Hi list,

Can somebody (I'm sure there are many out there) explain to me in simple words some of the concept about ac-drives vs motor torque at very low RPM.

There were drives that vendors claimed could go to zero Hz at 100% torque for 1/2 seconds. I understood that these, coupled with an encoder, would detect a movement and then drive the oposite way for a very short time. Hence you
would think the load would not move but it would. In such small step that perception is that the motor is static. I have experience this and the load is really held for this short time.

Now there are drives that will go zero Hz, full torque,... full duty. That is a motor stopped at full torque ALL THE TIME.

Can someone shed some light on me about how this is done. What about heat, the fan, etc. What type of wavw is used for this and why?

Tanks again.

-Pierre Desrochers

 

Pierre,

100% torque at stall for 24Hrs is easy to accomplish and will not harm the motor or overheat the motor in any way. Using a closed loop vector, there are two components to the current in the motor stator.

One component is a magnetizing current. This is simply using the well understood method of using current through coils to create an electromagnetic field in the rotor. When the rotor rotates, the encoder tracks the rotation and causes the magnetizing field to track synchronously with the rotor position.

The second component is a torque producing current. This current passes through the same stator and creates a magnetic field that is oriented at 90 degrees to the magnetizing current. The magnitude and direction of this magnetic field determines the torque and direction that the torque is applied.

Having an encoder report the rotor position at all times ensures that the orientation of these two components are always at 90 degrees to one another for maximum efficiency of force moment.

NEMA rated Induction motors are designed for 600% inrush at starting under full load. As such, they are very robust and capable of much abuse relative to current / flux saturation. Operating at rated current (and dramatically reduced voltages - due to the fact that there is no back EMF at stall). The actual power applied to the motor while at full torque and stall conditions is ZERO + losses due to efficiency.

If you have the drive placed in current mode (i.e. you feed a +/-10VDC signal to get +/- rated current) and control the drive from a motion controller that monitors the encoder, you can indeed lock a motor shaft in a position just like holding position with a servo motor.

I have placed standard TEFC design motors in a stall condition at 100% current rating for days at a time and the motors rarely get above room temperature. A lot of folks play the fear factor and insist on cooling fans and TENV designs . . . from my experience the fears are baseless.

Ken Brown Applied Motion Systems, Inc. http://www.kinemation.com

 

I'm not sure that this will answer your question completely, but there's a nice resource by Motorola at: "http://e-www.motorola.com/collateral/MOTORPRINTUT.html":http://e-www.motorola.com/collateral/MOTORPRINTUT.html

(In fairness, I found that link through "HowStuffWorks.com":http://www.HowStuffWorks.com , which also has some information on how electric motors work.)

There is also a nice passage at "http://www.theproductfinder.com/amplifiers/vecdri.htm":http://www.theproductfinder.com/amplifiers/vecdri.htm

The Association of International Motion Engineers has a web site ( "http://www.aime.net":http://www.aime.net ) that I did not find especially useful, but you might be able to get a hold of someone who could point you in the right direction.

If you have a relationship with a drives vendor, they will probably be able to provide some information. Rockwell has classes on drives at their annual trade show, for example.

-James Ingraham
Sage Automation, Inc.

 

Pierre:

There are a couple of issues here. One is algorithmic; the other relates to issues such as heat. First the algorithmic:

Let's say our induction motor produces rated (100%) torque at a 2Hz slip frequency (pretty typical). For a 4-pole motor running directly off a 60Hz line, this would correspond to a 1740rpm rotational speed (or 1450rpm for 50Hz). In this open-loop case, the load torque slows down the motor, creating slip relative to the fixed electrical frequency; the generated torque increases with the slip until it matches the load torque and you get equilibrium operation.

A closed-loop vector drive in essence does the opposite. It gets a command for how much torque it is supposed to produce, and calculates how much slip it needs to produce this. It adds this frequency to the mechanical speed (usually derived from the encoder) to determine what the electrical frequency it will generate. If the mechanical frequency is zero (e.g. a locked rotor or holding a vertical load) and the command is for rated torque, in our example it will produce a 2Hz waveform. In either case, there is no need for any change in the encoder to do this.

Conceptually, it can keep this up indefinitely. The question then becomes whether it can dissipate the heat generated. If you buy an induction motor for off-the-line operation, it has a fan welded to the shaft that generates nice cooling at the rated speed. Of course, this fan would do nothing on a locked rotor.

If the fan barely kept the motor within temperature specs at the rated speed, then the motor would definitely overheat producing rated torque at zero speed (although it would take a lot longer than 1/2-second to do so). For this reason, you can buy induction motors for vector applications with separately driven fans.

Ken has a lot more experience than I do in the actual implementation of this type of system, so I will not contradict him. (Basically, he is saying that a lot of motors are overengineered as far as heat dissipation is concerned.) But I will say this is an issue you must definitely consider in depth for this type of application, if only to protect yourself against those who believe the conventional wisdom.

Curt Wilson
Delta Tau Data Systems

 

Ken,

So if I get it, a motor which should take 10 Amps at full tork, You just let it have its 10 Amp (by reducing the voltage) and it will give the full tork, whatever the rotation speed?

But what about the angle of the current. Is it not a result of the coils? Do the drive have any impact on it or does is just sends a wave of tension and gets any results from the electromechanical aspect of the motor system?

-Pierre

 

Ken Brown wrote:
> I have placed standard TEFC design motors in a stall condition at 100%
> current rating for days at a time and the motors rarely get above room
> temperature. A lot of folks play the fear factor and insist on cooling
> fans and TENV designs . . . from my experience the fears are baseless.

I once ran across a vector drive driven motor that had an external cooling fan on it get VERY warm (the case was so warm it was untouchable) when run at near full load at low RPM when the external cooling fan motor starter failed. I am sure the motor would have eventually failed if it had run this way very long except that by random chance I nearly burned my hand leaning up against
the motor trying to look at something under the machine. This caused me to investigate just why the motor was so hot.

Incidentally, we did not shut the machine down, but rather brought in a big overhead fan and blew air on the motor to cool it off until we could replace the external cooling fan motor starter.

We had an overload contact wired from the IEC starter to the PLC on the fan, so if the overload had tripped it would have alarmed. However, we did not have running feedback to the PLC, and the motor starter coil failed. Not the way I would have done it, but the guy designing the hardware wanted to save money on PLC inputs, and the seven inputs he saved by not bringing the cooling fan running feedback into the PLC saved him an input card. After all, "the fan motor starter coils are always energized so why do you need any
indication that the fan is running"?

BTW. They saved money by not having any overtemp sensing in the motor as well.

Bob Peterson

 

Bob,

I don't doubt what you say is true, but I would ask whether the drive was set up correctly. If the mag current is set too high you can saturate the iron and waste current and create heating. Or if set too low you can pour all kinds of current into the motor and not develop rated torque. I have witnessed the scenario you describe with an open loop vector (encoderless vector) and also with a VF drive with no Field Oriented Control.

To quantify my experience, I have set up 20HP Reliance and Baldor TEFC motors with a lever on the end of the motor shaft and used this lever to press against a bathroom scale with a two foot moment arm. I dialed up enough torque to produce a 30 lb reading on the scale (60 ft-lbs) and let it operate like that from one day to the next. After 24 hours, Motor current was slightly above rated nameplate and the motor did not get significantly warmer than ambient. Based on these tests, these same motors have been implemented in winching applications keeping cables tight on dock side winches through tidal cycles where maximum number of motor revolutions over a 24 hour period are fewer than 100 revolutions - typically done at 50% - 100% load depending on river currents / wind load / etc.

I am unaware of any significant design issues related to these two manufacturers that would make them better at heat dissipation than other manufacturers and I doubt there is much difference unless you start looking at low cost off-shore motors (SEW, TECO, etc.)

I have thought it would be interesting to conduct tests of various manufacturers products in this fashion and write comparisons of performance much like those reports produced by Consumer Reports. I think you would see a huge disconnect between what various manufacturers claim and reality.

Ken Brown Applied Motion Systems, Inc. http://www.kinemation.com

 

Responding to Ken Brown's Mon, Nov 11, 10:30 am, comments:

While I can't impute your observations, I have some reservation about the general conclusions regarding TEFC capability under standstill conditions:

a) Stator Winding Loss. Based on full current, even at standstill, the loss is still I^2xRs.

b) Stator Core Loss. There is still hysteresis and eddy-current effects which can be estimated at close to their nominal loss at normal speed.

c) Rotor Winding Loss. The rotor winding, i.e., copper in slots or cast-aluminium, the heat generated will have a detrimental effect on the integrity of stator insulation as heat is transferred across the air-gap.

Based on the above, I conclude, that, while you haven't observed any noticeable short-time effects, the long-term affect will be shortening of the stator winding's life-expectancy. Of course at the lower standstill supply voltage stated, +/- 10 Vdc, there is virtually no risk of voltage breakdown.

Have you made any heat-rise measurements of the motor carcass? Perhaps a low ambient temperature may be the saving grace in your situation! It is reasonable to assume that the insulation's rated hot-spot temperature, te value upon which its design-rating is derived, will increase accordingly. This is the basis for my assertion that insulation integrity will be jeopardized.

On the other hand, if the observed full-load current is not RMS, then your observation is reasonable.

Regards, Phil Corso, PE (Boca Raton, FL)

 

Hi there,

just some remarks on stator core loss.

Fundamental flux caused eddy current and hysteresis losses are proportional respectively to 2nd and 1st power of flux frequency (saturation not accounted for). Overall fundamental flux stator core loss is practically proportional to 1.7-1.8 power of frequency. This loss component is low at standstill.

Another stator core loss component you should be aware of today is PWM caused loss. Experiments show 3 times and more overall stator core loss increase compared with pure sinusoidal supply at certain operating conditions.

Fortunately, PWM core loss is also low at standstill (low modulation index).

As it happens, I am now in US giving a series of academy and industry seminars on PWM iron loss theory (see abstract below).

-Alex
[email protected]

========================

Electrical Machine PWM Core Loss Evaluation Basics

Additional PWM induced machine core loss may cause 20-25% power derating. Experimental PWM loss investigation showed that induction motor overall core loss may be increased 3 times and more compared with pure sinusoidal supply.

Our machine PWM core loss theory covers:
- PWM loss dependencies on modulation frequency and motor voltage (speed);
- different motor types - DC, 2- and 3-phase AC brushless, induction motor, switched reluctance etc;
- various modulation types - bipolar / unipolar, sinusoidal vector, random and more;
- power stage common and special topologies - 4-switch 3-phase inverter, multi(three)-level, Vienna unity power factor PWM rectifier.

We suggest very simple and effective experimental PWM loss characterization procedure.

PWM core loss evaluation and reduction is important for energy efficiency critical applications and may be beneficial for industrial ones - less power derating, easier cooling, lower laminations cost, longer bearings life etc.

 

Bob,

If the only reason for running the overload contact to the PLC was confirmation of operation, the prescription for this problem would have been better engineering (and wouldn't have cost a cent, unless you had to hire a better engineer).

Try to find a parameter that's a more definitive measure of fan operation (auxilliary contact on the motor starter, voltage on one of the output phases, some sort of actual verification, like a prox that sees the fan blade motion,
etc.) This way there will be fewer failure modes that will fool the PLC logic. Still only one input per motor.

PS: I worked on a machine where a relay with extra poles was used to run a subfractional hp pump motor, so "operation" could be sensed. It struck me as a little odd and when I asked about it, I was told they needed to sense
motor operation, because they had had problems with the pumps tripping their breakers. It didn't occur to them that monitoring the relay state would not inform them of a breaker trip anyway, as they were in effect just monitoring the state of the output (and the relay). Using the motor power lead as an input would have supervised not just the breaker, but also the relay, without having to pay extra for a bigger relay. Or better yet, install a
flow switch to see that the pump motor is really working. Or, just put in the right breaker! Sometimes people don't think.

 

Responding to Alex Ruderman's Tue, Nov 19, 5:29 pm:

Thank you for contributing your expertise to the ALIST's store of information on VFDs. Are you published? And, is it available for public purchase?

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]