Back EMF on DC motor

[rainman]

Hi all,

I have some questions regarding the back EMF problem on the DC motor.

I have a 2 wire leads 160VDC motor that is connected to a electric cyclinder, which then extends and retracts vertically with a load on top, subjected to gravity.

In order to regulate the speed, I uses a 24VDC 20A switching power supply for the motor.

Extending forward was ok, however when the cyclinder retracts, the speed was alot faster than extending, speed was somehow uncontrollable.

To my horror, when I measure the voltage on the 24V PS, I got 50-60VDC, increasing as the speed goes.

Out of curiousity, I used a 5V power supply, the retracting speed was somehow controlled. Similarly I measured across the voltage across the power supply, I got 10-12VDC.

My guess is the retracting speed follows the voltage it received from the power supply + the Back EMF.

Is Back EMF a common problem for DC motor application?

How can I eliminate this problem, in terms of electronics? Adding of zener diode helps?

By the way, the 2 power supplies are conked up, burnt but still working (strange)

Thanks for all help!

Rainman

[Al_The_Man]

The BEMF on a DC motor is what restricts the speed for a given input voltage. IOW, if you you apply a certain voltage to the motor, the motor will increase speed until the BEMF voltage is equal (or slightly less) than the applied voltage.
If you are using a 160 v motor, then if this motor were to be rotated at its rated rpm, the output would be 160vdc.
If you are using it in an application where the speed is not being controlled, i.e. no feedback, then the amount of BEMF is unpredictable, and will depend on what effect the back feeding will have on the rpm.
A couple of options is either to use a controller that takes tach feed back, (requires a tach on the motor).
Use dynamic braking, this is usually a braking resistor switched in by a HexFet or similar device, when the detected voltage reaches a certain point.
Another method is to reduce the voltage to the motor when descending, or simply switch a resistor in parallel to the motor when descending also.
The value may have to be arrived at by trial and error.
Al.

[rainman]

Thks Al_The_Man

I will try putting a resistor parallel to the motor, how abt putting diode? It supposed to prevent reverse voltage?

[Al_The_Man]

Originally Posted by rainman how abt putting diode? It supposed to prevent reverse voltage?

You are thinking of reverse induced voltage when the field of an inductive load collapses, in your case, this is a voltage generated in the same polarity as the supply.
Al.

[gar]

071110-1555 EST USA

rainman:

A permanent magnet DC motor is a motor or generator depending upon the direction of energy flow.

If you truely had a regulated DC supply that could either source or sink current and regulate while doing this, then you could control speed in both directions. If a + raised the cylinder, then a - would lower it.

If you had a storage battery and a double pole double throw switch, then you could get near the same speed up and down, A little slower on up and slightly faster on down. With your current setup use a single pole double switch. One connection is to the power supply for raising and the other to an appropriate resistor for lowering.

.

[Al_The_Man]

Incidentally a DC motor is both motor and generator simultaneously, the voltage the motor sees is the difference between the applied voltage and the generated back emf, if a load is place on the motor, the rpm drops and the difference voltage increases, this results in higher current (torque), albeit, lower rpm.
e.g. if the voltage is applied to an armature with a locked rotor, the back emf is zero, the resultant current will depend on the resistance of the armature winding.
If the motor armature is rotated at a rate that generates a voltage higher than the applied voltage then some low impedance means has to be used to reduce the back emf in the form of an electrical load across the armature .
Al.

[Rhodan]

Yes, there's really no fundamental difference between an electric motor and electric generator.

In the Navy one of my earlier jobs was turbo-alternator operator. The TAs were 750KW alternators run by steam turbines. One day the Chief of the Watch told me to shut down the port TA which had been taken off-load already. I went and hit the trip and lights started dimming while the TA made altogether frightening noises! I immedialty reset the trip and things went back to normal.

It turned out the TA was not actually offline yet. Tripping it caused it to fall behind the other TA and the phase difference started changing the alternator into an AC motor (one TA driving the other).

The electricians had some 'splainin to do that day!

[rainman]

Hi,

I had tried diodes and zener diodes and both didnt work at all. I had also tried putting resistor parallel across the motor and it didnt worked as well (I cant get the values correct)

The only way to control the down speed was putting in a 5V PS, however the back EMF causes it to be 10-12V, which resulted damage to the PS (swollen capacitor)

I thinking if I could just upsize the capacitor so that it is more tolerable to the back EMF and also if adding a 7805 regulator would help?

[gar]

071125-1014 EST USA

rainman:

In place of your 5 V power supply place a short circuit and see if the descent is slow enough. If too slow, then select an appropriate resistance to use in place of the short. You must consider the power dissipation in the resistor. If the descent is too fast with the short, then you need an adjustable current source, or a more complex system.

.

[Al_The_Man]

This is an adaptation of a DC generator regulator I built some years ago, the original was for 240vdc, but this may work, I have changed a few components to suit, the components are not expensive so time would be the biggest loss if it doesn't work out. What could happen is it may have to be adjusted for optimum if the descent is jerky due to voltage collapse and build up, but it might be worth the try.
Al.

[vtxstar]

Is it possible to reduce the gear ratio to allow a slower descent?

This looks like a typical linear actuator (Satellite dish, etc). How critical is the position of the load when driving down? Is it just to unload the platform or is the "load" raw material that is being worked?

If the shorted-leads-while-going-down idea fails, you don't have many options if you must use that actuator.

Al's schematic is a controlled clamp - quite clever actually.
-=Doug

[rainman]

Thanks for the replies.

Yes it is linear actuator, the position is not that accurate, I have 2 reed switches that is mounted on the side of the actuator for positioning control (2 positions only)

It took me 3+ mins to extend to the max and 40 secs to retract fully (with 1 tonne load)
using 24VDC PS (back EMF til 60VDC)

With that kind of load, it was pretty scary to see the descent, not to mention about the sound the actuator created.

I'm caught in the situation whereby I can only solve the problems in terms of pure electronics.

I have yet to try out AI_the_Man method and the short method.
Desperately I will have to temporary stick to 5V PS and upgrade the capacitor and finger-crossed it may last.

Rgds,
Rainman