What is the efficiency level of Direct drive PM motor for washing machine during Laundry mode (High torque at low speed) and SPIN-DRYING mode (High speed with low torque)?

There are many variables in play here, but this will get you in the ballpark: (1) The efficiencies at low speed "agitate" (well below the motor's base speed) are in the 20-30% range, but fluctuate significantly since the load is highly volitile. (2) High speed efficiencies are around 80% or more. Again, there are many variables such as: loading conditions, agitate stroke profile, control algorithm, etc.. Hope this helps.

Ricks answer is a good one; and they are not (surprised if they were) designed for max efficiency, but low cost.

Actually the opposite is true in my experience as a washing machine direct drive developer. The PMSM/BLDC motor for a washing machine can be designed so that its "base" speed is just above the maximum agitate (wash) speed, typically around 250rpm, to achieve 70-90% efficiency during wash, where the phase currents and heating effects are the highest, therefore high efficiency is important. Then in spin, (up to 1000rpm+) field weakening (phase advance) drive techniques are used to spin the motor above it's normal design speed. In field weakening mode, the efficiency is compromised, but high efficiency is not needed so much in high speed spin since you have forced convection from windage, and the load torque is not as high. All this applies to the Fisher and Paykel Smartdrive system, which is what I am familiar with, and also to the Whirlpool Calypso and Kenmore high-end top loaders, which also use the Smartdrive system. Cheers.

Hi Adam, are you certain you can get to 70-90% efficiency, under load, at agitate speeds below 150 RPM? In my experience using F&P direct drive motors for the last 3 years, I've never seen those kind of numbers for agitate.

Here are some data points from an actual F&P direct drive motor dynamometer test:

- 50 RPM / 39 Nm / 20% eff
- 100 RPM / 36 Nm / 40% eff
- 150 RPM / 30 Nm / 55% eff
- 200 RPM / 25 Nm / 70% eff

The 50 and 100 RPM data points above are taken while driving the motor slightly above the linear torque region of the motor (some saturation), but even if we reduced the torque I don't see how we could get to 70-90% efficiency.

Adam makes a good point about being able to set design base speed to max the efficiency for the application you want. if you are switching quadrants of operation during agitataion; could be that is where the losses are coming from.

The interesting thing is that Adam and I are basically talking about the same motor from Fisher & Paykel with a base speed relatively close to the peak agitate speeds. Additionally, we are only operating in quadrant 1 (+ torque, + velocity) when running a dynamometer test, so changing from motoring to generating would not account for the efficiency differences that we are seeing.

I am running some dynamometer tests as we speak to see if I can get higher efficiencies for agitate speeds, but I still don't see how it could possibly get as high as 70-90%.

Great discussion by the way ... thanks for the comments!

Update:
By limiting my agitate torque and staying in the linear torque region, I can get up to 70% efficiency at 150 RPM, but it falls quickly below 150 RPM where most of the agitate cycles spend their time. For example, efficiency falls to 55% at 100 RPM and 40% at 50 RPM. This is certainly an improvement assuming you don't need more torque, but still not in the 70-90% range.

Yeah, great discussion. I know power electronics and controls effect efficiency, but not by 20% +.

I'm intrigued why a GE engineer would be experimenting with an F&P smartdrive motor .....but anyway....

I spent about 10 years developing and fine-tuning the drive controls, both software and hardware, for the Smartdrive motor for F&P. There's a lot of proprietory information tied up in it, which of course I can't discuss. A lot of which was to do with maximising efficiency and reducing noise. I can't discuss application-specific information which has commerical value to F&P. Only generic motors and controls info.

You didn't say what type of controller you're using, or which generation of motor, but to answer what I can....

Your torque figures are higher than what a standard F&P motor controller module will typically deliver to this motor (more like 25-28Nm), hence you are running it at slightly higher currents than intended. Max power delivered is around 500W.

Of course no motor will maintain the same efficiency over every operating condition. The 70-90% efficiency I mentioned is not at max torque for every speed. As you say, the efficiency will increase at higher speeds and lower torque loads. At max torque and low speeds (50rpm) yes the efficiency will drop to around 50%. But at 150rpm and higher, even at 25Nm, you should be able to exceed 70%, using an efficient drive method.

Very large wash loads, which are your worst case design scenario for heating, normally use higher peak agitate speeds, around 150rpm or so. Trying to hold 50rpm with such a wash load is a bit pointless, so a dyno test at this low speed doesn't tell you a lot.

If you really wanted more efficiency at <150rpm, you'd need to increase the Back-EMF constant of the motor, by adding turns, using stronger magnets etc, but this would of course lower the base speed, at the expense of max spin speed, and even more reduced efficiency during spin. Like everything in life, it's a trade-off!

Cheers!

Adam, Rick is listed with GE appliance, not transportation. GE is more know over here for small electrics (consumer goods) than all the other stuff they do.