Basic motor/drive questions... 1

[dgauthier]

We use 460V 3ph squirrel-cage motors here, and I need to get up to speed regarding motor basics. I'm a controls guy with a heavy electronics background, well versed in basic ac/dc theory but little motor experience other than the control end. I don't speak calculus.

1. Is there a relationship between torque and horsepower? If so, can you describe? All else being equal, will a higher HP motor necessarily deliver more torque?

1B. What about torque and speed relationships?

2. Are there constant-speed and/or constant-torque motors or is this a reference to VFD control schemes?

3. Given a "standard" 460V motor, what voltage limits should be respected? Typically, how hard can they be pushed, voltage-wise? (As applied to, say 5 to 20 HP motors). Am I correct that increasing voltage will increase torque? Will current rise in proportion to the voltage increase?

4. In heavy machinery applications (as opposed to fan/pump),when should DC motors be considered? Chief electrician here turns pale when I mention DC motors. What is the significant performance difference between DC vs. AC motors? What about "brushless" DC motors: maintenance only, or is there a performance difference between brushed and brushless?

5. Do you know of any good websites or books (that aren't written in equations)that might help?

Thanks in advance,
DG

 

[electricuwe]

1. Mechanical Power = 2*PI*speed*torque

But be careful to use the correct units and conversion factors. I'm used to SI-units, so I will not provide an equation in ancient units used elsewhere.

2. Most types of motors run at nearly constant speed when operating on a constant voltage. In a VFD-configuration some motos are thermally capabale of operating at constant torque in the whole speed range and some are not.

3. If you are talking about an induction motor operating on the line it will usually work with a small percentage of overvoltage without damage. In a VFD the voltage can be increase as long as V/f is kept constant, isolation isn't overstressed and the mechanical speed limit is not violated.

4. A VFD with induction motor and vector vontrol working well is equal or even superior to every dc-motor, but sensorless vector control has some drawbacks at low speeds.

5. Yes I have, but only in German.

The questions you are asking lead me to an idea what could be the problem with your conveyor application descirbed in another of your posts:

If you operate a 460 V motor on 460 V line using an inverter and you operate with frequencies in excess of the line frequency you operate the motor wuth reduced flux leading to significantly reduced torque!

Torque is proportional to flux² !

 

[electricpete]

great answers by electricuwe. I'll add a few more miscellaneous thoughts

1 - I likewise prefer metric, and prefer not to add units until I start substituting in values to equations. But some folks may prefer the following form
P = N*T/5250
where
P in horsepower
N in rpm
T in ft-lbf

All else being equal... operating torque is primarily determined by the load, unless the motor approaches breakdown torque. Higher HP motor will have higher torque capability (without approaching breakdown torque). Likewise higher voltage provides higher torque capability.


3 - increasing voltage increases torque capability, IF you don't change speed. Motor torque speed curve is roughly proportional to (V/f)^2

5 - This site is good for basics on induction motors (although no info on variable speed drives)

http://www.reliance.com/mtr/mtrthr.htm

 

[jraef]

dgauthier,
Some additional info to the above posts.
Re: #2. Constant Torque (CT) and Variable Torque (VT) refer to the load conditions, but are used by VFD manufacturers to describe performance criteria. For instance, a VFD rated for 20HP in a Constant Torque application may be capable of 30HP in a Variable Torque application such as a centrifugal pump or fan. This is because in a CT application, in order to maintain torque at low speeds, the VFD will need to deliver higher current. Since a centrifugal pump does not start pumping until, lets say 60% speed, the VFD is not required to deliver that higher current at low speeds. The power devices then are capable of higher overall HP since they only need to work at higher speeds where there is greater efficiency. There would be no difference in the motor.

Re: #3. Although it is correct that torque will increase with voltage, voltage beyond the nameplate rating on the motor will also be expressed as heat, and more heat = lower motor life. The rule is keep the voltage + or - 10%.

Re: #4. DC is still good for loads that need variable speed under 2HP. The DC motor and drive combo is still less expensive than AC in this size. For single speed, use AC all the time. If you already have DC it will always be cheaper to fix than replace with AC, but AC motors do not need as much maintenance as DC so consider long term ownership costs. Permanent magnet is too expensive to be realistic in high HP applications now that AC vector drives can match performance.
Re: #5.This site is great for motor information...

http://www.joliet-equipment.com/instructions_tech-tips.htm

Subvert the dominant paradigm... Think first, then act!

 

[tmahan]

Dgauthier,

With Respect to your question 2:

The is a difference in the motor for a constant torque/variable torque application. As Jraef correctly points out for variable torque applications the current required is very low (a relative term here) for the constant torque application the current drawn by the motor is "not as low". In most cases for a variable torque load motor cooling is not an issue at low speeds. For a constant torque load motor cooling may become an issue.

When requesting a motor quote for a variable speed drive application I suggest that you provide, in addition to the usual information (HP, voltage and so on, the application nature (constant torque or variable torque) as well as the expected speed range for the application (either as the ratio i.e. 4:1 or the actual range 450RPM to 1800RPM)

The motor manufacturer may either up size the motor, recommend a cooling blower or tell you that it is not a problem.

 

[electricpete]

Here is a link on vfd motor application published as a NEMA standard:

http://nema.org/engineering/icsstandards/acadjustable.pdf

(don't ask me why it's available for free)

 

[vanstoja]

Power Magazine used to publish special reports on motors, switchgear, etc. every few years which had simple, good information on the subject. An example is Special Report - Powerplant Motors, Power, March 1986 (16pp). It may still be possible to order reprints from Power Reprint Dept. 1221 Avenue of the Americas, New York, N.Y. 10020. Also Machine Design and Electrical & Electronics put out Reference Issues loaded with good info on motors and switchgear that were worth saving for eternity. Unfortunately most of those good old magazine sources of engineering info have dried up and become either "news of the day" or pure advertising publications. Fortunately, I was around back then to collect that bonanza of free engineering information which I cherish and preserve to this day.

 

[jbartos]

Suggestion to dgauthier (Electrical) Aug 30, 2002 marked ///\\We use 460V 3ph squirrel-cage motors here, and I need to get up to speed regarding motor basics. I'm a controls guy with a heavy electronics background, well versed in basic ac/dc theory but little motor experience other than the control end. I don't speak calculus.

1. Is there a relationship between torque and horsepower? If so, can you describe?
///Visit

http://www.reliance.com/mtr/flaclcmn.htm

for basic requested relationships.\\All else being equal, will a higher HP motor necessarily deliver more torque?
///Yes.\\1B. What about torque and speed relationships?
///Visit

http://www.reliance.com/mtr/flaclcmn.htm

for basic requested relationships.\\2. Are there constant-speed and/or constant-torque motors or is this a reference to VFD control schemes?
///This is more or less related to the VFDs. Visit

http://www.reliance.com/prodserv/motgen/b7097_2.htm

etc. for more info.\\3. Given a "standard" 460V motor, what voltage limits should be respected?
///According to NEMA Std MG-1, 10% overvoltage as a continuous maximum, 10% continuous undervoltage as a minimum, 85% motor starting minimum voltage if the motor is design for this percents, else 75% is available for example, motor stall voltage, e.g. 85% depending on the motor design.\\ Typically, how hard can they be pushed, voltage-wise? (As applied to, say 5 to 20 HP motors).
///It is good to respect the industry standards, e.g. NEMA, IEEE, IEC, BS, DIN, etc. to be on the safe side since many motor manufacturers tend to meet or exceed these industry standards.\\ Am I correct that increasing voltage will increase torque?
///Yes, including the starting torque and maximum torque.\\ Will current rise in proportion to the voltage increase?
///Not within the first 5% or 10% depending on the motor design.\\4. In heavy machinery applications (as opposed to fan/pump),when should DC motors be considered?
///Only when justified, e.g. speed torque characteristics, DC power supply is available, smaller motor availability is acceptable, etc.\\ Chief electrician here turns pale when I mention DC motors.
///That is good since they may cause a lot of problems.\\ What is the significant performance difference between DC vs. AC motors?
///Speed torque characteristics, significant difference in the speed and starting control concepts, max and min RPMs, efficiency, etc.\\\
What about "brushless" DC motors: maintenance only, or is there a performance difference between brushed and brushless?
///Yes, there is the performance difference since the brushless may be of a permanent magnet type or electronically commutated, both with all associated advantages and disadvantages.\\5. Do you know of any good websites or books (that aren't written in equations)that might help?
///Some are already posted, other are

http://www.pacificliquid.com/motorintro.pdf

http://shop.store.yahoo.com/elevatorbooks/motcirantheo.html

http://ecs

http://www.baylor.edu/faculty/williams/paper/daepaper.htm

etc.\\