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Overrating a power supply for a DC motor?

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Hollowpoint22/10/2020 22:08:22
354 forum posts
37 photos

Need some help guys.

I have a small DC motor I want to use on a project. The motor is 24v 200w and "rated" 11 amps.

I suspect the amp rating is at full speed with no load but not sure? Obviously if that is correct with heavy load it will draw more amps.

How do I choose the correct amp power supply? Do I need to go bigger? how much bigger?

Andy Gray 322/10/2020 22:43:31
46 forum posts
Posted by Hollowpoint on 22/10/2020 22:08:22:

Need some help guys.

I have a small DC motor I want to use on a project. The motor is 24v 200w and "rated" 11 amps.

11 amps will be at the rated output (11 x 24 = 240W). It will use very little current with no load.

IanT22/10/2020 22:46:57
1680 forum posts
163 photos

My days of knowing this kind of thing are long past HP - but a 200W motor being run with 24V is in theory passing 8.3 Amps. I imagine the 11A rating is the maximum current.

I am sure there are other important things to consider, such as continuous current rating, motor loading etc... others will no doubt clarify further...

Regards,

IanT

Hollowpoint22/10/2020 22:51:22
354 forum posts
37 photos

I've just been doing a bit more research and the 11 amp figure is indeed the maximum current.

With that in mind, what amp power supply should I buy?

This is the motor I have.

http://www.motiondynamics.com.au/unite-my8216-200w-12v-or-24v-dc-motor-3200-rpm.html

Edited By Hollowpoint on 22/10/2020 22:54:52

Ian P22/10/2020 22:58:51
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2439 forum posts
101 photos

Bit of a mixture there. Motors are not usually 'rated' by current. 24V 200" is about 8 Amps but as you say the current drawn depends on load but many many other factors are involved.

The highest current the motor can draw is when it is stalled, You can work that out if you know the applied voltage and the resistance of the windings. When the motor is switched on from stationary (even without a load) it will momentarily draw a relatively high current (relative to its running current). Depending on what the motor is driving and what type of power supply is used this high initial current may have to be allowed for, a switch mode PSU which otherwise might be more than capable of operating the motor and its load not even get started properly as it may think its output is shorted.

A little more information is needed to be able to suggest an appropriate PSU.

Ian P

Hollowpoint22/10/2020 23:15:39
354 forum posts
37 photos
Posted by Ian P on 22/10/2020 22:58:51:

Bit of a mixture there. Motors are not usually 'rated' by current. 24V 200" is about 8 Amps but as you say the current drawn depends on load but many many other factors are involved.

The highest current the motor can draw is when it is stalled, You can work that out if you know the applied voltage and the resistance of the windings. When the motor is switched on from stationary (even without a load) it will momentarily draw a relatively high current (relative to its running current). Depending on what the motor is driving and what type of power supply is used this high initial current may have to be allowed for, a switch mode PSU which otherwise might be more than capable of operating the motor and its load not even get started properly as it may think its output is shorted.

A little more information is needed to be able to suggest an appropriate PSU.

Ian P

I'm thinking of using the motor for a sherline lathe. It's a tiny lathe so its not likely to have a large starting current I wouldn't have thought? I was more concerned about blowing the PSU if the cutter digs in and stalls the motor.

Emgee22/10/2020 23:30:20
1764 forum posts
237 photos

Posted by Hollowpoint on 22/10/2020 23:15:39:

I'm thinking of using the motor for a sherline lathe. It's a tiny lathe so its not likely to have a large starting current I wouldn't have thought? I was more concerned about blowing the PSU if the cutter digs in and stalls the motor.

There is plenty of choice for a controller on eBay and Farnell or Amazon list 24v 15A PSU suitable for your motor.

If you arrange the power to the controller or motor to pass via a contactor (or relay) with maintaining contacts for the coil you could have a lock off stop fitted into the coil circuit to use in case of a dig-in that stalls the motor.

Emgee

Steviegtr22/10/2020 23:31:12
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1643 forum posts
197 photos

I bought a 20amp dc power supply from fleabay for around £10. Unfortunately for me i should have gone for the 10A version because my x axis power feed wiper motor only takes 8A full load. Which means the speed control pot does not vary the speed very well. But the power supply has held up great.

Steve.

Maurice Taylor22/10/2020 23:40:39
151 forum posts
23 photos
Posted by Steviegtr on 22/10/2020 23:31:12:

I bought a 20amp dc power supply from fleabay for around £10. Unfortunately for me i should have gone for the 10A version because my x axis power feed wiper motor only takes 8A full load. Which means the speed control pot does not vary the speed very well. But the power supply has held up great.

Steve.

How does the maximum output current affect the speed control ?

Maurice

Hollowpoint22/10/2020 23:41:57
354 forum posts
37 photos

Thinking of using this power supply.

Mean Well LRS-350-24 (24v 14.6 amp)

It is the same one I have fitted to my 3d printer, which has been reliable.

Steviegtr23/10/2020 00:04:38
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1643 forum posts
197 photos
Posted by Maurice Taylor on 22/10/2020 23:40:39:
Posted by Steviegtr on 22/10/2020 23:31:12:

I bought a 20amp dc power supply from fleabay for around £10. Unfortunately for me i should have gone for the 10A version because my x axis power feed wiper motor only takes 8A full load. Which means the speed control pot does not vary the speed very well. But the power supply has held up great.

Steve.

How does the maximum output current affect the speed control ?

Maurice

Well i am not sure but think the speed pot is current limiting. As the device draws nowhere near it output the applied current limit does not work correctly. My thoughts were to buy a 10A version & try that. I thought the pot was faulty & tried another + some with different values, but to no avail.

Steve.

not done it yet23/10/2020 08:06:22
5141 forum posts
20 photos
Posted by Andy Gray 3 on 22/10/2020 22:43:31:
Posted by Hollowpoint on 22/10/2020 22:08:22:

Need some help guys.

I have a small DC motor I want to use on a project. The motor is 24v 200w and "rated" 11 amps.

11 amps will be at the rated output (11 x 24 = 240W). It will use very little current with no load.

I expect that if the motor says 200W that is what it will deliver, at maximum load. At that maximum power it will draw 11 amps, so those extra 64W supplied will be dissipated as heat within the motor and will be needed to be lost to the surroundings so that the motor does not overheat - and fail.

Power will be reduced at lower speed - torque may well be maintained to a good degree but power is ‘torque x speed’. Can’t get round that. It is Physics.

If the motor is run at full speed (24V) a power supply in excess of 11A would be preferred (the amount depending on whether that item is rated conservatively, or with minimal surplus capacity). So always go bigger if buying cheap.

How much bigger will depend on how the PSU is protected. Don’t expect too much protection if it is a really cheap chinese offering! Read the detailed spec - you may find it could have a run-time limit at full power, if a cheap one. Just like cheaper motors, just like cheaper welders, etc.

The one thing you must not do is buy a simple PSU with both higher amperage at higher voltage - unless it is a better supply than cheap chinese. Both voltage and current need to be controlled to the maximum allowable values, to avoid destroying the motor by overheating, over-speeding, etc.

Matching items from different suppliers is not always easy. There are advantages of purchasing a matched system from one supplier - particularly for those that do not understand the conflicting specifications.

Clive Brown 123/10/2020 09:01:32
537 forum posts
18 photos

Not my field, but I thought that a DC motor system, as suggested by the OP would show considerable speed variation with load. Not the best feature in a lathe.

I realise a lot of machines do have DC motors nowadays, but isn't there some sort of speed control feedback system?

Hollowpoint23/10/2020 10:37:46
354 forum posts
37 photos
Posted by Clive Brown 1 on 23/10/2020 09:01:32:

Not my field, but I thought that a DC motor system, as suggested by the OP would show considerable speed variation with load. Not the best feature in a lathe.

I realise a lot of machines do have DC motors nowadays, but isn't there some sort of speed control feedback system?

My understanding, (and I'm no expert) is that a regulated supply will increase the amps to try and maintain the speed.

Martin Connelly23/10/2020 10:46:46
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1518 forum posts
170 photos

Regulated power supplies usually try to maintain a constant voltage at the output when the load (amps) goes up and down. An unregulated power supply usually has an output voltage drop for increase current draw. You can get bench power supplies with constant current output but they are a bit specialist for most cases.

Martin

SillyOldDuffer23/10/2020 10:50:59
Moderator
6461 forum posts
1422 photos

No need to overthink it! It's an ordinary DC motor. The type has been around forever, but modern ones feature a sooper-dooper permanent magnet and high-temperature insulation. Smaller than the originals, and able to run safely rather hot.

All that's needed is a 24Vdc supply and controller capable of delivering about 11A. I'd overate the supply slightly to allow for stalls etc, but it isn't critical. The motor will work at reduced power output with less than 11A. I'd go for 12 or 15A, but 10A would be close enough. Amazon, ebay and others all sell PWM Speed controllers from 3A up. 15A should do.

The power supply is more of a gamble. Cheapest are the PSU's sold to drive LED Lighting Strips. Try searching Amazon for '24v led power supply' for many possibles. Second on my page was "DC Switching Power Supply, 24V 14.6A 350W for 3D Printer, LED Strip Light, Industrial Control System NES/SE/S", $28.50   Note these are components, not ready to go.  They need to be boxed with attention to electrical safety.

Cheap LED switch mode power supplies are clever. Voltage regulation is good and they have over-current protection. The reason I suggest they're a gamble is a particular model might have been designed specifically for a constant current load from LED lighting, which is very different from the wildly fluctuating current drawn by a motor. It's regulator might have a nervous breakdown! In practice all four different units I've bought to power motors have been fine. The design appears to be general-purpose, not LED specific, but there's no specification.

NDIY mentioned heat, and that's what matters most. It's important the motor and power supply don't get too hot. Manufacturer ratings are guidance to that end. Anyway, using this motor to drive a Sherline means it will be lightly stressed because hobby lathes spend more time stopped or idling than they do cutting. Unless Hollowpoint is going to hammer his lathe, the motor should be fine because on average it has plenty of time to cool off.

Clive Brown questions if DC motors are unsuitable for lathes due ti iffy speed control. Universal AC/DC Motors are indeed dreadful, though many small lathes and sewing machines use them successfully. Otherwise DC motors are generally more suitable than AC single-phase motors, but they're 'good-enough' too. Hollowpoint's motor seems entirely suitable to me.

Dave

 

 

Edited By SillyOldDuffer on 23/10/2020 10:54:13

Emgee23/10/2020 10:51:42
1764 forum posts
237 photos

For the simple 2 wire DC motors nearly all very low cost 12-48v DC motor speed controllers use PWM to control motor speed, this provides good torque at lower speeds and the potentiometer controls the length of and interval between pulses of the max voltage to vary the motor speed.

Higher priced units from many sources have more features and offer motor protection.

Emgee

john fletcher 123/10/2020 11:10:00
630 forum posts

If you are not going to need speed control, then two old car batteries will be adequate. But if you need speed controller then one of the PWM units from Ebay together with the two 12 volt batteries. Charge one up for say 4 hours then the other for 4. Most folk have a car battery charger. John

Ronald Morrison23/10/2020 11:46:10
49 forum posts
Posted by Hollowpoint on 22/10/2020 22:08:22:

Need some help guys.

I have a small DC motor I want to use on a project. The motor is 24v 200w and "rated" 11 amps.

I suspect the amp rating is at full speed with no load but not sure? Obviously if that is correct with heavy load it will draw more amps.

How do I choose the correct amp power supply? Do I need to go bigger? how much bigger?

How fat is your wallet? Ideally you want a power supply that can produce 24 volts (motor is rated for that voltage) and infinite amperage. That would keep the voltage constant over any load possible. Now you know that the motor can only draw 11 amps maximum so in theory a power supply that can produce 24 volts and 11 amps should be sufficient but in the real world, as the power supply approaches that maximum load the voltage drops. I'd probably be looking for a supply that could produce twice the rated amperage as that should be fully capable of maintaining the voltage at 24V but then I would compare that supply cost to how my wallet looks and think about a compromise. How often would I need that full power? What will be the consequences if the voltage drops a bit under full load?

Robert Atkinson 223/10/2020 13:18:28
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823 forum posts
17 photos

NDIY and SOD have pretty much covered it.
The Meanwell 24V 14.6A power supply wil run the motor fine with a bit of reserve.
The motor data plate clearly says 200W OUTPUT so 264W input is 76% efficency which is reasonable.

Note that this motor will run at maximum speed (3200 RPM) on this power supply so adequte gearing or belt reduction will be requied. If you want speed control, buy a PWM controller from ebay or amazon. That WILL need overating to get reliability I'd suggest at least 36V 25A "china rating" for a cheap PWM controller.

Speed on a brushed DC motor is directly proportional to the applied VOLTAGE, Current is directly proportional to the output torque (load). Current will rise to maintain speed. For a non ideal mtor the speed will reduce slightly due to the voltage drop across the winding and brush resistance.

With a PWM control the inductance of the windings causes the apparent voltage across the motor to be the supply voltage x PWM percentage so at 50% PWM and 24V supply the motor "sees" 12V and you get half speed.

Robert G8RPI.

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