Image forming apparatus with a controller to control an alternating transfer bias

An image forming apparatus, comprising:

• an image bearer configured to bear a toner image;
• a transfer device configured to contact the image bearer to form a transfer nip;
• a transfer bias power source configured to output an output voltage to transfer the toner image from the image bearer onto a recording medium in the transfer nip;
• and
• a controller configured to control the transfer bias power source,
• wherein the transfer bias power source is configured to output the output voltage that alternates between a transfer-directional voltage and a return-directional voltage, to transfer the toner image from the image bearer to the recording medium, the transfer-directional voltage having a polarity to transfer the toner image from the image bearer to the recording medium and the return-directional voltage having an opposite polarity to the polarity of the transfer-directional voltage, and
• wherein the controller is configured to reduce a frequency of the output voltage as a ratio of A to B decreases, where A is a time period of application of the return-directional voltage within one cycle of the output voltage, and B is a time period of the one cycle of the output voltage.