Discussion Analysis of a REEM inadvertently used in a reverse power flow condition.

[timocharis]

Originally Posted by IneptOne

...both use induction motors because the partial load efficiency is higher ...

You say that like you understand it. I saw that song on the Tesla website and wondered, "why is that?" I'm willing to go along with the gag; I'd just like to get the punch line.

So: why is that?


Dave

[IneptOne]

Originally Posted by gkamysz

What software are you using to generate those plot graphics?

Greg

This is just a program I wrote to generate the plots. I like the plots because they give you the big picture, and it is easer to tell if a motor is well matched to the application. Then you can pick an operating point or region. Next you choose a Kv to fit the power source you have available. I like being able to overlay the power curves of various propellers as well. It is easy to see the props that will work well for a given power level.

I still like the traditional dyno style plots like I have been generating in Excel, and the fixed voltage plots like most of the RC motor system programs produce. But first I like to see the efficiency map.

The first one I saw was years ago from the AC Propulsion website. It was showing the induction motor used in the T-Zero electric car, now used in the Wrightspeed and Tesla electric cars.

Dan

[IneptOne]

Originally Posted by timocharis

So: why is that?

Dave

This is from a few posts back

“A second benefit is that the motor efficiency for any power level can be optimized for the operating RPM. You use high field currents when you need max power for acceleration, but once up to speed and you need a fraction of the power for cruising, the motor would be very inefficient if the field were left at the maximum value. If the field is reduced, the core losses drop for a given RPM. Look at the 2A efficiency map at 4800RPM. At 3kW output power the efficiency is in the mid 80s, but say you drop back to 500W for cruising, the efficiency drops to around 65% or so. Now look at the 1A efficiency map, 4800RPM and 500W output power. The efficiency for this case is around 78%. A map of field current vs RPM and Power Level can be created for maximum efficiency. The variable field motor has a much wider operating area than a permanent magnet motor! The downside is more complicated controls for optimum operation, less reliability because of the field brushes and of coarse the power required for the field. The field power is really only an issue at the lower power levels. At the highest output powers the field power is insignificant.”

You can see that at high RPM and low torque the efficiency is higher with lower field current. Adjusting the field current is the same as using different strength magnets.

You wind lots of motors, so you may have the parts to perform this experiment.

Take a small motor, it need not have any windings on it, and install a rotor with no magnets. Spin the motor by hand, it probably turns very freely. If you give it a twirl, it will probably spin for several seconds. Now install a rotor with magnets. Spin the motor again. In addition to the cogging torque that resists and then enhances the turning, you will feel a lot more resistance. Give the motor a twirl. It will likely stop in under a second.

The increased drag you feel is because of the core losses. At low speed, the losses are dominated by hysteresis. At high speeds the losses are dominated by eddy currents.

Both these effects set up magnetic fields that oppose the rotation of the magnets in the rotor. The faster the rotor turns the more the induced fields resist the rotation. At high speeds the torque needed to spin the motor can be quite high. The eddy currents cause I^2*R heating in the core and it will get hot. In normal operation the torque required just to spin the motor comes from current in the motor windings. This current causes I^2*R heating in the windings. When you measure The Io of a motor you are essentially measuring the torque required to overcome these magnetic losses.

These losses are there the whole time the motor is spinning, even when you are not drawing any power from the motor. If the motor is spinning fast and the output power is low the efficiency is very poor.

Now if you suddenly took the magnets away, there would be no magnetic losses. There would be not be any torque production either, so the trick is to use weaker magnets so the core losses drop, but not too weak so there is no torque production. For every RPM and torque combination there is a magnet strength that will be the most efficient. You do this with the Alternator Motor by changing the field current. In an induction motor you control the strength of the magnetic field by adjusting the slip speed. If the rotor spins the same speed as the rotating field in the stator and the stator current is reduced, there is no rotor magnetic field produced and the core losses drop to zero. No torque either, so again, the trick is to use the optimum slip speed for the RPM and torque the motor is running at.

The core losses are proportional to the strength of the magnetic field squared. So cutting the field in half reduces the core losses to one quarter.

In a permanent magnet motor, the magnetic field is not adjustable so there is no way to adjust it for low torque at high RPM. The permanent magnet motor will have a higher peak efficiency than the induction motor, but only at the optimum operating point.

Traction applications need lots of torque at low speeds for acceleration, and then a lot less torque at high speed for cruising. It is hard for a permanent magnet motor to be optimized for both conditions.

Don’t know it this helped or not?

Dan

[timocharis]

Originally Posted by IneptOne

This is from a few posts back ... Don’t know it this helped or not?

Perfect! I feel rather like a dope for not putting 2+2 together. Then again, I've been ratting around the desert lately and thinking of other things (back now, so no excuses).

But your explanation is golden. Thanks!


Dave

[salty9]

Has anyone considered what would need to be done to effectively control the "REEM"?