# Wave form of the phase currents

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#### elecLear78

##### Member
What will be the waveform of the phase currents of 3 phase synchronous motor, if the stator winding are 120 degree apart. I hope it should always be sine waves with 120 degrees apart. Am I correct? Please advise

##### Well-Known Member
Yes, the below is an example of 60 Hz. Three Phase.

The currents are IR1 Green, IR2 Blue and IR3 Red. The peak currents are 10 Amps in the example.

Ron

#### elecLear78

##### Member
Thank you for the reply. I am trying to drive a motor using H bridge. Could you please clarify me that if i am giving fixed duty cycle of say some 60% of around 1 khz frequency to stator legs H1,H2,H3 with low legs being made complementary still i get 120 phase shift currents. My exact question is i want to switch the legs as per the phase currents. Since the phase currents are 120 i get sinusoidal pwm. But how should i start means which pwm should i start with? Can i start with some x duty cycle and immediately start reading the phase currents? I am sorry if i confused you.

##### Well-Known Member
QUOTE="elecLear78, post: 1296756, member: 247756"]Thank you for the reply. I am trying to drive a motor using H bridge. Could you please clarify me that if i am giving fixed duty cycle of say some 60% of around 1 khz frequency to stator legs H1,H2,H3 with low legs being made complementary still i get 120 phase shift currents. My exact question is i want to switch the legs as per the phase currents. Since the phase currents are 120 i get sinusoidal pwm. But how should i start means which pwm should i start with? Can i start with some x duty cycle and immediately start reading the phase currents? I am sorry if i confused you.[/QUOTE]

You mention a H-Bridge and you mention PWM? Neither one of which would apply to an AC motor, let alone a 3 phase or poly phase AC motor. To reverse rotation on an AC motor two of the three phases are reversed, so for example rather than L1, L2 and L3 it would be come L1, L3 and L2 applied. Poly phase AC motor speed is generally controlled using a VFD (Variable Frequency Drive) or any of several Triac Speed Control circuits.

An H-Bridge circuit or module is designed to reverse the polarity on a DC motor to reverse direction and PWM (Pulse Width Modulation) is one method of controlling the Speed of a DC motor. You can't use an H-Bridge to control an AC motor. Additionally with any motor, AC or DC you simply can't just reverse rotation direction. The motor needs to be stopped and then rotation direction changed. When reversing a poly phase AC motor the phase currents will reverse of the reversed phases.

In the above illustration IR1, leads IR2 leads IR3. The phases are 120 degrees apart. If I reverse two phases I get this:

Note the phase rotation change.

Anyway PWM and H-Bridge are not used for any control of an AC motor.

Ron

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#### elecLear78

##### Member
Sorry i was referring to control of bldc motors using pwm and giving it to gate circuits the outputs of which will control the motor. I was worried about the commutation based on the feedback of phase currents on startup without hall sensors. Is it possible to drive to a known location that is 1 step etc.

##### Well-Known Member
A BLDC (Brushless DC Motor) can't be driven to a step. You can control motor direction with a H-Bridge and control the speed with PWM (Pulse Width Modulation). There is no "step". If you want a motor which is driven to location then use a "Stepper Motor".
For a BLDC Motor the L298 is a nice little prebuilt driver which will handle up to 2 Amps.
Motor Driver 2A Dual L298 H-Bridge
Now if you want a positional motor then you want a Stepper Motor.

Either motor can be driven by an Arduino uC (micro-controller).

Ron

#### elecLear78

##### Member
Ok. This is final clarification i am using only bldc motor. Based on some external command i start and stop the motor. I am clear on stopping the motor, i will switch off the pwm hence it is going to get stopped. Starting is the problem. I will be rotating the motor based on feedback of phase currents. I am assuming that if i stop i will not be having phase currents. Shall i give some fixed duty and start?

##### Well-Known Member
I am guessing this is what you have:
An Introduction to Brushless DC Motor Control
What exactly do you have for a motor driver?
You are correct in that when you stop there is no feedback or phase currents. Start and Stop should all be handled by your controller.

Ron

#### elecLear78

##### Member
Yes I have the same circuit. Rotor position is given by an encoder with mark flag and 1000 pulses for revolution. I do not know the exact rotor position until i start rotating it in such cases how should i proceed? Can you briefly tell me some algorithm to do it. I have a microcontroller micro chip PWMs interfaced with driver circuit of gates with outputs voltages going to the motor (3 terminals). The feedback of rotor position is through encoder. I am reading the phase currents through A/D converter.

##### Well-Known Member
Yes I have the same circuit. Rotor position is given by an encoder with mark flag and 1000 pulses for revolution. I do not know the exact rotor position until i start rotating it in such cases how should i proceed? Can you briefly tell me some algorithm to do it. I have a microcontroller micro chip PWMs interfaced with driver circuit of gates with outputs voltages going to the motor (3 terminals). The feedback of rotor position is through encoder. I am reading the phase currents through A/D converter.
You got me at this point. The units I have worked with in the past would stop rotation but the motor was on and would continue sending position from the Hall Effect sensors which also were decoded for direction information. Hopefully another member has a thought on your setup and how to do what you want to do. I simply do not have an answer or solution.

Anyone?

Ron

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