Simplest circuit to drive a 24V DC motor from the mains?

[Foo]

Hi, let's say that I have this hypothetical motor that is rated for
24V DC and draws an unknown amount of current (say around 1A).

Input is 230V AC.

I'm wondering if the motor will work properly with a step-down transformer
to 24V AC and a rectifier but no smoothing.

Is that going to make it noisy or break it in the long run?

Thanks.

 

[Rich Grise]

Hi, let's say that I have this hypothetical motor that is rated for 24V DC
and draws an unknown amount of current (say around 1A).

Input is 230V AC.

I'm wondering if the motor will work properly with a step-down transformer
to 24V AC and a rectifier but no smoothing.

Is that going to make it noisy or break it in the long run?

No, as long as the output of the rectifier is 24VRMS. It might have a
bit of 120HZ hum (I'm assuming a full-wave rectifier), but the motor
itself has a lot of inductance, which sort of makes it its own filter.

Hope This Helps!
Rich

 

[Foo]

No, as long as the output of the rectifier is 24VRMS. It might have a
bit of 120HZ hum (I'm assuming a full-wave rectifier), but the motor
itself has a lot of inductance, which sort of makes it its own filter.

Hope This Helps!

Yes, thanks!

 

[Jamie]

Hi, let's say that I have this hypothetical motor that is rated for
24V DC and draws an unknown amount of current (say around 1A).

Input is 230V AC.

I'm wondering if the motor will work properly with a step-down transformer
to 24V AC and a rectifier but no smoothing.

Is that going to make it noisy or break it in the long run?

Thanks.

If its just a commutated brush type motor, it should work fine.

 

[petrus bitbyter]

Yes, thanks!

Beware. The operational current to drive a 24V/24W motor wil be about 1A as
you might expect. The inrush current however will be a multiple of that 1A
and your rectifier should be able to handle that current. A 10A full bridge
rectifier is no overkill.

petrus bitbyter

 

[Foo]

Firstly, a transformer will deal with voltage level and isolation from
the AC
line, but it'll cost you. A 230VAC motor might be a cheaper way
to go. The transformer can be 48V center-tapped, with two diodes, or
24V with a bridge of four diodes.

Unfortunately, I don't think this motor can be replaced.

I didn't include this in my first post, but was actually thinking
about such a transformer _and_ a four diodes bridge to allow reverse
operation of the motor with a two-way switch.

The nominal "24VDC" rating probably means anything from 18V to 28V
is acceptable. If you don't know the current requirement, DO include
a fuse.
Transformers are often made short-circuit-safe with fusible wire, and
burning out the transformer makes for one expensive bit of
experience, but a fivepak of fuses gives you that experience
several times, for the cost of a candy bar.

Thanks for the tip. Maybe a fuse on the 230V side is a good idea too?

 

[Rich Grise]

Thanks for the tip. Maybe a fuse on the 230V side is a good idea too?

Yes, but size it such that it won't blow unless there's a transformer
failure. IOW, the fuse on the motor side should blow first.

Hope This Helps!
Rich

 

[Rich Grise]

Beware. The operational current to drive a 24V/24W motor wil be about 1A
as you might expect. The inrush current however will be a multiple of that
1A and your rectifier should be able to handle that current. A 10A full
bridge rectifier is no overkill.

Also, that's what "Slo-blo" fuses are for.

Cheers!
Rich

 

[Foo]

Hi, let's say that I have this hypothetical motor that is rated for
24V DC and draws an unknown amount of current (say around 1A).

Input is 230V AC.

I'm wondering if the motor will work properly with a step-down transformer
to 24V AC and a rectifier but no smoothing.

Thanks for your replies.

Now, if I happen to add a capacitor to smooth the DC output somewhat, do I
need to replace my transformer with one that outputs 18V instead, or will the
voltage drop take care of that?

Which setup would you recommend? My goal is to keep this circuit as simple
and efficient as possible. I'm willing to add a capacitor if it improves
efficiency.

 

[Foo]

---
Since 24VRMS is about 34V peak, DC, adding a capacitor will cause the
input of the motor to rise to 34V if capacitor is large enough.

I'd stick with a full-wave bridge and no cap.

Okay. One last question: for a 25W motor, is a 30VA transformer appropriately
sized?

 

[Rich Grise]

Okay. One last question: for a 25W motor, is a 30VA transformer
appropriately sized?

Yes, with the full-wave bridge and NO capacitor.

Cheers!
Rich

 

[Foo]

Yes, with the full-wave bridge and NO capacitor.

Thanks!