Driving a 220-240v DC motor

[thiago]

Hi,

I'm building a solution to automate my sit/stand desk. I have got a 220-240v DC motor from eBay (link below), and a full bridge rectifier to convert from 220v AC from UK mains.

On my first attempt, the motor just blew up upon the first connection. The only thing I have to take measures, is a cheap multimeter, which indicated that, on the output of the rectifier, I was getting 220v DC, but also getting over 400v AC, which could explain the issue.

I bought a new motor, and instead of connecting directly to the mains, I've used a 220v/110v 2:1 45vac transformer, I get 110v DC on the rectifier, and something around 350v AC. It looks like this is OK with the motor. Without load, it drains around 80mA, and the DC voltage rises to 160v, and the AC stays at 350v.

I'm assuming the DC voltage from the multimeter is the median, and the AC is the RMS considering peak-to-peak (~400v). Is that correct?

Is it safe to use it like this?
Any alternative circuit suggestions to drive the motor without the transformer?
Should I add any other component for protection?

Thank you!

DC Motor: https://www.ebay.co.uk/itm/202698113844
Rectifier: https://www.ebay.co.uk/itm/283499985397

[soldar]

You might be interested in this thread: https://www.eevblog.com/forum/chat/230v-ac-motor-speed-control-looking-for-simple-circuit/

[Zero999]

Your circuit should work. The motor should be able to be driven from 230VDC unsmoothed from a bridge rectifier. I've seen a similar motor used in a vacuum cleaner to drive the brush.

Did you use a smoothing capacitor? If so, then perhaps it's that which caused it to blow up, since it would increase the RMS voltage to the motor.

Did you measured the voltages with a true RMS meter? If not, then you can't trust the reading.

Unless the motor is in a plastic box, with the load driven with plastic gears, then the motor's case should be connected to earth, otherwise it will present a shock hazard.

Lastly, the motor's specification doesn't add up. It says it's rated for 500W, when the current draw is only 0.22A, which would be 50.6W of power draw and the power output will be be 31.6W, as the efficiency is just 62.4%, which is typical for a small, cheap, brushed motor.

[amyk]

What do you mean exactly by "blew up"? A 240VAC supply will be closer to 340V rectified, and that is a pretty significant overvoltage, but non-electronic motors like these will usually tolerate short overvoltages for some time without catastrophic failure. (See Photonicinduction YouTube videos, for example...) Maybe you just got a bad one at first.

Lastly, the motor's specification doesn't add up. It says it's rated for 500W, when the current draw is only 0.22A, which would be 50.6W of power draw and the power output will be be 31.6W, as the efficiency is just 62.4%, which is typical for a small, cheap, brushed motor.

0.22A no load current, when it's running at 15k RPM. The current at rated load should then be 2.2A.

[Zero999]

Lastly, the motor's specification doesn't add up. It says it's rated for 500W, when the current draw is only 0.22A, which would be 50.6W of power draw and the power output will be be 31.6W, as the efficiency is just 62.4%, which is typical for a small, cheap, brushed motor.

0.22A no load current, when it's running at 15k RPM. The current at rated load should then be 2.2A.

Oh, I missed that.

It's not clear whether 500W is the input or output power. I still think 500W is a bit optimistic, given the lack of a fan and the small ventilation holes. I think it's the peak, rather than continuous power rating.