Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

DC Motor Power Supply

Status
Not open for further replies.

stagnant7

New Member
Hi,

I have a problem of increased brush wear on a 110VDC permag motor. The motor is driven by a DC/DC Single Output Converter.

The input to the converter is from a 48V battery and the output is supplied to the motor. The output rating of the converter is 125V 8A. The motor is operated intermittently for 10sec at a time and draws 6A typically.

This same type of motor has been used in similar situations for many many years with normal brush wear. The difference here is the DC/DC converter setup.

My question is what impact the converter could have on brush wear. Can different types of supply voltages (i.e. switchmode supplies like the converter mentioned) affect the DC motors brushes?

Thanks.
 
How are you stoping and starting the motor? Before or after the motor? Can you enable and disable the Converter?
 
Was the previous supply a simple transformer, rectifier, and filter?

I assume the converter's operating frequency is well above audio frequencies and that it is filtered. That in itself should not present any problems. You can run PMDC motors at 16KHz without filtering as the motor's inductance will keep the current looking like it's DC. We've had problems keeping switching devices cool above about 30KHz. There isn't much advantage going higher for a PMDC motor load and all the extra aluminum required to keep the switching devices cool gets expensive. Inverter drives generally impact the integrity of the insulation system and bearing life more than brush life.

How stable is the power supply's output when driving a highly reactive load? PMDC motors are highly reactive on startup and having current lag voltage can cause feedback problems within the supply, especially during startup and also after speed changes. Within reasonable limits, brush life is typically a function of the inverse amperage squared so big current spikes will eat brushes.

Is the output being ramped up or just switched on? How many commutator segments are there and what is the RPM? Is there a flywheel diode across the motor?
 
Last edited:
Thanks for the replies.

- Yes the other configurations use a bridge rectifier
- The Motor 2 pole. There are 48 commutator segments. The RPM of the motor at rated load is about 1200RPM. There is no flywheel diode across the motor
- The 48V to 110V DC/DC converter is just switched on as far as I know. I am only at an early investigation stage, so I'll be finding out more about the setup.

I imagine measuring the output voltage and current waveforms of the DC/DC converter when the motor is energised will give some indication of how the DC/DC converter is handling the load

Thanks Again.
 
I'd say you definitely need the flywheel diode. Although I doubt it's the real source of the problem, the lack of one is likely increasing the arcing of the brushes and could well be affecting power supply stability too.

There is probably some impedance protection but the sudden application of 110VDC is where the preponderance of brush wear is likely occuring. Power switched on -> motor jumps but PS shuts down due to overcurrent -> motor is spinning slowing when it tries to start again 50 ms later -> gets it moving faster -> PS shuts down again -> starts again -> shuts down again -> eventually makes it up to speed after a second or so equals multiple sudden applications of 110VDC, maybe 10 times in the first half second, maybe more, equals 10 times the brush wear.

The only indication that it's happening would be a rough start and that may not even be at an audible frequency. Try including a big 2.0Ω 100 Watt resistor in series for a little while and see if it changes things. It could reduce starting current by 20% and shorten the rough start. It's not a fix but it could shorten the rought start and help to isolate the cause.
 
Last edited:
The 48V to 110V DC/DC converter is just switched on as far as I know. I am only at an early investigation stage, so I'll be finding out more about the setup.
You may what to leave the converter on and switch power to the motor. If you switch the converter on and off it may be sending some strange stuff to the motor before it stablizes.
 
That might work better but it's hard to say for sure. If there's pretty much nothing else for a load when the motor is idle, the power supply is probably running in burst mode or marginally stable. Instantly switching from 0.0015% to 150% of the power supply's rated output with a highly reactive load could still result in what is essentially the same behavior.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top