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I am trying to drive brushed DC motor rated at 7 volts in only one direction. The motor can sink high currents under load (I expect up to 20A). My genius plan is to run it from rectified mains electricity in an attempt to use very few converters/parts and thick wires. I know that the coil inside the motor in conjunction with a PWM voltage source and a flyback diode acts like a buck converter.

My questions are:

  1. Is this going to destroy the motor by applying a much, much higher voltage than the motor expects, even if it's in short and frequent pulses?

  2. If 220V will destroy the motor, how high can I go voltage wise?

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    \$\begingroup\$ Highly unlikely. At least not for extended periods of time. The field windings are most probably of the same lacquer as the higher voltage counterparts but the brushes will not like it. Without any smoothing, your source will need to take the full peak current too. \$\endgroup\$ – winny Apr 23 at 13:04
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    \$\begingroup\$ How high you can go to have a reliable and safe motor: use the rated voltage. \$\endgroup\$ – Huisman Apr 23 at 13:12
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    \$\begingroup\$ The motor becomes a live part and everything connected to the motor case. Just don't use 220Vdc. That would be my genius advise in an attempt to use very few real lives (you don't have 9, just 1). \$\endgroup\$ – Huisman Apr 23 at 13:16
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    \$\begingroup\$ I do not like "Is is possible.." questions. It is possible to connect the motor directly to 220 V DC but very likely the motor will be damaged. Likewise it is also possible to power the motor using short high voltage pulses but no one will guarantee that the motor will not be damaged by doing that. No one here will tell you that what you propose is a good idea because if (or when) your motor brakes you will say: but <insert name> said it was OK. We're engineers here, we use components at their proper operating voltage. \$\endgroup\$ – Bimpelrekkie Apr 23 at 13:17
  • \$\begingroup\$ @Bimpelrekkie I'm also a fan of using components at their rated values. However in this case I don't fully understand whether 7V is the maximum pulse peak or the maximum continuous rating. \$\endgroup\$ – tiftik Apr 23 at 13:49
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As you can see from the comments, 220 volts is too high for any serious consideration. That voltage is almost certain to arc over the commutator and likely also arc to the motor housing. It may also break down the winding insulation causing a turn-to-turn short or winding-to-frame short.

The 7-volt motor rating is the rated average voltage that would result in the rated motor speed. Exceeding the rated average voltage would result in exceeding the rated speed. That may be permissible to some extent. A 5 or 10 percent over-voltage tolerance is generally expected for most electrical products. A small brushed DC motor might tolerate 50 percent over-voltage, but bearing and commutator wear rates would be increased.

For typical PWM control, you might expect the peak voltage to be as much as 50% above the average voltage. Peak voltage that is twice the average voltage would probably be ok. Controlling the speed of the motor by changing the average voltage over a small percentage of the supply voltage is generally not a very attractive strategy even if the motor will withstand the peak voltage.

For a 7-volt motor, something like a 12 to 15 volt power supply would probably be a good choice for PWM control. Anything more than 50 volts would probably be asking for trouble.

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