Electric motor type relative figures 1

Hi everybody,

I took some time to compare some motor series catalogs from ABB, Leroy Somer and WEG (see link below), so to gain some insight in present days industrial grade motors, comparing motor type and size vs nominal ratings.

In efficiency you can find IE4 options of any type, for IE5 it seems you shall go for paired motor-converter, usuallly some type of syc. reluctanc or hybrid magnet motor.

I took 250kW nominal power and 4 pole/400V for comparison.

My naive conclusions are:

In volumetric power density:
[ul]
[li] Best figures are for PM Synchronous, PM+Sync. reluctance, and pure Sync reluctance.Water cooled IM are near.[/li]
[/ul]

In mass power density:
[ul]
[li] Best: PM+Sync. reluctance, followed by PM Synchronous.[/li]
[/ul]

In maximun Torque volumetric and mass density:
        [ul]
[li] Best: Water cooled IM, then pure Sync reluctance and Self-cooled IM (specially high efficiency IM)[/li]
[/ul]

One question I would like to get some insight, is why non IM types seems to have a lower nominal and maximun torque, for same pole number and power/voltage.
Thanks for any comment or discussion.

Regards

Link

 

[Gr8blu]

Machine volume (and usually mass) will be affected by operating conditions (ambient temperature, altitude, cooling method) as well as mechanical considerations (single speed vs variable torque vs constant torque). In addition, operation in a classified (hazardous) environment may also impact the design - often by requiring increased volume for a given power/speed combination.

As to why "synchronous" machines exhibit SLIGHTLY lower rated torque compared to asynchronous (i.e. Induction) designs boils down to operating speed. Torque is (power/speed)*(factor). The factor changes depending on the units used: for example, Imperial units have power (horsepower), speed (revolutions per minute), and factor (5252) to get pound-feet of torque. An asynchronous motor will ALWAYS be running a bit slower than synchronous speed, which means the power/speed number will be marginally higher than for a true synchronous design. Maximum (i.e. "peak") torque is dependent on the internal geometry, among other things. The design criteria is process-dependent as well: some processes require high starting torque, moderate peak torque, and relatively low operating torque. Other processes may have low starting torque, very high peak torque, and moderate-to-high operating torque. And then there's everything in between.

Converting energy to motion for more than half a century

 

[waross]

I applaud your research.
Torque:
Rated, working or running torque is a function of rated HP and rated speed.
Maximum torque is a function of design and is generally outside the continuous operating range of the motor. (there are exceptions)
For an Induction Motor (IM) maximum torque depends mostly on the design of the rotor, particularly the squirrel cage winding.
The resistance of the squirrel cage winding and the depth of the winding below the surface of the rotor both affect the maximum torque.
For good basic information on induction motors see the Cowern Papers.
The Cowern Papers
Look particularly at page 3; Design Letters. This shows the torque curves of different designs of induction motors.
For added research search for "double squirrel cage windings".

Synchronous types of motors;
The maximum torque of a synchronous motor may have two values.
The first limit will be the torque at which the motor drops out of synchronism.
At that point, the motor may stall or the motor may revert to the starting torque curve.
Starting of synchronous type motors;
Starting at grid frequency;
Historically, synchronous motors were started as induction motors.
The synchronous motors used a squirrel cage winding for starting as an induction motor and as a stabilizing winding when running in synchronism.
The curves on page three of the Cowern Papers show just four of the possible squirrel cage designs.
The squirrel cage winding on a synchronous motor when used for starting will be designed for short time use and may not be suitable for continuous use.

Running an induction motor at variable frequency.
What is much more important to the torque of an induction motor than the applied frequency is the slip frequency or slip speed.
The slip frequency determines the frequency seen by the squirrel cage winding which determines the torque.
I will use a 1760 RPM motor for illustration:
This motor develops rated torque when the load pulls the speed down to 1760 RPM.
That is at no load the motor runs at 1800 RPM (neglecting parasitic losses, windage and bearing friction)
As the load is increased the speed drops until at full load the motor is at 1760 RPM and rated torque.
The slip speed is 40 RPM and the slip frequency or rotor frequency is 40RPM/1800RPM x 60Hz = 1.33 Hz.
For any applied frequency, the motor will develop rated torque at a speed corresponding to applied frequency minus 1.33 Hz.
Remember that torque is an effect, not a cause.
There will be no torque until a load is applied and then the actual torque will be that demanded by the load.
I will leave a discourse on the other types of motors you mention to others.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[onatirec]

Lots of words up here.

I guess I'm the only one surprised to see higher torque density in induction machines vs pmsm?

I looked at your comparison sheet and don't understand it. E.g. I see columns for Nm and Volume [m3] and torque density [Nm/m3]. But the torque density column does not equal the [Nm] column divided by the [m3] column? Same with the torque density calculations. Where do these last two columns come from?

Edit: Ah, I see the Max Nm column was used. Didn't see this before. Still surprised frankly.

 

[xj25]

Hi,

I didn´t know the Cowern Papers, it is a remarkable reference with interesting historic perspective.

Induction motors selected in the comparison are general purpouse, so I assume curve B is a good assumption.

Reviewing again, I see that most of non IM in the comparison, seems to have a higher speed working range that the 4-pole IM selected, so that could explain the big nominal torque difference.

i.e.: for 250kW
ABB IM: 1500rpm 4pole, 1600Nm
ABB Sync reluctance: 1500-2200rpm 4pole, 1591Nm
WEG PM Sync: max. 3000rpm 4pole, 701Nm. Likely rpms are the factor here
Dyneo PM Sync reluctance: max. 3600rpm 4pole 663Nm. Similar to the former

Regards

 

[waross]

When the Cowern papers were written the common integral horsepower (1 HP and larger) motors in use were the Induction Motor, the Wound Rotor Motor, the Synchronous Motor and the DC Motor.
Solid state drives such as the VFD and PWM controllers were not yet available.
The induction motor served well over 90% of applications.
Wound rotor motors were used for high inertia loads and when rough but dependable speed control was needed.
Wound rotor motors were often used for bridge crane applications.In the wound rotor motor both the speed/torque curve and the speed/current curve vary as the rotor resistance is varied.
Torque motors such as are used on cable take-up reels may spend much of their energized life at zero speed or locked rotor.
Wound rotor motors were often used as torque motors with suitable values of rotor resistance.
In larger sizes, synchronous motors were often oversized and used for Power Factor control as well as driving a load.
DC motors were used when more precise speed control than possible with a wound rotor motor was needed.
DC motors were typically driven by Motor-Generator sets.
As an example, a 40 HP DC drive would consist of a 40 HP DC motor in series with a 40 HP DC generator turned by a 40 HP or 50 HP induction motor and controlled by a 1500 Watt Amplidyne.
(The next time you are worried about the extra cost of a VFD for a speed controlled application, reflect on the cost of the old DC drives.)

When the development of high power/high voltage semi-conductors and the ready availability of VFDs DC controllers and PWM controllers, a lot of hardware became obsolete overnight.

We had to learn new characteristics of VFD controlled motors versus grid fed motors.

A tip to comparing motor curves between grid supply and VFD supply.
The important figure when studying the torque/speed curve of a VFD motor is the slip frequency or the slip speed.
Looking at the right hand side of the torque speed curve of an induction motor, you will see the torque peaking at about 2 times or 2 1/2 times rated torque.
A VFD driven motor should always operate in this portion of the curve. That is zero slip to about 2 to 2.5 times rated slip.
It helps to relabel the curve from RPM to slip RPM, or slip as a percentage of full load rated slip.
So for a 1760 RPM motor, 1800 RPM becomes zero slip and 1760 rpm becomes 40 RPM slip or 100% slip.
Now convert the supply frequency from Hz to equivalent RPM and you can see where on the curve you are operating for available torque.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!