Hi, I make PWM drives for DC motors. The PWM is about 20Khz. When I use these H-bridge PWM drives for motors with very low inductance such as flat pancake style DC motors the amplifiers do not work well and I have to add a couple of inline inductances to increase the impedance. I know that some drives do work with low inductance motors without requiring such inductances, does anyone know how they do this? Or why is it a problem for me?

Regards
Assaff

 

The high current ripple produced by 20kHz PWM current control for low inductance motor (except additional choke) maybe decreased with higher (up to 80kHz) frequency or lower current gain loop.

The linear amp (APEX) maybe uses for low power motors.

Leonid

 

Assaff,

here are the remedies along with their deficiencies:

1. PWM frequency increase - increases semiconductor switching loss and reduces maximum voltage due to switching dead time (shoot-through delay)

2. Serial smoothing choke - this additional component has associated (copper) loss and causes torque-speed area reduction (especially, for 3-phase motors)

3. Multi-level PWM - 3-level will reduce maximum PWM current ripple 2 times for the same PWM frequency (4-level - 3 times and so on); power stage topology / control become more complex, increased semiconductor count and conductivity loss

4. Split Phase Approach

This is described for 3-phase AC motor at
http://www.acpropulsion.com/Split_phase_motor_drives.pdf

and involves spliting power stage H-bridges and splitting / coupling serial chokes along with proper PWM phasing in splited H-bridges.

Some years ago I investigated into similar approach for DC motor. I found motor voltage waveforms similar to 3-level PWM. However, advantages of this approach over 2. and 3. are questionable and require careful comparison.

-Alex
[email protected]

 

Hi,

First you need to know what is the bus voltage assume U (volts dc).

Second select a form factor you want to have lets say FF=1.05. FF is define as sqrt [1 +(Irmsac/Iave)^2].
Assume f is switching frequency in Hz.

Your current has a DC component Iaver + an AC component Ip-p. This AC component can be approximate to a traingular profile (saw tooth type).Inductance is given by Lt = U*FF/[4*1.732*f*Irms*[sqrt(ff^2-1)]

If I assume f= 20000 hz, U = 130 Vdc, FF=1.05, Irms = 5 Amps (motor rms current) this gives you Lt = 0.00062 Henries or 0.62 millihenries. Lets assume againg you motor has 0.1 mH inductance so you need an additional inductance of: 0.61-0.1 = 0.51 mH.

A good way to do that is to use an inductance with 2 windings and have 0.26 millihenries on each leg. The advantage is to better protect the motor. If the amplifier do not see enough inductance you can have conduction of the 2 output transistor of the same side and lew up it up.