How can the motor avoid getting extremely hot?

[Ola.L]

Hi,

I am running a motor using the PCI 7344 motion control board and a MID-7654 servo with encoder feedback. Running the motor is not a problem but when it has stopped (there is no load) the servo system still puts out a lot of voltage (+-48V) all the time. This makes the motor really hot. I am using very smooth regulator parameters so as far as I know it should´nt have to make these corrections. In the next step there will be a load on the motor so I can´t just disable the axes (which otherwise would be the easiest solution).

//Ola

[Rodj]

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@Ola.L wrote:
Hi,

I am running a motor using the PCI 7344 motion control board and a MID-7654 servo with encoder feedback. Running the motor is not a problem but when it has stopped (there is no load) the servo system still puts out a lot of voltage (+-48V) all the time. This makes the motor really hot. I am using very smooth regulator parameters so as far as I know it should´nt have to make these corrections. In the next step there will be a load on the motor so I can´t just disable the axes (which otherwise would be the easiest solution).

//Ola

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Ola,

The bus voltage of the drive is 48V so you will always read that. What I need you to look at is the current going from the drive to the motor. When the motor is not moving, this current should be near 0 in the millivolts range.

Also have you set the drive's continuous and peek current settings correctly for the motor you are using?

You can also use the Step Response tab in our Servo Tune panels in MAX. Check that you are not oscillating around the set point at the end of a move.

When a drive and motor are properly sized and tuned, the motor should not remain or get hot when at rest.

Rodger S.
National Instruments

[Ola.L]

Thank You,

I have solved the problem. The encoder I used was disturbed by the motor so some "extra" pulses were created (I have not yet created a serious coupling with shielded cables and so on). With a new encoder the problem was gone.

//Ola

[Ola.L]

Rodger,

Now I have received the motor and encoder I'm supposed to use for the project and again the motor gets really hot.
I have looked at the motor signal when standing still (the position counts in 1D-interactive in MAX is not changing) and the servo puts out a lot of power. The current when I measure AC RMS is 4-5 Ampere when the motor is standing still.

I have two servos, two motion controllers and two motors and all combinations gives the same result. The motor I have problems with is "Parvex F12M4R Axem series" see "http://www.parvex.com/pdf/english/com/3564-GB_axem.pdf" or "http://www.parvex.com/pdf/francais/motinstal/3385.pdf" for more information.

When I do the same measure with another motor the current is approx. 400 mAmpere current. So it seems like the servo doesn't like my parvex motor.



//Ola

[Brian_Beal]

I just want to clarify something. When you say that the motor is still, is it also under no load? For example, I want to make sure that it isn't being used in a vertical application. When comparing the "good" motor to the "bad" motor, are all conditions the same?

[Ola.L]

The motor has no load at all now when testing but it will have later in the project. It is just a motor with an encoder fixed to it. The other motor which is a smaller one has no load either.

One more thing: I have the Dip switches put to minimum current so the servo should not even leave more than max 1.7 A.

//Ola

[Ola.L]

There was a problem in the order of the messages in this thread but this is the current status:

Now I have received the motor and encoder I'm supposed to use for the project and again the motor gets really hot.
I have looked at the motor signal when standing still (the position counts in 1D-interactive in MAX is not changing) and the servo puts out a lot of power. The current when I measure AC RMS is 4-5 Ampere when the motor is standing still.

I have two servos, two motion controllers and two motors and all combinations gives the same result. The motor I have problems with is "Parvex F12M4R Axem series" see "http://www.parvex.com/pdf/english/com/3564-GB_axem.pdf" or "http://www.parvex.com/pdf/francais/motinstal/3385.pdf" for more information.

When I do the same measure with another motor the current is approx. 400 mAmpere current. So it seems like the servo system doesn't like my parvex motor. I have the Dip switches put to minimum current so the servo should not even leave more than max 1.7 A!

The motor has no load at all now when testing but it will have later in the project. It is just a motor with an encoder fixed to it. The other motor which is a smaller one has no load either.



//Ola

[ChadE]

Hello Ola,

I noticed that the motor has a brake. I wonder if the brake isn't "somehow" engaging when the motor is seemingly idle. This could be what is providing the load that is causing the control loop to become unstable. I couldn't find any documentation describing the operation of the brake on their web site. It may be helpful to find a manual that describes the brake's operation or any special connections for the brake.

I hope this helps,

[Ola.L]

Hello Chad,

the brake is optional and we don't have it on our motors. When the servo is off or axis disabled the motor is moved very easy by hand.
Even when the servo is on I can rotate the motor a few pulses away by hand from the current position without the servo trying to maintain the last position but it's continuously putting out the power which makes the motor hot. When I rotate a little more the servo tries to put the motor back in the last position.

Is there a way to see the motor signal that is sent out from the 7344 card in MAX for example?
Shouldn't it be sending out 0V as motor signal when standing still and no move is expected?
And if it does send out 0V how can the servo amplifier then put out a +-48V, 4-5A current signal to the motor?

//Ola

[ChadE]

Hello Ola,

1. Use Read DAC.flx to read the value being output to the DAC. The voltage of the DAC is controlled by the PID loop. A well tuned motor should go to 0 when it is standing still or something very close to 0.

2. Also read the following error using Read Following Error.flx. Following error is the input to the PID. This may help explain the value on the DAC. You may want to try multiple reading of both the DAC and following error to see if the output is oscillating.

3. What are your PID values?

To answer you last question, the drive should output a current that is proportional to the output of the DAC. The drive uses a 48V Pulse Width Modulated output to control current using the inductance of the motor to average out the voltage and Ohms law to translate voltage to current.

4. How are you measuring voltage and current?