Variable speed of DC motor

Thread Starter

Alexvatt

Joined Aug 24, 2013
31
I have a pump that is driven by DC motor. The pump has to work in a pulsed mode: it pumps for 2s and then it has to be idle for next 2s. I release the excess pressure with a solenoid valve but I also want to reduce the motor speed as well. I’m planning to run the motor at max speed for 2s and then reduce its speed by 50% for 2s. Will pulsed mode of operation burn the motor quickly?
Thank you.
 

MaxHeadRoom

Joined Jul 18, 2013
28,576
One of the KB/Baldor drives should do it, they are available on ebay, they have settable accel/decel so it is easier on the motor, if under constant load it is better rather than applying full voltage at that frequency of switching.
If it is an impeller pump, the current/load will be lower under pressure.
Max.
 

Thread Starter

Alexvatt

Joined Aug 24, 2013
31
One of the KB/Baldor drives should do it, they are available on ebay, they have settable accel/decel so it is easier on the motor, if under constant load it is better rather than applying full voltage at that frequency of switching.
If it is an impeller pump, the current/load will be lower under pressure.
Max.
I use PWM to control speed.
 

Thread Starter

Alexvatt

Joined Aug 24, 2013
31
I use Arduino UNO to generate PWM signal and default frequency is 500Hz. In theory I can increase it to max of 60kHz but I’m not sure if it would make any sense.
 

MaxHeadRoom

Joined Jul 18, 2013
28,576
No I meant what is the frequency/period between the switching of the 2sec hi and 2sec low.
Or is this a constant repetition?
Max.
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
I have a pump that is driven by DC motor. The pump has to work in a pulsed mode: it pumps for 2s and then it has to be idle for next 2s. I release the excess pressure with a solenoid valve but I also want to reduce the motor speed as well. I’m planning to run the motor at max speed for 2s and then reduce its speed by 50% for 2s. Will pulsed mode of operation burn the motor quickly?
Thank you.
if, at max speed, your motor is rated for the load, and you then run it at some reduced loading for some duty, you'll be fine. Note however that reduced speed, may not necessarily mean reduced load. A simple approach would be to monitor the temp rise, possibly employing a thermistor if there isn't one already.
 

MaxHeadRoom

Joined Jul 18, 2013
28,576
Just keep in mind that when applying full voltage to a stationary DC motor, maximum current will flow until the motor produces enough counter EMF via RPM.
If it is started at full voltage over a short period of time, it may not produce sufficient to heat the motor to any degree, but worth monitoring.
Max.
 

Thread Starter

Alexvatt

Joined Aug 24, 2013
31
if, at max speed, your motor is rated for the load, and you then run it at some reduced loading for some duty, you'll be fine. Note however that reduced speed, may not necessarily mean reduced load. A simple approach would be to monitor the temp rise, possibly employing a thermistor if there isn't one already.
In the first 2 s it pumps into a pressure vessel and it’s a pretty heavy load (although it will never reach max rating). In the next 2s it pumps into the atmosphere so a load variation is quite significant.

It’s a miniature diaphragm pump and the motor is really small. It consumes around 7W with the max load. It will run for 10 min at a time and I don’t think it will ever overheat. I can check it without a temperature sensor.

Actually I’ve got two reasons why I want to reduce the speed. One is to reduce the load variation, second to reduce pressure. I have one valve that controls pressure and the flow rate that it provides is not enough to reduce the pressure in the system to a desired level. I have two options: to install another valve in parallel or to reduce pump output.
 

Thread Starter

Alexvatt

Joined Aug 24, 2013
31
I monitor pressure with enough precision via pressure sensor. I'm not sure if there is and advantage in employing a pressure relief valve. I think it will be just an unnecessary expense when additional solenoid valve costs only$15 and I got it anyway.
 

MaxHeadRoom

Joined Jul 18, 2013
28,576
What type of pressure sensor? Is it monitored by the Arduino?
If so you could control the motor accordingly?
Without knowing all the details and facts of the system and the desired results, this is all guess work.
Max.
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
In the first 2 s it pumps into a pressure vessel and it’s a pretty heavy load (although it will never reach max rating). In the next 2s it pumps into the atmosphere so a load variation is quite significant.

It’s a miniature diaphragm pump and the motor is really small. It consumes around 7W with the max load. It will run for 10 min at a time and I don’t think it will ever overheat. I can check it without a temperature sensor.

Actually I’ve got two reasons why I want to reduce the speed. One is to reduce the load variation, second to reduce pressure. I have one valve that controls pressure and the flow rate that it provides is not enough to reduce the pressure in the system to a desired level. I have two options: to install another valve in parallel or to reduce pump output.
modulate your diaphragm armature.

Oops, motorized, I missed that.
 
Last edited:

strantor

Joined Oct 3, 2010
6,781
when applying full voltage to a stationary DC motor, maximum current will flow until the motor produces enough counter EMF via RPM.
In the first 2 s it pumps into a pressure vessel and it’s a pretty heavy load (although it will never reach max rating). In the next 2s it pumps into the atmosphere so a load variation is quite significant.


As Max has pointed out, starting the motor, especially with high load, is the worst possible scenario; it is the time of the highest current draw, and therefore the highest heating. And this is what you're going to be doing every 4 seconds. The heat will be cumulative, and on such a small motor, it won't have a very high thermal mass. It should be easy to test out. If it doesn't burn up in the first couple of minutes, then it probably won't ever burn up. Can you afford to lose a motor as a test?
 

Thread Starter

Alexvatt

Joined Aug 24, 2013
31
I know that it will work. I run it for about 5 min at a time many times and did not have problems with overheting.
My question was about long-term impact of varaiable speed operation of DC motor’s longevity.
 

MaxHeadRoom

Joined Jul 18, 2013
28,576
DC motors/servo's have traditionally been in motion control & CNC control etc for decades, in applications where the motor is in constant and variable rpm many hours at a time.
There are two rating, continuous torque rating and peak torque, naturally the peak torque should only occur very briefly as in applying full power to stationary motor or high load etc.
In extreme conditions, applying full power to a locked rotor for a period of time can de-magnetize the field, this is why I suggested a soft start from zero rpm.
The maximum continuous torque usually occurs at zero rpm for a DC motor.
Max.
 
Last edited:

strantor

Joined Oct 3, 2010
6,781
I know that it will work. I run it for about 5 min at a time many times and did not have problems with overheting.
My question was about long-term impact of varaiable speed operation of DC motor’s longevity.
When you say you ran it 5 min at a time, do you mean continuous, or in your pulsed mode?
 

Thread Starter

Alexvatt

Joined Aug 24, 2013
31
DC motors/servo's have traditionally been in motion control & CNC control etc for decades, in applications where the motor is in constant and variable rpm many hours at a time.
There are two rating, continuous torque rating and peak torque, naturally the peak torque should only occur very briefly as in applying full power to stationary motor or high load etc.
In extreme conditions, applying full power to a locked rotor for a period of time can de-magnetize the field, this is why I suggested a soft start from zero rpm.
The maximum continuous torque usually occurs at zero rpm for a DC motor.
Max.
Thank you for the advice. I can do soft start and just pump into the atmosphere during this period. What would be an optimal time for acceleration form 0rpm to max?
 
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