Supression the initial current drawn by a motor

[jophuds]

Hello,

The situation I'm in is that I am using a Motor I/O Card to control two different motors. The card has two outputs to control the motors. The card is being used in conjunction with a PC104 card, which will control the motion of the motors through the Motor I/O Card.

The problem I have is that the Motor I/O card can only output a maximum of 3A and an absolute maximum of 6A for very short transients --- however each motor draws upto 11A initially before settling to steady-state of 1-1.5A.

So what I want to do is find a way to supress that initial current needed by the motor.

It has been suggested to me that the use of capcitors across the battery would be an effective solution and the use of Power Mosfets was also suggested.

Does anyone have any comments on these or any other solutions to this problem.
Thanks in advance,
joe.

 

[busbar]

One cannot get blood from a turnip. In ANSI regions, a magnetic starter is the accepted method for controlling motors, for either PLC-based I/O or 'passive' control components.

Also, it is imperative to incorporate motor-overload time-current characteristics in the motor-power circuit.

 

[electricpete]

absolutely valid concerns raised by busbar.

I assume it is a dc motor. I believe a help capacitor across the battery/supply output would maintain the voltage during the high current.

You have the responsbility to make a safe choice.

 

[jophuds]

electricpete and busbar -- thank you very much for your replies.
My apologies for failing to mention it is a DC Motor !

Can either of you shed some light on choosing an appropriate capcitor value -- as this seems the most suitable solution given the situation I'm in.

Thank You

 

[busbar]

Whether AC or DC, the magnetic starter would be considered effectively a very desirable “interposing relay.”

 

[electricpete]

I can get you started.

i = c * dv/dt
integral i dt = integral c *dv = c* integral dv = c * deltaV

C = i*t / deltaV

Let's say your power supply is 10vdc and you want to keep it above 90% voltage (deltaV=1vdc) while supply I=11A (neglect current provided by battery) for t=2sec

C =i*t / deltaV = 11A*2sec / 1V = 22F

Maybe you should check my math and my units. That's a pretty big cap.

 

[Nandos]

The starting current of the motor needs to be analyzed if the motor is loaded or not.

Though, the motor will require additional current to start the run and it is going to be difficult to have it reduced.
The problem that I had, somewhat close to yours, was solved by having an additional circuit that detected the initial arrival voltage and added a circuit to apply at 3 times the incoming current for about 4 seconds.

I had designed the circuit for it.

Nando

 

[GusD]

Hello jophuds .

Capacitors are a great device but unfortunately they don't perform miracles.Your card relays are good for 3 Amps, max 6 amps and your motors draw 11 .You cannot change those conditions.Just to take a page out of Busbar's book ,You cannot get blood out of a Turnip.The best course of action is the use of interposing relays or contactors with high enough current rating to run your motor.Most relays/contactors are probably good for at least 15 amps Inductive.

GusD

 

[jophuds]

Thanks GusD,

Can anyone recommend a site or a place where I can learn more about relays and the possibility of using them.

Does anyone know if it is possible to build a circuit that will only involve the use of the relay when the motor is stopped and needs to be restarted ?

Thanks in advance,
joe.

 

[cbarn24050]

Hi, all you need is a current limiter in the motor cicuit. Cheapest is a resistor, if that degrades the performance too much then a power transitor can be used. look up constant current circuits to find out how to connect it.

 

[jbartos]

Suggestion: Depending on nature of motor control needed, the following might be considered:
1. The output from the controller could be connected into the input of a suitable amplifier. There is a variety of amplifiers with various characteristics.
2. If the motor control is simple, e.g on/off, the controller amplification might be implemented via a suitable contactor, e.g. solid state contactor or relay.

 

[jophuds]

jbartos,

I am concerned about the use of a relay as the motor must be able to accelerate and have differenct velocities. My understanding of a relay indicates that it can only be on or off -- is this correct ??

If possible could you detail more about the use of an ampflier and state any possible pitfalls with this method.

Thanks in advance,
joe

 

[jbartos]

Comment:
1. On/Off amplification of the I/O control card is implemented over a suitably sized contactor or relay that would be rated for the maximum 11A with some design margin added. I/O card output controls the contactor or relay coil. Contactor or relay contacts control the input from a suitable power supply controlling the motor.
2. The amplifier approach is more flexible since the I/O card output is possible to adjust, thus implementing the variable input to the motor control over the amplifier output controlling the power to the motor.
Visit

http://www.aveox.com/lcbmc.html

etc. for more info on the amplifier

 

[electricpete]

Yes, I think I misunderstood your setup. I thought that the power supply was limiting your current, not electronic switch capability. Even in that case there would be a problem with inrush to the cap upon power-up.

 

[Marke]

Hello jophuds
It sounds to me as though you are using a card that is designed to control the voltage applied to the motor, probably using a PWM technique, rather than an OFF/ON switching arrangement. If this is the case, then you should try to program it to apply a ramp up in voltage rather than a step to full voltage. This will allow the motor to accelerate to full speed more gradually and will reduce the "starting" current.
Best regards,

Mark Empson

http://www.lmphotonics.com