instantaneous current limits in dc motors?

[Brandon]

Wondering if there is some rule of thumb for instantaneous current
limits in DC motors? I think I read somewhere that 3 times the
constant current was a good guess, but, even if that is true, that
doesn't address how long the higher current can be applied.

 

[Paul E. Schoen]

Wondering if there is some rule of thumb for instantaneous current
limits in DC motors? I think I read somewhere that 3 times the
constant current was a good guess, but, even if that is true, that
doesn't address how long the higher current can be applied.

For AC induction motors, without brushes, the maximum current is
self-limited to about 3-5 times the rated current, and a thermal motor
controller will limit that "stall current" to about 5 seconds.

But for series wound or permanent magnet DC motors, much higher currents
can occur. You are probably safe to use a circuit breaker or fuse rated at
the normal current value, although a slow blow fuse or a time delay circuit
breaker could allow 3x current for as long as 60-90 seconds, which may be
too much.

If a motor has an overload spec, you might be able to use that for an I^2t
rating. So a motor that is rated at 10 amps, and can run at 20 amps for 10
seconds, has an I^2t of 4000. So you can run 32 amps for 4 seconds, and 63
amps for 1 second. You must also allow a cooling off period between bursts,
and this is also a square law function. At 2x the duty cycle,
ton/(ton+toff), is 1/2^2 = 25%, at 3x it is about 10%, and 5x it is 4%.

But the brushes might limit the safe overload to a lower value. At some
point there will be excessive arcing and localized heating which could
cause damage and reduced life of the brushes and commutator. These figures
are related to I^2R heating effects, and are commonly applied to
transformers.

Brushless DC motors have their own limitations due to the solid state
components, and should have overcurrent protection built into the
controller.

BTW, there is no such thing as a DC motor. It must be converted to AC by
some means to make it continue to run. The only true DC machine is a
solenoid. There is also no such thing as DC. The current had to start at
zero at some point in time, and then it took a finite time to reach some
level, and eventually it will return to zero. So it's really a rectangular
waveform of (perhaps very long) period.

Paul

 

[Phil Allison]

"Brandon"

Wondering if there is some rule of thumb for instantaneous current
limits in DC motors? I think I read somewhere that 3 times the
constant current was a good guess, but, even if that is true, that
doesn't address how long the higher current can be applied.

** DC motors draw current - you apply a voltage to the terminals.

You should not exceed the max voltage rating at all.

High current will flow if the motor is stalled or overloaded.

Just how much and for how long is OK depends on the motor.


....... Phil

 

[Tim Wescott]

Wondering if there is some rule of thumb for instantaneous current
limits in DC motors? I think I read somewhere that 3 times the constant
current was a good guess, but, even if that is true, that doesn't
address how long the higher current can be applied.

I can't tell you details, because getting such information from the motor
manufacturers can be like pulling teeth (or perhaps finding hen's teeth).

I can tell you that graphite brushes are more tolerant of overload than
precious metal, and that AlNiCo magnets will demagnetize with too much
current, probably instantaneously.

I can also tell you that in one application we routinely subjected some
of the Maxon 16mm graphite-brushed motors to the full stall current with
a slow PWM voltage. The only failure mechanism that I know about was the
magnetic encoder wheels spinning off of the shaft from the angular
acceleration.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[Jamie]

Wondering if there is some rule of thumb for instantaneous current
limits in DC motors? I think I read somewhere that 3 times the
constant current was a good guess, but, even if that is true, that
doesn't address how long the higher current can be applied.

It's specified on the motor..

http://webpages.charter.net/jamie_5"

 

[Stephen J. Rush]

BTW, there is no such thing as a DC motor. It must be converted to AC by
some means to make it continue to run. The only true DC machine is a
solenoid. There is also no such thing as DC. The current had to start at
zero at some point in time, and then it took a finite time to reach some
level, and eventually it will return to zero. So it's really a
rectangular waveform of (perhaps very long) period.

Well, there are homopolar motors, but they are rare except as lab
demonstrations.

 

[Paul E. Schoen]

"Paul E. Schoen PITA Fucking IDIOT"




** Shame how there are millions of the darn things.




** Still called a DC motor cos it runs on DC current.




** Now *that* is just plain absurd.

It is merely a PHIL-osophical point, meant to be humorous.

I'll let you have the last word.

Paul

 

[John Fields]

BTW, there is no such thing as a DC motor. It must be converted to AC by
some means to make it continue to run.