What does this motor namplate tell? - Page 1
Forum section: electronics / general

author: onemilimeter
author_rank: Frequent Contributor
timestamp: October 10, 2010, 01:45:33 pm
content: Hi...Got a very old brushed DC shunt machine. The nameplate of the motor is attached. As you can see, the nameplate states that "RPM: 750/2250". May I know why there are two different rpm values? What do they mean?Thank you very much

author: rossmoffett
author_rank: Regular Contributor
timestamp: October 10, 2010, 03:39:12 pm
content: Load/no load RPMs.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 10, 2010, 06:14:20 pm
content: Quote from: onemilimeter on October 10, 2010, 01:45:33 pmAt the named voltage 200V the motor will spin at 2250 rpm.The safest lower voltage are the one that it will operate at 750 rpm.Any voltage lower than that it will effect the gain in Horse Power .. and it will over heat the coil in it.

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2010, 06:18:11 pm
content: It doesn't say the minimum voltage.If I remember rightly, there normally isn't a lower voltage limit for DC motors, that only applies to AC motors.I would expect the field winding to be tapped or consist of two parts which can be paralleled for a lower speed with more torque or connected in series for a higher speed with less torque.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 10, 2010, 06:48:38 pm
content: Basically if the current is maintained near to 19A and no more , the volts have an second importance .

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2010, 07:41:34 pm
content: Yes, you're right, the losses are mainly, therefore the heat dissipated, due to the current flowing I2R, although there will be a maximum speed which is what is probably shown on the plate.There's also the lower speed of 750 but DC motors don't have a lower speed limit which is what made me guess a tapped winding or something like that.

author: rossmoffett
author_rank: Regular Contributor
timestamp: October 10, 2010, 10:00:26 pm
content: You guys, please do not risk providing misinformation if you are unsure of what you are speaking about. Industrial electrical systems are not to be guessed at, it is a matter of human safety and life in many situations. The standards that apply in industrial systems are often confusing to people who are normally working only with low-power electronics. If this offends you, please put aside your pride and understand that people can die or be seriously injured when industrial parts are hooked up incorrectly.As I said, the lower RPM is the speed under a full load (one which requires 4 HP). The upper RPM is the unloaded speed (the motor is unconnected to any physical load). DC motors will spool up to much higher speeds with no load than they will when connected to a load.Obviously it will spin slower under lower voltages as well, but that is not the intention of the nameplate ratings.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 10, 2010, 10:22:53 pm
content: I find it odd that a shunt DC motor would slow down that much under load.QuoteAt lower speeds, the motor would have less effective cooling, reducing the rated continuous torque. On a torque-speed plot, it shows up as "Gouni curves", making the plot look something like an hourglass. (As a joke, I came up with that term in ECEN 441 class, naming it after my friend Susmitha Gouni who is very skinny. I suppose I could also call those "Yep curves", but then nobody in class would get the joke.)

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 10, 2010, 10:36:53 pm
content: Here's some good info on DC shunt motors:http://zone.ni.com/devzone/cda/ph/p/id/54

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 10, 2010, 11:06:32 pm
content: Quote from: rossmoffett on October 10, 2010, 10:00:26 pmRULE NO1Never say to others what to say or not !!Read the label , and say what you know ... this is the question of the topic .

author: osmosis321
author_rank: Regular Contributor
timestamp: October 10, 2010, 11:37:07 pm
content: Personally, I would never try anything dangerous because some dude I've never met before told me it was ok. Likewise, I wouldn't expect anyone to put themselves at risk on my say-so either.This just goes without saying, no? Hopefully we're all capable of sound judgement.

author: rossmoffett
author_rank: Regular Contributor
timestamp: October 11, 2010, 05:32:10 am
content: Quote from: NiHaoMike on October 10, 2010, 10:22:53 pmThat's actually true.. my motor book says that 3-5% should be the difference between full load and no-load for a DC shunt motor, and 750 is 1/3 of 2250.How many wires are in the junction box?QuoteProbably a better one for me in this case would be to read more carefully before jumping to a conclusion, because I totally skipped over the fact it is a shunt motor. On the other hand, you don't make the rules here either, brother. Consider the paradox of you telling me what I can't say while telling me not to tell others what they can't say.

author: AvrFan65
author_rank: Newbie
timestamp: October 11, 2010, 05:51:44 am
content: Can the motor be configured for two different speeds by wireing?Jan

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2010, 07:06:11 am
content: Quote from: rossmoffett on October 10, 2010, 10:00:26 pmThen perhaps you should also follow that rule yourself?QuoteA speed drop of 662/3for a shunt wound motor? As others have said it's not possible, it looks like you're guessing there.QuoteI think the name plate doesn't give enough information. There's obviously more to it than that, for example, the original poster said it's shunt wound but the name plate doesn't say that. I think he should examine the connections to the field coils.

author: sonicj
author_rank: Frequent Contributor
timestamp: October 11, 2010, 12:39:17 pm
content: Quote from: Hero999 on October 11, 2010, 07:06:11 am

author: Excavatoree
author_rank: Frequent Contributor
timestamp: October 11, 2010, 12:45:23 pm
content: Time to crack open my machinery text.

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 11, 2010, 01:02:58 pm
content: If I came across this nameplate at work, I would probably assume that the two speeds are maximum and minimum operating speeds. These motors are designed to be used with a speed control of some kind and the amps will remain constant as the torque decreases and speed increases (if it were an AC motor, it would indicate that there is a double winding for multiple speeds). That being said, an electronics forum is probably not the best place to ask about motor nameplate ratings, especially on UK plates (probably) predating WWII. I'll post it on an electrical forum with UK participants and see if I can get a solid answer.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 11, 2010, 04:42:50 pm
content: There is only too areas about electrical installations,that I do not have a clue about the special stuff that they use .Elevators and electrical stuff in the navy ...This 200V DC its not easily found on the land ...

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2010, 05:48:12 pm
content: DC isn't used much nowadays but this motor probably pre-dates most modern electrical infrastructure and DC was still used in many areas back then.http://en.wikipedia.org/wiki/War_of_Currents#Remnant_and_existent_DC_systemsFortunately it's possible to power this motor directly off 230V AC. It's a universal motor so will work of both AC and DC, although the performance will probably be poorer on AC because there will be greater core losses, especially in the field winding which might not have a laminated core and the current will be lower due to the inductance. A better option would be to use a bridge rectifier but unfiltered 230VDC might be too higher voltage so you could reduce to 200VAC before by using a 30VAC mains transformer with the secondary connected in anti-phase with the live.Using unfiltered DC shouldn't be a problem anddon't add filter capacitors to unfiltered 200VDCbecause the RMS voltage will creep nearer to the peak voltage of 282V which could burn out the motor. If vibration and noise are a problem, then a more smooth supply is a good idea but it will be a pain to do, for 200VDC you'll need 140VAC to 150VAC, (a 110VAC or 120VAC and a 30VAC transformer in series might do) and you'll need a huge filter capacitor to get a low ripple voltage 4700µF for 25V which is going to be expensive, maybe go for 180VAC (50VAC secondary connected anit-phase with the live) and 1000µF which will give more ripple but have an RMS of nearer 200V.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 11, 2010, 07:15:56 pm
content: I do not think that any real expert needs the Wiki , so to write two words as reply .We all know where wiki are ...

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2010, 10:47:57 pm
content: Quote from: Kiriakos-GR on October 11, 2010, 07:15:56 pmWhat do you mean? Wikipedia is as good as source of information as any, go and read about the history of DC power distribution somewhere else if you don't believe me.

author: tecman
author_rank: Frequent Contributor
timestamp: October 11, 2010, 11:35:58 pm
content: Quote from: Hero999 on October 11, 2010, 05:48:12 pmNormally shunt wound motors can not be run on AC. You refer to a universal motor, which is a series wound motor, and will generally run on AC or DC. The high inductance of a shunt wound field prevents them from running effectively on AC.I suspect that the numbers are an RPM range. You can control the speed of a shunt wound motor by weakening the field. Usually (in the old days) with a big rheostat in series with the field coil.paul

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 11, 2010, 11:48:18 pm
content: I'm still waiting for a decent answer from the UK guys on the other forum. Unfortunately, electricians tend to be sophists and answer questions with more questions

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 11, 2010, 11:53:36 pm
content: Quote from: tecman on October 11, 2010, 11:35:58 pmAncient elevators with rheostat with contacts ... stop - slow - fast - faster , and some one to handle the stick on the elevator ..

author: Zero999
author_rank: Super Contributor
timestamp: October 12, 2010, 07:17:57 am
content: Quote from: tecman on October 11, 2010, 11:35:58 pmYes, that makes sense. I'm not sure about running a series wound motor designed for AC on DC for the same reason, the inductance of the field will be lower so the field will be weaker so it might spin too fast but that depends on the load, if it's a fan (which is normally the case) it might not make much difference.QuoteYes and you get less torque at higher speeds (weaker field) and more at lower speeds (stronger field).

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 12, 2010, 11:08:19 am
content: So far the best answer provided by the other guys was this linkhttp://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee3/bdeee3_1-1.aspxThe 3:1 speed ratio mentioned matches that described on the nameplate pretty wellAlso this was offered: "No excitation voltage is given on the nameplate. This might indicate that the speed range given was intended to be by shunt field (excitation) control."