Please tell me the relationship between the motor speed and pwm frequency. ...
There is NO
relationship, per se
. You are confusing a method of power
control with methods of motor speed
PWM, or Pulse-Width Modulation, is a means to control the delivery of an average
current to a low-impedance load from a low source-impedance voltage source. If such a voltage source were applied in a steady continuous fashion, instead of with PWM, excessive current would flow in the load. This current would then somehow need to be limited to avoid overheating in the load or overloading of the voltage source.
PWM is one solution to the problem, but it requires a switch in series with the load to periodically open very quickly and close very quickly, without a significant voltage drop across the closed
switch, or a significant current flow through the open
switch. Such a switch theoretically dissipates zero power, whether open or closed, while being able to control large currents from a relatively high voltage source. Does this sound like your average wall-mounted light switch? Well, except for how fast
it is turned on and off, that is exactly what it is.
current controlled by the switch is equal to the duty cycle of the PWM frequency multiplied by the load current that would flow if the switch remained closed all the time. So, if a motor winding has, say, one ohm of resistance and requires 10 amperes of current to reach full torque on its rotor, how could we provide power to this winding from a low-impedance 100 V source?
If the 100 V source were connected directly to the motor winding, then clearly 100 amperes of current would flow through the one ohm resistance of the winding, probably burning out the winding after just a few moments of duration. However, we can drop this down to an average current of 10 amperes, by rapidly switching a connection that applies the 100 V source to the winding for just 0.1 seconds, and then waits 0.9 seconds before repeating the connection. During that 0.1 seconds 100 A flows, but for 0.9 seconds 0 A flows. Therefore 100 A flows for 10% of the time, and the remaining 90% of the time zero current flows.
So the average current is 10 A, which is 10% of 100 A. The interval of 0.1 seconds, when an excessive current of 100 A flows, is short enough to prevent excessive heat build-up in the motor winding, while the interval of 0.9 seconds provides sufficient "cool down" time to dissipate the heat that builds up once each second.
... To increase the speed can i simply increase the frequency? How much duty cycle should be set? ...
Nowhere in the above description of PWM power control is frequency specifically mentioned as a parameter to be controlled. It is the duty cycle, the ratio of the ON time to the sum of ON time plus OFF time, that is important. You set the duty cycle to control the power delivered to the motor windings.
The PWM duty cycle can vary from zero (always off) to one hundred percent (always on) while the PWM frequency can be just about anything. However, if the frequency is too low (the period of the ON/OFF cycle is too long), then excessive heating or power supply overload will occur during the ON part of the cycle. Although the cooling time is extended at lower frequencies, it may not be long enough for sufficient cooling during the OFF part of the cycle. OTOH, if the frequency is too high, insufficient current may flow during the ON part of the cycle because of inductive reactance in the motor winding.
There are other considerations in picking an appropriate PWM frequency that are related to how the PWM duty cycle varies as a function of time. Some other forms of BLDC motors are the multi-pole stepper motor, multi-phase synchronous motor, and various forms of variable reluctance motors. With these motor types, a multi-phase sinusoidal current waveform is often desirable. It is created by sinusoidal modulation of the PWM duty cycle at a much lower frequency than the PWM frequency, usually with negative feedback from a current-sensing resistor or Hall-effect transducer in series with each motor winding.
It is the modulation frequency of the PWM duty cycle that determines how fast the motor runs. Note that the modulation cycle must be synchronous to the motor shaft revolutions, so some form of position sensing along with a fast-responding phase-lock loop is required to generate the PWM duty cycle modulation. You could implement this with an H-bridge driver and a bunch of external components, but there are integrated circuits available
that handle all the details. Just add a microprocessor, some MOSFET or IGFET switches, and appropriate feedback sensors for current and shaft position.
... Please help. It is for a BLDC motor.
If you are serious about BLDC motor control, please prepare to step off into the deep end of the pool
. There has been more than fifty years of research, development, and product refinement in this field. Don't try to re-invent the wheel here because most of the best wheels already exist and are very affordable. You might want to start by selecting a motor and then discovering what is available off-the-shelf to drive it. This Texas Instruments DRV8313
device may be a good place to begin.