Motor Overloading

No.

Any motor can be overloaded. It all depends on the design of the motor and motor driver and how well it was designed for a particular application.

A linear actuator delivering peanuts will work fine until an elephant climbs aboard, then problems start occurring. e.g. too much mass.

dats right

any motor can be heated or overloaded if u dont monitor ur input voltage and ur load ie maximum current it should drwa at full load

That is rubbish.

Motors, either DC or AC can have and will have an overloading condition due to increase of load, during voltage drop due to faults in the control system, faults in the distribution line, efficiency of the motor decrease, aging of the motor copper winding coil, faults in the load that the DC motor is connected and e.t.c.

Whether it is an AC motor or a DC motor it required some forms of protection relays such as overload relay, Fuse protection, over current protection and breakers.

For example:

In most of the rolling mills, especially for steel mills, AC and DC motors are used especially for the rolling rollers. DC motor is used due to the constant speed and constant torque are needed and it is a one of the most importance parameters in the rolling section.

During rolling of the billets down size from 120mm to 5mm size, with smooth rolling, the current for the DC motor are at the normal running operating current. But if the rollers miss rolled and the steel bar stuck inside the rollers, the DC current will increase up to a point where the Inverter will trip the DC motor as to protect the DC motor overheated.

Any motors either AC or DC will have an overloading and the overloading will cause motors to be overheated due to excessive of the current passing thru the motor coil winding. Heat will generate due to the magnetic flux in the core.

<...That is rubbish....>

Beware the disapproving Guest!

There are various types of motors, such as torque motors, that can be stalled without overloading. This includes AC for sure, and I would guess that there are DC types that would also work.

I was thinking at the same type of motor. A torque motor used to strech paper in a printing machine.

This type of motor has a very high resistance and a good cooling. It will warm up, but not as high that is will burn.

So no overload is possible.

I have seen a similar application in an automatic door close system. There were no limit switches to stop the motor, it just applied a certain torque to hold the door closed.

ha.. thsnk guys !

i had been debating this ( i said we need some overloading protection on the motors in our design ), but my boss said that DC motors do need that protection coz it will never happen.

Debated for 1 hr.... he still insists... ha ~

If you are still having problems convincing your boss (protection IS necessary for this type of DC motor by the way) you could actually prove it to him with an example using a linear actuator, a power supply and an ammeter.

Sometimes it takes something visual and hands-on to prove a valid point that if ignored can potentially cause a lot of grief and expense further on down the track.

Normally, a dc motor is supplied and controlled from a dc inverter. Within this unit there is a current limiting setting to be adjusted by the user. Also, there are fuses etc.

Therefore, maybe your boss is referring to this fact (without knowing exactly...?) and therefore he could be right in saying that you do not need an extra protection device downstream from the dc inverter driving the dc motor. the protection will be before the dc driving board to protect against failure of this board. Don't forget that the board has a bridge rectifier (controlled with SCRs) and if these fail, then a short circuit will appear on the supply to the board (there should be some fuses inside the board, but a set of fuses should also protect the supply...).

There is more than one way to protect a motor or other load based on limiting the input. The problem becomes one of controlling the amount of damage you are willing to incur. If your power supply or source is only capable of delivering the voltage and current that matches the specifications of the motor then you will reduce the possibility of an overload, but the motor performance will suffer.

Putting all of your faith on the motor manufacturer or the installer or the application may be acceptable for a quick test, but it would be foolish for the long term.

If you perform a "failure mode & effects analysis" (FMEA) you can start to see what I am talking about. Consider the locked rotor current at full voltage. Because there is no rotation in a locked rotor, there is no cooling. Consequently the heat will build up until insulation starts to melt. Shortly after that you will have short circuits between the loops of wire and it will just get hotter, faster. Then you will draw all the current the supply can deliver to the new lower impedance circuit. At this point, you are definitely in an overload condition.

Or some DC motors use brushes rubbing against commutators and they slowly turn into conductive dust covering all parts of the guts of the motor. Most of the time that is not a big problem. But consider the brave electrician who has no concept of safety and installs the motor while the electricity is still on. One little slip and he has a disaster in front of him.

In short, you should consider the effects of a dead short inside the motor. If you think that never happens, you should talk to one of the many firms that re-wind motors as their business.

The point is this: If you are prepared for a disaster, then you can mitigate the consequences. Otherwise a component failure inside the motor will propagate toward the power source until something burns up and stops the disaster. So, how much burning are you willing to tolerate? After all, most equipment runs until something fails, and an open circuit is not always the failure mode.

It's good to point out the possibility of a direct short in the motor, but that is not the same as an over load condition. The OP did not specifically say that the motor is not to have any type of current limiting devices such as fuses, which would act in the case of shorted windings.

The term "overload" is a very general description of what can happen. But if the motor is expensive I would definitely want something to intervene in case there was a condition where the mechanical load (connected to the rotor) suddenly seized up. Chains or belts can break or even get hung up in the machinery causing the motor to stall. When that happens, it causes an overload condition.

I don't think I have ever seen a motor that costs less than the chain or belt to connect it to some other device. Consequently, I would choose to protect the expensive stuff first. I would also want to prevent damage to the wiring because that kind of damage can escalate very quickly.

Try not to fall into the trap of assuming that the motor has been properly sized for the job to be performed. It is an expensive and potentially dangerous set of conditions where the motor is so oversized that can not stall.

That being said, there are other forms of protection such as a clutch or shear pin but as an electrical engineer I admit to being biased on using good design techniques rather than relying on some mechanical device to cover my butt.

For the novice motor overload protector like myself?

What would constitute a 'level' of protection?

fuses?.. resettable fuses? circuitry? thermal cut off switch? PLC?

____________________________________

..for the linear actuator.. I assume this must be an argument on whether or not to provide this in the design for an OEM product rather than to add protection to a single unit.

A boss that believes he's right when he's not? ...unheard of

thsnk a lot guys !

appreaciate advices from all of u ~