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I have an actuator which is rated at 12 VDC input voltage (0-15) with a maximum allowed duty cycle of 20%. The already existing voltage levels in my system are 5V and 22V, and since I am weight constrained I would rather not add another step-down converter.

What would be the implications of running at a higher voltage (22 Volts) and a lower duty cycle, say 11%, resulting in the same average voltage and current? My PWM-frequency is about 20kHz and the motor is a simple brushed, dc motor.

More specifically, would I get away with it, without damaging my motor and severely reducing its Lifespan?

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  • \$\begingroup\$ There are ways to do this, yes. But first you need to characterize the load (explain the type of motor/actuator) - if it is inductive you will want to take that into consideration - and it can actually be a good thing since it can make it easy to do current regulation. \$\endgroup\$ – Chris Stratton Dec 28 '18 at 17:39
  • \$\begingroup\$ specify L and DCR of actuator or link datasheet \$\endgroup\$ – Sunnyskyguy EE75 Dec 28 '18 at 18:01
  • \$\begingroup\$ WHich supply source has the best load regulation error for the surge power level needed? \$\endgroup\$ – Sunnyskyguy EE75 Dec 28 '18 at 18:26
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If the 20% duty-cycle limit is based on power dissipation, then you need to understand that power is proportional to the square of the voltage. If you want to raise the voltage by a factor of 1.8, then you need to drop the duty cycle by a factor of 3.36 — to no more than 6%.

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  • \$\begingroup\$ Power is only proportional to the square of the applied voltage for a resistive load. For an substantially inductive one, the current rise time may be need to be taken into account. \$\endgroup\$ – Chris Stratton Dec 28 '18 at 17:36
  • \$\begingroup\$ @ChrisStratton: A motor is "substantially" resistive. \$\endgroup\$ – Dave Tweed Dec 28 '18 at 17:38
  • \$\begingroup\$ Depends on the type... for something electronically commutated like a stepper (or in the degenerate case solenoid) inductance is intimately coupled to the duty cycle. \$\endgroup\$ – Chris Stratton Dec 28 '18 at 17:40
  • \$\begingroup\$ And I'm not convinced that even a brushed DC motor looks resistive at the mentioned 20 KHz PWM rate. \$\endgroup\$ – Chris Stratton Dec 28 '18 at 17:43
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    \$\begingroup\$ The point is that with a PWM rate where the inductive reactance is non-trivial (this is far above the brush commutation rate) part of each on-time is spent building up current, so the power delivered is no longer in linear relation to the duty cycle, but rather less than a linear relation would predict. \$\endgroup\$ – Chris Stratton Dec 28 '18 at 17:51
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You would be operating your actuator beyond the maximum voltage specified by the manufacturer, so it is possible that you will damage the actuator. The damage may occur immediately or only after a significant time of operation.

Without more information we can't know why the manufacturer said that 15 V is the maximum voltage. If the only real concern is average power then you could use PWM and a low duty cycle. But we don't know if average power is the true limiting factor. There may be some component in the actuator that will fail if 22 V is applied for a millisecond. Some people will be happy to guess or provide an opinion, but the truth is that we cannot know.

Ask yourself how far you are willing to go with this argument. Do you think you could use 100 V at a very low duty cycle? 1000 V? How would you know where to draw the line between reliable and unreliable operation?

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  • \$\begingroup\$ A less serious answer to your question would be 3072 V, since I want to use an 8-bit timer on my microcontroller for the PWM, and 3072/256 is 12. A more serious answer would be, that as a hobbyist I stay away from Voltages above 60 Volts, but yes, I get your point. Without knowledge of L, C, or R of my motor I cannot know how much it really "sees" of the 22V and without internal knowledge I cannot know which components are sensitive to higher voltages. But I know that both L and C are not equal to zero, so I'll try and maybe need to buy another motor soon. \$\endgroup\$ – joelsa Dec 29 '18 at 22:30

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