Motor Understanding

Tim Ganz

Member
Join Date
Dec 2010
Location
Dallas, Texas
Posts
674
Where I work we use a lot of Maratho electric motor. I am trying to understand how to select them. Looking at the pdf link below on page 4 how would I choose between the motor type as in rolled steel, aluminum or cast iron type?

Which is best and where would each be used?

I also see the micromax described as constant torque? What does that mean? In the other things I have read on drive the torque is constand anyway up to th base speed on the name plate correct?

I have seen CT / VT or constant torue / varible torque used in motor and drive documents but gave no real understanding of it?

On some of the blue max info it says it does constant HP up to 1.5 times base speed. Would this be the same as constant torque up to 1.5 times base speed?

Sorry for so many questions but I am really trying to gain a better understanding of motors and drives and I am really struggling.
 
On some of the blue max info it says it does constant HP up to 1.5 times base speed. Would this be the same as constant torque up to 1.5 times base speed?

HP = HorsePower => power unit
so constant HP = constant power
P = Mw => so if P = const. then torque is reverse proportional to speed
 
The load will do a lot of the determination of what motor is needed. First off is it constant torque ie conveyer or wheeled device (assuming its load is not changed) OR "increasing torque ie centrifugal pump or squirrel cage blower.
THEN determine torque demanded by load input shaft at maximum load. The old style torque wrenches work well for this - you will quite likely need to make an adapter unless two nuts jammed together will work.
NEXT what is max RPM?

HP = (T x RPM)
IF driven on VFD then motor will deliver maximum torque (if demanded by load) and it will be constant. As RPM goes up HP goes up assuming constant torque load to baseline RPM.
Above baseline voltage held constant and torque decreases with increasing speed.

All of above was assuming constant torque load. IF driving a centrifugal pump get the pump graphs and determine HP and torque at the speed you want to drive the pump.

As far as frame materials that is dependent mostly on the motors operating environment and whether stationary motor or on mobile equipment (cast iron gets heavy aluminum may be better choice) Also whether in washdown or other harsh envirnments (stay away from cast unless you love painting motors and aluminum may be degraded by cleansing agents). In very dusty environment TEFC may be best choice. If flammable gases or liquids or dusts then you need to consider explosion proof moters

Dan Bentler
 
This is the minimum info you need to spec a motor:
20 hp 1760 rpm 230/460 VAC 3 Phase
Open Drip Proof Enclosure
Design B, Insulation Class F
Frame Size 256T, 21.69” Long (including shaft),
Weight = 237 lbs.
1.15 Service Factor
Inverter Duty NEMA MG1 Part 31.4.4.2.
Full Load: Efficiency 93.0%, 86.0% PF
23.5 FLA (Full Load Amps), 140 LRA (Locked Rotor Amps,
7.8 NLA (No Load Amps)
60 ft-lb FLT (Full Load Torque) 165 ft-lb Break Down
Torque, 120 ft-lb Locked Rotor Torque

I suggest you get a couple of motor suppliers in your office and discuss this with them. The topic is quite complex.
 
Tom
You cheated - you read the nameplate on the one in your back pocket

Well there were a couple that are not on the nameplate but who is picky?

Dan Bentler
 
Tom
You cheated - you read the nameplate on the one in your back pocket

Dan Bentler

I cheated, but not with the motor in my back pocket. (My back can't handle more than 10 hp nowadays!) This is an example I use in one of the classes I teach at UW Madison.

Besides, you talk like cheating is a bad thing! Harrumph!
 
Necessity was NOT the Mother of invention - laziness was.
Smart people work no harder than they have to which is why they use tools and machinery. Also called cheating

Nothing wrong with cheating. Especially when you cheat and avoid getting back strain

Dan Bentler
 
I am really tempted to dive in here and again offer a brief lesson on three phase induction motors and their application but, it seems to me that the OP could offer a bit more in initial effort first.

For example, he could easily search this forum for earlier discussions on exactly this same topic. He could at least search the web for some manufacturers' white papers on the subject which, as Tom Jenkins mentions, is large and complex.

And, to help the OP get going on this, may I suggest that a web search be done for a NEMA Design B torque-speed curve. Spend a little time studying this and then come back for further embellishment of the subject.

I'll watch for that as I am not unwilling to go through this discussion again if some preliminary work has been done.
 
I am really tempted to dive in here and again offer a brief lesson on three phase induction motors and their application but, it seems to me that the OP could offer a bit more in initial effort first.

For example, he could easily search this forum for earlier discussions on exactly this same topic. He could at least search the web for some manufacturers' white papers on the subject which, as Tom Jenkins mentions, is large and complex.

And, to help the OP get going on this, may I suggest that a web search be done for a NEMA Design B torque-speed curve. Spend a little time studying this and then come back for further embellishment of the subject.

I'll watch for that as I am not unwilling to go through this discussion again if some preliminary work has been done.

I have done a lot of study from MFG documents and this site but I am still having trouble understanding CT/VT and how you would choose between the different steel type per my initial post.

I was hoping someone could help me understand this minus a lot of the jargon you get with MFG documents.
 
OK, Tim, I'll try to lay out some of the technical aspects of three phase induction motors in ordinary terms in the best way that I can.

First, I assume that you have examined a NEMA Design B torque-speed curve. It shows the motor shaft speed at no load being right at the synchronous rotation speed of the stator's magnetic field at 60Hz (ie. 3600,1800,1200,or 900rpm depending upon the number of magnetic poles wound into the stator coils. Then, as some load begins drawing torque out of the motor rotor, the rotor begins to slow down slightly (this is called "slip") until, at the nameplate rated torque level, the rotor has slowed to the nameplate speed. For example, in a common 4 pole motor at 60hz the stator field is spinning at 1800rpm and at no load the rotor is alsos spinning very close to 1800rpm as well. If the motor is loaded up to its nameplate output (let's just say 10hp), the shaft speed will drop to the nameplate speed (let's say 1760rpm). That 40rpm slip (1800-1760=40) is directly proportional to shaft torque. So, at 20rpm slip you would be at 1/2 torque, at 30rpm it would be 3/4 torque, and so on. Since HP=T x rpm/5250, in the above 10hp motor the full load torque would be about 30ft-lbs. You will also notice that, on the NEMA B curve, the motor can actually deliver up to 220% of nameplate torque before it begins to enter the breakdown region where current continues upward but torque fall off. This is due to the magnetic circuit of the motor beginning to saturate. When that happens, magnetic chaos ensues, the motor becomes much less efficient, and, if left to operate in this region would overheat rapidly and destroy itself.

It is important to see that the NEMA B curve and everything I've mentioned above about it pertains to a power supply of 60hz and proper nameplate voltage.

If the frequency of the incoming power is reduced, the stator field begins to spin slower and the resulting speed of the motor shaft is similarly reduced. In fact, the motor's ability to make torque in its shaft is dependent upon the ratio of the voltage to the frequency. For example, a 480V 60hz motor needs a ratio of 8/1 to produce nameplate torque. A 240V 60hz motor has a 4/1 ratio and a 400V 50hz European motor has an 8/1 ratio.

If we reduce the power frequency fed to a motor and simultaneously reduce the voltage to maintain the same ratio (480/60, 400/50,240/30,120/15 etc), we can get the motor to slow down and continue to produce the same torque as long as we maintain the same volts per hz ratio. This is exactly how an inverter gets a motor to change speed.

From the above, it should be clear that, as long as we hold the v/hz ratio constant, the motor will be able to source constant torque levels as we slow down. This is actually the case down to about 1.5hz where the motor becomes impossible to control using simple v/hz speed control.

So, we have a motor (in fact, any induction motor) that can source constant torque down to nearly zero speed as long as we hold the v/hz ratio constant. The problem with this is that, while magnetically able to do this, the motor is NOT thermally able to do it. As the motor slows down, the shaft mounted cooling fan (internal in ODP motors and external in TEFC motors) also slows down and the motor's ability to cool itself goes away. So, it can be said that all induction motors are capable of constant torque output over the speed range of near zero up to motor nameplate or base speed magnetically but not thermally. Most motor manufacturers will list the "speed turndown" ratio of their motors which would typically be 4/1 Constant Torque. This means that the cooling system designed into the motor will keep the motor within its design temperature limit from 60hz down to 15hz (4/1) while working over the speed range at full torque. Sometimes you will see motors listed at 10/1 or even 1000/1. Motors with very high turndown ratios will generally not have a shaft fan but some other method of cooling. Auxiliary fans which have their own power source and run at constant speed regardless of motor shaft speed or TENV (Totally Enclosed Non-Vented) motors would be like that.

As you mention, Tim, sometimes you will see motor speed turndown ratios listed as, for example, 4/1 CT, 10/1 VT. The CT is Constant Torque loading as described above but the VT is variable torque and clearly has a wider speed turndown range. The difference is in the load connected to the motor, not the motor itself.

A variable-torque load is normally a load like a fan or centrifugal pump that does not draw the same amount of torque at all speeds. Typically, the torque begins at essentially zero at zero speed and increases by the square of the speed up to the design load and speed.
 
Continuing, a variable torque load would typically load at 100% torque at 100% speed, 81% load at 90% speed, 25% load at 50% speed and so on down to zero at zero. Clearly, motors connected to this kind of load are not working at full torque in the slower speeds so you can slow them down further before they will overheat. Thus, a motor listed as 10/1 VT could be slowed down to 6hz (10/1) when connected to a centrifugal fan or pump and not experience overheating problems. Note that the motor hasn't changed---it can still produce the same full torque at all speeds. It's the load that's changed and therefore the speed can be brought down slower without thermal problems.

Let me know if you are with me up to this point and then we can talk about what happens when we take a motor over its nameplate or base speed.
 

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