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Slow Running a BLDC

Posted: Sat, 13 Nov 2010, 12:56
by T1 Terry
This is a general type question. Is there any down sides to running a BLDC at slow speeds? The reason for the question is Danfoss refridgeration compressors use BLDC motors but there COP efficiency can be improved by running them slower than the normal speed they run at using their electronic control. There must be a reason for it, I'm guessing it's to protect the motor or control from over load but I don't understand enough of how the control circuit works.

T1 Terry

Slow Running a BLDC

Posted: Sat, 13 Nov 2010, 15:41
by coulomb
T1 Terry wrote: This is a general type question. Is there any down sides to running a BLDC at slow speeds?

Two issues. To get the same power at lower speed, the motor will be running at lower voltage and higher current. Copper losses are proportional to the square of the current, so half speed means four times the copper loss. (You make up for it a bit with less iron loss, but probably not enough to compensate).

The other issue is that there could be a cooling fan attached to the rotor, so at half speed, you might get considerably less cooling. With the increased winding losses, you might get temperature problems.

However, I would assume that air conditioning is a moderately low duty application (say 60% duty cycle, total guess, on the hottest days), so it should have time to cool down.

I would say give it a try, but monitor the temperature. Maybe you can just glue on a temperature sensor and watch it from the cabin, or if it's accessible, use an IR thermometer to record temperatures after a commonly used route. If the case of the motor is below say 60C, maybe a little higher, that should be OK.

Ideally, you'd want a sensor on the windings, but you are most unlikely to be able to get to these inside an air conditioner, I would think.

Slow Running a BLDC

Posted: Sat, 13 Nov 2010, 17:19
by acmotor
The question is related to the compressor and refigerant ?

The BLDC motor will (as per the coulomb copper current note) likely run at better efficiency at higher RPM if the power level remains the same.
However, the power level of a compressor is generally linked directly to the RPM. It will increase as the delivered pressure increases so if power required is 1kW at 1000RPM it can be 3kW at 2000RPM.
There will be a minimum RPM for correct operation of the heat pump process at any given temperature differential between condensor and evaporator. Basically the refirgerant must enter the evaporator as a liquid, obviously.

Run the compressor too slow (not enough compression rate) and there's no heat pump. Pump too hard and the compressor efficiency will drop as the fluid pressure is too high and too hot and hot fluid enters the evaporator.

Slowing the compressor speed is a technique used in inverter aircons as pumping losses reduce as RPM comes down, but only as the temperature differential also drops i.e. system not needing to run at full capacity.

The trap can be that with a high differential pressure and lower RPM the torque (and current) will rise to produce the required power. You need to know where you stand before making changes. I'm with coulomb, monitor the temperatures (and power).   Image

Slow Running a BLDC

Posted: Sat, 13 Nov 2010, 21:12
by T1 Terry
Thanks Coulomb and acmotor, the compressor in question is for a fridge/freezer, the specs are here, it's a Danfoss DB35 and by the specs the COP improves at lower pumping speeds as does the current draw. It has been suggested on another forum that the compressor speed could be dropped even lower than the 2,000rpm min given in the specs but I felt there had to be a good reason why they would not drop the speed down to the lowest possible point to gain the best COP. There is every chance that 2,000rpm is that point, I can't see any other reason why a company would spend all that money on developing an electronic control if a lower pumping speed would be beneficial.
The other reason was I'm looking at converting a domestic split system unit that is already fitted to my bus to a DB50 compressor and possibly making my own control to minimise current draw over night so knowing all the downsides will help me not make a very expensive mistake, hopefully.

T1 Terry