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difference between a dc motor and generator

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danrogers

Member
Hi all,

Untill now I've been under the impression that a dc generator is the same in construction to a motor. However a friend of mine is certain that they are not the same but doesnt know how they differ?

Can anyone put us straight please :)
 

MikeMl

Well-Known Member
Most Helpful Member
Anything that has permanent magnets in it can be either. However, the design is optimized to be one or the other. A good motor may not be a very good generator, and vice versa.
 

danrogers

Member
thanks mike, would you know what the main differences would be? I cant really think of any apart from torque / resistance values differing between powering and generating?
 

Dean Huster

Well-Known Member
The International Cub Cadet garden tractor of the mid-1960s used a full-size Delco automotive DC generator for its intended purpose and then switched the circuitry around to use it as a starter motor for the 7hp Kohler engine. The front of the engine had a pulley of about 10-inch diameter to provide better starting torque against the engine compression and a higher RPM rate for the generator when the engine was running. It was a very trouble-free system.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The 1970's Yamaha RD200 used a dynamo (rather than an alternator), and it included a second pair of brushes to operate it as a starter motor. Very interesting system, no clashes of gears as it was obviously permanently connected.
 

crutschow

Well-Known Member
Most Helpful Member
I know the old series wound DC generators in cars would act as both. I remember as a teenager, playing with the voltage regulator on our car with the engine stopped and accidentally closing the cutout relay (which normally disconnects the generator from the battery when the engine stops). This caused the generator to start turning (the pulley was slipping in the V-belt). I quickly opened the relay thinking I had damaged something. But everything still worked fine after that.

The differences may include the number of winding turns and wire size.
 
Can anyone put us straight please :)

The design criteria for DC motors and generators are completely different. For a motor, start under load characteristics might be more important, and, of course, it will be designed to operate at some design nominal speed. A motor used as a generator will seldom produce the output its name plate data suggests. If start under load is what it was designed for, then it's probably a series connected stator. That makes for a generator with horrible voltage regulation. Parallel wound stators, or permanent magnet stators, stand a better chance of working at least a bit right.

Most generators have stator coils optimized for producing strong magnetic fields. When used as motors, they typically don't rotate at over a few hundred RPMs -- if that's what you want, OK. If not, the results will disappoint (and probably require a stator re-wind).

Then there is the possibility of a mechanical failure. A DC motor will have to run at above its design speed if it is to produce the same output voltage (since its stator magnetic field will usually be weaker). Cutting down the magnetic field strength to increase speed of a generator might push it above its design nominal speed, if you need more RPMs. You could be looking at ruining the rotor by slinging off the rotor windings and/or commutator bars.

About the only exception is the starter motor/generator that jet and turbojet engines use. These are independent field devices whose characteristics can be tailored for either engine starting or power generation.
 

mbarazeen

Member
you can say there is no different, normally frame size and its core charecteristics are the optimization for any motors capacity for optimum usage.
a DC motor either a permenent megnet one or the one with field winding will perfectly work as a generator.
in motor operation field control will controll the speed,
in generator mode it will control the voltage out put.

when a motor is used as a generator and if the RPM exceeds its design value then core saturation will limit the out put. but normally its not used so.

in case of AC, large generators are some time used as starter motor by the use of vriable speed drives to bring it to self sustaining speed, especially those are coupled with combustion turbines. and of cause it can be driven to its full capacity to pull loads. but the speed drive will have to have the cpacity for that.

tell to your friend in construction there is no such difference between large DC motors and generators ( especially with no megnet)
but if you look at small motors, you can see some difference like in motor megnet is on stator and in generator megnet may rotate.

charecteristic of a certain machine as a motor and as a generator at rated RPM doesnt differ. but starting charecteristics for both cases will have different concerns such as starting torque in motors and voltage build up in egenrators.
 

tcmtech

Banned
Most Helpful Member
The International Cub Cadet garden tractor of the mid-1960s used a full-size Delco automotive DC generator for its intended purpose and then switched the circuitry around to use it as a starter motor for the 7hp Kohler engine. The front of the engine had a pulley of about 10-inch diameter to provide better starting torque against the engine compression and a higher RPM rate for the generator when the engine was running. It was a very trouble-free system.

I have seen that used all the way up to the 16 HP Kohler's they used on some of the cub cadets. I have it on my 12 and 14 hp Kohler's and a friend has the 16 HP one. The old Wisconsin engines had it on some of the 19.6 HP two cylinder THD series engines too.

As far as using a generator as a motor that depends entirely on application. Some are only one or the other to any great effect but there are countless other applications where they serve full function dual purpose as both motors and generators.

In a way its a very weighted question. So yes, No, Sometimes, Very well, and Not at all apply. It all comes down to the actual application.
 

crutschow

Well-Known Member
Most Helpful Member

Sceadwian

Banned
So a generator is not REQUIRED to have different construction, but for optimal power conversion in a useful zone they are. So basically you're both right. You could use Nigel and Deans examples as proof that different construction isn't required, just typical.
 

tcmtech

Banned
Most Helpful Member
It gets more complicated (or maybe simpler) when you get into the large scale power systems.
One system that has been long used for the high power stuff is Ward - Lenard system with is basically a large synchronous AC motor/generator (part 1) that drives a large DC generator/motor (Part 2) that supplies variable voltage and current for a large variable speed DC motor/generator (part 3) in industrial applications.

When the end Motor (part 3) is under load it works just like any other DC motor but then when the load is reduced and starts to over run the motor it works like a variable rate dynamic brake which then back feeds it power to the big DC generator/motor (part 2) which in turns tries to speed up which changes the synchronous motor/generator into a synchronous generator (part 1).

Motor -generator - motor going from the power source to the mechanical load but then its a generator - motor - generator going from the mechanical load back to the power source. ;)
 
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danrogers

Member
Wow, thanks to everyone who has replied! In short then, there's no significant differences UNLESS efficiency is an issue in which case coils and so on would be optimized?

Thanks again to you all
 

BrownOut

Banned
Hybrid automobile technology uses units which are both motors and generators.
 

mbarazeen

Member
even efficiency would NOT differ when you use a particular machine within its design limits. in motor operation you give electric power and get mechanical out put, and in generator viseversa. in both cases for full load or a partial load, at a certain speed all losses are nearly same. you can say mechanical losses, losses in windings, in core etc.
 

Albercook

New Member
Students built DC motor - horrible generator

So I wanted some elementary students that I teach to have a more hands on experience of electricity. We took apart small brush DC motors and looked at the parts. Next we built some simple motors. They are made of PVC pipe as the rotor with three 1" carriage bolts wrapped in magnet wire for the electric magnets. They used scrap copper for the three segment commutator and two Neodymium magnets for the stationary magnets on the outside.

They work amazingly well as motors. As long as you keep the friction down and the magnets close to the bolts. They have about 10 Ohms of resistance which is typical of a small battery powered DC motor. At 12 Volts they draw about 1 amp.

But they are horrible generators. Spinning one with a drill motor will barely get you 0.5 volts and the current is in the 10s of mA range.

I put the motor on a scope and saw some strange results. The voltage is almost zero across the motor. I do see a noisy AC signal but the DC offset is zero. I checked and the voltage drop across the leads it was zero as well so it seems that the 12 volts was being dropped across the internal resistance of the battery (lead acid). That seemed very strange.

I checked and while a commercial DC motor is running at no load the voltage is almost equal to the applied voltage (that was with alkaline batteries if it matters).

Does anyone have any ideas about what is going on here and how I can change the design so that these motors are at least passable generators.

Thanks
Albercook
 

mneary

New Member
A lot of your losses are in the commutator. Home made (science lab) generators seem to show off better if the magnets rotate inside stationary windings.

Look on the internet for some designs featuring this technique. Somebody actually built a generator that uses a gerbil to light some LEDs.
 

mbarazeen

Member
check the theory of DC motor, since you have 10 Ohms across its the series internal resistor of the motor, but the equation I=V/R will not work, since you have to consider back emf (induced at certain RPM) to work out the right value of current.
you will get like

I= (V-E) / R and E = kw (constant multiplied by RPM decided by the characteristic)

you have spin your motor as a generator with varying speed to get the right characteristic.

also as mnearly told, make sure proper pressure is there at slip rings to have good contact.
 
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