed) the magnetizing current will be drastically reduced compared with the torque producing component.

Friction and windage is also going to fall fairly dramatically though this loss will not tend to affect the winding temperature. At rated speed most of the f&w loss probably comes from the fan.

So for sine wave operation, your losses will fall to something under 100W, say 20% of full load losses. Although the fan air volume will be 10% of that at the rated speed, the motor will be able to dissipate far more than 10% of the full load losses. For example, see fig.5 in radarrays link - assuming the speed axis goes down to zero, this shows that at zero speed the 180T motor can still operate at about half power (the curve is for TEFC motors but I don't think this matters).

Even with no forced air movement, a 184T frame has enough dissipative surface to shift about 125W with a 60degC temperature rise, based on 12W/m^2 degC in metric units.

So I don't think there will be a problem, even with some loss added due to inverter harmonics. If anything is going to suffer it will be the winding, so if it's accessible you could always put a thermocouple on the end-winding to check.

 

[electricpete]

TEFC is much better at heat dissipation without fan than ODP. NEMA figure 30-2 shows approx 10% more torque derating for ODP than TEFC over the range 15-30hz.

That curve does not go down to 10% speed (6hz) where gcaudill wants to operate. It stops of 15hz (1/4 speed). If we were to project those curves to the zero axis it would show 40% torque derating for odp and 50% for TEFC, but I'm not sure if we're allowed to extend it (if it's valid all the way down then why didn't they draw it?). Also as mentioned this particular curve accounts only for effect of reduced cooling, not increased heating from harmonics.

UKPete - what is the origin of 12W/m^2 degC?

 

[electricpete]

Here are the NEMA MG-1 figures referenced in the above excerpts:

http://reliability-magazine.com/pub/NEMA_CoolingFigures.jpg

 

[electricpete]

For completeness, I need to say that I clipped the "notes" off of Figure 30-2. Here they are:

"NOTES
I-Curve identification
a. Limit for Class B 80°C or Class F 105°C rise by resistance, 1 .O service factor.
b. Limit for Class B 90°C or Class F 115°C rise by resistance, 1 .I 5 service factor
2-All curves are based on a sinusoidal wave shape, rated air-gap flux. Additional derating for harmonic voltages should be applied as a multiplier to the above limits.
3-All curves are based on non-injurious heating which may exceed rated temperature rise.
4-Curves are applicable only to frame sizes and design types indicated. For larger frames or other design types consult the motor manufacturer.

 

[electricpete]

One more correction, I clipped part of the Title of Fig 30-2. It is "THE EFFECT OF REDUCED COOLING ON THE TORQUE CAPABILITY AT REDUCED SPEEDS OF 60 HZ NEMA DESIGN A, B, AND E MOTORS"

OK, all done now.

 

[UKpete]

electricpete, my figure for heat transfer coefficient was based on ESDU data (this is an engineering data service in the UK). It is a very distant memory and perhaps it is over-optimistic, 6W/m^2 K may be a more realistic value, e.g. see fig.8 in the following link:

http://www.motor-design.com/Flotherm_Conf_Staton_1996.pdf

Regarding the NEMA graph, I have had a look at this and you are correct, it does not support my argument. I think be pessimistic data, the purpose of the standard being to represent part of the contract between manufacturer and customer rather than a source of typical engineering data. It will have to represent the worst case.

Obviously gcaudill you will be aware of the risks; if you are from an OEM supplier about to deliver this drive to customers, you will need to investigate further. At the other extreme, if it a one off for a non-essential process in-house, you may as well run it and see how hot the frame gets.

 

[electricpete]

UKPete - that's a great link. Definitely a good approach you suggest to apply a heat transfer coefficient to the area. One thing I believe that the "hc" tabulated in fig 8 is a convection coefficient related to differential temperature between the ambient and the outer metal surface. Isn't there an additional differential temperature going from the winding to the outer metal surface. I notice they don't mention it at all in the article so I'm not sure whehter it's important or not.

 

[gcaudill]

Thanks everyone for the information. We are going to give it a go and see what happes. We will monitor temperature to prevent any thermal suicide.