Hi,

I have a couple of questions about choosing the correct size transformer to drive a DC motor that was used in a stair lift. It was previously powered by two 12V batteries but I would like to replace this with a mains power supply.

The motor data plate is marked:
19V DC
18.5 A
Output = 240 Watts.

Am I correct in thinking that a 500 VA transformer would provide sufficient power for the motor to work as designed?
[19V x 18.5A = 350 VA , increase by 40% to allow for inrush current = 500VA]

Also, am I correct in thinking that a 24VAC secondary once rectified will provide approximately 20VDC ?

Thanks in advance

 

Also, am I correct in thinking that a 24VAC secondary once rectified will provide approximately 20VDC ?

hi ropak.
The 24Vac when rectified and smoothed by a suitable capacitor will be approx (24Vac * 1.414) -(2 *Diode Voltage drops) = ~32Vdc. Off Load.

E

 

The 24Vac when rectified and smoothed by a suitable capacitor will be approx (24Vac * 1.414) -(2 *Diode Voltage drops) = ~32Vdc. Off Load.

Thanks Eric for correcting me.

If I were to use a transformer with an 18Vac secondary winding, then according to the above calculation I would get approx. 24 Vdc. As the motor previously ran from 24 Vdc battery supply, I assume this would be ok.

I planned to use full wave bridge rectifier to convert the ac to dc. Does this method alter my output voltage calculation in any way?

 

The motors may work okay with the rectified DC from the bridge rectifier without any filtering.
In that case you would use a transformer with a 24Vrms ac output.

Does the lift have any electronic controls?
If so, that would require filtered DC.

 

Hi crutschow,

I would like to try the motor without any filtering and see how it goes using 24Vrms ac secondary.

There are no electronics involved as such, but I was planning to use switches and relays to turn the motor on/off and for change of direction. Do you think these components would require a filtered, or possibly even a regulated, supply?

 

Buy a 24V switchmode power supply.

It will be cheaper than a transformer, all things considered.

 

The problems with SMPS types for controlling motors such as this is reliability, IMO the linear supply is far more rugged and does not need regulation for a motor.
Max.

 

The problems with SMPS types for controlling motors such as this is reliability, IMO the linear supply is far more rugged and does not need regulation for a motor.
Max.

No doubt this is true.

I have found that transformers are getting expensive and the shipping costs are often the deal-killer for a big heavy chunk of iron.
Then you have to struggle with building the rest of the PSU circuit, all adding to the costs- time and money.

De-rating the SMPS is a good idea, to reduce the stress and increase reliability.

 

The switch mode supplies look very convenient, but at this stage I'm not sure how big a supply I would need.

I estimated a 500VA transformer would be enough. SMPS in this range seem relatively expensive. Am I over estimating how much power I need? As I understand, the amount of current required to start the motor can be up to 6 times the current used to keep the motor going.

Thanks

 

The motors may work okay with the rectified DC from the bridge rectifier without any filtering.
In that case you would use a transformer with a 24Vrms ac output.

Does the lift have any electronic controls?
If so, that would require filtered DC.

The motor is working just fine under load, straight from the rectifier, using a 500VA transformer with a 24Vrms ac secondary. I now need to filter and regulate the supply going to the control relays. Thanks for the advice.

 

The motor is working just fine under load, straight from the rectifier, using a 500VA transformer with a 24Vrms ac secondary. I now need to filter and regulate the supply going to the control relays. Thanks for the advice.

You should not need any filtering for DC relays, or even regulation if the voltage is withing reason of the relay rated V.
Max.

 

Thanks, I will keep that in mind.

 

From your recommendations, it seems the simplest thing to do is to run the motor and relays straight from the bridge rectifier.

More as an exercise in understanding, is there any benefit in filtering and regulating the supply at least to the control circuit and leaving the motor running from raw dc?

24 Vac secondary -> FWB Rectifier -> 2,200uf Capacitor -> LM7824 Voltage regulator.

Some articles have suggest that when using an IC regulator, the secondary voltage from the transformer should be a few volts higher than the desired dc output because of the voltage drop across the regulator. But if there ~ 34 volts after filtering, isn't that plenty of voltage to cover the drop?

I've attached a circuit diagram, not perfect but indicative.

 

More as an exercise in understanding, is there any benefit in filtering and regulating the supply at least to the control circuit and leaving the motor running from raw dc?

Depends upon what's in the control circuit.

 

There are 3 power relays, one signal relay, 4 momentary switches with LEDs and two limit switches (as per the attached diagram).

Edit: ... I also need to incorporate a motor brake, not shown in the diagram.

 

I don't see anything that particularly would benefit from a steady DC as compared to the pulsating DC.

 

I will keep it nice and simple and leave off any regulation and or filtering.

 

I'm afraid I still have to get my head around this.

I have a motor rated at 19 VDC and 18.5 A.

I have a 240:24 Vac transformer rated at 500VA to power the motor.

I would like to use the transformer to power the relays that control the motor which are rated for 24Vdc.

I would also like to use the transformer to power a LED indication circuit which is 12Vdc.

There must be a simple way to do this but I keep getting tied up in smoothing, regulation and ripple current calculations that are confusing to me. From the above discussion I take it there is no need to smooth the supply for the relays. I was only heading down this path again because I thought it would be easier to get my 12V supply from a regulated 24V by means of a DC-DC converter and that it would probably improve the operation of the relays anyway.

Any thoughts on a simple solution. I've thought about buying a 24Vdc wall adaptor to run the control circuit separately but it feels like an awkward solution and I won't be learning anything.

Thanks, in advance.

 

If you obtain a toroidal style transformer it is very easy to add an overwind for 12v, average addition would be ~24 turns.
As already mentioned you don't need the smoothing, bridge only.
Max.

 

Thank you Max, that is a very streamline solution. The main transformer is toroidal so I look forward to trying this out. Excellent!